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Where Is Everybody_ The Fermi Paradox · Night Science Stories

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Speaker 1: Good evening and welcome to the Sleepy Scientist. I'm very
0:03
Speaker 1: glad you're here tonight. I'm always curious where these quiet
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0:13
Speaker 1: the world you're listening from. Perhaps you're already tucked into bed.
0:17
Speaker 1: Perhaps you've reached the hour where the rest of the
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Speaker 1: house has gone silent, and you've decided to stay awake
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Speaker 1: a little longer anyway. Wherever you happen to be, I
0:27
Speaker 1: hope you're comfortable, settled, and ready to leave the day
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Speaker 1: behind for a while. A small thing to ask, I know.
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Speaker 1: But given tonight's subject, a reply from somewhere out there
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Speaker 1: in the dark would be rather reassuring. And if you'd
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0:52
Speaker 1: close your eyes, and simply listen. If you enjoy these
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Speaker 1: slow journeys through science, Our like and a subscribe are
1:00
Speaker 1: always enormously appreciated. And if you've been joining me a
1:04
Speaker 1: while now, thank you. It's rather nice knowing that somewhere
1:08
Speaker 1: out there, people are drifting off while I quietly explain
1:12
Speaker 1: increasingly unreasonable things about the universe. Tonight, though, we're dealing
1:17
Speaker 1: with a question so simple it can be asked in
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Speaker 1: three words. Where is everybody? Because when you look at
1:24
Speaker 1: our galaxy... With its hundreds of billions of stars, its
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Speaker 1: countless planets and its billions upon billions of years of history,
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Speaker 1: it seems only reasonable to wonder whether somebody else ought
1:36
Speaker 1: to have turned up by now. Possibly quite a lot
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Speaker 1: of somebodies. And yet, as far as we can tell,
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Speaker 1: the galaxy is remarkably quiet. So tonight we'll wander slowly
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Speaker 1: through one of the strangest, unanswered questions in all of science.
1:53
Speaker 1: A physicist's offhand remark over lunch, which has troubled people
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Speaker 1: ever since. The staggering number of worlds that appear capable
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Speaker 1: of holding life. Decades of patient listening for a signal,
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Speaker 1: any signal at all. And a few of the more
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Speaker 1: unsettling explanations for the silence. I'll do my best not
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Speaker 1: to make any of them quite alarming enough to keep
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Speaker 1: you awake. So, shall we begin? If the universe has
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Speaker 1: had billions of years to produce intelligent life, then where
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Speaker 1: is everybody? In the summer of 1950, a small group
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Speaker 1: of physicists were walking across the grounds of the Los
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Speaker 1: Alamos Laboratory in New Mexico on their way to lunch.
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Speaker 1: Among them was Enrico Fermi, one of the most celebrated
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Speaker 1: physicists of the 20th century. Walking with him were Emil Konopinski,
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Speaker 1: Edward Teller and Herbert York. all scientists working in a
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Speaker 1: place that had already become inseparably connected with some of
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Speaker 1: the most consequential physics ever attempted. Their destination was Fuller Lodge,
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Speaker 1: a large wooden building that served as a social centre
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Speaker 1: and dining room for the Los Alamos community. And as
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Speaker 1: scientists sometimes do on the way to lunch, they were
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Speaker 1: discussing aliens. There had been a great deal of public
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Speaker 1: interest in flying saucers round this time, Reports of strange
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Speaker 1: objects in the sky had become newspaper material, and the
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Speaker 1: subject had inspired jokes as readily as serious speculation. A
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Speaker 1: cartoon in the New Yorker had even played with the
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Speaker 1: idea of extraterrestrial visitors being responsible for the mysterious disappearance
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Speaker 1: of rubbish bin lids. The conversation wandered naturally from flying
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Speaker 1: saucers into something rather more interesting. Could beings actually travel
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Speaker 1: between the stars? It was not an entirely ridiculous question,
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Speaker 1: even in 1950. Interstellar distances were enormous, but the known
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Speaker 1: laws of physics did not obviously forbid a sufficiently advanced
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Speaker 1: civilisation from crossing them. A journey to another star might
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Speaker 1: take decades, centuries, or considerably longer depending on the technology available,
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Speaker 1: but difficulty was not quite the same thing as impossibility.
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Speaker 1: Fermi was particularly good at taking questions like this and
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Speaker 1: stripping them down to their essentials. He had a remarkable
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Speaker 1: reputation for making rough calculations in his head. Rather than
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Speaker 1: demanding perfect information before attempting an answer, he would identify
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Speaker 1: the quantities that mattered most, estimate them, and see roughly
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Speaker 1: where the numbers led. Problems solved in this manner are
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Speaker 1: still often called Fermi Problems, The point is not to
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Speaker 1: produce an answer accurate to ten decimal places. The point
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Speaker 1: is to discover whether something is roughly the size of
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Speaker 1: a house, a mountain or a planet, before becoming terribly
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Speaker 1: concerned about the wallpaper. So the conversation continued. Eventually the
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Speaker 1: physicists arrived at Fuller Lodge, sat down for lunch and
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Speaker 1: moved on to other subjects. Then, apparently without warning, Fermi
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Speaker 1: asked a question. Where is everybody? The people around the
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Speaker 1: table laughed, not because the question was foolish, but because
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Speaker 1: they immediately knew what he meant. Fermi was asking where
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Speaker 1: the extraterrestrials were. The exact wording and details of that
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Speaker 1: afternoon come to us mainly through recollections recorded years later.
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Speaker 1: There was no stenographer sitting quietly beside the lunch table,
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Speaker 1: preserving every sentence for future historians. Decades afterwards, Eric Jones
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Speaker 1: collected accounts from several of the people who had been present,
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Speaker 1: allowing the episode to be reconstructed reasonably well while leaving
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Speaker 1: some details inevitably uncertain. But the essential moment survived. Fermi
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Speaker 1: had reduced an extraordinary problem to an almost absurdly ordinary question.
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Speaker 1: Where is everybody? There is an important distinction here. because
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Speaker 1: Fermi did not sit down that afternoon and formally construct
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Speaker 1: everything now known as the Fermi Paradox. He did not
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Speaker 1: publish a grand theory demonstrating that extraterrestrial civilisation must exist.
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Speaker 1: Nor did he produce a list containing every explanation for
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Speaker 1: why we might not have encountered one. Much of that
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Speaker 1: came later. Other researchers developed the argument by combining several observations.
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Speaker 1: The Milky Way contains an enormous number of stars. Many
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Speaker 1: stars are far older than the Sun. Planetary systems, as
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Speaker 1: astronomers would eventually discover, are extremely common. And if even
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Speaker 1: a small fraction of inhabited worlds eventually produced technological civilizations
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Speaker 1: capable of travelling between stars, some of those civilizations could
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Speaker 1: potentially have had an extraordinary amount of time to spread.
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Speaker 1: Not merely thousands of years. Not merely millions. In some cases, billions.
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Speaker 1: That changes the character of the question. Imagine that somewhere
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Speaker 1: in the Milky Way, a technological civilization appeared hundreds of
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Speaker 1: millions of years before human beings. It would not need
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Speaker 1: magical technology. It would not need faster-than-light travel. It would
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Speaker 1: not even need particularly fast spacecraft by the standards of
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Speaker 1: theoretical physics. If it could establish settlements around neighbouring stars,
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Speaker 1: and if those settlements could eventually establish others, then expansion
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Speaker 1: could proceed gradually through the galaxy. One journey at a time,
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Speaker 1: one star at a time, and the Milky Way has
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Speaker 1: existed for vastly longer than humanity has been capable of
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Speaker 1: building radios, rockets, or telescopes. So Fermi's little lunchtime question
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Speaker 1: begins to acquire some weight. It is one thing to
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Speaker 1: imagine that intelligent life exists somewhere extremely far away. It
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Speaker 1: is another to ask, what should happen if technological life
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Speaker 1: has appeared repeatedly throughout a galaxy that has been producing
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Speaker 1: stars and planets for billions of years? If that has happened,
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Speaker 1: why does our immediate neighbourhood appear so quiet? There are
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Speaker 1: no confirmed extraterrestrial spacecraft in the solar system. No unambiguous
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Speaker 1: artificial structures have been discovered on nearby worlds. No message
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Speaker 1: from another civilisation has been verified. No telescope has found
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Speaker 1: a piece of technology around another star that the scientific
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Speaker 1: community can confidently say was constructed by intelligent beings. This
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Speaker 1: does not mean that nothing is there. Silence is not
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Speaker 1: proof of emptiness. Humanity has examined only a tiny portion
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Speaker 1: of the galaxy, and our ability to recognise extraterrestrial technology
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Speaker 1: may be considerably poorer than we like to imagine. But
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Speaker 1: that is precisely why Fermi's question remains so effective. It
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Speaker 1: does not require certainty. It simply points towards attention. The
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Speaker 1: universe appears to offer an astonishing number of opportunities for life.
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Speaker 1: The Milky Way alone contains hundreds of billions of stars,
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Speaker 1: and we now know that planets are common around them.
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Speaker 1: Many of those systems existed long before the Sun had
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Speaker 1: even formed, yet here we are, living on a small
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Speaker 1: rocky world beside one fairly ordinary star, listening to a
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Speaker 1: galaxy that, so far, has offered no confirmed reply. Four
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Speaker 1: physicists went to lunch in New Mexico and found themselves
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Speaker 1: confronting a mystery spanning the entire Milky Way. More than
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Speaker 1: three quarters of a century later, the question is still
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Speaker 1: waiting on the table. Where is everybody? Enrico Fermi's question
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Speaker 1: becomes much stranger when we leave the lunch table behind
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Speaker 1: and begin to consider the scale of the place surrounding us.
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Speaker 1: The Milky Way is a vast spiral galaxy containing somewhere
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Speaker 1: in the region of 100 billion to several hundred billion stars.
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Speaker 1: The precise number is difficult to determine because we are
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Speaker 1: trying to count the contents of a galaxy while sitting
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Speaker 1: inside it, surrounded by dust, gas and and an inconvenient
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Speaker 1: number of stars. Its main stellar disk stretches for roughly 100,000
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Speaker 1: light-years from one side to the other, although the galaxy's
10:28
Speaker 1: more diffuse outer regions extend farther still. Even light, moving
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Speaker 1: at the fastest speed permitted by nature, takes around 100,000
10:38
Speaker 1: years to cross that distance, and scattered throughout this enormous
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Speaker 1: structure are stars of almost every imaginable age and character.
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Speaker 1: There are enormous blue stars that burn through their fuel
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Speaker 1: in only a few million years. There are small red
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Speaker 1: stars capable of shining for vastly longer than the Sun.
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Speaker 1: There are stellar remnants left behind by stars that died
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Speaker 1: before our own planetary system had even begun to form.
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Speaker 1: For much of human history, however, there was one crucial
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Speaker 1: thing we did not know. We had no idea whether
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Speaker 1: planets were common. The solar system might have been normal.
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Speaker 1: or it might have been some extraordinary accident. That uncertainty
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Speaker 1: mattered enormously. A galaxy containing hundreds of billions of stars
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Speaker 1: sounds promising if we are wondering about extraterrestrial life, but
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Speaker 1: not if almost none of those stars possess planets. Then
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Speaker 1: astronomers began finding them. The first confirmed planets orbiting ordinary
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Speaker 1: sun-like stars were discovered during the 1990s, and what followed
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Speaker 1: changed our picture of the Milky Way. Planet after planet appeared,
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Speaker 1: Some were enormous gas giants racing around their stars in
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Speaker 1: only a few days. Others followed highly elongated orbits. Some
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Speaker 1: systems contained planets packed together far more tightly than anything
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Speaker 1: in our own solar system. The details were often surprising.
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Speaker 1: The general lesson was simpler. Planets are not rare decorations
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Speaker 1: surrounding an unusual handful of stars. They appear to be
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Speaker 1: a normal consequence of the process that creates stars themselves.
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Speaker 1: When stars form from collapsing clouds of gas and dust,
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Speaker 1: material left over around the young star can flatten into
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Speaker 1: a rotating disk. Within these protoplanetary disks, dust grains collide
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Speaker 1: and accumulate, eventually building larger objects and, under suitable conditions,
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Speaker 1: entire planetary systems. Observations now suggest that the Milky Way
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Speaker 1: contains at least 100 billion planets, and quite possibly considerably more.
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Speaker 1: That does not mean 100 billion Earths. Far from it.
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Speaker 1: Many planets are places where human beings would last for
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Speaker 1: an impressively short amount of time, but once planets became common,
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Speaker 1: the arithmetic behind Fermi's question became hard to ignore. One
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Speaker 1: of the most important pieces of evidence came from the
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Speaker 1: National Aeronautics and Space Administration's Kepler Space Telescope. Kepler was
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Speaker 1: launched in 2009 with a wonderfully patient job, It stared
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Speaker 1: at a region containing more than 100,000 stars and watched
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Speaker 1: their brightness. If a planet happened to pass between one
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Speaker 1: of those stars and the telescope, it could block a
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Speaker 1: tiny fraction of the star's light. By measuring these repeated dips,
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Speaker 1: astronomers could infer the presence of planets that were otherwise
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Speaker 1: far too faint and distant to see directly. Kepler discovered
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Speaker 1: thousands of planetary candidates and confirmed planets, but its greatest
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Speaker 1: contribution was statistical. it allowed astronomers to begin estimating how
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Speaker 1: common different kinds of planets must be throughout the galaxy.
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Speaker 1: One analysis based on Kepler observations suggested that there may
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Speaker 1: be roughly 300 million rocky worlds in the habitable zones
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Speaker 1: of stars broadly similar to the Sun within the Milky Way.
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Speaker 1: Depending on exactly which types of stars and planets are included,
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Speaker 1: estimates can vary substantially. And the phrase habitable zone needs
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Speaker 1: a little care. A circumstellar habitable zone is usually defined
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Speaker 1: as the range of distances around a star where conditions
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Speaker 1: might allow liquid water to exist on a planet's surface.
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Speaker 1: Provided that planet has a suitable atmosphere and other favourable properties,
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Speaker 1: it is not a magical ring that automatically produces forests,
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Speaker 1: oceans and someone sitting beside a radio telescope. Venus provides
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Speaker 1: a useful warning. It lies close to the inner region
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Speaker 1: normally associated with the Sun's habitable zone yet its surface
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Speaker 1: is hot enough to melt lead beneath an atmosphere dominated
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Speaker 1: by carbon dioxide. Mars offers another. It sits near the
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Speaker 1: outer region of the Sun's conventional habitable zone, but today
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Speaker 1: it is cold, dry and wrapped in a very thin atmosphere.
14:54
Speaker 1: Distance from a star is important. It is not the
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Speaker 1: whole story. Planetary mass matters. Atmospheric composition matters. Geological activity
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Speaker 1: may matter. Magnetic environments, stellar radiation, water inventories, chemical cycles,
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Speaker 1: and the long-term stability of a climate may all influence
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Speaker 1: whether a world remains suitable for life. And even a
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Speaker 1: perfectly habitable planet is not necessarily inhabited. That distinction is essential.
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Speaker 1: Astronomy can increasingly estimate how many worlds might possess suitable conditions.
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Speaker 1: It cannot yet tell us how often those conditions actually
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Speaker 1: produce life. Still, the numbers are enormous, and there is
15:34
Speaker 1: another complication. Time. Earth formed around 4.5 billion years ago.
15:40
Speaker 1: The Milky Way is much older. Some of its oldest
15:43
Speaker 1: stars formed more than 13 billion years ago, and planetary
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Speaker 1: systems could have existed billions of years before the Sun appeared.
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Speaker 1: That means another inhabited world would not necessarily have had
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Speaker 1: only a slight head start on us. It might not
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Speaker 1: be a civilisation resembling humanity a few hundred years from now.
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Speaker 1: It could, in principle, have emerged a million years earlier,
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Speaker 1: or a hundred million. or perhaps several billion. Those intervals
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Speaker 1: are difficult to picture because human civilization occupies such a
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Speaker 1: tiny portion of geological time. Modern human beings have existed
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Speaker 1: for only a few hundred thousand years. Radio technology has
16:21
Speaker 1: existed for little more than a century. Our ability to
16:25
Speaker 1: launch machines beyond Earth is younger still. A civilization merely 10,000
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Speaker 1: years ahead of us would already possess a period of
16:32
Speaker 1: technological development far longer than our own recorded history. A
16:37
Speaker 1: civilisation a million years older becomes almost impossible to imagine responsibly.
16:43
Speaker 1: And that is where the large numbers become interesting. They
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Speaker 1: do not prove that extraterrestrial intelligence exists. Multiplying an enormous
16:52
Speaker 1: number of planets by a probability we do not know
16:55
Speaker 1: can still produce almost any answer, if the origin of
16:59
Speaker 1: life is fantastically unlikely. The galaxy could contain billions of
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Speaker 1: apparently suitable planets and still produce only one biosphere. If
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Speaker 1: complex life is rare, the same problem appears again. If
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Speaker 1: intelligence is rare, again. If technological civilisation is rarer still,
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Speaker 1: the enormous planetary population may tell us surprisingly little. But
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Speaker 1: each newly discovered world removes one possible explanation for the silence.
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Speaker 1: We can no longer easily say that perhaps planets themselves
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Speaker 1: are almost non-existent. The Milky Way seems to manufacture them enthusiastically.
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Speaker 1: There are planets around stars younger than the Sun, and
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Speaker 1: planets around stars far older. There are rocky worlds, giant worlds,
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Speaker 1: frozen worlds, scorched worlds, and probably countless types we have
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Speaker 1: not yet encountered. Somewhere among them may be environments that
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Speaker 1: resemble early Earth, perhaps many of them, perhaps almost none.
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Speaker 1: We do not yet know. And so Fermi's simple question
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Speaker 1: grows larger. Hundreds of billions of stars. At least hundreds
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Speaker 1: of billions of planets. Potentially hundreds of millions of rocky
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Speaker 1: worlds in regions where liquid water might be possible. Billions
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Speaker 1: of years during which life could, in principle, have begun
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Speaker 1: before humanity appeared. None of this demonstrates that somebody else
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Speaker 1: must be there. But it gives the silence of the
18:25
Speaker 1: galaxy a very different character. because the mystery is no
18:29
Speaker 1: longer whether the universe has provided enough places for life
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Speaker 1: to try, it seems to have provided chances almost everywhere.
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Speaker 1: The extraordinary number of planets in the Milky Way gives
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Speaker 1: us one half of the puzzle, the other half is time.
18:43
Speaker 1: The galaxy is not merely large, it is old. The
18:47
Speaker 1: Milky Way began forming more than 13 billion years ago,
18:51
Speaker 1: and although its structure has changed enormously since then, Generations
18:56
Speaker 1: of stars have been appearing and disappearing for almost the
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Speaker 1: entire history of the universe. Against that background, the age
19:03
Speaker 1: of human technology is barely noticeable. We have been capable
19:07
Speaker 1: of sending radio signals beyond Earth for little more than
19:10
Speaker 1: a century. We have been launching spacecraft for only a
19:13
Speaker 1: few decades more than half a century. Even the oldest
19:17
Speaker 1: surviving human cities represent almost nothing beside astronomical timescales. This
19:23
Speaker 1: difference becomes important when people imagine interstellar travel. It is
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Speaker 1: natural to picture a spacecraft attempting to cross the entire
19:30
Speaker 1: Milky Way in one heroic journey. That would indeed be daunting.
19:35
Speaker 1: The galaxy is roughly 100,000 light years across and even
19:38
Speaker 1: travelling at 10% of the speed of light, which is
19:41
Speaker 1: enormously faster than any spacecraft humanity has built. A direct
19:45
Speaker 1: crossing would take around one million years. But an expanded
19:50
Speaker 1: civilisation would not need to do that. It could move
19:53
Speaker 1: in stages. Imagine a civilisation reaching a nearby star ten
19:57
Speaker 1: light years away. There it establishes a permanent settlement, or
20:02
Speaker 1: perhaps simply builds autonomous machines capable of gathering resources and
20:06
Speaker 1: manufacturing new spacecraft. The new system eventually sends expeditions to
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Speaker 1: other nearby stars. Those new settlements do the same. The
20:15
Speaker 1: civilisation does not need one spacecraft to visit everything. It
20:20
Speaker 1: merely needs each new outpost to become another starting point.
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Speaker 1: The result is something like a slowly expanding wave spreading
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Speaker 1: through the stellar population. This distinction makes an enormous difference.
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Speaker 1: Suppose each stage takes thousands of years. A spacecraft spends
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Speaker 1: centuries travelling to another star. A settlement then takes centuries
20:44
Speaker 1: or millennia to become established. Eventually it launches new expeditions.
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Speaker 1: To human beings, this sounds painfully slow. To the Milky Way,
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Speaker 1: it barely counts as waiting. Even if the process pauses repeatedly,
21:01
Speaker 1: expansion can continue outward over very long periods. Under optimistic assumptions,
21:08
Speaker 1: models of this kind have suggested that a civilisation could
21:11
Speaker 1: spread through much of the Milky Way in only a
21:13
Speaker 1: few million years, or perhaps a few tens of millions.
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Speaker 1: That sounds enormous until it is compared with the age
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Speaker 1: of the galaxy. Ten million years is less than one-tenth
21:25
Speaker 1: of one percent of ten billion years. So if a
21:29
Speaker 1: technological civilisation had appeared a billion years before humanity …
21:34
Speaker 1: It could theoretically have had enough time for this entire
21:36
Speaker 1: process to occur many times over. That does not require
21:41
Speaker 1: spacecraft moving anywhere close to the speed of light. It
21:44
Speaker 1: does not require wormholes. It does not require new physics.
21:48
Speaker 1: It only requires persistence. This idea became a major part
21:53
Speaker 1: of later discussions of what is now called the Fermi Paradox.
21:57
Speaker 1: Astronomer Michael Hart argued during the 20th century that if
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Speaker 1: technologically advanced civilizations commonly arose and spread between stars, then
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Speaker 1: the absence of any obvious extraterrestrial presence in the solar
22:09
Speaker 1: system required explanation. Physicist Frank Tipler developed related arguments involving
22:16
Speaker 1: self-replicating machines. The idea was that an advanced civilization might
22:21
Speaker 1: not even need to send biological settlers everywhere. It could
22:25
Speaker 1: instead build robotic probes capable of travelling to another planetary system,
22:30
Speaker 1: using local materials to construct copies of themselves and then
22:34
Speaker 1: sending those copies onward. In principle, the process could continue
22:39
Speaker 1: again and again. These hypothetical machines are sometimes discussed alongside
22:43
Speaker 1: the broader concept of self-replicating probes, although nobody has ever
22:48
Speaker 1: built anything remotely capable of doing this autonomously across interstellar space.
22:55
Speaker 1: The thought experiment is useful. If even one civilisation became
22:59
Speaker 1: capable of launching machines that could reproduce and continue travelling,
23:03
Speaker 1: then the number of probes might grow rapidly. One becomes several.
23:08
Speaker 1: Several become dozens. Dozens become hundreds. And given enough time,
23:13
Speaker 1: the expanding network could theoretically reach an enormous number of stars.
23:19
Speaker 1: Some published models have produced galactic settlement times of only
23:22
Speaker 1: a few million years under favourable assumptions. Other models produce
23:27
Speaker 1: much longer timescales. The exact number is not the important part.
23:32
Speaker 1: The important part is that even very long settlement times
23:35
Speaker 1: can remain surprisingly short compared with the age of the
23:39
Speaker 1: Milky Way. And this creates the uncomfortable question. If technological
23:44
Speaker 1: civilisations have appeared many times before us, And if even
23:48
Speaker 1: a small fraction of them expand, where is the evidence?
23:52
Speaker 1: The solar system contains no confirmed alien outpost. No known
23:57
Speaker 1: object in the asteroid belt has turned out to be
23:59
Speaker 1: ancient extraterrestrial machinery. No probe has appeared in orbit around
24:05
Speaker 1: Earth and politely explained that it has been waiting here
24:08
Speaker 1: since before mammals existed. Nothing of the sort has been verified.
24:13
Speaker 1: But there is an important danger here. it is easy
24:16
Speaker 1: to turn a theoretical possibility into an expectation. The fact
24:21
Speaker 1: that galactic expansion may be physically possible does not mean
24:24
Speaker 1: every civilisation would attempt it. A species might have no
24:29
Speaker 1: interest in colonising other stars. It might prefer virtual environments,
24:33
Speaker 1: local exploration, or forms of existence that require very little territory.
24:39
Speaker 1: Interstellar travel might be technically possible, but economically pointless. Perhaps
24:45
Speaker 1: biological life finds long voyages intolerable. Perhaps machines cannot reproduce
24:51
Speaker 1: reliably enough without enormous support. Perhaps civilizations frequently change their priorities,
24:59
Speaker 1: long before they begin expanding across thousands of star systems.
25:03
Speaker 1: Even the idea of endless growth may reflect something very human.
25:08
Speaker 1: We know that some organisms and societies expand when resources
25:12
Speaker 1: allow it. That does not prove that every intelligent species
25:16
Speaker 1: in the galaxy would behave the same way. Technology matters too.
25:21
Speaker 1: A spacecraft travelling at a meaningful fraction of the speed
25:24
Speaker 1: of light requires enormous amounts of energy. At high speeds,
25:28
Speaker 1: tiny grains of interstellar dust become dangerous projectiles. Radiation becomes
25:34
Speaker 1: a serious problem. Machines must continue functioning for journeys that
25:38
Speaker 1: could last centuries. Settlements would need to survive without assistance
25:44
Speaker 1: from their homeworld, and if self-replicating probes are involved, they
25:48
Speaker 1: must somehow extract raw materials, manufacture extraordinarily complicated components, repair themselves,
25:57
Speaker 1: and avoid accumulating errors over generation after generation. None of
26:02
Speaker 1: this is simple. The Fermi Paradox, therefore, does not say
26:06
Speaker 1: that aliens definitely should have colonised the Milky Way. Nor
26:10
Speaker 1: does it prove that extraterrestrial life cannot exist because we
26:14
Speaker 1: do not see spaceships outside the window. It is better
26:18
Speaker 1: understood as a tension between several ideas that each appear plausible.
26:22
Speaker 1: Planets are common. The galaxy is extremely old. Technological development
26:28
Speaker 1: is possible because humanity has demonstrated at least one example.
26:33
Speaker 1: Interstellar travel does not obviously violate known physics. Expansion could,
26:39
Speaker 1: in principle... proceed gradually, and the timescales required may be
26:43
Speaker 1: much shorter than the age of the Milky Way. Yet
26:47
Speaker 1: there is still no confirmed evidence that another technological civilisation
26:51
Speaker 1: has ever reached us, contacted us, or left something unmistakable behind.
26:57
Speaker 1: That is the paradox, not a contradiction in the laws
27:00
Speaker 1: of physics, not proof that anyone must exist, simply a
27:04
Speaker 1: mismatch between what some reasonable assumptions appear to suggest and
27:09
Speaker 1: and what we actually observe. And the more time we
27:12
Speaker 1: give the galaxy, the stranger that mismatch begins to feel.
27:16
Speaker 1: Because the Milky Way has not merely had enough time
27:19
Speaker 1: for one civilization to become older than humanity. It has
27:23
Speaker 1: potentially had enough time for entire technological histories to begin, flourish, spread, disappear,
27:32
Speaker 1: and be forgotten before the Earth itself was fully formed.
27:36
Speaker 1: If Enrico Fermi had reduced the mystery of extraterrestrial intelligence
27:41
Speaker 1: to one remarkably simple question, Frank Drake tried to do
27:45
Speaker 1: something almost opposite. He took that question apart. In 1961,
27:51
Speaker 1: a small group of scientists gathered at the National Radio
27:55
Speaker 1: Astronomy Observatory in Greenbank, West Virginia. Their purpose was unusual
28:00
Speaker 1: for the time. They wanted to discuss whether it was
28:04
Speaker 1: possible to search scientifically for intelligent life beyond Earth. Not
28:08
Speaker 1: simply to speculate about it. Not to write stories about
28:12
Speaker 1: visitors from other worlds. But to ask whether astronomy and
28:16
Speaker 1: radio technology had reached the point where another civilisation might
28:20
Speaker 1: actually be detectable. The meeting was organised by the American
28:24
Speaker 1: radio astronomer Frank Drake. Drake had already carried out one
28:28
Speaker 1: of the first modern searches for extraterrestrial radio signals. He
28:33
Speaker 1: understood that before researchers could decide where to look, how
28:36
Speaker 1: to listen, or what sort of signal might be meaningful,
28:40
Speaker 1: they first needed some sense of how many detectable civilizations
28:44
Speaker 1: might reasonably exist. The difficulty was obvious. Nobody knew. So
28:49
Speaker 1: Drake did not try to guess one enormous number all
28:52
Speaker 1: at once. Instead, he divided the problem into smaller pieces.
28:57
Speaker 1: That became the Drake Equation. It is sometimes presented as
29:01
Speaker 1: though it were a machine into which scientists can feed
29:04
Speaker 1: a handful of measurements and receive the number of alien
29:07
Speaker 1: civilizations in the Milky Way. It is not. Its real
29:12
Speaker 1: value is much more subtle. Drake created the equation as
29:16
Speaker 1: a way of organizing the Green Bank meeting. Each term
29:20
Speaker 1: represented a separate scientific question that the researchers could discuss.
29:24
Speaker 1: The quantity at the end was the number of technological
29:27
Speaker 1: civilizations in the Milky Way. whose activity might, in principle,
29:32
Speaker 1: be detectable. To reach that number, Drake began with stars.
29:37
Speaker 1: How rapidly do suitable stars form in the galaxy? Then
29:41
Speaker 1: comes the question of planets. What fraction of those stars
29:45
Speaker 1: possess planetary systems? Once planets exist, how many of them
29:50
Speaker 1: provide environments that could potentially support life? Then the equation
29:55
Speaker 1: leaves astronomy and enters much less certain territory. On how
29:59
Speaker 1: many suitable worlds does life actually begin? Of those inhabited worlds,
30:05
Speaker 1: how often does intelligence evolve? Of those intelligent species, how
30:10
Speaker 1: many develop technologies that produce signals or other evidence that
30:14
Speaker 1: can be detected across interstellar distances? And finally comes a
30:19
Speaker 1: term that may be among the most important of all.
30:22
Speaker 1: How long does that detectable phase last? Written conceptually, the
30:27
Speaker 1: reasoning is simple.
30:53
Speaker 1: There is nothing mysterious hiding in
30:58
Speaker 1: the mathematics. The mystery is in the numbers. Some parts
31:02
Speaker 1: of the equation have become far less uncertain since 1961.
31:07
Speaker 1: Astronomers now know that planets are common. Thousands of worlds
31:11
Speaker 1: have been confirmed around other stars, and statistical studies indicate
31:16
Speaker 1: that planetary systems are a normal feature of the Milky Way.
31:19
Speaker 1: That means the fraction of stars with planets is no
31:22
Speaker 1: longer the almost completely unknown quantity it was when Drake
31:26
Speaker 1: first wrote his equation. The same is partly true of
31:29
Speaker 1: the number of potentially habitable planets. Missions such as the
31:34
Speaker 1: National Aeronautics and Space Administration's Kepler Space Telescope have allowed
31:39
Speaker 1: astronomers to estimate how frequently rocky planets occur at distances
31:43
Speaker 1: where liquid water might be possible. Those estimates are still uncertain.
31:49
Speaker 1: but they're based on real observations. Then the confidence falls
31:53
Speaker 1: away rather quickly. We do not know the probability that
31:56
Speaker 1: life begins on a suitable planet. We have exactly one
32:00
Speaker 1: confirmed inhabited world from which to judge it. Earth. Life
32:05
Speaker 1: appears to have emerged relatively early in Earth's history, which
32:08
Speaker 1: might suggest that the process is not terribly difficult under
32:12
Speaker 1: suitable conditions, or it might tell us very little. We
32:16
Speaker 1: are observing the one planet on which the process definitely worked,
32:20
Speaker 1: because if it had not worked here, nobody would be
32:23
Speaker 1: around to discuss the matter. The uncertainty becomes even greater
32:27
Speaker 1: when we ask how frequently life produces intelligence. Life existed
32:32
Speaker 1: on Earth for billions of years before anything appeared that
32:35
Speaker 1: could build a radio telescope. For most of this planet's
32:39
Speaker 1: biological history, there were no cities, spacecraft, television transmitters, Or
32:44
Speaker 1: physicists wondering where everybody else was. Even complex multicellular life
32:49
Speaker 1: is comparatively recent. Technological civilisation is extraordinarily recent. That creates
32:56
Speaker 1: another unknown. Perhaps intelligence capable of advanced technology is an
33:01
Speaker 1: almost inevitable consequence of evolution given enough time. Perhaps it
33:06
Speaker 1: is a rare accident. With one example, there is no
33:09
Speaker 1: reliable way to tell. And intelligence alone is not enough
33:13
Speaker 1: for the Drake equation. A species must also become detectable.
33:18
Speaker 1: That could mean transmitting powerful radio signals. It could mean
33:21
Speaker 1: building enormous structures. It could mean altering a planet's atmosphere
33:25
Speaker 1: through industry. It could mean using technology in some way
33:29
Speaker 1: that produces a recognisable signature over astronomical distances. A civilisation
33:35
Speaker 1: could be highly intelligent and still remain practically invisible to us.
33:40
Speaker 1: Then comes the final question. How long does it remain detectable?
33:45
Speaker 1: Suppose technological civilizations appear frequently, but their detectable phases last
33:51
Speaker 1: only a few centuries. The chance that two nearby civilizations
33:55
Speaker 1: exist at the same moment becomes much smaller. Suppose instead
33:59
Speaker 1: that advanced civilizations survive for millions of years, then even
34:03
Speaker 1: a relatively low rate of emergence could leave the galaxy
34:06
Speaker 1: containing many of them at once. This is why different
34:10
Speaker 1: choices for the Drake equation can produce wildly different answers.
34:15
Speaker 1: Optimistic assumptions can suggest a galaxy containing large numbers of
34:20
Speaker 1: detectable civilizations. Pessimistic assumptions can reduce the answer to almost none.
34:26
Speaker 1: The equation does not settle the argument because too many
34:29
Speaker 1: of its terms remain unknown. But that is not a failure.
34:33
Speaker 1: In a sense, identifying exactly what you do not know
34:37
Speaker 1: is one of the most useful things an equation can do.
34:40
Speaker 1: It separates the problem into regions of confidence and ignorance.
34:46
Speaker 1: Astronomy can increasingly tell us how many stars exist, how
34:50
Speaker 1: frequently planets form, and how common some types of planet
34:54
Speaker 1: may be. Biology cannot yet tell us how often life begins.
34:59
Speaker 1: Evolutionary science cannot tell us how frequently intelligence comparable to
35:03
Speaker 1: our own develops elsewhere. and no branch of science can
35:07
Speaker 1: currently provide a reliable average lifetime for technological civilizations when
35:13
Speaker 1: humanity has observed only one and ours is still in progress.
35:18
Speaker 1: Frank Drake therefore turned Fermi's question into something more structured.
35:22
Speaker 1: Instead of saying that the Milky Way contains an enormous
35:26
Speaker 1: number of stars and wondering why nobody has appeared, the
35:29
Speaker 1: Drake equation asks us to follow every step between a
35:33
Speaker 1: newly formed star and a civilisation capable of being noticed.
35:38
Speaker 1: A star must exist. Planets must form. At least one
35:43
Speaker 1: world must be suitable. Life must begin. Intelligence must develop.
35:49
Speaker 1: Technology must become detectable, and that detectable period must last
35:54
Speaker 1: long enough to overlap with somebody else who happens to
35:57
Speaker 1: be listening. Every stage narrows the possibilities. Some may be easy,
36:02
Speaker 1: Some may be extraordinarily difficult. We simply do not yet
36:06
Speaker 1: know which. And that is perhaps the most revealing thing
36:10
Speaker 1: the Drake equation tells us. Not how many civilizations are
36:14
Speaker 1: out there, but exactly how much of the path between
36:17
Speaker 1: an ordinary star and another voice in the darkness remains unknown.
36:21
Speaker 1: Some parts of Frank Drake's equation remain almost completely mysterious.
36:26
Speaker 1: Others have changed enormously since 1961. When Drake first wrote
36:31
Speaker 1: down his framework, astronomers did not know whether planets were
36:34
Speaker 1: common around other stars. They did not know how frequently
36:38
Speaker 1: rocky worlds formed, how often planetary systems resembled our own,
36:44
Speaker 1: or how many stars might possess environments remotely suitable for life. Today,
36:50
Speaker 1: those questions are no longer entirely speculative. The first term
36:54
Speaker 1: in the Drake equation concerns the formation of stars. The
36:58
Speaker 1: Milky Way is still producing new ones. Deep inside cold
37:02
Speaker 1: clouds of molecular gas, gravity slowly draws material together until
37:07
Speaker 1: dense regions collapse. As they contract, their centres become hotter
37:11
Speaker 1: and denser, eventually reaching conditions where nuclear fusion can begin.
37:16
Speaker 1: A new star is born. Across the Milky Way, the
37:20
Speaker 1: total amount of gas converted into stars each year is
37:23
Speaker 1: roughly equivalent to the mass of a few suns. That
37:26
Speaker 1: may not sound particularly dramatic for an entire galaxy, but
37:30
Speaker 1: star formation has been continuing for billions of years. And
37:34
Speaker 1: not every newly formed star offers the same possibilities. Massive
37:39
Speaker 1: stars can produce enormous amounts of light and heat, but
37:42
Speaker 1: they live quickly. The most massive stars may survive for
37:46
Speaker 1: only a few million years before exhausting their nuclear fuel
37:50
Speaker 1: and ending their lives in violent explosions. That is not
37:54
Speaker 1: necessarily enough time for complex biology to develop around them,
37:58
Speaker 1: at least if Earth is any guide. Smaller stars are
38:02
Speaker 1: very different. Red dwarf stars are by far the most
38:05
Speaker 1: common type of star in the Milky Way. They are cooler,
38:09
Speaker 1: dimmer and much less massive than the Sun. Because they
38:13
Speaker 1: consume their nuclear fuel so slowly, the smallest red dwarfs
38:18
Speaker 1: can remain active for extraordinarily long periods. potentially trillions of years.
38:24
Speaker 1: On paper, that sounds rather promising. A planet orbiting such
38:28
Speaker 1: a star could have an almost absurd amount of time
38:31
Speaker 1: available for biology. But there are complications. Many red dwarfs
38:37
Speaker 1: are magnetically active, particularly when they are young. They can
38:41
Speaker 1: produce powerful stellar flares and bursts of high-energy radiation capable
38:46
Speaker 1: of affecting the atmospheres of nearby planets. And because red
38:50
Speaker 1: dwarfs are faint, A planet must orbit relatively close to
38:54
Speaker 1: receive enough energy for liquid water to be possible. That
38:58
Speaker 1: close orbit can introduce additional effects, including strong tidal interactions.
39:04
Speaker 1: So even something as apparently simple as choosing the right
39:07
Speaker 1: kind of star becomes complicated remarkably quickly. Then comes one
39:11
Speaker 1: of the parts of the Drake equation that has changed
39:14
Speaker 1: most dramatically. How many stars have planets? For much of
39:19
Speaker 1: the 20th century, nobody knew. the solar system might have
39:22
Speaker 1: been typical, or it might have been an extraordinary exception.
39:27
Speaker 1: That uncertainty began to disappear as astronomers discovered planets orbiting
39:32
Speaker 1: other stars. The National Aeronautics and Space Administration's Kepler mission
39:37
Speaker 1: transformed the subject. Kepler monitored the brightness of enormous numbers
39:42
Speaker 1: of stars, looking for the tiny repeated dimming caused when
39:46
Speaker 1: a planet crossed in front of its star from our
39:48
Speaker 1: point of view. One planet produced a small dip, then
39:51
Speaker 1: another appeared, then another. Thousands of worlds were eventually identified.
39:57
Speaker 1: And from those detections, astronomers could estimate how many planets
40:01
Speaker 1: must exist beyond the ones whose orbits happen to line
40:05
Speaker 1: up conveniently with Earth. The conclusion was striking. Planets are
40:10
Speaker 1: extremely common. They may be an ordinary consequence of star formation.
40:15
Speaker 1: The Milky Way probably contains at least as many planets
40:19
Speaker 1: as it contains stars, meaning there may be hundreds of
40:22
Speaker 1: billions of worlds scattered throughout the galaxy. Some stars have
40:26
Speaker 1: one known planet, others possess several, and because our detection
40:30
Speaker 1: methods favour certain sizes and orbital arrangements, many smaller or
40:35
Speaker 1: more distant planets almost certainly remain undiscovered. So one of
40:40
Speaker 1: the great uncertainties facing Frank Drake in 1961 has changed dramatically.
40:46
Speaker 1: The question is no longer whether planetary systems are rare.
40:50
Speaker 1: They do not appear to be. The harder question is
40:53
Speaker 1: how many of those planets might actually be suitable for life.
40:57
Speaker 1: This is where astronomers often discuss potentially habitable worlds. A
41:02
Speaker 1: planet generally needs to be rocky if we are imagining
41:05
Speaker 1: something broadly resembling Earth. It needs an appropriate temperature range.
41:10
Speaker 1: Liquid water is usually considered especially important because every known
41:14
Speaker 1: form of life on Earth depends upon it. But a
41:18
Speaker 1: planet's distance from its star is only the beginning. A
41:21
Speaker 1: world can sit within the conventional circumstellar habitable zone and
41:26
Speaker 1: still be profoundly hostile. Its atmosphere might be too thick
41:30
Speaker 1: or too thin. It might contain almost no water. It
41:35
Speaker 1: could be subjected to intense radiation. Its climate might swing
41:38
Speaker 1: between extremes. Its mass might be so low that it
41:42
Speaker 1: struggles to retain an atmosphere for billions of years. Or
41:46
Speaker 1: it could acquire an atmosphere so dense that a runaway
41:49
Speaker 1: greenhouse effect transforms the surface into something more closely resembling
41:53
Speaker 1: Venus than Earth. The habitable zone is therefore better understood
41:57
Speaker 1: as a useful first filter. It identifies an orbital region
42:01
Speaker 1: where surface liquid water might be possible under suitable conditions.
42:06
Speaker 1: It does not identify planets where life definitely exists. Even so,
42:11
Speaker 1: the numbers remain impressive. Using observations from Kepler and estimates
42:15
Speaker 1: of how frequently Earth-sized planets occur around stars broadly similar
42:19
Speaker 1: to the Sun, researchers have estimated that the Milky Way
42:23
Speaker 1: could contain roughly 300 million potentially habitable rocky worlds around
42:28
Speaker 1: those kinds of stars alone. The exact figure depends on
42:32
Speaker 1: how the boundaries are defined. Change the definition of an
42:35
Speaker 1: Earth-like planet, alter the assumed range of stellar temperatures, or
42:39
Speaker 1: include additional types of stars and the estimate changes. That
42:44
Speaker 1: uncertainty matters, but so does the scale. Even if the
42:48
Speaker 1: true number were substantially lower, there could still be an
42:51
Speaker 1: enormous number of rocky worlds occupying regions where liquid water
42:55
Speaker 1: is physically possible, and the discoveries have revealed something else.
42:59
Speaker 1: Planetary systems are wonderfully untidy. Before the first extrasolar planets
43:04
Speaker 1: were found, astronomers naturally had one well-studied example. Our own.
43:10
Speaker 1: It was tempting to imagine other planetary systems being arranged
43:14
Speaker 1: in broadly similar ways, with small rocky planets close to
43:17
Speaker 1: their stars and giant planets farther out. Nature did not
43:22
Speaker 1: appear particularly interested in following that template. Astronomers discovered giant
43:27
Speaker 1: planets skimming extremely close to their stars. They found planets
43:31
Speaker 1: on highly elongated orbits. They found worlds larger than Earth,
43:36
Speaker 1: but smaller than Neptune. a common class with no direct
43:40
Speaker 1: equivalent in our own solar system. They discovered systems with
43:44
Speaker 1: several planets packed into regions smaller than the orbit of Mercury.
43:49
Speaker 1: So the search for extraterrestrial life has been strengthened and
43:52
Speaker 1: complicated at the same time. Planets turned out to be
43:56
Speaker 1: far more common than once feared, but planetary systems also
44:00
Speaker 1: turned out to be far more varied. This matters for
44:03
Speaker 1: the Fermi Paradox. One possible explanation for the silence of
44:08
Speaker 1: the galaxy might have been that suitable places for life
44:11
Speaker 1: almost never form. Modern astronomy has made the simplest version
44:15
Speaker 1: of that argument harder to maintain. Stars are abundant. Planets
44:20
Speaker 1: are abundant. Rocky planets are not unique to the solar system.
44:24
Speaker 1: Potentially temperate worlds may number in the hundreds of millions.
44:28
Speaker 1: The astronomical side of Frank Drake's equation has therefore become
44:32
Speaker 1: much less mysterious. Not solved, but measurable. Telescopes can count stars,
44:38
Speaker 1: they can detect planets, they can estimate planetary sizes, orbital periods,
44:44
Speaker 1: stellar temperatures, and sometimes even begin probing atmospheres. The problem
44:49
Speaker 1: is that eventually the equation reaches a boundary. Astronomy can
44:53
Speaker 1: tell us that a rocky planet exists. It can tell
44:57
Speaker 1: us how much energy that planet receives from its star.
45:00
Speaker 1: In favourable cases, it may eventually tell us something about
45:03
Speaker 1: its atmosphere. But there is one transformation our telescopes cannot
45:08
Speaker 1: yet assign a reliable probability to. Somewhere on a lifeless world,
45:13
Speaker 1: ordinary chemistry must become biology. And for that step, the
45:17
Speaker 1: Milky Way has so far given us only one confirmed example.
45:21
Speaker 1: The astronomical parts of Frank Drake's equation have become steadily clearer.
45:25
Speaker 1: We can count stars. We can estimate how often planets form.
45:30
Speaker 1: We can identify worlds that appear rocky, measure their orbits.
45:34
Speaker 1: and sometimes even begin to examine the gases surrounding them.
45:38
Speaker 1: And then, rather suddenly, we reach the edge of what
45:40
Speaker 1: we know. Because finding a planet that could support life
45:44
Speaker 1: is not the same thing as finding a planet where
45:47
Speaker 1: life actually begins. For that question, humanity possesses one confirmed example. Earth.
45:56
Speaker 1: Every organism we have ever discovered belongs to the same
45:58
Speaker 1: terrestrial biosphere. From enormous blue whales to bacteria living deep
46:03
Speaker 1: beneath the ground, all known life shares basic features of
46:06
Speaker 1: chemistry and ancestry. We do not yet have a second example.
46:11
Speaker 1: That makes one of the most important terms in the
46:13
Speaker 1: Drake equation extraordinarily difficult to estimate. How often does life begin?
46:19
Speaker 1: The origin of life from non-living chemistry is usually discussed
46:23
Speaker 1: under the term a biogenesis. Somewhere in Earth's distant past,
46:27
Speaker 1: chemical systems that were not alive eventually gave rise to
46:31
Speaker 1: systems capable of reproduction, variation and evolution. Exactly how that
46:37
Speaker 1: happened remains unresolved. Scientists investigate possibilities involving organic molecules, mineral surfaces,
46:45
Speaker 1: hydrothermal environments, shallow pools, chemical cycles and molecules capable of
46:51
Speaker 1: storing information or assisting their own replication. But there is
46:56
Speaker 1: still no complete historical record of the transition. The earliest
47:00
Speaker 1: Earth has been transformed repeatedly by geology, erosion, impacts and
47:05
Speaker 1: plate tectonics. Whatever delicate chemical trail led towards the first
47:10
Speaker 1: living systems has largely disappeared. We do know that life
47:14
Speaker 1: seems to have appeared relatively early in Earth's history. Earth
47:18
Speaker 1: formed roughly 4.5 billion years ago, and evidence indicates that
47:22
Speaker 1: life was present more than 3.5 billion years ago, with
47:26
Speaker 1: some proposed evidence suggesting an even earlier beginning. At first glance,
47:31
Speaker 1: that might sound encouraging. If life appeared fairly quickly once
47:35
Speaker 1: Earth became suitable, perhaps biology tends to emerge naturally whenever
47:39
Speaker 1: the chemistry is right. But there is another interpretation. Perhaps
47:44
Speaker 1: the origin of life is extraordinarily unlikely and it simply
47:48
Speaker 1: happened unusually early here. We cannot easily distinguish between those
47:53
Speaker 1: possibilities because every observer capable of asking the question must
47:58
Speaker 1: already live on a planet where life succeeded. If a
48:01
Speaker 1: biogenesis normally takes a hundred billion years, we would never
48:05
Speaker 1: find ourselves on an ordinary planet waiting for it to happen.
48:10
Speaker 1: We can only observe a world where the necessary sequence
48:12
Speaker 1: occurred early enough for intelligent observers to appear before conditions
48:16
Speaker 1: changed too drastically. One successful example, therefore, tells us surprisingly
48:22
Speaker 1: little about the underlying probability. And even if simple life
48:26
Speaker 1: turns out to be common, another enormous uncertainty follows. Simple
48:31
Speaker 1: life is not the same thing as complex life. For
48:35
Speaker 1: a tremendous portion of Earth's biological history, life consisted entirely
48:40
Speaker 1: of microscopic organisms. The earliest cells appeared billions of years
48:44
Speaker 1: before anything resembling an animal, tree, bird or human being.
48:50
Speaker 1: One particularly important transition was the appearance of eukaryotic cells.
48:55
Speaker 1: These are the complex cells found in animals, plants, fungi
48:59
Speaker 1: and many single-celled organisms. They contain highly organised internal structures,
49:04
Speaker 1: including a nucleus, and their evolutionary history involved major biological innovations.
49:10
Speaker 1: Complex multicellular organisms appeared much later still. For most of
49:15
Speaker 1: the time, life has existed on Earth. There has been
49:18
Speaker 1: nothing here capable of looking at the night sky and
49:21
Speaker 1: wondering whether anybody else was doing the same thing. Then
49:24
Speaker 1: there is intelligence. Even that word requires care. Many animals
49:29
Speaker 1: demonstrate impressive cognitive abilities. Crows solve complicated problems. Octopuses manipulate
49:36
Speaker 1: objects and navigate their environments. Chimpanzees use tools. Dolphins communicate
49:42
Speaker 1: socially in sophisticated ways. Intelligence is not a single switch
49:47
Speaker 1: that evolution simply turns on. The particular form of intelligence
49:51
Speaker 1: that produced technological civilization on Earth required an unusual combination
49:56
Speaker 1: of abilities. Human beings possess complex language. We transmit enormous
50:02
Speaker 1: amounts of information between generations. We accumulate knowledge rather than
50:07
Speaker 1: forcing every individual to rediscover everything from the beginning. We
50:11
Speaker 1: have hands capable of extremely precise manipulation. We cooperate in
50:16
Speaker 1: groups containing people who are not closely related. We control
50:20
Speaker 1: external sources of energy. We manufacture tools that help us
50:25
Speaker 1: manufacture better tools. And eventually, after hundreds of thousands of
50:29
Speaker 1: years of anatomically modern humans existing on Earth, Some of
50:33
Speaker 1: those tools became radio transmitters and telescopes. There is no
50:37
Speaker 1: known law of evolution stating that this outcome must occur.
50:41
Speaker 1: Natural selection does not aim towards radio astronomy. It favours
50:46
Speaker 1: traits that improve reproductive success within particular environments. For the dinosaurs,
50:52
Speaker 1: extraordinary success did not require building satellites. They dominated terrestrial
50:58
Speaker 1: ecosystems for more than 150 million years without producing anything
51:03
Speaker 1: that we would recognise as technological civilisation. So even if
51:08
Speaker 1: life begins frequently, and even if complex life is common,
51:12
Speaker 1: the fraction of inhabited worlds that eventually produce a technological
51:16
Speaker 1: species remains unknown. Then another filter appears. A technological species
51:22
Speaker 1: does not automatically become detectable across interstellar distances. The Drake
51:27
Speaker 1: equation includes the fraction of intelligent civilizations that develop technologies
51:32
Speaker 1: capable of producing detectable evidence. Traditionally, much attention has focused
51:38
Speaker 1: on radio transmissions. Human civilization has been producing radio waves
51:43
Speaker 1: for only a tiny fraction of its existence. Some powerful
51:46
Speaker 1: transmissions can escape into space, although much ordinary communication is
51:51
Speaker 1: far more difficult to detect over interstellar distances. than popular
51:55
Speaker 1: descriptions sometimes suggest. Other civilisations might never follow the same
52:00
Speaker 1: technological path. They could use communication methods that produce almost
52:04
Speaker 1: no radiation we could recognise. They might abandon strong radio
52:09
Speaker 1: broadcasting after a brief period. They might communicate through tightly
52:13
Speaker 1: directed signals that never happen to point towards Earth. Or
52:17
Speaker 1: their most obvious evidence might not be communication at all.
52:21
Speaker 1: Astronomers increasingly use the broader term technosignature for observable evidence
52:26
Speaker 1: produced by technology. A technosignature might be a deliberately transmitted
52:31
Speaker 1: radio signal. It might be an artificial pulse of light.
52:35
Speaker 1: It could, in principle, involve unusual atmospheric chemicals, enormous engineered structures,
52:43
Speaker 1: excess heat, or some other phenomenon that natural processes struggle
52:47
Speaker 1: to explain. But once again, We do not know how
52:50
Speaker 1: common any of these things are. And then Frank Drake's
52:54
Speaker 1: equation reaches its final term, longevity. How long does a
53:00
Speaker 1: detectable civilisation remain detectable? This number can transform the answer.
53:06
Speaker 1: Imagine that technological civilisations appear quite frequently throughout the Milky Way,
53:11
Speaker 1: but remain detectable for only 100 years. The galaxy might
53:16
Speaker 1: produce many of them across its history while very few
53:19
Speaker 1: overlap in time. One civilisation could disappear a million years
53:24
Speaker 1: before another one begins transmitting. Their worlds might be separated
53:28
Speaker 1: by only a few dozen light years and still never
53:32
Speaker 1: know that the other existed. Now imagine instead that a
53:36
Speaker 1: technological civilisation remains detectable for 10 million years. Suddenly the
53:42
Speaker 1: probability of overlap becomes enormously greater. A long-lived civilization has
53:48
Speaker 1: time to be noticed. It may also have time to spread,
53:51
Speaker 1: construct durable technology, alter planetary systems, or leave evidence that
53:57
Speaker 1: persists long after the civilization itself changes. This final term
54:02
Speaker 1: is particularly awkward, because humanity cannot yet provide a completed example.
54:08
Speaker 1: Our own technological civilization is still extremely young. We do
54:13
Speaker 1: not know whether societies like ours normally remain detectable for centuries, millennia,
54:18
Speaker 1: millions of years, or something much longer. We do not
54:24
Speaker 1: even know whether advanced civilisations become easier to detect as
54:27
Speaker 1: they develop or quieter, and this explains why the Drake
54:31
Speaker 1: equation can produce such dramatically different results. The arithmetic itself
54:37
Speaker 1: is straightforward. The uncertainty is not. If life begins easily,
54:42
Speaker 1: complex cells commonly evolve, intelligence appears frequently, technological societies announce
54:48
Speaker 1: themselves clearly, and those societies survive for enormous periods, then
54:53
Speaker 1: the Milky Way could contain many detectable civilizations. If even
54:58
Speaker 1: one or two of those transitions are fantastically improbable, the
55:02
Speaker 1: expected number can collapse. Both outcomes can be produced without
55:06
Speaker 1: abusing the mathematics. They emerge because several of the quantities
55:10
Speaker 1: being multiplied together are almost completely unknown. Astronomy has brought
55:15
Speaker 1: us surprisingly far. We can look across hundreds or thousands
55:20
Speaker 1: of light years and discover planets that human beings will
55:23
Speaker 1: probably never visit. We can measure their sizes and years
55:28
Speaker 1: and estimate their temperatures. But eventually every calculation reaches the
55:33
Speaker 1: same quiet boundary. We know that matter can become alive
55:37
Speaker 1: because it happened here. We know that life can become
55:40
Speaker 1: intelligent because it happened here. We know that intelligence can
55:44
Speaker 1: produce technology capable of searching the stars because, for a
55:49
Speaker 1: very brief moment in cosmic history, it has happened here.
55:53
Speaker 1: Whether those are ordinary steps in the life of a
55:56
Speaker 1: suitable planet, or a sequence of astonishing accidents, remains almost
56:01
Speaker 1: entirely unknown. Once the possibility of other technological civilizations is
56:07
Speaker 1: taken seriously, a practical question follows. How would we know
56:12
Speaker 1: they were there? For most of human history, there was
56:16
Speaker 1: no realistic way to find out. People could watch the
56:19
Speaker 1: night sky, speculate about inhabited worlds, and wonder whether other
56:24
Speaker 1: beings existed beneath distant suns, but there was no obvious
56:29
Speaker 1: method for detecting them. Radio astronomy changed that. In 1959,
56:35
Speaker 1: physicists Giuseppe Cocconi and Philip Morrison published a paper in
56:40
Speaker 1: the scientific journal Nature, examining whether radio communication across interstellar
56:45
Speaker 1: distances might actually be possible. Their reasoning was surprisingly straightforward.
56:51
Speaker 1: Radio waves are electromagnetic radiation, just like visible light. They
56:57
Speaker 1: travel through empty space at the speed of light, and
57:00
Speaker 1: sufficiently powerful radio transmissions can cross enormous distances. Cocconi and
57:06
Speaker 1: Morrison realised that the radio technology already available during the
57:11
Speaker 1: 1950s was, at least in principle, capable of communication over
57:16
Speaker 1: distances measured in light years. That meant a civilisation elsewhere
57:20
Speaker 1: in the Milky Way might be doing exactly the same thing.
57:25
Speaker 1: And if someone was transmitting... Perhaps humanity could listen. Almost independently,
57:33
Speaker 1: the American radio astronomer Frank Drake had been thinking along
57:36
Speaker 1: very similar lines. In 1960, at the National Radio Astronomy
57:42
Speaker 1: Observatory in Greenbank, West Virginia, Drake began an experiment that
57:46
Speaker 1: became known as Project Osmer. The name came from the
57:51
Speaker 1: fictional land of Oz created by Lyman Frank Baum. a
57:55
Speaker 1: distant place populated by unfamiliar beings. It was an appropriately
58:00
Speaker 1: whimsical name for an experiment attempting something nobody had seriously
58:05
Speaker 1: done before. Drake used the 85-foot Howard-Totell radio telescope at
58:10
Speaker 1: Green Bank. By modern standards, the equipment was modest. The
58:14
Speaker 1: telescope was capable, but the receiving system could examine only
58:18
Speaker 1: a tiny slice of the radio spectrum at a time.
58:22
Speaker 1: Data did not pour into enormous computer clusters. Drake and
58:26
Speaker 1: his small team listened through a loudspeaker and watched a
58:30
Speaker 1: chart recorder draw lines across paper. The targets were two
58:35
Speaker 1: nearby stars, Tau Ceti and Epsilon Eridani. Both lie only
58:40
Speaker 1: about a dozen light-years from Earth, practically next door by
58:44
Speaker 1: galactic standards. They were broadly sun-like stars and seemed reasonable
58:49
Speaker 1: places to begin, although nobody knew at the time whether
58:52
Speaker 1: either possessed planets. Drake tuned the telescope close to a
58:56
Speaker 1: particularly important natural radio frequency. Neutral hydrogen atoms naturally produce
59:03
Speaker 1: radio emission with a wavelength of about 21 centimetres. This
59:08
Speaker 1: corresponds to a frequency of roughly 1,420 megahertz. Hydrogen is
59:15
Speaker 1: the most abundant element in the universe. Drake reasoned that
59:19
Speaker 1: any civilisation capable of radio astronomy would almost certainly know
59:24
Speaker 1: about this hydrogen emission. It was not a frequency chosen
59:27
Speaker 1: according to a human language, calendar or culture. It came
59:32
Speaker 1: from physics itself. That made it an appealing candidate for
59:36
Speaker 1: a kind of universal meeting place. Two civilisations that knew
59:40
Speaker 1: nothing about one another might independently decide that the natural
59:44
Speaker 1: hydrogen frequency was an obvious region of the radio spectrum
59:48
Speaker 1: in which to listen. For several weeks, Drake alternated between
59:53
Speaker 1: Tau Ceti and Epsilon Eridani. There was an early moment
59:57
Speaker 1: of excitement when an apparent signal appeared, but it was
1:00:01
Speaker 1: eventually attributed to interference, probably from terrestrial radar. No extraterrestrial
1:00:07
Speaker 1: transmission was found. Project OSMA ended quietly. And yet, scientifically,
1:00:15
Speaker 1: it had accomplished something important. The search for extraterrestrial intelligence
1:00:20
Speaker 1: had moved from abstract speculation into observation. Humanity had pointed
1:00:27
Speaker 1: a scientific instrument at another star and deliberately listened for technology.
1:00:32
Speaker 1: The answer had been silence. But for the first time,
1:00:37
Speaker 1: the question had been asked experimentally. Other searches followed. Radio
1:00:42
Speaker 1: telescopes examined more stars, more frequencies, and larger areas of
1:00:47
Speaker 1: the sky. Receivers became more sensitive. Computers became vastly more powerful.
1:00:54
Speaker 1: Digital electronics allowed astronomers to divide wide stretches of radio
1:00:59
Speaker 1: spectrum into enormous numbers of narrow channels and search them simultaneously.
1:01:04
Speaker 1: The Search for Extraterrestrial Intelligence Institute, founded in 1984, became
1:01:10
Speaker 1: one of the major organisations devoted to this work. Its
1:01:14
Speaker 1: researchers have used many observatories over the decades and have
1:01:18
Speaker 1: helped expand the search beyond the simple idea of waiting
1:01:22
Speaker 1: for a deliberate radio greeting. One particularly important instrument is
1:01:27
Speaker 1: the Allen Telescope Array at Hat Creek Radio Observatory in
1:01:31
Speaker 1: Northern California. Rather than using one enormous dish, the Allen
1:01:36
Speaker 1: Telescope Array consists of dozens of smaller antennas working together.
1:01:41
Speaker 1: Its 42 dishes can search large areas of the sky
1:01:44
Speaker 1: across a broad range of radio frequencies, and the facility
1:01:48
Speaker 1: can devote substantial amounts of observing time specifically to searches
1:01:54
Speaker 1: for technological signals. Modern searches also make use of enormous
1:01:59
Speaker 1: radio observatories built primarily for conventional astronomy. The Carl Guter
1:02:05
Speaker 1: Jansky Very Large Array in New Mexico, for example, consists
1:02:10
Speaker 1: of 27 large radio antennas operating together as a single
1:02:15
Speaker 1: astronomical instrument. A specialised digital system now allows data from
1:02:21
Speaker 1: observations at the Very Large Array to be searched for
1:02:24
Speaker 1: possible technological radio signals, while ordinary astronomical research continues at
1:02:30
Speaker 1: the same time. It is an elegant arrangement. The telescope
1:02:34
Speaker 1: can study the natural radio universe, while, quietly in the background,
1:02:40
Speaker 1: another system listens for something artificial. Radio is no longer
1:02:45
Speaker 1: the only possibility. Researchers have also searched for extremely brief
1:02:50
Speaker 1: flashes of visible or infrared light, A sufficiently advanced civilisation
1:02:56
Speaker 1: might use powerful lasers to send tightly directed signals between stars.
1:03:01
Speaker 1: Such pulses could, under the right circumstances, momentarily outshine the
1:03:06
Speaker 1: natural light of a star at a particular wavelength. Other
1:03:10
Speaker 1: searches look for broader technosignatures. Instead of asking whether somebody
1:03:15
Speaker 1: is deliberately calling us, astronomers can ask whether technology might
1:03:19
Speaker 1: reveal itself accidentally. An advanced civilisation might alter the chemistry
1:03:24
Speaker 1: of its planet's atmosphere. It might produce unusual amounts of
1:03:28
Speaker 1: waste heat. It could build structures large enough to change
1:03:32
Speaker 1: the way starlight reaches us. None of these ideas guarantees
1:03:37
Speaker 1: that such technology exists. They simply widen the range of
1:03:41
Speaker 1: things astronomers know how to look for. Among the largest
1:03:45
Speaker 1: modern efforts is Breakthrough Listen. Launched in 2015 and now
1:03:51
Speaker 1: based at the University of Oxford, Breakthrough listens searches nearby stars,
1:03:56
Speaker 1: the plane and centre of the Milky Way, and even
1:03:59
Speaker 1: galaxies far beyond our own. It uses major radio observatories,
1:04:05
Speaker 1: including the Green Bank Telescope in West Virginia, the Parkes
1:04:09
Speaker 1: Radio Telescope in Australia, and the Mir Cateray in South Africa.
1:04:14
Speaker 1: The difference between these searches and Project Osmer is extraordinary.
1:04:18
Speaker 1: Frank Drake originally scanned a comparatively narrow range of frequencies
1:04:23
Speaker 1: using equipment that would now seem almost charmingly simple. Modern
1:04:27
Speaker 1: digital systems can process enormous streams of information while examining
1:04:32
Speaker 1: billions of radio channels. Computers automatically search for signals that
1:04:37
Speaker 1: are unusually narrow, unusually persistent, or changing in ways that
1:04:41
Speaker 1: might indicate a transmitter located on a moving planet. Machine
1:04:46
Speaker 1: learning techniques can help separate potentially interesting signals from the
1:04:51
Speaker 1: tremendous amount of interference produced by human technology, and interference
1:04:56
Speaker 1: is everywhere. Satellites transmit, aircraft transmit, mobile networks transmit, radar
1:05:04
Speaker 1: systems transmit, spacecraft transmit. Human beings have surrounded themselves with
1:05:11
Speaker 1: so much radio technology that searching for an extraterrestrial signal
1:05:16
Speaker 1: can sometimes feel less like listening for a whisper in
1:05:19
Speaker 1: a quiet room and more like trying to identify one
1:05:23
Speaker 1: unfamiliar conversation in a railway station. Yet there is a
1:05:27
Speaker 1: curious contrast hidden inside all this technological progress. Our ability
1:05:33
Speaker 1: to listen has improved beyond anything Frank Drake could have
1:05:37
Speaker 1: attempted in 1960. But the Milky Way is enormous. There
1:05:43
Speaker 1: are hundreds of billions of stars. There are vast ranges
1:05:47
Speaker 1: of possible radio frequencies. Signals could be weak or powerful,
1:05:51
Speaker 1: continuous or intermittent, broad or narrow, directed towards Earth or
1:05:56
Speaker 1: pointed somewhere else entirely. A civilisation might transmit for one
1:06:01
Speaker 1: minute every decade. We could observe the correct star for
1:06:05
Speaker 1: nine years, 364 days, 23 hours and 59 minutes and
1:06:07
Speaker 1: still miss it. So after more than six decades of searching,
1:06:16
Speaker 1: Humanity has become extraordinarily better at listening. But we should
1:06:21
Speaker 1: be careful about what the silence means. We have searched
1:06:25
Speaker 1: far more than Frank Drake ever could. We have not
1:06:28
Speaker 1: searched anything approaching everything. For now, the telescopes continue to turn.
1:06:34
Speaker 1: Thousands of stars pass through their beams. Computers sift through
1:06:39
Speaker 1: oceans of radio noise. And somewhere beneath all the electronics
1:06:43
Speaker 1: and algorithms remains the same simple experiment that began at
1:06:46
Speaker 1: Green Bank. Point towards another sun, listen carefully, and see whether,
1:06:53
Speaker 1: among all the natural sounds of the universe, somebody else
1:06:56
Speaker 1: has learned to make a noise of their own. On
1:06:59
Speaker 1: the 15th of August 1977, the Big Ear Radio Telescope
1:07:04
Speaker 1: at Ohio State University was quietly scanning the sky. The
1:07:09
Speaker 1: telescope was not especially glamorous. It was a large fixed
1:07:13
Speaker 1: radio observatory, designed to survey broad regions of the sky
1:07:17
Speaker 1: as Earth rotated beneath them. It could not swing around
1:07:21
Speaker 1: rapidly to chase a target. Instead, the sky drifted through
1:07:25
Speaker 1: its observing beam. That evening, something unusual passed through. The
1:07:31
Speaker 1: telescope recorded a strong narrow band radio signal. It did
1:07:35
Speaker 1: not immediately announce itself with flashing lights or dramatic alarms,
1:07:39
Speaker 1: It appeared in the data. Later, astronomer Jerry Eamon was
1:07:43
Speaker 1: examining a computer printout containing a sequence of letters and
1:07:47
Speaker 1: numbers representing signal strength. One particular sequence stood out. The
1:07:53
Speaker 1: signal had become much stronger than the surrounding background. Eamon
1:07:57
Speaker 1: circled the sequence and wrote a single word beside it. Wow!
1:08:02
Speaker 1: The name remained. The wow signal became one of the
1:08:05
Speaker 1: most famous unexplained events in the history of the search
1:08:08
Speaker 1: for extraterrestrial intelligence. And part of what made it so
1:08:12
Speaker 1: intriguing was not simply that it was strong. It behaved,
1:08:16
Speaker 1: in several ways, like something coming from the sky. As
1:08:21
Speaker 1: a fixed celestial source drifted through the Big Ear telescope's beam,
1:08:26
Speaker 1: the detected signal would be expected to rise in strength,
1:08:30
Speaker 1: reach a maximum near the centre of the beam, and
1:08:33
Speaker 1: then fade again as the source moved away. That is
1:08:37
Speaker 1: broadly what happened. The signal lasted for a little over
1:08:41
Speaker 1: one minute, its strength increased and then decreased over the
1:08:45
Speaker 1: period in a way that was consistent with the rotation
1:08:48
Speaker 1: of Earth carrying the telescope beam across a source at
1:08:52
Speaker 1: a fixed position in the sky. That did not prove
1:08:56
Speaker 1: the signal was extraterrestrial, but it made it interesting. The
1:09:00
Speaker 1: signal was also narrow in frequency. Natural astrophysical radio sources
1:09:06
Speaker 1: often emit over broader ranges of frequencies, while a very
1:09:10
Speaker 1: narrow signal can be suggestive of technology. Human transmitters are
1:09:15
Speaker 1: very good at producing narrow band radio emission. So if
1:09:19
Speaker 1: another civilisation were deliberately sending a radio beacon, something narrow
1:09:24
Speaker 1: and concentrated would be one plausible form for it to take.
1:09:29
Speaker 1: The wow signal also appeared near a frequency associated with
1:09:32
Speaker 1: neutral hydrogen. That made it even more tantalising. Hydrogen is
1:09:37
Speaker 1: the most abundant element in the universe, and its characteristic
1:09:41
Speaker 1: radio emission has long been considered a natural place where
1:09:45
Speaker 1: technologically advanced astronomers might choose to communicate. The temptation was obvious.
1:09:51
Speaker 1: Perhaps after years of listening, somebody had finally answered. But
1:09:55
Speaker 1: science has a habit of becoming less romantic at exactly
1:09:59
Speaker 1: the moment when romance would be most convenient. The signal
1:10:03
Speaker 1: was never reliably detected again. This is the central problem.
1:10:09
Speaker 1: Astronomers returned to the region of sky where the wow
1:10:12
Speaker 1: signal had appeared. They listened again, and again. Nothing equivalent returned.
1:10:19
Speaker 1: Without repetition, there was no way to perform the observations
1:10:22
Speaker 1: needed to establish what the source actually was. Scientists could
1:10:27
Speaker 1: not confirm that it originated from a distant star. They
1:10:31
Speaker 1: could not show that it was artificial. They could not
1:10:34
Speaker 1: even identify with certainty whether it had been produced by
1:10:37
Speaker 1: some unusual natural phenomenon, an unrecognised form of interference, or
1:10:43
Speaker 1: something else entirely. The wow signal therefore remains unexplained in
1:10:48
Speaker 1: the most literal sense. Unexplained does not mean extraterrestrial. It
1:10:53
Speaker 1: simply means that the available evidence was not sufficient to
1:10:56
Speaker 1: establish a cause. And this distinction is important. The history
1:11:01
Speaker 1: of searches for extraterrestrial technology contains many interesting signals. Almost
1:11:07
Speaker 1: all of them become much less interesting after careful investigation.
1:11:12
Speaker 1: One modern example came from the direction of Proxima Centauri,
1:11:16
Speaker 1: the nearest known star to the Sun. Data gathered as
1:11:19
Speaker 1: part of the Breakthrough Listen programme revealed a narrow band
1:11:23
Speaker 1: signal that initially appeared unusual. Because Proxima Centauri has planets
1:11:29
Speaker 1: and lies only a little more than four light-years away,
1:11:32
Speaker 1: the possibility attracted enormous attention. The signal was designated as
1:11:37
Speaker 1: a candidate of interest and examined closely. The researchers did
1:11:42
Speaker 1: not immediately announce the discovery of another civilisation. They tested it.
1:11:47
Speaker 1: They compared it with other observations. They searched for similar signals.
1:11:52
Speaker 1: They examined how its frequency changed. And eventually, the evidence
1:11:56
Speaker 1: showed that it was almost certainly radiofrequency interference produced by
1:12:00
Speaker 1: human technology. That is how the process is supposed to work.
1:12:05
Speaker 1: A strange signal is not the conclusion. It is the
1:12:08
Speaker 1: beginning of the investigations. Any genuine extraterrestrial detection would need
1:12:14
Speaker 1: to survive an exhausting series of tests. It should ideally
1:12:18
Speaker 1: be detected repeatedly. Independent observatories would need to confirm it.
1:12:23
Speaker 1: Its position should correspond to a point in the sky
1:12:27
Speaker 1: rather than a local transmitter. Its behaviour would need to
1:12:31
Speaker 1: resist ordinary explanations involving satellites, aircraft, radar, electronics and other
1:12:38
Speaker 1: human sources. And even then, Scientists would need to ask
1:12:43
Speaker 1: whether some natural astrophysical process could produce the same thing.
1:12:47
Speaker 1: The standard is high because the claim would be extraordinary.
1:12:51
Speaker 1: There is also an interesting reversal hidden inside all of this.
1:12:55
Speaker 1: While humanity is listening for others, humanity itself is producing technosignatures.
1:13:01
Speaker 1: Earth is no longer completely quiet. Powerful planetary radars can
1:13:06
Speaker 1: send concentrated radio energy into space. Deliberate messages have occasionally
1:13:11
Speaker 1: been transmitted towards other stars. Lasers can, in principle, be
1:13:16
Speaker 1: used as artificial beacons. Industrial activity changes the chemistry of
1:13:21
Speaker 1: Earth's atmosphere. Artificial satellites surround the planet. At some point,
1:13:27
Speaker 1: a sufficiently advanced distant civilisation with sufficiently powerful instruments might
1:13:32
Speaker 1: be able to distinguish some of these effects from natural processes.
1:13:36
Speaker 1: but it is important not to exaggerate how easy we
1:13:39
Speaker 1: would be to detect. Ordinary radio leakage from television, mobile
1:13:44
Speaker 1: communications and many other everyday systems becomes extremely faint as
1:13:49
Speaker 1: it spreads through space. Radio waves expand outward, distributing their
1:13:55
Speaker 1: energy over larger and larger areas. By the time weak
1:13:59
Speaker 1: leakage has travelled across hundreds or thousands of light-years, Detecting
1:14:04
Speaker 1: it would require extraordinarily sensitive equipment. Even a technological civilisation
1:14:10
Speaker 1: could therefore be difficult to notice unless it produces very
1:14:14
Speaker 1: powerful or deliberately directed signals. And that creates another problem.
1:14:19
Speaker 1: Perhaps many civilisations are detectable only briefly. Perhaps their strongest
1:14:24
Speaker 1: radio emissions last for a few centuries before their technology
1:14:28
Speaker 1: becomes quieter, more efficient or more tightly directed. Perhaps they
1:14:33
Speaker 1: never produce powerful broadcasts at all. Perhaps we are listening
1:14:38
Speaker 1: for the cosmic equivalent of smoke signals, while everyone else
1:14:42
Speaker 1: has moved on to something we have not imagined. That
1:14:45
Speaker 1: possibility does not make the search pointless. It makes the
1:14:49
Speaker 1: search difficult. And difficulty is the theme that runs through
1:14:54
Speaker 1: every strange candidate ever found. The wow signal remains fascinating.
1:15:00
Speaker 1: because it briefly looked like something that might have fit
1:15:03
Speaker 1: several expectations. It was strong. It was narrow. It appeared
1:15:08
Speaker 1: to come from the sky. It rose and fell in
1:15:11
Speaker 1: a way consistent with a fixed celestial source moving through
1:15:16
Speaker 1: the telescope beam. And then it vanished. One event. One observation.
1:15:21
Speaker 1: No confirmation. No repeat. That is not enough. Over the decades,
1:15:27
Speaker 1: radio telescopes have become more sensitive. receivers can examine wider
1:15:32
Speaker 1: stretches of spectrum, computers can inspect enormous numbers of channels
1:15:37
Speaker 1: at once, observatories can revisit promising targets with instruments far
1:15:42
Speaker 1: superior to anything available in 1977. Still, no signal has
1:15:48
Speaker 1: yet crossed the line from intriguing candidate to confirmed extraterrestrial technology.
1:15:54
Speaker 1: That does not mean the galaxy is empty. It means
1:15:57
Speaker 1: that after more than half a century of serious listening,
1:16:01
Speaker 1: Every apparent whisper has either faded, failed to repeat, or
1:16:05
Speaker 1: found a more ordinary explanation. And in that sense, the
1:16:09
Speaker 1: silence becomes the observational version of Enrico Fermi's old question.
1:16:14
Speaker 1: We have built better ears. We have listened longer. We
1:16:18
Speaker 1: have searched more stars. We have learned to recognise interference
1:16:23
Speaker 1: more carefully than ever before. Yet nothing has unmistakably answered.
1:16:28
Speaker 1: The strange part is not that we have heard nothing
1:16:31
Speaker 1: at all. The strange part is that, after decades of listening,
1:16:35
Speaker 1: we have heard just enough peculiar things to keep turning
1:16:38
Speaker 1: the telescopes back towards the sky. And so the listening continues,
1:16:44
Speaker 1: not because the silence has proved that nobody is there,
1:16:48
Speaker 1: but because so far nothing has proved that somebody is.
1:16:52
Speaker 1: There is another way to approach the silence of the galaxy.
1:16:55
Speaker 1: Perhaps the reason we do not see technological civilisations everywhere
1:16:59
Speaker 1: is not that they all disappear. Perhaps they hardly ever
1:17:03
Speaker 1: appear in the first place. This idea is often associated
1:17:06
Speaker 1: with the rare earth hypothesis, developed especially by Peter Ward
1:17:10
Speaker 1: and Donald Brownlee. The basic proposal is not necessarily that
1:17:15
Speaker 1: life itself is almost impossible. Simple life might emerge on
1:17:19
Speaker 1: many worlds. Microbes may turn out to be common. But
1:17:22
Speaker 1: the long chain from simple cells to complex animals, intelligence,
1:17:27
Speaker 1: technology and eventually radio telescopes may require such an unusual
1:17:31
Speaker 1: combination of circumstances that worlds like Earth are genuinely rare.
1:17:37
Speaker 1: At first, Earth can look almost ordinary. It is a
1:17:40
Speaker 1: rocky planet orbiting a fairly typical star in one spiral
1:17:45
Speaker 1: galaxy among billions. But when we examine its history closely,
1:17:49
Speaker 1: a large number of favourable conditions begin to accumulate. The
1:17:53
Speaker 1: Sun is one of them. It is relatively stable and long-lived.
1:17:58
Speaker 1: It does not belong to the class of very massive
1:18:01
Speaker 1: stars that burn through their fuel in only a few
1:18:03
Speaker 1: million years. Instead, it provides billions of years during which
1:18:07
Speaker 1: planetary conditions can remain broadly suitable. Earth itself orbits at
1:18:12
Speaker 1: a distance where liquid water can persist on the surface.
1:18:16
Speaker 1: provided the atmosphere behaves in the right way. That matters
1:18:20
Speaker 1: because every known form of life depends on liquid water.
1:18:24
Speaker 1: But water alone is not enough. Earth also has an
1:18:27
Speaker 1: atmosphere thick enough to regulate temperature and shield the surface
1:18:30
Speaker 1: from some harmful radiation, yet not so thick that the
1:18:34
Speaker 1: planet entered the extreme runaway greenhouse state seen on Venus.
1:18:39
Speaker 1: Its climate has changed dramatically over geological time. There have
1:18:43
Speaker 1: been ice ages. There have been much warmer periods. There
1:18:47
Speaker 1: have been enormous volcanic episodes and major impacts. And yet,
1:18:51
Speaker 1: through all of this, liquid water has persisted somewhere on
1:18:55
Speaker 1: or beneath the surface for billions of years. That long
1:18:59
Speaker 1: continuity may be important. Complex evolution requires time. If a
1:19:05
Speaker 1: planet repeatedly sterilises its surface, loses its oceans or swings
1:19:10
Speaker 1: permanently between extreme climates, then life may never receive the
1:19:14
Speaker 1: long uninterrupted intervals needed for greater complexity to develop. One
1:19:20
Speaker 1: process that may have helped Earth maintain long-term climate stability
1:19:24
Speaker 1: is plate tectonics. Earth's outer rocky shell is broken into
1:19:28
Speaker 1: moving plates. These plates interact, collide, separate, sink into the
1:19:34
Speaker 1: mantle and reshape the surface over immense periods. Plate tectonics
1:19:39
Speaker 1: is connected to the carbon cycle. Carbon dioxide can be
1:19:43
Speaker 1: removed from the atmosphere through weathering and eventually stored in
1:19:46
Speaker 1: sediments and rocks. Subduction can carry carbon into Earth's interior.
1:19:52
Speaker 1: Volcanism can later return some of that carbon to the atmosphere.
1:19:56
Speaker 1: Over very long timescales, this exchange can help regulate climate.
1:20:01
Speaker 1: If temperatures rise, chemical weathering may accelerate under some conditions
1:20:05
Speaker 1: and remove more carbon dioxide. If the planet cools, weathering
1:20:09
Speaker 1: may slow, allowing volcanic carbon dioxide to accumulate. This is
1:20:14
Speaker 1: sometimes described as part of a planetary thermostat. It would
1:20:18
Speaker 1: be tempting to conclude that plate tectonics is therefore essential
1:20:21
Speaker 1: for complex life. But that conclusion goes too far. We
1:20:27
Speaker 1: only know that plate tectonics has played an important role
1:20:30
Speaker 1: on Earth. We do not yet know whether another planet
1:20:33
Speaker 1: could achieve long-term climate regulation through different geological processes. That
1:20:39
Speaker 1: distinction matters throughout the rare earth discussion. Earth tells us
1:20:44
Speaker 1: what worked here. It does not automatically tell us what
1:20:47
Speaker 1: must work everywhere. The moon is another frequently discussed factor.
1:20:53
Speaker 1: Relative to the size of earth, our moon is unusually large.
1:20:57
Speaker 1: It produces powerful tides. It has gradually slowed earth's rotations.
1:21:02
Speaker 1: and its gravitational influence affects the behaviour of Earth's rotational axis.
1:21:08
Speaker 1: One argument is that the Moon helps stabilise Earth's axial tilt,
1:21:13
Speaker 1: reducing the degree to which the planet's orientation changes over
1:21:16
Speaker 1: long periods. Large changes in axial tilt could produce major
1:21:21
Speaker 1: climatic effects. From this, some researchers have suggested that a
1:21:25
Speaker 1: large Moon may make complex life more likely, But here again,
1:21:29
Speaker 1: the evidence does not justify treating the moon as an
1:21:32
Speaker 1: absolute requirement. Models of planetary rotation are complicated. A planet
1:21:37
Speaker 1: without a large moon does not automatically become climatically chaotic,
1:21:42
Speaker 1: and a world experiencing larger variations might still support life.
1:21:47
Speaker 1: The moon may have helped create the particular environmental history
1:21:50
Speaker 1: that Earth experienced. Whether something similar is necessary elsewhere remains uncertain.
1:21:57
Speaker 1: Then there is Jupiter. For years Jupiter has often been
1:22:01
Speaker 1: described as Earth's great cosmic protector. The picture is appealing.
1:22:06
Speaker 1: Jupiter is enormously massive. Its gravity can capture, deflect or
1:22:11
Speaker 1: eject comets and asteroids that might otherwise travel into the
1:22:15
Speaker 1: inner solar system. And in some cases it certainly does
1:22:18
Speaker 1: remove potentially dangerous objects. But the full story is more complicated.
1:22:24
Speaker 1: Jupiter's gravity can also redirect small bodies onto new trajectories.
1:22:29
Speaker 1: Some of those trajectories can bring objects towards the inner planets.
1:22:33
Speaker 1: Depending on the population of comets or asteroids being considered,
1:22:37
Speaker 1: a giant planet can sometimes reduce impact risk and sometimes
1:22:41
Speaker 1: increase it. So Jupiter is not simply standing guard beside
1:22:45
Speaker 1: Earth with a gravitational shield. Its influence is dynamic. It
1:22:49
Speaker 1: rearranges orbits. Sometimes that helps us. Sometimes it can do
1:22:54
Speaker 1: the opposite. Even if every astronomical and geological condition were favourable,
1:22:59
Speaker 1: biology itself may contain the rarest steps. Life on Earth
1:23:04
Speaker 1: began with simple cells. For billions of years, that was
1:23:08
Speaker 1: essentially the whole story. Then, at some point, far more
1:23:12
Speaker 1: complex cells appeared. These are known as eukaryotic cells. They
1:23:17
Speaker 1: contain internal structures including a nucleus, and they form the
1:23:21
Speaker 1: basis of animals, plants, fungi and many other organisms. One
1:23:26
Speaker 1: of the most important events in their origin involved a
1:23:29
Speaker 1: profound biological partnership. An ancestral cell incorporated a bacterium that
1:23:35
Speaker 1: eventually became the mitochondrion, the energy-producing structure found in most
1:23:39
Speaker 1: eukaryotic cells. That merger appears to have happened only once
1:23:44
Speaker 1: in the history of life that led to all known
1:23:46
Speaker 1: complex eukaryotes. Perhaps such events are common on inhabited planets.
1:23:51
Speaker 1: Perhaps they are incredibly rare. We do not know. Multicellularity
1:23:56
Speaker 1: then had to develop. Cells began cooperating, specialising and building
1:24:01
Speaker 1: larger organisms. Atmospheric oxygen levels changed over geological time, eventually
1:24:08
Speaker 1: allowing forms of metabolism capable of supporting larger and more
1:24:12
Speaker 1: energetic organisms. Nervous systems evolved. Sensory organs became increasingly sophisticated.
1:24:20
Speaker 1: Some animals developed complex social behaviour and advanced learning. And still,
1:24:25
Speaker 1: None of that guaranteed technological civilisation. Dinosaurs inhabited Earth for
1:24:30
Speaker 1: more than 150 million years. They were enormously successful. Some
1:24:36
Speaker 1: were highly specialised. Some had advanced senses. Some were social.
1:24:41
Speaker 1: Yet none, as far as we know, built radio telescopes.
1:24:46
Speaker 1: The appearance of human technological intelligence required another sequence of
1:24:50
Speaker 1: evolutionary developments. Large and adaptable brains. Fine motor control, complex language,
1:24:58
Speaker 1: cumulative culture, the ability to teach knowledge across generations, the
1:25:03
Speaker 1: capacity to manipulate materials, control of fire, eventually agriculture, cities, writing, mathematics, industry, electricity, radio.
1:25:16
Speaker 1: None of those steps appears written into the laws of biology.
1:25:20
Speaker 1: Evolution does not aim towards intelligence. It does not have
1:25:23
Speaker 1: technological civilisation as a final destination. It simply favours traits
1:25:29
Speaker 1: that help organisms survive and reproduce within particular environments. That
1:25:33
Speaker 1: makes the history of humanity look rather less inevitable. Perhaps
1:25:38
Speaker 1: something similar would happen again on another Earth. Perhaps not.
1:25:42
Speaker 1: This is the strongest careful form of the rare Earth argument.
1:25:46
Speaker 1: It does not require every planet to be sterile. It
1:25:49
Speaker 1: does not require microbes to be unique to Earth. The
1:25:52
Speaker 1: galaxy could contain countless worlds where chemistry has crossed into biology.
1:25:57
Speaker 1: There could be oceans containing simple organisms beneath unfamiliar skies.
1:26:02
Speaker 1: There could be entire planets covered in microbial ecosystems. And still,
1:26:07
Speaker 1: the Milky Way might remain almost completely silent. Because between
1:26:11
Speaker 1: a living planet and a technological civilization lies an enormous
1:26:15
Speaker 1: chain of transitions. A stable environment. Complex cells. Multicellular organisms.
1:26:22
Speaker 1: suitable energy sources, long-term ecological persistence, advanced nervous systems, general intelligence, language,
1:26:31
Speaker 1: cumulative culture, technology, and enough time for all of them
1:26:35
Speaker 1: to occur, perhaps none of these steps is individually impossible,
1:26:39
Speaker 1: perhaps most are not even extraordinarily unlikely. But if several
1:26:44
Speaker 1: are difficult, multiplying their probabilities together can produce something very
1:26:49
Speaker 1: rare indeed, The Milky Way may therefore be full of planets.
1:26:53
Speaker 1: It may even be full of life. And yet the
1:26:56
Speaker 1: transformation from a wet, rocky world covered in chemistry to
1:27:01
Speaker 1: a species capable of looking into the night and asking
1:27:04
Speaker 1: where everybody else is may be one of the rarest
1:27:06
Speaker 1: things the universe ever does. The Great Filter is one
1:27:09
Speaker 1: of the most useful ways of organising the mystery raised
1:27:12
Speaker 1: by Enrico Fermi. The idea was developed by economist Robin
1:27:16
Speaker 1: Hansen in 1998. It does not claim to identify one
1:27:20
Speaker 1: specific event that destroys civilizations. It does not say that
1:27:25
Speaker 1: every species reaches the same point and then disappears. Instead,
1:27:29
Speaker 1: it offers a broader way of thinking about the journey
1:27:32
Speaker 1: from lifeless matter to a civilization capable of surviving for
1:27:36
Speaker 1: immense periods and perhaps spreading through the galaxy. Imagine that
1:27:42
Speaker 1: journey as a long sequence of steps. At the beginning
1:27:45
Speaker 1: is an ordinary planet. It contains rocks, water, gases, organic molecules,
1:27:51
Speaker 1: and all the familiar ingredients of chemistry, but nothing is alive. Somehow,
1:27:56
Speaker 1: chemistry must become capable of self-replication. Those early replicating systems
1:28:01
Speaker 1: must become more reliable. Eventually, simple cells appear. Simple cells
1:28:06
Speaker 1: must then develop greater complexity. Complex cells have to emerge.
1:28:11
Speaker 1: Multicellular organisms may follow. Nervous systems develop. some organisms become
1:28:16
Speaker 1: increasingly capable of learning, communication, cooperation and manipulating their environment.
1:28:23
Speaker 1: Eventually intelligence appears, then technology, then a technological civilisation, and
1:28:30
Speaker 1: after that comes an entirely different challenge. That civilisation must
1:28:34
Speaker 1: continue existing long enough to become stable on astronomical timescales.
1:28:39
Speaker 1: Perhaps it develops interstellar travel. Perhaps it builds self-sustaining settlements
1:28:43
Speaker 1: around other stars. Perhaps it creates machines capable of surviving
1:28:47
Speaker 1: for millions of years. Perhaps it spreads far enough that
1:28:51
Speaker 1: no single planetary disaster could eliminate it. At first glance,
1:28:56
Speaker 1: none of those stages necessarily looks impossible. After all, every
1:29:00
Speaker 1: stage up to technological civilisation has occurred at least once.
1:29:04
Speaker 1: It happened here. But the Great Filter asks a more
1:29:07
Speaker 1: difficult question. How likely was each step? Suppose the Milky
1:29:11
Speaker 1: Way contains billions of planets where liquid water can exist.
1:29:16
Speaker 1: Suppose simple organic chemistry occurs almost everywhere. Even then, if
1:29:21
Speaker 1: one later transition has a probability of only one in
1:29:23
Speaker 1: a billion … Almost every promising world may still fail
1:29:27
Speaker 1: to produce technological intelligence. The filter does not need to
1:29:31
Speaker 1: be dramatic. It does not need to involve explosions, wars
1:29:35
Speaker 1: or planetary catastrophe. It could simply be a biological transition
1:29:40
Speaker 1: that almost never happens. Perhaps life itself is difficult to start.
1:29:46
Speaker 1: Perhaps the origin of the first self-replicating chemistry is so
1:29:49
Speaker 1: unlikely that nearly every suitable planet remains lifeless forever. In
1:29:55
Speaker 1: that case, the Great Filter would lie very early in
1:29:58
Speaker 1: the chain. Earth would have passed through it billions of
1:30:01
Speaker 1: years ago, or perhaps simple life begins easily. Maybe oceans
1:30:06
Speaker 1: throughout the galaxy are filled with microorganisms. The difficult step
1:30:11
Speaker 1: might then be the appearance of complex cells. On Earth,
1:30:14
Speaker 1: simple life existed for an enormous period before complex eukaryotic
1:30:18
Speaker 1: cells appeared. Perhaps that transition is exceptionally rare. Or perhaps
1:30:23
Speaker 1: complex cells are common, but multicellular organisms are not. Perhaps
1:30:28
Speaker 1: multicellular organisms evolve frequently, but sophisticated intelligence almost never does.
1:30:35
Speaker 1: Or perhaps intelligence develops, but only rarely produces cumulative culture
1:30:40
Speaker 1: and advanced technology. There are many places where the chain
1:30:43
Speaker 1: could narrow, and that creates one of the most unsettling
1:30:46
Speaker 1: questions in the entire Fermi paradox. Is the great filter
1:30:50
Speaker 1: behind us, or is it ahead? If the most improbable
1:30:54
Speaker 1: stages have already happened, then humanity may have been extraordinarily fortunate.
1:30:59
Speaker 1: Perhaps the origin of life was the difficult step. Perhaps
1:31:03
Speaker 1: the emergence of complex cells was the difficult step. Perhaps
1:31:07
Speaker 1: technological intelligence is itself astonishingly rare. If so, the silence
1:31:12
Speaker 1: of the Milky Way becomes easier to understand. Other planets
1:31:16
Speaker 1: may exist in enormous numbers. Some may even be covered
1:31:19
Speaker 1: in life. But very few ever produce anything capable of
1:31:23
Speaker 1: building radio telescopes and spacecraft or interstellar probes. In that case,
1:31:28
Speaker 1: humanity has already crossed the narrower section of the bridge.
1:31:32
Speaker 1: The dangerous probability lies somewhere in our past. But there
1:31:36
Speaker 1: is another possibility. Perhaps the early steps are easy. Perhaps
1:31:40
Speaker 1: life appears frequently. Perhaps complex organisms often develop. Perhaps intelligence
1:31:47
Speaker 1: and technology are not especially unusual. then the absence of
1:31:51
Speaker 1: visible ancient civilisations becomes much harder to explain. If technological
1:31:57
Speaker 1: societies arise regularly, why do we not see any that
1:32:00
Speaker 1: have survived for millions of years? Why has none apparently
1:32:04
Speaker 1: expanded across a significant portion of the galaxy? Why are
1:32:08
Speaker 1: there no obvious signs of engineering on astronomical scales? Under
1:32:12
Speaker 1: the Great Filter Framework, one possible answer is that the
1:32:15
Speaker 1: difficult step lies later. Perhaps technological civilisations usually fail before
1:32:21
Speaker 1: they become long-lived. That failure could take many forms. A
1:32:25
Speaker 1: civilisation might destroy itself. Its environment could become unstable. Its
1:32:30
Speaker 1: technology could create risks that it fails to manage. It
1:32:34
Speaker 1: could simply collapse socially or economically. Or perhaps the difficulty
1:32:38
Speaker 1: has nothing to do with destruction at all. Maybe surviving
1:32:42
Speaker 1: for millions of years is harder than we imagined. Perhaps
1:32:45
Speaker 1: technological societies tend to change into forms that no longer
1:32:49
Speaker 1: expand or remain detectable. The Great Filter does not specify
1:32:53
Speaker 1: the cause. It simply points towards the possibility that somewhere
1:32:56
Speaker 1: between a habitable planet and a long-lived galactic civilisation, the
1:33:01
Speaker 1: probability of continuing becomes extremely small. This is why the
1:33:06
Speaker 1: framework connects so closely to Fermi's question. If every step
1:33:10
Speaker 1: were easy, then the Milky Way would present a puzzle.
1:33:13
Speaker 1: There are vast numbers of stars. Planets are common. Some
1:33:17
Speaker 1: planetary systems are billions of years older than ours. Even
1:33:21
Speaker 1: relatively slow expansion could, under certain assumptions, spread technology across
1:33:27
Speaker 1: enormous regions of the galaxy in a fraction of its lifetime.
1:33:32
Speaker 1: If life commonly became intelligence, and intelligence commonly became durable
1:33:36
Speaker 1: technological civilisation, then some evidence might reasonably be expected. But
1:33:42
Speaker 1: we do not see it. The Great Filter is an
1:33:45
Speaker 1: attempt to explain why something, somewhere in the chain, may
1:33:50
Speaker 1: be rare enough to prevent the galaxy from becoming visibly crowded.
1:33:55
Speaker 1: There is another subtle problem, however. Human beings are not
1:33:58
Speaker 1: neutral observers of this process. We necessarily find ourselves on
1:34:03
Speaker 1: a planet where every earlier stage succeeded. This is related
1:34:07
Speaker 1: to anthropic selection. We could not have appeared on a
1:34:10
Speaker 1: world where life never began. We could not be discussing
1:34:14
Speaker 1: complex cells if complex cells had never evolved. We could
1:34:18
Speaker 1: not wonder about technological intelligence from a planet where intelligence
1:34:22
Speaker 1: never developed. Our own existence therefore guarantees that the chain
1:34:27
Speaker 1: behind us worked at least once. That makes it difficult
1:34:30
Speaker 1: to judge how improbable those steps really were. Imagine a
1:34:34
Speaker 1: person who survives an extraordinarily unlikely sequence of accidents. From
1:34:39
Speaker 1: their perspective, every survival event happened. That does not mean
1:34:44
Speaker 1: those events were likely. It simply means that someone who
1:34:47
Speaker 1: did not survive would not be present to comment on
1:34:50
Speaker 1: the outcome. The same logic makes Earth a difficult statistical sample.
1:34:55
Speaker 1: Life began here. Complex cells emerged here. Multicellular organisms evolved here.
1:35:02
Speaker 1: Intelligence appeared here. Technology followed. But none of those facts
1:35:07
Speaker 1: tells us directly how often the same sequence occurs elsewhere.
1:35:11
Speaker 1: One example... cannot reveal whether a transition happens on almost
1:35:15
Speaker 1: every suitable planet or only once in a trillion. This
1:35:19
Speaker 1: is why the Great Filter should not be treated as
1:35:22
Speaker 1: a discovered law of nature. There is no instrument that
1:35:25
Speaker 1: has detected it. There is no known physical barrier called
1:35:29
Speaker 1: the Great Filter waiting somewhere in the future. It is
1:35:32
Speaker 1: a logical framework, a way of arranging our ignorance. It
1:35:37
Speaker 1: asks us to look at the path from chemistry to
1:35:39
Speaker 1: lasting technological civilisation and wonder where the probability might collapse.
1:35:45
Speaker 1: Perhaps the unlikely event happened billions of years ago, when
1:35:49
Speaker 1: non-living chemistry first became capable of reproduction. Perhaps it occurred
1:35:54
Speaker 1: when simple cells became complex. Perhaps it happened when one
1:35:58
Speaker 1: branch of life developed language, cumulative culture and technology. Or
1:36:04
Speaker 1: perhaps the narrowest part of the path has not yet arrived.
1:36:07
Speaker 1: That uncertainty is what gives the great filter its peculiar power.
1:36:12
Speaker 1: The silence of the Milky Way can be comforting or
1:36:14
Speaker 1: disturbing depending on where the filter lies. If almost nobody
1:36:18
Speaker 1: reaches our stage, then simply being here may mean humanity
1:36:21
Speaker 1: has already achieved something astonishingly rare. If many worlds reach
1:36:26
Speaker 1: our stage, but almost none progress beyond it, then the
1:36:29
Speaker 1: silence carries a very different implication. At the moment, we
1:36:33
Speaker 1: cannot tell which picture is closer to reality. We see
1:36:37
Speaker 1: only one completed portion of the pathway, our own. Behind
1:36:42
Speaker 1: us lies a sequence of biological transitions stretching back billions
1:36:46
Speaker 1: of years. Ahead lies a future we have barely begun
1:36:49
Speaker 1: to explore. And somewhere, either behind us or still hidden
1:36:53
Speaker 1: beyond the horizon, there may be a step that almost
1:36:56
Speaker 1: nobody manages to cross. Mars is one of the most
1:37:00
Speaker 1: obvious places to search for life beyond Earth. It is nearby,
1:37:04
Speaker 1: at least by astronomical standards. It is a rocky planet.
1:37:09
Speaker 1: Its surface preserves ancient landscapes that were shaped by flowing water.
1:37:14
Speaker 1: Spacecraft have discovered dried river valleys, sedimentary rocks, minerals formed
1:37:19
Speaker 1: in the presence of water and evidence that lakes once
1:37:22
Speaker 1: existed on its surface. Today, Mars is cold, dry and
1:37:28
Speaker 1: exposed to a harsh radiation environment. But billions of years ago,
1:37:33
Speaker 1: it was a different world. There were periods when liquid
1:37:36
Speaker 1: water flowed across parts of its surface. Rivers fed lakes,
1:37:41
Speaker 1: groundwater moved through rock. The atmosphere was thicker than it
1:37:45
Speaker 1: is today. That does not mean ancient Mars was covered
1:37:49
Speaker 1: in life. But it means there were environments in which life,
1:37:53
Speaker 1: at least as we understand it, may have had a
1:37:56
Speaker 1: chance to exist. This makes the search for ancient Martian
1:38:01
Speaker 1: microbes one of the most exciting scientific investigations humanity has
1:38:06
Speaker 1: ever attempted. And strangely, within the logic of the Great Filter,
1:38:12
Speaker 1: discovering them could make the wider mystery of extraterrestrial intelligence
1:38:16
Speaker 1: more complicated. The reasoning begins with one of the earliest
1:38:20
Speaker 1: unknown steps in the history of life, abiogenesis. Somehow, on
1:38:26
Speaker 1: the young Earth, non-living chemistry became capable of producing systems
1:38:30
Speaker 1: that could reproduce, vary and evolve. We do not know
1:38:35
Speaker 1: how difficult that transition was. Perhaps it happens quite naturally
1:38:39
Speaker 1: whenever a rocky planet has liquid water, suitable chemistry and
1:38:43
Speaker 1: enough time. If so, microbial life might be common throughout
1:38:48
Speaker 1: the galaxy. Or perhaps the transition is fantastically improbable. Perhaps
1:38:53
Speaker 1: billions of habitable planets can remain biologically empty for their
1:38:57
Speaker 1: entire histories because chemistry almost never crosses the boundary into life.
1:39:03
Speaker 1: Earth alone cannot tell us which possibility is correct. But
1:39:07
Speaker 1: a second independent example would change the situation. Suppose a
1:39:13
Speaker 1: rover on Mars eventually discovered an unmistakable fossil microorganism buried
1:39:19
Speaker 1: inside ancient rock. Suppose careful analysis showed that this organism
1:39:24
Speaker 1: was genuinely Martian, and had appeared billions of years ago.
1:39:28
Speaker 1: That alone would be extraordinary. But the most important question
1:39:33
Speaker 1: would be whether Martian life represented a completely independent origin.
1:39:38
Speaker 1: This distinction matters because Earth and Mars have not always
1:39:42
Speaker 1: been biologically isolated from one another. Large asteroid impacts can
1:39:46
Speaker 1: strike a planet with enough energy to blast pieces of
1:39:49
Speaker 1: its surface into space. Some of those rocks can escape
1:39:53
Speaker 1: the planet entirely. We know this has happened on Mars
1:39:57
Speaker 1: because meteorites found on Earth have been chemically identified as
1:40:00
Speaker 1: pieces of Martian rock. They were launched from Mars by
1:40:04
Speaker 1: ancient impacts, travelled through interplanetary space, and eventually fell onto
1:40:09
Speaker 1: our planet. The reverse process can also occur. material blasted
1:40:14
Speaker 1: away from Earth can, in principle, reach Mars. This creates
1:40:19
Speaker 1: a possibility known as lithopanspermia. The idea is that microorganisms,
1:40:24
Speaker 1: or material containing them, could potentially travel between planets while
1:40:28
Speaker 1: protected inside rocks. Whether living organisms could survive every stage
1:40:33
Speaker 1: of such a journey would depend on impact forces, heating,
1:40:37
Speaker 1: radiation exposure, travel time, and conditions after arrival. But experiments
1:40:44
Speaker 1: and modelling have shown that transfer of rocky material between
1:40:47
Speaker 1: Earth and Mars is physically possible. So imagine that ancient
1:40:51
Speaker 1: Martian fossils were discovered and turned out to use exactly
1:40:55
Speaker 1: the same fundamental biochemistry as life on Earth. Perhaps they
1:41:00
Speaker 1: used the same basic genetic machinery. Perhaps their molecular structures
1:41:04
Speaker 1: strongly suggested a common ancestry. That would still be one
1:41:08
Speaker 1: of the greatest discoveries in science. but it might not
1:41:12
Speaker 1: tell us that life originated twice. It could mean that
1:41:15
Speaker 1: life began on Earth and later reached Mars, or that
1:41:19
Speaker 1: life began on Mars and reached Earth, or that both
1:41:22
Speaker 1: planets were seeded from an earlier common source. In that situation,
1:41:27
Speaker 1: the solar system might still contain only one known origin
1:41:30
Speaker 1: of life. Now imagine something more remarkable. Suppose Martian life
1:41:35
Speaker 1: were based on fundamentally different biochemistry. Perhaps it stored biological
1:41:40
Speaker 1: information in a different molecular system. Perhaps its basic chemical
1:41:44
Speaker 1: organisation clearly showed that it could not share a common
1:41:48
Speaker 1: ancestor with terrestrial organisms. Then humanity would have discovered two
1:41:53
Speaker 1: independent origins of life in one planetary system. That would
1:41:57
Speaker 1: begin to change our estimate of how difficult a biogenesis
1:42:01
Speaker 1: might be. Earth and Mars would represent two separate experiments.
1:42:06
Speaker 1: Both would have produced life. And if life began independently
1:42:09
Speaker 1: on two neighbouring planets, it would become harder to argue
1:42:13
Speaker 1: that the origin of biology is an almost impossibly rare event.
1:42:18
Speaker 1: This is where the great filter becomes slightly uncomfortable. If
1:42:22
Speaker 1: the beginning of a life is easy, then the explanation
1:42:25
Speaker 1: for the apparent absence of ancient technological civilisations may have
1:42:30
Speaker 1: to move farther along the chain. Perhaps simple life is everywhere,
1:42:34
Speaker 1: then perhaps complex cells are the difficult step If complex
1:42:38
Speaker 1: cells also prove common, perhaps multicellular life is the barrier.
1:42:44
Speaker 1: If complex organisms are common, perhaps intelligence is rare. And
1:42:48
Speaker 1: if technological intelligence turns out to arise frequently, then the
1:42:53
Speaker 1: unexplained difficulty may lie later still. This is why Robin
1:42:57
Speaker 1: Hanson and others discussing the Great Filter have pointed out
1:43:00
Speaker 1: a counterintuitive implication. Under certain assumptions … Finding independent, simple
1:43:07
Speaker 1: life elsewhere could make the long-term silence of the galaxy
1:43:10
Speaker 1: more puzzling. It would remove one possible explanation. If a
1:43:15
Speaker 1: biogenesis is extremely difficult, then perhaps Earth is simply one
1:43:20
Speaker 1: extraordinarily fortunate planet. But if life begins readily whenever conditions permit,
1:43:26
Speaker 1: we need another reason why the Milky Way does not
1:43:29
Speaker 1: appear to contain numerous ancient technological societies. That does not
1:43:34
Speaker 1: mean discovering Martian microbes would literally be bad news. That
1:43:39
Speaker 1: description is far too simple. The conclusion depends heavily on assumptions. First,
1:43:45
Speaker 1: Martian life would need to represent a genuinely independent origin. Second,
1:43:50
Speaker 1: we would need to understand how representative Earth and Mars
1:43:53
Speaker 1: are of potentially habitable planets elsewhere. Third, the Great Filter
1:43:58
Speaker 1: may consist of several moderately difficult steps. rather than one
1:44:03
Speaker 1: impossibly difficult transition. And finally, the absence of obvious extraterrestrial
1:44:09
Speaker 1: technology may have explanations that do not require civilisation to
1:44:13
Speaker 1: fail at all. So a Martian bacterium would not somehow
1:44:17
Speaker 1: predict disaster for humanity. It would simply change one piece
1:44:21
Speaker 1: of an enormous probability puzzle. The same logic extends beyond Mars.
1:44:27
Speaker 1: Jupiter's moon Europa is thought to contain a vast ocean
1:44:30
Speaker 1: beneath its icy surface. Saturn's moon, Enceladus, also possesses a
1:44:36
Speaker 1: subsurface ocean, and jets of water-rich material erupt through fractures
1:44:41
Speaker 1: near its south pole and escape into space. Both worlds
1:44:45
Speaker 1: are important targets in the search for extraterrestrial biology. If
1:44:50
Speaker 1: life were discovered independently on Mars, Europa and Enceladus, then
1:44:55
Speaker 1: a solar system containing several separate origins of biology would
1:44:58
Speaker 1: strongly suggest that simple life can emerge relatively readily, when
1:45:02
Speaker 1: suitable conditions exist. That would be astonishing. It would imply
1:45:07
Speaker 1: that the universe may be biologically much richer than we
1:45:10
Speaker 1: currently know. But once again, it would raise the next question.
1:45:14
Speaker 1: If simple life begins so easily, why do we not
1:45:19
Speaker 1: see what comes after it? Perhaps the answer is simply
1:45:22
Speaker 1: that most living worlds remain microbial, There is nothing unsuccessful
1:45:27
Speaker 1: about that. Microorganisms have dominated enormous portions of Earth's history
1:45:33
Speaker 1: and remain among the most resilient and abundant forms of
1:45:36
Speaker 1: life on the planet today. Evolution has no requirement to
1:45:40
Speaker 1: produce brains, spacecraft or radio transmitters. A galaxy containing countless
1:45:47
Speaker 1: microbial worlds could therefore remain completely silent. And that is
1:45:52
Speaker 1: what makes the search for life on Mars so interesting.
1:45:56
Speaker 1: The discovery of even one genuine Martian organism would answer
1:46:01
Speaker 1: one enormous question, while immediately creating several new ones. It
1:46:06
Speaker 1: might show that Earth is not the only living world
1:46:09
Speaker 1: in the solar system. If that life had an independent origin,
1:46:13
Speaker 1: it might suggest that the transition from chemistry to biology
1:46:17
Speaker 1: is not especially rare. And if that earlier step becomes easier,
1:46:22
Speaker 1: then somewhere farther along the path from microbe to civilisation,
1:46:25
Speaker 1: another difficulty may be waiting to explain the silence. Finding
1:46:30
Speaker 1: life beyond Earth would therefore not solve the Fermi Paradox.
1:46:34
Speaker 1: It might deepen it. We could discover that the universe
1:46:37
Speaker 1: is full of biology while still having no idea why
1:46:41
Speaker 1: almost none of it seems to be talking. There is
1:46:44
Speaker 1: one term in the Drake equation that may matter more
1:46:47
Speaker 1: than almost any other. Not how many stars there are.
1:46:51
Speaker 1: not how many planets form, not even how often life begins.
1:46:56
Speaker 1: How long does a technological civilisation remain detectable? It is
1:47:00
Speaker 1: an uncomfortable question, because humanity has only one example to
1:47:04
Speaker 1: examine– ourselves. And our own technological phase has lasted for
1:47:10
Speaker 1: an astonishingly short time. For almost all of human history,
1:47:14
Speaker 1: Earth produced no radio transmissions, no artificial satellites, No industrial
1:47:20
Speaker 1: atmospheric signatures and no spacecraft leaving the planet. Then, within
1:47:26
Speaker 1: little more than a century, everything changed. Radio appeared. Aircraft
1:47:32
Speaker 1: crossed continents. Nuclear physics transformed energy and warfare. Spaceflight began.
1:47:38
Speaker 1: Computers became widespread. Human industry altered the atmosphere on a
1:47:43
Speaker 1: global scale. From an astronomical point of view, this happened
1:47:47
Speaker 1: almost instantly. That matters because Frank Drake's equation does not
1:47:52
Speaker 1: merely ask how often technological civilizations appear, it asks how
1:47:57
Speaker 1: long they remain detectable. A civilization could arise quite frequently,
1:48:02
Speaker 1: and still be extremely difficult to find if its detectable
1:48:05
Speaker 1: period is brief. Imagine that intelligent species develop advanced technology
1:48:10
Speaker 1: on many planets, but their radio-emitting phase lasts only a
1:48:14
Speaker 1: few hundred years. two civilisations could exist in the same
1:48:18
Speaker 1: region of the galaxy. and miss each other completely. One
1:48:22
Speaker 1: might disappear 10,000 years before the other invents radio. Their
1:48:27
Speaker 1: planets could orbit neighbouring stars, separated by what astronomers would
1:48:31
Speaker 1: consider a very modest distance, and neither would ever know
1:48:34
Speaker 1: the other had existed. This possibility became especially difficult to
1:48:39
Speaker 1: ignore during the 20th century. For the first time in
1:48:43
Speaker 1: Earth's history, a single species developed technology capable of causing
1:48:47
Speaker 1: destruction on a global scale. The clearest example was nuclear weapons.
1:48:52
Speaker 1: The first nuclear weapons were developed during the Second World War,
1:48:56
Speaker 1: and by the decades that followed, the combined arsenals of
1:48:59
Speaker 1: nuclear-armed states had grown large enough to make large-scale nuclear
1:49:04
Speaker 1: conflict a genuine planetary concern. That did not mean human
1:49:08
Speaker 1: extinction became inevitable. It did mean something historically new had happened.
1:49:13
Speaker 1: Technological power had reached a level where decisions made by
1:49:17
Speaker 1: a relatively small number of people could potentially affect the
1:49:20
Speaker 1: lives of billions. A sufficiently large nuclear war could destroy cities,
1:49:26
Speaker 1: disrupt infrastructure, contaminate environments and alter climate through the injection
1:49:31
Speaker 1: of smoke and soot into the atmosphere. Researchers have modelled
1:49:35
Speaker 1: scenarios in which severe nuclear conflict could cool parts of
1:49:38
Speaker 1: the planet and disrupt agriculture for years. The precise outcomes
1:49:42
Speaker 1: depend heavily on the scale and targets of a conflict,
1:49:45
Speaker 1: but the broader lesson is simple. A civilisation can become
1:49:49
Speaker 1: powerful enough to threaten the systems on which it depends,
1:49:53
Speaker 1: and nuclear weapons are only one example. Biotechnology creates another
1:49:57
Speaker 1: category of risk. Human beings have become increasingly capable of
1:50:01
Speaker 1: manipulating microorganisms, altering genomes and designing biological systems. Most of
1:50:07
Speaker 1: that capability is used for medicine, agriculture and research. But
1:50:12
Speaker 1: the same general progress raises concerns about engineered pathogens. A
1:50:16
Speaker 1: naturally occurring disease can already spread across much of the planet.
1:50:20
Speaker 1: A deliberately altered pathogen could, in principle, be made more dangerous. Again,
1:50:26
Speaker 1: this does not mean such an event is inevitable. It
1:50:30
Speaker 1: does not even mean that it would necessarily threaten the
1:50:32
Speaker 1: survival of the species, but it illustrates the same pattern.
1:50:37
Speaker 1: Our ability to change the world can advance faster than
1:50:40
Speaker 1: our ability to guarantee that every use of that power
1:50:43
Speaker 1: will remain safe. Environmental change provides a slower example. Human
1:50:48
Speaker 1: activity has already altered the composition of Earth's atmosphere. The
1:50:52
Speaker 1: concentration of carbon dioxide has risen dramatically since the beginning
1:50:56
Speaker 1: of industrialisation. largely because of fossil fuel use and land
1:51:01
Speaker 1: use change. This strengthens the greenhouse effect and drives global warming.
1:51:06
Speaker 1: Climate change can increase heat extremes, alter rainfall patterns, raise
1:51:11
Speaker 1: sea levels, disrupt ecosystems, and place growing stress on agriculture, infrastructure,
1:51:18
Speaker 1: and societies. These are serious risks. But it is important
1:51:22
Speaker 1: to separate the possibility of severe disruptions from the much
1:51:25
Speaker 1: stronger claim that climate change by itself will inevitably cause
1:51:29
Speaker 1: human extinction. Those are not the same thing. Civilisations can
1:51:33
Speaker 1: be destabilised without every individual dying. Economic systems can fail.
1:51:38
Speaker 1: Governments can collapse. Populations can move. Conflicts can intensify. Infrastructure
1:51:45
Speaker 1: can deteriorate. A technological civilisation may therefore become less capable,
1:51:50
Speaker 1: less connected, or less detectable without disappearing biologically. that distinction
1:51:56
Speaker 1: matters for the Fermi paradox. The lifetime in the Drake
1:52:00
Speaker 1: equation is often treated as the duration of a civilisation's detectable,
1:52:05
Speaker 1: technological phase. That is not necessarily the same as the
1:52:09
Speaker 1: lifetime of the species itself. Human beings could survive for
1:52:13
Speaker 1: hundreds of thousands of years, while radio broadcasting, as we
1:52:17
Speaker 1: currently understand it, occupies only a tiny fraction of that period.
1:52:21
Speaker 1: A civilisation may become quieter, as its technology advances. Communication
1:52:28
Speaker 1: could shift towards tightly directed beams. Energy use could become
1:52:32
Speaker 1: more efficient. Large amounts of radio leakage might disappear. Cities
1:52:37
Speaker 1: might become harder to distinguish at interstellar distances. A society
1:52:42
Speaker 1: could remain healthy, prosperous and technologically sophisticated, while becoming less
1:52:47
Speaker 1: obvious to distant observers. So when astronomers talk about civilisation longevity,
1:52:53
Speaker 1: they're really asking several different questions at once. How long
1:52:57
Speaker 1: does the species survive? How long does organised technological society survive?
1:53:03
Speaker 1: How long does it remain capable of astronomy and communication?
1:53:06
Speaker 1: And how long does it produce techno-signatures that somebody else
1:53:10
Speaker 1: can actually detect? Those intervals may be very different. There
1:53:15
Speaker 1: is a broader principle hiding beneath all of these examples.
1:53:18
Speaker 1: Technological capability can grow faster than wisdom. coordination, and institutions.
1:53:25
Speaker 1: A species can learn how to release nuclear energy before
1:53:28
Speaker 1: learning how to eliminate war. It can learn how to
1:53:31
Speaker 1: alter biology before establishing perfect safeguards. It can gain the
1:53:35
Speaker 1: power to transform a planetary climate before developing flawless global cooperation.
1:53:41
Speaker 1: None of this is unique to humanity in any proven sense,
1:53:45
Speaker 1: but if technological intelligence elsewhere develops through similar processes, then
1:53:50
Speaker 1: this mismatch between power and control could become one possible
1:53:53
Speaker 1: explanation for cosmic silence. A young civilisation may pass through
1:53:58
Speaker 1: a vulnerable period. Its technology becomes powerful. Its systems remain
1:54:03
Speaker 1: politically fragmented. Its decisions still depend on individuals and institutions
1:54:08
Speaker 1: shaped by much older evolutionary pressures. If enough civilisations fail
1:54:14
Speaker 1: during this period, then the galaxy could produce technological species repeatedly.
1:54:19
Speaker 1: while rarely producing ones that survive long enough to become ancient.
1:54:23
Speaker 1: This is one possible location of the Great Filter, but
1:54:26
Speaker 1: only one. There is no evidence that every civilisation must
1:54:30
Speaker 1: destroy itself. There is no known law of nature stating
1:54:33
Speaker 1: that intelligence inevitably becomes self-destructive. That would be far too
1:54:38
Speaker 1: strong a conclusion. Humanity has also demonstrated the opposite tendency.
1:54:43
Speaker 1: International agreements have limited certain weapons. Public health systems have
1:54:48
Speaker 1: contained dangerous diseases. Environmental regulations have reduced some forms of pollution.
1:54:54
Speaker 1: Scientific cooperation often crosses political boundaries. Technological societies are capable
1:55:00
Speaker 1: of recognising risks and responding to them. The question is
1:55:04
Speaker 1: whether they can continue doing so as their capabilities grow.
1:55:08
Speaker 1: And there is an important statistical point. Suppose most technological
1:55:12
Speaker 1: civilisations are short-lived. Perhaps 99 out of every 100 disappear,
1:55:17
Speaker 1: or become undetectable within a few thousand years. That sounds
1:55:21
Speaker 1: like an effective explanation for the silence. But what about
1:55:25
Speaker 1: the remaining one? If even a tiny fraction of civilizations
1:55:29
Speaker 1: become stable for millions of years, those rare survivors could
1:55:33
Speaker 1: dominate the galaxy's technological history. A civilization lasting 10 million
1:55:38
Speaker 1: years exists 1,000 times longer than one lasting 10,000 years.
1:55:43
Speaker 1: It has vastly more time to be detected. It may
1:55:46
Speaker 1: have time to send probes to neighbouring stars. Those probes
1:55:50
Speaker 1: may establish new settlements. The civilisation may spread beyond the
1:55:53
Speaker 1: vulnerability of any single planet. Once that happens, destroying the
1:55:58
Speaker 1: civilisation completely becomes much harder. A disaster on one world
1:56:03
Speaker 1: would no longer be enough. And this brings the Fermi
1:56:06
Speaker 1: Paradox back into view. Short lifetimes can explain silence, only
1:56:11
Speaker 1: if short lifetimes are common enough. If even a modest
1:56:15
Speaker 1: number of technological civilisations survive for geological timescales, their long
1:56:20
Speaker 1: presence should make them much easier to notice. Perhaps they
1:56:24
Speaker 1: would leave radio signals, perhaps artificial structures, perhaps altered planetary atmospheres,
1:56:30
Speaker 1: perhaps settlements across multiple star systems, perhaps something we have
1:56:35
Speaker 1: not yet learned to recognise. This is why civilisation lifetime
1:56:39
Speaker 1: is such a powerful term in the Drake equation. A
1:56:43
Speaker 1: galaxy could create intelligence frequently and still look empty if
1:56:46
Speaker 1: technology tends to vanish quickly. Or technological life could arise
1:56:51
Speaker 1: only rarely, yet a handful of very long-lived civilisations could
1:56:55
Speaker 1: remain visible for millions of years. At the moment, humanity
1:56:59
Speaker 1: does not know which pattern is more realistic. Our own
1:57:03
Speaker 1: experiment has barely begun. We have had powerful technology for
1:57:07
Speaker 1: only a tiny sliver of our species history. We do
1:57:11
Speaker 1: not yet know whether technological civilisation is naturally fragile. We
1:57:16
Speaker 1: do not know whether advanced societies tend to collapse, stabilise, transform, spread,
1:57:22
Speaker 1: or simply become quiet. Self-destruction therefore remains a possible explanation
1:57:27
Speaker 1: for the silence. Not a prediction. Not a destiny. and
1:57:32
Speaker 1: certainly not evidence that every intelligent species eventually destroys itself,
1:57:37
Speaker 1: it is simply one possible answer to a difficult question.
1:57:41
Speaker 1: Perhaps civilizations appear. Perhaps they shine brightly for a while.
1:57:45
Speaker 1: Perhaps they become visible across a few dozen light years.
1:57:49
Speaker 1: And then, for reasons that could be biological, political, environmental, technological,
1:57:54
Speaker 1: or completely unfamiliar to us, the signal fades. If that
1:57:59
Speaker 1: happens often enough, The Milky Way could be full of
1:58:01
Speaker 1: worlds where somebody once looked up at the stars, and
1:58:04
Speaker 1: almost none where somebody is still looking now. There is
1:58:08
Speaker 1: one explanation for the silence of the galaxy that is
1:58:12
Speaker 1: less a scientific theory and more a disturbing thought experiment.
1:58:16
Speaker 1: It is known as the Dark Forest. The idea became
1:58:20
Speaker 1: widely known through the Chinese science fiction author Liu Chixin
1:58:24
Speaker 1: and his novel The Dark Forest. It is important to
1:58:27
Speaker 1: treat it carefully. The Dark Forest is not an established
1:58:30
Speaker 1: scientific explanation for the Fermi Paradox. There is no observational
1:58:35
Speaker 1: evidence that extraterrestrial civilizations are deliberately hiding from one another. Instead,
1:58:41
Speaker 1: it is a speculative argument about risk, uncertainty, and what
1:58:46
Speaker 1: might happen when intelligent societies are separated by enormous distances.
1:58:51
Speaker 1: The metaphor is simple. Imagine the universe as a dark forest.
1:58:56
Speaker 1: Every civilization is like a hunter moving quietly between the trees.
1:59:00
Speaker 1: Nobody knows who else is nearby. Nobody knows whether another
1:59:04
Speaker 1: hunter is peaceful, frightened, aggressive, or simply cautious. And because
1:59:10
Speaker 1: revealing your position could expose you to danger, the safest
1:59:13
Speaker 1: strategy may seem to be silence. In this picture, the
1:59:17
Speaker 1: galaxy is not quiet because nobody is there. It is
1:59:21
Speaker 1: quiet because everybody is trying not to be noticed. The
1:59:24
Speaker 1: idea becomes more interesting when we add the scale of
1:59:27
Speaker 1: interstellar distance. Suppose two technological civilizations discover one another. They
1:59:34
Speaker 1: are separated by 1,000 light years. One sends a message.
1:59:39
Speaker 1: The other receives it 1,000 years later. It replies immediately.
1:59:43
Speaker 1: The answer takes another 1,000 years to return. A single
1:59:47
Speaker 1: exchange has now taken two millennia. That makes ordinary trust
1:59:52
Speaker 1: extraordinarily difficult. On Earth, two groups can communicate repeatedly. They
1:59:58
Speaker 1: can observe one another. Agreements can be tested. Promises can
2:00:02
Speaker 1: be broken or honoured. Intentions can be revised as new
2:00:06
Speaker 1: information arrives. Across interstellar distances, almost none of that is
2:00:11
Speaker 1: possible quickly. By the time one civilisation receives a message,
2:00:16
Speaker 1: the society that sent it may have changed completely. Governments
2:00:20
Speaker 1: may have disappeared. Political systems may have transformed. Technology may
2:00:26
Speaker 1: have advanced. Entire cultures could have risen and fallen. A
2:00:30
Speaker 1: peaceful civilisation today might not remain peaceful for the next
2:00:34
Speaker 1: thousand years. And the civilisation receiving the message has no
2:00:38
Speaker 1: reliable way to know. This creates what is sometimes described
2:00:42
Speaker 1: as a chain of suspicion. Imagine civilisation A discovers civilisation B.
2:00:48
Speaker 1: Civilisation A knows that it is peaceful. But Civilization A
2:00:54
Speaker 1: also knows that Civilization B cannot be certain of that.
2:00:58
Speaker 1: Civilization B therefore has a reason to be cautious. Civilization
2:01:02
Speaker 1: A knows that Civilization B has a reason to be cautious.
2:01:06
Speaker 1: So Civilization A may now worry that Civilization B could
2:01:10
Speaker 1: act offensively. Civilization B may reason in exactly the same way.
2:01:15
Speaker 1: Neither side needs to be aggressive. Neither side needs to
2:01:18
Speaker 1: want conflict. The problem arises from uncertainty. Each civilisation knows
2:01:24
Speaker 1: that the other cannot fully know its intentions, and when
2:01:27
Speaker 1: communication takes centuries or millennia, correcting misunderstandings becomes extremely difficult.
2:01:34
Speaker 1: In the darkest version of the argument, this uncertainty could
2:01:37
Speaker 1: encourage pre-emptive behaviour. A civilisation might decide that revealing itself
2:01:43
Speaker 1: is too dangerous because another society could eventually become powerful
2:01:47
Speaker 1: enough to threaten it. If technology can advance dramatically over
2:01:51
Speaker 1: relatively short astronomical timescales, then a civilisation that appears harmless
2:01:57
Speaker 1: today could become much more capable before any meaningful dialogue
2:02:01
Speaker 1: is completed. The safest action, according to this reasoning, might
2:02:06
Speaker 1: be to remain hidden. Do not broadcast. Do not build
2:02:10
Speaker 1: obvious structures. Do not send unnecessary signals. Do not announce
2:02:15
Speaker 1: your location to strangers whose motives you cannot verify. Seen
2:02:19
Speaker 1: this way, cosmic silence becomes a strategy. Perhaps advanced civilisations
2:02:25
Speaker 1: deliberately reduce their radio emissions. Perhaps they rely on tightly
2:02:30
Speaker 1: directed communication rather than broadcasting into space. Perhaps they observe
2:02:35
Speaker 1: other planetary systems while avoiding contact. Perhaps they detect young
2:02:40
Speaker 1: technological societies and simply choose not to reveal themselves. This
2:02:45
Speaker 1: turns one of the central features of the Fermi paradox
2:02:47
Speaker 1: on its head. Instead of asking why an advanced civilisation
2:02:51
Speaker 1: has not made itself obvious, the dark forest asks why
2:02:55
Speaker 1: it ever would. If visibility creates risk, then silence may
2:02:59
Speaker 1: be rational. But the argument depends on a large number
2:03:03
Speaker 1: of assumptions. The first is that many independent civilisations would
2:03:08
Speaker 1: reach similar conclusions. The Milky Way could contain species with
2:03:12
Speaker 1: completely different evolutionary histories, social structures, values, technologies … and
2:03:19
Speaker 1: attitudes towards risk. It is not obvious that all of
2:03:23
Speaker 1: them would behave cautiously. Even if most remained silent, one
2:03:27
Speaker 1: unusually curious or expansionist civilization might still announce itself, and
2:03:32
Speaker 1: over billions of years, even rare exceptions could matter. The
2:03:37
Speaker 1: second problem is that hiding may not be easy. A
2:03:40
Speaker 1: technological civilization could produce detectable signatures whether it intends to
2:03:45
Speaker 1: or not, Its planet may have unusual atmospheric chemistry. Large-scale
2:03:51
Speaker 1: industry could produce waste heat. Artificial structures might alter the
2:03:55
Speaker 1: light from a star. Spacecraft could leave traces. A society
2:04:00
Speaker 1: attempting perfect silence might discover that technology itself is difficult
2:04:05
Speaker 1: to conceal completely. The dark forest also assumes that aggression
2:04:10
Speaker 1: or pre-emptive action would provide some reliable strategic advantage. That
2:04:15
Speaker 1: is far from obvious across interstellar distances. If another civilisation
2:04:20
Speaker 1: is hundreds of light years away, attacking it would involve
2:04:24
Speaker 1: enormous practical difficulties. Any weapon or spacecraft would take centuries
2:04:29
Speaker 1: to arrive unless it travelled at an extraordinary fraction of
2:04:33
Speaker 1: the speed of light. During that time, the target might change.
2:04:38
Speaker 1: It might disappear. It might become far more technologically advanced.
2:04:43
Speaker 1: and launching an attack could reveal information about the attacker.
2:04:46
Speaker 1: The logic becomes complicated very quickly. There is also the
2:04:50
Speaker 1: question of resources. Two civilisations separated by hundreds of light
2:04:55
Speaker 1: years are not competing for the same nearby river, farmland,
2:05:00
Speaker 1: mineral deposit or energy source. The galaxy contains enormous quantities
2:05:05
Speaker 1: of matter and energy. If travelling between stars is extremely difficult,
2:05:11
Speaker 1: There may be little practical reason for distant societies to
2:05:14
Speaker 1: see one another as immediate threats. So the dark forest
2:05:18
Speaker 1: should not be treated as the natural or inevitable behaviour
2:05:21
Speaker 1: of intelligent life. It is one possibility among many. Its
2:05:25
Speaker 1: appeal comes from the way it changes the meaning of silence.
2:05:29
Speaker 1: A quiet galaxy no longer needs to be an empty one.
2:05:33
Speaker 1: Silence could itself be behaviour. That is what makes the
2:05:37
Speaker 1: idea unsettling. We usually imagine that finding another civilisation would
2:05:42
Speaker 1: begin with a signal, a message, a beacon, something obvious
2:05:47
Speaker 1: enough to announce that somebody else is there. The Dark
2:05:50
Speaker 1: Forest asks us to imagine the opposite. Perhaps the most
2:05:54
Speaker 1: advanced societies are the ones that learned not to call
2:05:57
Speaker 1: attention to themselves. Perhaps the galaxy contains listeners rather than broadcasters.
2:06:04
Speaker 1: Perhaps everyone is waiting for someone else to speak first.
2:06:07
Speaker 1: and perhaps that is why nothing appears to happen. But
2:06:11
Speaker 1: there is a very important final point. At present, this
2:06:15
Speaker 1: remains speculation. We have no evidence that extraterrestrial civilisations exist.
2:06:21
Speaker 1: We have no evidence that any are hiding. We have
2:06:24
Speaker 1: no evidence that intelligent life commonly behaves according to dark
2:06:28
Speaker 1: forest reasoning. The idea is powerful because it exposes a
2:06:33
Speaker 1: genuine problem of trust across enormous distances. It is compelling
2:06:38
Speaker 1: because it offers a way for a populated galaxy to
2:06:41
Speaker 1: remain quiet. But it does not solve the Fermi paradox.
2:06:47
Speaker 1: It simply gives the silence another possible meaning. Perhaps there
2:06:52
Speaker 1: is nobody in the forest, or perhaps there are many,
2:06:56
Speaker 1: and if the dark forest idea were ever correct, the
2:06:59
Speaker 1: most unsettling possibility would be that the silence is not
2:07:03
Speaker 1: accidental at all. It is the sound everyone has agreed,
2:07:07
Speaker 1: independently to make. There is another possibility that sits somewhere
2:07:12
Speaker 1: between the unsettling and the strangely comforting. Perhaps somebody already
2:07:17
Speaker 1: knows we are here. In 1973, the astronomer John Ball
2:07:22
Speaker 1: published an idea in the scientific journal Icarus that became
2:07:25
Speaker 1: known as the Zoo Hypothesis. The comparison is exactly what
2:07:30
Speaker 1: the name suggests. Imagine Earth as a kind of protected reserve.
2:07:35
Speaker 1: Humanity is allowed to develop naturally, without direct interference from
2:07:39
Speaker 1: more advanced civilisations, in much the same way that people
2:07:42
Speaker 1: sometimes observe wild animals while trying not to disturb their behaviour.
2:07:47
Speaker 1: Under this idea, the silence around us does not mean
2:07:50
Speaker 1: nobody has noticed Earth. It means that anybody who has
2:07:54
Speaker 1: noticed has deliberately chosen not to make contact. Perhaps they
2:07:58
Speaker 1: watch from a distance. Perhaps they avoid transmitting towards us.
2:08:03
Speaker 1: Perhaps they have rules preventing interference with younger technological societies.
2:08:08
Speaker 1: The idea has obvious appeal because it offers a simple
2:08:11
Speaker 1: explanation for one part of the Fermi Paradox. If advanced
2:08:15
Speaker 1: civilisations are capable of finding inhabited planets, but intentionally leave
2:08:20
Speaker 1: them alone, then the absence of visitors is no longer surprising.
2:08:25
Speaker 1: We would not expect obvious spacecraft. We would not expect
2:08:29
Speaker 1: large structures in the solar system. We would not necessarily
2:08:33
Speaker 1: expect messages. We might simply be living inside a carefully
2:08:37
Speaker 1: maintained silence. There is, however, an enormous problem. A zoo
2:08:42
Speaker 1: requires someone to maintain it. If the Milky Way contains
2:08:46
Speaker 1: many independent technological civilisations, then all of them would need
2:08:51
Speaker 1: to respect the same broad policy of non-interference. That is
2:08:55
Speaker 1: not impossible. But it is a very strong assumption. Imagine
2:08:59
Speaker 1: thousands of societies arising at different times around different stars,
2:09:04
Speaker 1: with completely different biological histories, political systems, technologies, values and
2:09:10
Speaker 1: attitudes towards curiosity. For Earth to remain perfectly isolated, every
2:09:16
Speaker 1: civilisation capable of reaching or contacting us would somehow need
2:09:20
Speaker 1: to agree not to do so. One civilisation breaking the
2:09:24
Speaker 1: rule would be enough. One curious probe, one careless transmission,
2:09:30
Speaker 1: one group deciding that introducing themselves was harmless. After millions
2:09:35
Speaker 1: or billions of years, maintaining complete silence would require remarkable consistency.
2:09:41
Speaker 1: Perhaps one extremely old civilisation dominates a large region of
2:09:45
Speaker 1: the galaxy and enforces such a policy. Perhaps advanced societies
2:09:51
Speaker 1: converge on similar ethical principles. Perhaps communication with younger civilisations
2:09:56
Speaker 1: is regarded as irresponsible. But these are additional assumptions rather
2:10:01
Speaker 1: than observations. There is currently no evidence that any such
2:10:05
Speaker 1: policy exists, and perhaps deliberate hiding is not required at all.
2:10:11
Speaker 1: Another possibility is simply that we do not know what
2:10:14
Speaker 1: to listen for. Humanity's earliest serious searches for extraterrestrial intelligence
2:10:20
Speaker 1: concentrated heavily on radio. That made sense. Radio waves travel
2:10:25
Speaker 1: through interstellar space. They can be produced deliberately. Their frequencies
2:10:31
Speaker 1: can be measured with extraordinary precision, and human civilization had
2:10:35
Speaker 1: already discovered how useful radio was for communication. But this
2:10:40
Speaker 1: creates a possible bias. We may be assuming that another
2:10:44
Speaker 1: civilization would communicate in ways that resemble our own technological history.
2:10:49
Speaker 1: Imagine somebody waiting beside a 19th century telegraph machine for
2:10:53
Speaker 1: a modern smartphone call. Nothing arrives. The conclusion should probably
2:10:59
Speaker 1: not be that nobody is trying to communicate. The equipment
2:11:02
Speaker 1: may simply be wrong. The same could apply on a
2:11:05
Speaker 1: much greater scale. An advanced civilization may use technology that
2:11:10
Speaker 1: we have not invented or or perhaps not even imagined.
2:11:14
Speaker 1: Its communication could be highly directional. Its transmissions might occupy
2:11:19
Speaker 1: frequencies we rarely observe. It could use extremely brief pulses
2:11:23
Speaker 1: rather than continuous signals. Or communication itself might be almost
2:11:28
Speaker 1: irrelevant to our chances of detecting it. This is why
2:11:33
Speaker 1: the search for extraterrestrial technology has gradually expanded beyond radio.
2:11:39
Speaker 1: Astronomers now discuss a much broader category known as technosignatures.
2:11:45
Speaker 1: A technosignature is any observable evidence that could indicate the
2:11:50
Speaker 1: presence of technology. One possibility is powerful laser light. A
2:11:55
Speaker 1: civilisation could use lasers to send very short, intense optical
2:12:00
Speaker 1: pulses between stars. Under suitable conditions, Pulse could briefly stand
2:12:06
Speaker 1: out against the natural light of the star around which
2:12:09
Speaker 1: the civilisation lives. Searching for such flashes is known as
2:12:14
Speaker 1: optical search for extraterrestrial intelligence. Another possibility lies in planetary atmospheres.
2:12:21
Speaker 1: Human industry has altered the chemistry of Earth's atmosphere. Some
2:12:25
Speaker 1: artificial compounds are difficult to produce naturally in large quantities.
2:12:30
Speaker 1: In principle, astronomers studying the atmosphere of a distant planet
2:12:34
Speaker 1: might one day detect unusual chemical combinations that suggest industrial activity.
2:12:40
Speaker 1: Heat offers another possibility. Technology uses energy, and ultimately much
2:12:46
Speaker 1: of that energy becomes heat. A civilization operating on an
2:12:50
Speaker 1: enormous scale might produce more infrared radiation than expected from
2:12:55
Speaker 1: natural processes alone. Even if it never sent a message,
2:12:59
Speaker 1: its energy use could leave a detectable thermal signature. then
2:13:04
Speaker 1: there are the most ambitious possibilities. A sufficiently advanced civilisation
2:13:09
Speaker 1: might build structures on scales far beyond anything humanity has attempted.
2:13:15
Speaker 1: One famous concept is the Dyson Sphere, associated with physicist
2:13:20
Speaker 1: Freeman Dyson. The idea is not necessarily a solid shell
2:13:25
Speaker 1: around a star, despite the way it is often portrayed.
2:13:28
Speaker 1: A more realistic version could involve vast numbers of orbiting
2:13:32
Speaker 1: structures designed to capture a significant fraction of a star's energy.
2:13:37
Speaker 1: Such a system could alter the way the star appears.
2:13:41
Speaker 1: Visible light might be partially blocked, while large amounts of
2:13:44
Speaker 1: energy are re-emitted as infrared radiation. Astronomers can therefore search
2:13:49
Speaker 1: for stars with unusual combinations of brightness and heat that
2:13:53
Speaker 1: might be difficult to explain through ordinary astrophysics. No convincing
2:13:58
Speaker 1: example has been confirmed, But the important point is that
2:14:02
Speaker 1: radio signals are only one possible sign of technology. The
2:14:06
Speaker 1: wider the search becomes, the more careful we need to
2:14:09
Speaker 1: be about interpreting silence. There is also another problem that
2:14:13
Speaker 1: receives less attention than it deserves. Timing. Suppose a technological
2:14:19
Speaker 1: civilisation appears around a star 5,000 light-years away. It broadcasts
2:14:24
Speaker 1: powerful signals for 10,000 years. That sounds generous. 10,000 years
2:14:30
Speaker 1: is far longer than humanity has possessed radio. Yet the
2:14:34
Speaker 1: chances of overlap are still limited. If its technological period
2:14:39
Speaker 1: ended 2 million years ago, we could point every radio
2:14:43
Speaker 1: telescope on Earth towards that system today and hear nothing.
2:14:47
Speaker 1: Its signal would already have passed us. Now, reverse the timing.
2:14:53
Speaker 1: Suppose that same civilization does not appear until 2 million
2:14:57
Speaker 1: years from now. Again, we hear nothing. Two inhabited worlds
2:15:02
Speaker 1: can exist in the same galaxy and still miss one
2:15:05
Speaker 1: another completely. Their biological histories may overlap for billions of
2:15:10
Speaker 1: years while their detectable technological phases fail to coincide at all.
2:15:15
Speaker 1: This is one of the strangest consequences of astronomical time.
2:15:19
Speaker 1: Searching the sky is also searching the past. When we
2:15:23
Speaker 1: observe a star 100 light years away, we see it
2:15:27
Speaker 1: as it was 100 years ago. At 1,000 light years,
2:15:31
Speaker 1: we see 1,000 years into its past. At 10,000 light years,
2:15:36
Speaker 1: the picture we receive began travelling towards us before much
2:15:40
Speaker 1: of recorded human history had occurred. The farther we look,
2:15:45
Speaker 1: the older the information becomes. So even if we detect
2:15:49
Speaker 1: a planet with a civilisation, We do not see that
2:15:52
Speaker 1: civilisation as it exists now. We see whatever it was
2:15:57
Speaker 1: doing when the light began its journey. A society visible
2:16:01
Speaker 1: to us today may no longer exist. A society that
2:16:05
Speaker 1: exists today may not become visible to us for centuries.
2:16:09
Speaker 1: A message sent from the far side of the Milky
2:16:11
Speaker 1: Way before human civilisation began could still be travelling towards Earth.
2:16:17
Speaker 1: This makes the Fermi Paradox a problem not only of distance,
2:16:21
Speaker 1: but of synchronisation. The Milky Way could host many inhabited
2:16:26
Speaker 1: planets whose technological windows barely overlap. Some may have existed
2:16:31
Speaker 1: in the distant past. Others may lie in the distant future.
2:16:36
Speaker 1: Some could be transmitting now, but their signals have not
2:16:40
Speaker 1: yet reached us. Others may already have received the first
2:16:43
Speaker 1: faint evidence of humanity, while any reply remains centuries away.
2:16:49
Speaker 1: And perhaps some know that Earth exists and have deliberately
2:16:52
Speaker 1: remained quiet. The zoo hypothesis is one way to imagine
2:16:57
Speaker 1: that possibility. It is speculative. It requires assumptions about behaviour
2:17:03
Speaker 1: and coordination that we have no evidence for. But it
2:17:06
Speaker 1: reminds us of something important. Silence does not have only
2:17:10
Speaker 1: one interpretation. Perhaps nobody is speaking. Perhaps somebody is speaking
2:17:16
Speaker 1: in a way we do not recognise. Perhaps the conversation
2:17:19
Speaker 1: happened millions of years ago. Perhaps it has not begun yet.
2:17:24
Speaker 1: Or perhaps, somewhere beyond the reach of our instruments, somebody
2:17:28
Speaker 1: has already noticed this small blue planet and decided, for
2:17:33
Speaker 1: reasons we cannot know, simply to watch. There is another
2:17:37
Speaker 1: possibility that requires no hidden civilisation, no galactic conspiracy, and
2:17:43
Speaker 1: no catastrophic great filter. Perhaps we are simply early. The
2:17:49
Speaker 1: universe is... old by human standards. It is roughly 13.8
2:17:54
Speaker 1: billion years old, which is a span of time so
2:17:58
Speaker 1: large that ordinary intuition gives up almost immediately. Stars had
2:18:02
Speaker 1: already been forming for billions of years before the sun appeared.
2:18:07
Speaker 1: Galaxies had assembled, collided, and evolved. Generations of stars had
2:18:13
Speaker 1: lived and died. Heavy elements had been forged inside stellar
2:18:18
Speaker 1: interiors and scattered back into space. From our perspective, the
2:18:23
Speaker 1: universe can seem ancient. But there is another way to
2:18:28
Speaker 1: measure its age. Instead of asking how long it has existed,
2:18:32
Speaker 1: we can ask how much ordinary stellar history may still remain.
2:18:37
Speaker 1: And suddenly, 13.8 billion years begins to look surprisingly young.
2:18:43
Speaker 1: The reason is the smallest stars. The Sun is expected
2:18:46
Speaker 1: to spend roughly 10 billion years in its main hydrogen
2:18:50
Speaker 1: burning phase. It has already used around half of that time.
2:18:54
Speaker 1: Massive stars live much shorter lives. Some burn so intensely
2:18:59
Speaker 1: that they exhaust their nuclear fuel in only a few
2:19:02
Speaker 1: million years. But the smallest red dwarf stars behave very differently.
2:19:07
Speaker 1: They are dim. They are cool. They consume their hydrogen
2:19:11
Speaker 1: extraordinarily slowly. And because of the way material can circulate
2:19:16
Speaker 1: through much of their interiors, they can make very efficient
2:19:20
Speaker 1: use of their available fuel. The smallest red dwarfs may
2:19:24
Speaker 1: continue shining for trillions of years. Not billions. Trillions. The
2:19:31
Speaker 1: universe is therefore only a tiny fraction of the way
2:19:34
Speaker 1: through the potential lifetimes. of its longest-lived ordinary stars. No
2:19:41
Speaker 1: red dwarf born in the early universe has yet had
2:19:43
Speaker 1: enough time to reach the natural end of such an
2:19:46
Speaker 1: extraordinarily long main sequence lifetime. The oldest of them are,
2:19:52
Speaker 1: in a sense, still near the beginning. That creates an
2:19:55
Speaker 1: interesting shift in perspective. Human beings look around at a
2:19:59
Speaker 1: universe almost 14 billion years old and ask why technological
2:20:03
Speaker 1: life has not already filled it. but perhaps the more
2:20:07
Speaker 1: remarkable fact is that life like us appeared this soon.
2:20:11
Speaker 1: There may be vastly more stellar time ahead than behind.
2:20:15
Speaker 1: This does not mean most stars that will ever exist
2:20:18
Speaker 1: are still waiting to form, quite the opposite. The cosmic
2:20:22
Speaker 1: star formation rate reached its peak billions of years ago,
2:20:26
Speaker 1: when the universe was much younger than it is today.
2:20:30
Speaker 1: Galaxies were then producing stars far more rapidly than they
2:20:34
Speaker 1: generally do now. Since that peak, star formation across the
2:20:38
Speaker 1: universe has declined. Gas has been consumed. Some has been heated.
2:20:44
Speaker 1: Some has been expelled from galaxies. New stars still form today,
2:20:49
Speaker 1: including within the Milky Way, but the universe is no
2:20:52
Speaker 1: longer manufacturing them at anything like its former rate. So
2:20:56
Speaker 1: the argument is not that the main age of stars
2:20:59
Speaker 1: has yet to begin. It is that many of the
2:21:02
Speaker 1: stars already here can continue shining. for periods of time
2:21:06
Speaker 1: that dwarf the current age of the cosmos, and conventional
2:21:09
Speaker 1: star formation itself is expected to continue at ever-decreasing rates
2:21:15
Speaker 1: for an extremely long time before the necessary cold gas
2:21:19
Speaker 1: becomes too scarce or inaccessible. Farther still into the future,
2:21:23
Speaker 1: the last long-lived stars will continue burning after new star
2:21:27
Speaker 1: formation has effectively ended. On those scales, the present day
2:21:31
Speaker 1: becomes a remarkably early moment, There is another reason intelligent
2:21:35
Speaker 1: life may have needed time before it could appear at all.
2:21:39
Speaker 1: The earliest generations of stars formed from material dominated by
2:21:43
Speaker 1: hydrogen and helium, with only tiny amounts of heavier elements.
2:21:47
Speaker 1: Astronomers use the term metals rather broadly for elements heavier
2:21:51
Speaker 1: than helium, even though a chemist might reasonably object to
2:21:54
Speaker 1: calling oxygen a metal. Those heavier elements are essential for
2:21:58
Speaker 1: worlds like ours. Carbon is central to terrestrial biology. Oxygen
2:22:03
Speaker 1: is part of water and many biological molecules. Silicon and
2:22:08
Speaker 1: oxygen help form rocky planetary material. Iron contributes to the
2:22:13
Speaker 1: structure of Earth and its core. Elements like these were
2:22:16
Speaker 1: not present in large quantities immediately after the Big Bang.
2:22:20
Speaker 1: They had to be manufactured inside stars. Some were produced
2:22:24
Speaker 1: through nuclear fusion during stellar lifetimes. Others were created or
2:22:28
Speaker 1: distributed in explosive events such as supernovae. Over successive generations
2:22:34
Speaker 1: of star formation, galaxies gradually became enriched. New stars formed
2:22:39
Speaker 1: from clouds containing more of these heavier elements. Around those stars,
2:22:44
Speaker 1: rocky planets could form more readily from material that simply
2:22:47
Speaker 1: had not been abundant in the earliest cosmic era. In
2:22:50
Speaker 1: that sense, the universe had to prepare itself. Stars had
2:22:54
Speaker 1: to live before some kinds of planets could exist. Stars
2:22:58
Speaker 1: had to die before many of the ingredients of those
2:23:00
Speaker 1: planets could become widespread. galactic chemistry had to evolve. Only
2:23:06
Speaker 1: later did environments like the solar system become possible. That
2:23:10
Speaker 1: does not mean Earth sits at some uniquely privileged moment.
2:23:14
Speaker 1: Rocky planets appear to have existed billions of years before
2:23:18
Speaker 1: the solar system, but it does remind us that biological
2:23:21
Speaker 1: opportunity has not necessarily been uniform throughout cosmic history. The
2:23:26
Speaker 1: very early universe was a different place, and the far
2:23:29
Speaker 1: future will be different again. This raises an unusual possibility.
2:23:34
Speaker 1: Perhaps technological civilisation eventually becomes common. Perhaps life is not
2:23:40
Speaker 1: extraordinarily difficult to start. Perhaps intelligence is not a one-in-a-trillion accident.
2:23:46
Speaker 1: Perhaps technological societies do not usually destroy themselves. Even then,
2:23:51
Speaker 1: humanity could still belong to one of the earlier generations.
2:23:54
Speaker 1: Imagine a small red dwarf that will remain stable for
2:23:57
Speaker 1: several trillion years. A planet around it might not become
2:24:01
Speaker 1: biologically interesting now. Perhaps conditions suitable for complex life develop
2:24:06
Speaker 1: hundreds of billions of years in the future. To such
2:24:10
Speaker 1: a future civilisation, the present day would belong to an
2:24:13
Speaker 1: almost unimaginably ancient cosmic past. The Milky Way itself may
2:24:19
Speaker 1: have changed. The Sun would be long gone. Earth would
2:24:23
Speaker 1: no longer exist in anything resembling its current form. Humanity,
2:24:27
Speaker 1: if remembered at all, would belong to the earliest pages
2:24:30
Speaker 1: of technological history. If this picture is correct, then asking
2:24:35
Speaker 1: why we do not see countless ancient civilizations may contain
2:24:38
Speaker 1: a hidden assumption. We assume that the present is a
2:24:41
Speaker 1: typical time for technological life, but perhaps it is not.
2:24:46
Speaker 1: Perhaps most technological civilizations that will ever exist have not
2:24:50
Speaker 1: yet appeared. Not because their stars do not exist. Not
2:24:54
Speaker 1: because their planets have not necessarily formed. But because the
2:24:58
Speaker 1: amount of potentially habitable time still ahead is so enormous,
2:25:03
Speaker 1: a world might take 5 billion years to produce intelligence.
2:25:07
Speaker 1: Another might take 20 billion. Another might remain biologically simple
2:25:11
Speaker 1: for 100 billion years before something changes. There is no
2:25:15
Speaker 1: rule saying that Earth's timetable must be typical. Our planet
2:25:19
Speaker 1: happened to produce a technological species within roughly 4.5 billion
2:25:24
Speaker 1: years of its formation. That may be fast. It may
2:25:27
Speaker 1: be slow. With one example, we cannot know. This offers
2:25:32
Speaker 1: one of the gentler possible answers to the Fermi paradox.
2:25:36
Speaker 1: Perhaps the Milky Way is not silent because intelligence always
2:25:40
Speaker 1: destroys itself. Perhaps nobody is hiding. Perhaps there is no
2:25:44
Speaker 1: impossible biological barrier preventing complex life. Perhaps the cosmic schedule
2:25:50
Speaker 1: has simply not advanced very far. The universe may contain
2:25:53
Speaker 1: immense numbers of future opportunities for life. that have not
2:25:57
Speaker 1: yet had time to develop. Technological civilisations could eventually become
2:26:02
Speaker 1: far more common than they are today, while still being
2:26:06
Speaker 1: extremely rare at this moment. And if so, the silence
2:26:10
Speaker 1: around us would mean something very different. We would not
2:26:14
Speaker 1: be the survivors of some terrible universal test. We would
2:26:18
Speaker 1: not necessarily be alone forever. We might simply be among
2:26:21
Speaker 1: the early arrivals. the strange thing would no longer be
2:26:25
Speaker 1: that the universe has remained quiet for 13.8 billion years.
2:26:30
Speaker 1: The strange thing would be that with trillions of years
2:26:33
Speaker 1: potentially still ahead around the smallest stars, somebody became capable
2:26:37
Speaker 1: of asking the question this soon. There is a very
2:26:40
Speaker 1: simple possibility that can become surprisingly easy to forget. Perhaps
2:26:45
Speaker 1: the galaxy seems silent because we have hardly listened to it,
2:26:49
Speaker 1: Human beings have been conducting serious searches for extraterrestrial technology
2:26:54
Speaker 1: for only a little more than half a century. That
2:26:57
Speaker 1: feels like a long time when measured against a human life.
2:27:00
Speaker 1: Against the size and age of the Milky Way, it
2:27:03
Speaker 1: is almost nothing. There are hundreds of billions of stars
2:27:06
Speaker 1: in our galaxy. There are probably at least as many planets.
2:27:10
Speaker 1: Signals could arrive at countless different frequencies, with different strengths, durations, bandwidths, directions,
2:27:17
Speaker 1: repetition patterns, forms of polarisation, and we have examined only
2:27:22
Speaker 1: a tiny fraction of those possibilities. Astronomer Jill Tarter has
2:27:27
Speaker 1: often described the problem using the idea of a cosmic haystack.
2:27:31
Speaker 1: Searching for extraterrestrial technology is not simply a matter of
2:27:34
Speaker 1: pointing a telescope towards every star once. The thing we
2:27:38
Speaker 1: are looking for has many dimensions, We need to choose
2:27:41
Speaker 1: a position in the sky. We need to choose a
2:27:44
Speaker 1: range of radio frequencies. We need enough sensitivity to detect
2:27:49
Speaker 1: the signal. We need to observe at the correct time.
2:27:52
Speaker 1: If the signal repeats, we need to return at the
2:27:55
Speaker 1: correct interval. A transmitter might operate continuously. or once every minute,
2:28:01
Speaker 1: or once a year, or once every thousand years. It
2:28:05
Speaker 1: might occupy an extremely narrow radio frequency. It might spread
2:28:09
Speaker 1: its power across a broad range. It might be strongly polarised.
2:28:14
Speaker 1: It might drift in frequency because the transmitting planet is
2:28:17
Speaker 1: rotating or orbiting its star. It might be aimed directly
2:28:21
Speaker 1: towards Earth, or we might encounter only a weak signal
2:28:24
Speaker 1: leaking sideways from a transmission intended for somewhere else. Every
2:28:29
Speaker 1: one of these possibilities adds another dimension to the search.
2:28:32
Speaker 1: In 2018, astronomers Jason Wright, Shubham Kanodia and Emily Lubar
2:28:38
Speaker 1: attempted to quantify just how much of this enormous search
2:28:41
Speaker 1: space had actually been examined by previous radio searches. Their
2:28:46
Speaker 1: conclusion was rather humbling. Using one particular definition of the
2:28:50
Speaker 1: Cosmic Haystack, The portion searched was comparable to taking something
2:28:55
Speaker 1: like the volume of a large hot tub or small
2:28:57
Speaker 1: swimming pool from all the water in Earth's oceans, and
2:29:00
Speaker 1: then checking that sample for fish. You would learn something
2:29:03
Speaker 1: from the experiment, but announcing that the oceans contained no
2:29:07
Speaker 1: fish would be slightly premature. That comparison is not exact
2:29:12
Speaker 1: in every imaginable sense, because the search for extraterrestrial technology
2:29:16
Speaker 1: is not a perfectly uniform sampling exercise. Astronomers deliberately choose
2:29:21
Speaker 1: promising targets. They observe nearby stars. They concentrate on frequencies
2:29:26
Speaker 1: where signals may travel efficiently through interstellar space. They search
2:29:31
Speaker 1: regions associated with interesting planetary systems. They design algorithms specifically
2:29:37
Speaker 1: to identify signals that look artificial rather than natural. So
2:29:41
Speaker 1: the water we have sampled from the cosmic ocean has
2:29:44
Speaker 1: not been scooped out completely at random. Even so, The
2:29:48
Speaker 1: scale of what remains unsearched is extraordinary, and modern technology
2:29:53
Speaker 1: is allowing that volume to grow rapidly. Early searchers could
2:29:56
Speaker 1: examine relatively narrow frequency ranges. Modern receivers can observe much
2:30:01
Speaker 1: broader sections of the radio spectrum. Powerful computers can divide
2:30:06
Speaker 1: those observations into enormous numbers of channels and analyse them simultaneously.
2:30:12
Speaker 1: Large surveys can inspect many more stars than Frank Drake
2:30:15
Speaker 1: could realistically have attempted during Project OSMA. Data gathered for
2:30:20
Speaker 1: ordinary astronomy can sometimes be searched for technological signals as well.
2:30:25
Speaker 1: Each improvement expands the portion of the haystack humanity has examined,
2:30:30
Speaker 1: but the haystack expands in our imagination too. Radio is
2:30:34
Speaker 1: no longer the only possibility. There are optical laser signals,
2:30:38
Speaker 1: infrared waste heat, unusual planetary atmospheres, artificial structures, brief transient events,
2:30:46
Speaker 1: and potentially forms of technology we have not yet thought
2:30:49
Speaker 1: to search for. Our apparent silence may therefore say less
2:30:53
Speaker 1: about the galaxy than we instinctively assume. There is another
2:30:57
Speaker 1: assumption worth questioning. The strongest versions of the Fermi Paradox
2:31:02
Speaker 1: often imagine that once a technological civilisation becomes capable of
2:31:06
Speaker 1: interstellar travel, expansion should eventually follow. But perhaps that step
2:31:12
Speaker 1: is much more difficult than the simple model suggests. The
2:31:15
Speaker 1: nearest star system to the Sun is more than four
2:31:18
Speaker 1: light-years away. Our fastest spacecraft would require thousands of years
2:31:23
Speaker 1: to cross a distance like that, if aimed there using
2:31:26
Speaker 1: present technology. Future spacecraft could be much faster, but increasing
2:31:31
Speaker 1: speed creates enormous engineering challenges. A large interstellar spacecraft must
2:31:36
Speaker 1: first be accelerated. That requires energy. And the faster the
2:31:41
Speaker 1: spacecraft becomes, the more energy must be supplied. Then the
2:31:44
Speaker 1: vehicle must survive the journey. Interstellar space is extremely empty
2:31:49
Speaker 1: by terrestrial standards, but it is not perfectly empty. There
2:31:53
Speaker 1: are atoms, there are charged particles, there are grains of dust.
2:31:58
Speaker 1: At ordinary spacecraft speeds, these are manageable. At a significant
2:32:03
Speaker 1: fraction of the speed of light, even tiny particles can
2:32:06
Speaker 1: strike with enormous energy. Radiation poses another difficulty. A biological
2:32:12
Speaker 1: crew travelling for decades or centuries would need protection from
2:32:16
Speaker 1: energetic particles. A machine travelling alone might avoid the problems
2:32:20
Speaker 1: of food and psychology, but it would still need extraordinary reliability.
2:32:25
Speaker 1: Components fail, materials degrade, radiation damages electronics. Something designed to
2:32:32
Speaker 1: operate without repair crews for centuries would need to be
2:32:35
Speaker 1: far more durable. than almost anything humanity currently builds. Then
2:32:40
Speaker 1: there is the problem of slowing down. Accelerating towards another
2:32:44
Speaker 1: star is only half the journey. A spacecraft travelling at
2:32:48
Speaker 1: tremendous speed cannot simply arrive and apply the brakes as
2:32:52
Speaker 1: though it has found a convenient motorway exit. It must
2:32:55
Speaker 1: somehow lose an enormous amount of kinetic energy if it
2:32:58
Speaker 1: intends to enter the destination system rather than race straight
2:33:02
Speaker 1: through it. And arrival itself does not create a colony.
2:33:06
Speaker 1: Settlers would need energy, shelter, raw materials, food or some
2:33:12
Speaker 1: alternative biological support system, industry capable of producing replacement parts,
2:33:18
Speaker 1: the ability to survive without assistance from the civilization that
2:33:21
Speaker 1: launched them. If machines make the journey instead, those machines
2:33:25
Speaker 1: would need to build and maintain extraordinarily complicated technology using
2:33:30
Speaker 1: whatever resources are available locally. Interstellar expansion may therefore be
2:33:36
Speaker 1: physically possible while remaining unattractive. Those are not the same thing.
2:33:41
Speaker 1: Human beings can technically live in Antarctica. That does not
2:33:46
Speaker 1: mean every available square kilometre of Antarctica immediately fills with cities.
2:33:51
Speaker 1: A technological civilisation might be capable of reaching other stars
2:33:55
Speaker 1: and simply decide that there is little reason to do so.
2:33:58
Speaker 1: Its home system could contain enormous resources. Asteroids provide metals,
2:34:04
Speaker 1: stars provide energy. Artificial habitats could create living space without
2:34:09
Speaker 1: journeys lasting generations. Digital environments might eventually matter more to
2:34:14
Speaker 1: such a society than physical expansion. The assumption that every
2:34:19
Speaker 1: successful civilization eventually spreads in all directions may simply reflect
2:34:23
Speaker 1: our own ideas about exploration and growth. Even civilizations that
2:34:28
Speaker 1: do expand need not occupy everything. Models of galactic settlement
2:34:33
Speaker 1: can behave very differently when colonies are allowed to choose
2:34:37
Speaker 1: whether or not to launch further missions. Some systems may
2:34:40
Speaker 1: be attractive, others may be ignored. Expansion might stop for
2:34:45
Speaker 1: thousands of years and begin again. Separate regions could develop independently.
2:34:51
Speaker 1: A civilization could spread through part of the Milky Way
2:34:54
Speaker 1: while leaving enormous areas untouched. The result could resemble islands
2:34:59
Speaker 1: and empty seas, rather than a completely filled galaxy. That
2:35:04
Speaker 1: weakens one of the most dramatic forms of Fermi's question.
2:35:08
Speaker 1: If a single expansionist civilisation does not inevitably occupy every
2:35:13
Speaker 1: star system, then the absence of extraterrestrial visitors in the
2:35:17
Speaker 1: solar system becomes less surprising. Perhaps somebody has spread through
2:35:21
Speaker 1: another part of the galaxy. Perhaps they never came this way.
2:35:25
Speaker 1: Perhaps there is simply nothing here they particularly need. and
2:35:29
Speaker 1: perhaps the real answer to the Fermi paradox is not
2:35:32
Speaker 1: one spectacular explanation at all. We naturally look for a
2:35:36
Speaker 1: single missing piece, one great filter, one universal catastrophe, one
2:35:42
Speaker 1: hidden rule obeyed by every intelligent civilisation. But several smaller
2:35:47
Speaker 1: effects could work together. Life might be somewhat uncommon. Complex
2:35:52
Speaker 1: intelligence might be rarer still. Technological civilizations may appear at
2:35:57
Speaker 1: widely separated points in galactic history. Their easily detectable phases
2:36:02
Speaker 1: may be brief. Signals might be weak. They may use
2:36:06
Speaker 1: technologies we do not recognize. Interstellar journeys could be expensive
2:36:10
Speaker 1: enough that few societies attempt them. Those that do travel
2:36:14
Speaker 1: may settle selectively rather than filling the Milky Way. and
2:36:18
Speaker 1: humanity may have searched such a small fraction of the
2:36:21
Speaker 1: relevant possibilities that our present silence is not particularly remarkable.
2:36:26
Speaker 1: None of these ideas requires the galaxy to be completely empty.
2:36:31
Speaker 1: Imagine that only one technological civilisation exists for every million stars.
2:36:36
Speaker 1: That would still leave an enormous number across the Milky Way.
2:36:41
Speaker 1: Yet the average separation between them could be so large,
2:36:45
Speaker 1: their periods of activity so poorly synchronised, and their signal
2:36:49
Speaker 1: so difficult to detect that each civilization experiences the same
2:36:53
Speaker 1: apparent solitude. Everyone could look into the darkness and conclude
2:36:58
Speaker 1: that the galaxy seems strangely quiet. That may be the
2:37:02
Speaker 1: most useful caution surrounding the entire Fermi paradox. We should
2:37:06
Speaker 1: not confuse what we have failed to detect with everything
2:37:09
Speaker 1: that could possibly exist. Humanity has searched only briefly. Our
2:37:14
Speaker 1: instruments have been capable of serious interstellar listening for a
2:37:17
Speaker 1: tiny period of time. We have visited only a handful
2:37:21
Speaker 1: of worlds directly. The farthest human spacecraft have not come
2:37:25
Speaker 1: remotely close to another star. Almost every planet in the
2:37:29
Speaker 1: Milky Way remains unseen as an individual world. Almost every
2:37:34
Speaker 1: moment of possible transmission has passed without anyone on Earth watching.
2:37:38
Speaker 1: Perhaps technological life is exceptionally rare. Perhaps we genuinely are alone.
2:37:44
Speaker 1: Those possibilities remain open, but so does a much less
2:37:47
Speaker 1: dramatic explanation. The galaxy may seem silent because distances are enormous.
2:37:53
Speaker 1: Timing is unforgiving. Technology is difficult to recognise. Interstellar travel
2:37:59
Speaker 1: is harder than our thought experiments suggest, and our search
2:38:02
Speaker 1: has barely begun. We have lowered a small cup into
2:38:06
Speaker 1: an ocean and found nothing swimming inside it. For now,
2:38:10
Speaker 1: the scientifically careful response is not to declare the ocean empty.
2:38:14
Speaker 1: It is to keep looking. More than three quarters of
2:38:18
Speaker 1: a century ago, Enrico Fermi sat down to lunch at
2:38:21
Speaker 1: Fuller Lodge in Los Alamos and asked a question that
2:38:23
Speaker 1: was almost comically small compared with the subject it opened.
2:38:27
Speaker 1: Where is everybody? It was not delivered as a grand declaration.
2:38:32
Speaker 1: There was no dramatic announcement that one of the greatest
2:38:34
Speaker 1: mysteries in science had just been identified. It was simply
2:38:38
Speaker 1: a question dropped into an ordinary conversation. And yet it
2:38:42
Speaker 1: has endured because the more we learn about the Milky
2:38:45
Speaker 1: Way … the harder it becomes to dismiss. Our galaxy
2:38:50
Speaker 1: contains hundreds of billions of stars. Planets appear to be
2:38:54
Speaker 1: common around them. Some of those planets are rocky. Some
2:38:57
Speaker 1: orbit at distances where liquid water could potentially exist. Some
2:39:01
Speaker 1: planetary systems are billions of years older than the solar system.
2:39:05
Speaker 1: And under certain assumptions, a technological civilization capable of travelling
2:39:10
Speaker 1: between stars would not need anything close to the entire
2:39:13
Speaker 1: age of the galaxy. to spread through a large part
2:39:16
Speaker 1: of it. Even relatively slow interstellar expansion, repeated over millions
2:39:22
Speaker 1: of years, could cover enormous distances. The Milky Way has
2:39:27
Speaker 1: had vastly longer than that. And still, no extraterrestrial civilization
2:39:32
Speaker 1: has been confirmed. No artificial radio signal has survived every
2:39:37
Speaker 1: test required to establish that it came from another technological species.
2:39:42
Speaker 1: No probe has been discovered in the solar system. No
2:39:45
Speaker 1: structure around another star has been demonstrated to be artificial.
2:39:49
Speaker 1: No unambiguous technosignature has announced that somebody else is there.
2:39:54
Speaker 1: That is the tension at the heart of the Fermi Paradox.
2:39:58
Speaker 1: Not that alien life must exist. Not that the galaxy
2:40:01
Speaker 1: definitely should be crowded. But that several things which appear
2:40:05
Speaker 1: individually plausible seem, when placed together, to produce an uncomfortable question.
2:40:12
Speaker 1: Frank Drake approached that uncertainty in a different way. Rather
2:40:16
Speaker 1: than asking for one answer, he divided the problem into pieces.
2:40:20
Speaker 1: How often do suitable stars form? How many have planets?
2:40:24
Speaker 1: How many planets could support life? How often does life begin?
2:40:29
Speaker 1: How often does intelligence evolve? How often does intelligence become
2:40:33
Speaker 1: technologically detectable? And how long does that detectable phase last?
2:40:39
Speaker 1: More than 60 years later, That framework remains useful partly
2:40:43
Speaker 1: because it shows how uneven our knowledge still is. Astronomy
2:40:47
Speaker 1: has made extraordinary progress. We now know that planets are common.
2:40:52
Speaker 1: We can estimate how frequently certain kinds of rocky worlds occur.
2:40:57
Speaker 1: We can study planetary atmospheres and stellar environments with instruments
2:41:01
Speaker 1: Frank Drake could scarcely have imagined. But once the question
2:41:05
Speaker 1: moves from planets into biology, confidence disappears quickly. We do
2:41:11
Speaker 1: not know how often life begins. We do not know
2:41:13
Speaker 1: how often simple cells become complex. We do not know
2:41:17
Speaker 1: how often complex life produces intelligence. We do not know
2:41:21
Speaker 1: how often intelligence develops technology. And we have almost no
2:41:25
Speaker 1: idea how long technological civilizations normally survive or remain detectable.
2:41:30
Speaker 1: That leaves many possible answers. Perhaps Earth is unusually fortunate.
2:41:35
Speaker 1: Perhaps simple life is common, but the evolution of complex
2:41:37
Speaker 1: cells is extraordinarily rare. Perhaps intelligence almost never develops. Perhaps
2:41:43
Speaker 1: the great filter lies somewhere deep in our evolutionary past,
2:41:47
Speaker 1: and humanity has already passed through its narrowest point. Or
2:41:51
Speaker 1: perhaps the difficult step still lies ahead. Technological societies may
2:41:56
Speaker 1: frequently appear, and then fail to survive for very long.
2:42:00
Speaker 1: They may destroy themselves, they may collapse, they may become quieter.
2:42:05
Speaker 1: They may transform into something we would no longer recognise
2:42:08
Speaker 1: as a civilisation at all, or perhaps none of that
2:42:12
Speaker 1: is necessary. Civilisations might survive and simply choose not to broadcast.
2:42:17
Speaker 1: They may communicate using methods we do not know how
2:42:20
Speaker 1: to detect. Their technological periods may be separated from ours
2:42:24
Speaker 1: by millions of years. They may be scattered so widely
2:42:28
Speaker 1: that no signal has yet crossed the distance between us.
2:42:32
Speaker 1: Interstellar travel may be technically possible, while remaining so expensive,
2:42:37
Speaker 1: slow or unattractive that almost nobody attempts it. Humanity may
2:42:42
Speaker 1: have appeared unusually early in the long history of possible
2:42:45
Speaker 1: life around long-lived stars. Or perhaps the explanation is almost
2:42:49
Speaker 1: embarrassingly simple. We have barely searched. The cosmic haystack is enormous.
2:42:56
Speaker 1: We have examined only a tiny portion of the possible
2:42:58
Speaker 1: combinations of stars, frequencies, signal strengths, durations, repetition patterns, technologies,
2:43:06
Speaker 1: and moments in time where another civilisation could reveal itself.
2:43:10
Speaker 1: Several of these explanations could even be true at once.
2:43:14
Speaker 1: Life may be uncommon, intelligence may be rarer, civilisations may
2:43:18
Speaker 1: be short-lived, signals may be difficult to recognise, interstellar travel
2:43:24
Speaker 1: may be unpopular, and our search may still be far
2:43:27
Speaker 1: too small. There may be no single dramatic answer waiting
2:43:30
Speaker 1: to solve everything. The Great Filter is powerful precisely because
2:43:35
Speaker 1: it reminds us how little the silence itself can tell us.
2:43:40
Speaker 1: If the most difficult transition lies behind humanity, then our
2:43:44
Speaker 1: existence may represent an extraordinarily rare success. If the most
2:43:49
Speaker 1: difficult transition lies ahead, then the silence could have a
2:43:52
Speaker 1: more troubling meaning. But we cannot currently distinguish between those possibilities.
2:43:58
Speaker 1: The galaxy does not come with an explanation attached. Silence
2:44:02
Speaker 1: is simply silence. It is not proof that nobody exists.
2:44:07
Speaker 1: In the same way, the enormous number of planets in
2:44:10
Speaker 1: the Milky Way is not proof that somebody must. Both
2:44:14
Speaker 1: conclusions would go beyond the evidence. The science lives in
2:44:18
Speaker 1: the space between them. Arthur Charles Clarke is commonly associated
2:44:22
Speaker 1: with a simple observation that captures the strange symmetry of
2:44:25
Speaker 1: the problem. Either humanity is alone in the universe, or
2:44:29
Speaker 1: it is not. Both possibilities are astonishing. If we are
2:44:33
Speaker 1: not alone, then somewhere beyond Earth Matter has organised itself
2:44:37
Speaker 1: into life more than once. Somewhere else, chemistry has become biology,
2:44:43
Speaker 1: biology has become intelligence, and intelligence has looked outward. There
2:44:48
Speaker 1: may be other beings who have asked their own version
2:44:51
Speaker 1: of the same question. Perhaps they are asking it now.
2:44:55
Speaker 1: If we are alone, the implication is equally extraordinary. Then,
2:44:59
Speaker 1: in a galaxy containing hundreds of billions of stars and
2:45:02
Speaker 1: perhaps hundreds of billions of planets, One small world has
2:45:07
Speaker 1: produced the only known creatures capable of understanding that any
2:45:10
Speaker 1: of it exists. Either way, the result is difficult to
2:45:14
Speaker 1: make ordinary. And so we keep listening. Radio telescopes continue
2:45:19
Speaker 1: examining the sky. Optical observatories search for unusual flashes of light.
2:45:25
Speaker 1: Astronomers study planetary atmospheres for signs of biology and technology.
2:45:30
Speaker 1: Spacecraft travel towards Mars and the icy moons of the
2:45:33
Speaker 1: outer solar system. Future instruments will examine worlds that today
2:45:37
Speaker 1: are little more than faint changes in starlight. The search
2:45:41
Speaker 1: is still young. For almost all of human history, wondering
2:45:44
Speaker 1: about life elsewhere was philosophy, imagination or storytelling. Now it
2:45:50
Speaker 1: is also observation. We can test pieces of the question.
2:45:54
Speaker 1: We can discover planets. We can analyse atmospheres. We can listen.
2:45:59
Speaker 1: And perhaps one day, something will answer. Or perhaps centuries
2:46:04
Speaker 1: will pass and the silence will remain. That silence would
2:46:08
Speaker 1: still not automatically mean emptiness. It would simply make the
2:46:11
Speaker 1: question deeper. For now, the Milky Way continues turning quietly overhead.
2:46:17
Speaker 1: Hundreds of billions of stars follow their path through the galaxy.
2:46:21
Speaker 1: Around many of them are planets we have never seen.
2:46:24
Speaker 1: Those planets have histories we do not know. Some may
2:46:28
Speaker 1: be lifeless. Some may contain oceans. Some may have known
2:46:32
Speaker 1: life for billions of years, and somewhere in all that darkness,
2:46:37
Speaker 1: somebody had to be among the first beings capable of
2:46:40
Speaker 1: looking outward and wondering whether anyone else was there. Perhaps
2:46:44
Speaker 1: that happened long ago, somewhere beyond our reach. Perhaps it
2:46:49
Speaker 1: is happening tonight around another star. Or perhaps, for this
2:46:53
Speaker 1: brief moment in cosmic history, the unsettling answer to Enrico
2:46:57
Speaker 1: Fermi's question is not that everybody has disappeared. Perhaps almost
2:47:02
Speaker 1: everybody has yet to arrive. And perhaps, for now, we
2:47:06
Speaker 1: are simply the ones who became able to ask first.