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The Universe Is Mostly Empty · Boring Science

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Speaker 1: Hello, and welcome to Boring Science to Sleep, a quiet
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Speaker 1: place where science becomes slow, soft, and easy to drift through. Tonight,
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Speaker 1: you're going to gently leave the familiar warmth of Earth,
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Speaker 1: move beyond the planets you already know, and slowly enter
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Speaker 1: the enormous space between the stars. You are not here
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Speaker 1: to learn everything, and there is nothing you need to remember.
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Speaker 1: You can simply listen, breathe softly, and let the distance
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Speaker 1: unfold around you. Before you settle in, if you truly
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Speaker 1: enjoy these calm journeys through interesting facts about science and
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Speaker 1: the universe, you're welcome to like this video, and subscribe,
0:41
Speaker 1: only if it feels right for you. And when you're ready,
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Speaker 1: let me know in the comments where you're listening from,
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Speaker 1: and what time it is right now. It's always a
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Speaker 1: quiet joy to imagine all these different places becoming still together,
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Speaker 1: Now dim the lights, perhaps let a fan hum softly
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Speaker 1: somewhere nearby. Settle into whatever position feels comfortable, and let
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Speaker 1: yourself drift slowly outward, beyond the familiar planets, toward the
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Speaker 1: first quiet stars. You can begin by imagining that you
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Speaker 1: are simply floating above Earth. There is no need to
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Speaker 1: travel quickly. Below you, the planet turns quietly beneath a
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Speaker 1: thin layer of atmosphere, blue and white and softly illuminated
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Speaker 1: by the sun. The oceans reflect a little light. The
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Speaker 1: clouds move almost imperceptibly. Somewhere below them are cities, forests, mountains,
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Speaker 1: and quiet rooms where other people are also beginning to rest.
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Speaker 1: But from here, from this gentle distance, Earth already looks
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Speaker 1: very small. And as you slowly move farther away, the
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Speaker 1: familiar shape of your home begins to become less distinct.
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Speaker 1: The moon remains nearby for a while, following its patient
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Speaker 1: path around Earth. Then Earth becomes one small sphere among
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Speaker 1: the darkness, and the moon becomes only a tiny companion.
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Speaker 1: Farther still, the other planets begin to appear. Venus, bright
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Speaker 1: and cloud-covered. Mars, small and reddish. The distant giants, slowly
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Speaker 1: turning in the cold outer reaches of the solar system.
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Speaker 1: And then, beyond the planets, something begins to change. There
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Speaker 1: are fewer familiar objects. There is more darkness. The sunlight
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Speaker 1: becomes softer, not because the sun has stopped shining, but
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Speaker 1: because you are moving farther away from it, into a
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Speaker 1: region where its light spreads thinner and thinner across an
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Speaker 1: enormous space. You can still imagine the sun behind you,
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Speaker 1: shining steadily, sending light outward in every direction. But now
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Speaker 1: you are beginning to understand something quietly important about space.
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Speaker 1: The solar system is not a crowded place. The planets
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Speaker 1: occupy only tiny islands within an immense ocean of distance.
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Speaker 1: Between one planet and another, there can be millions of
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Speaker 1: kilometres of almost nothing. And beyond the outer planets, that
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Speaker 1: emptiness becomes even more profound. You continue drifting. There is
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Speaker 1: no sound here, no wind, no rushing air, no footsteps,
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Speaker 1: only the imagined feeling of weightlessness and the distant memory
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Speaker 1: of sunlight behind you. Eventually, you cross into interstellar space,
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Speaker 1: the broad and quiet region between the stars. It is
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Speaker 1: not completely empty. There are tiny amounts of gas and
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Speaker 1: dust scattered through it, including individual atoms of hydrogen, along
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Speaker 1: with particles that have travelled away from stars and clouds.
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Speaker 1: But compared with the spaces you know on Earth, it
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Speaker 1: is extraordinarily sparse. You could imagine a room larger than
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Speaker 1: any building you have ever seen. and then imagine placing
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Speaker 1: only a few grains of dust inside it. Even that
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Speaker 1: would be far more crowded than much of interstellar space.
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Speaker 1: And this is where the scale of the journey begins
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Speaker 1: to change. The planets are no longer your main landmarks. Now,
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Speaker 1: the stars become the distant lights you are moving toward.
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Speaker 1: One of the nearest stellar neighbourhoods lies around a group
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Speaker 1: of stars called Alpha Centauri. There are three stars here,
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Speaker 1: bound together by gravity. moving through space as members of
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Speaker 1: one small stellar family. Two of them, Alpha Centauri A
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Speaker 1: and Alpha Centauri B, are somewhat similar to our Sun
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Speaker 1: in their general nature, while the third, Proxima Centauri, is smaller,
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Speaker 1: cooler and much fainter. Proxima Centauri is the closest individual
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Speaker 1: star to our solar system, and even though it is
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Speaker 1: our nearest stellar neighbour, it is still unimaginably far away.
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Speaker 1: Its light takes a little more than four years to
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Speaker 1: reach Earth. So when you look toward Proxima Centauri, you
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Speaker 1: are not seeing it as it exists at this exact moment.
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Speaker 1: You are seeing light that began its journey more than
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Speaker 1: four years ago. You can let that thought become very quiet.
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Speaker 1: A single photon of light leaves the star. It travels
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Speaker 1: through the darkness. There are no roads, no signs, no walls.
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Speaker 1: Nothing to guide it. except the structure of space itself.
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Speaker 1: And after more than four years of travelling, that tiny
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Speaker 1: piece of light finally reaches Earth. Perhaps it enters a telescope.
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Speaker 1: Perhaps it touches a camera. Perhaps it reaches your eyes
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Speaker 1: on a clear night. And for just a moment, a
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Speaker 1: tiny message from another star becomes part of your own experience.
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Speaker 1: Around Proxima Centauri, scientists have found evidence for planets. One
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Speaker 1: of them, known as Proxima Centauri b, is especially interesting
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Speaker 1: because its orbit places it in a region where temperatures could,
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Speaker 1: under suitable conditions, allow liquid water to exist. But that
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Speaker 1: does not mean that life has been discovered there. It
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Speaker 1: does not mean that the planet is another Earth. It
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Speaker 1: simply means that, from the limited information we have, there
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Speaker 1: are circumstances that make the world worth studying, and perhaps
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Speaker 1: that is enough. You do not need an answer tonight.
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Speaker 1: You can simply imagine the planet moving quietly around its
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Speaker 1: small red star. The star rises in its own distant sky.
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Speaker 1: The planet continues along its orbit. Light travels outward. And somewhere,
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Speaker 1: many years later, that light may reach another world. There
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Speaker 1: is something strangely peaceful about this. A planet can circle
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Speaker 1: a star for billions of years without anyone watching. A
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Speaker 1: star can shine through the darkness for longer than any
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Speaker 1: human life, and the space between them can remain almost
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Speaker 1: completely silent. You continue drifting farther from Earth. Behind you,
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Speaker 1: our solar system becomes smaller. The Sun becomes one point
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Speaker 1: of light. Earth becomes impossible to distinguish without help. And
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Speaker 1: ahead of you, Proxima Centauri remains only a faint red star.
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Speaker 1: You have travelled an enormous distance, and yet you have
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Speaker 1: barely begun. There are other stars beyond it, other systems,
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Speaker 1: other planets, other quiet worlds that may never be visited
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Speaker 1: by a human being. For now you can simply remain
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Speaker 1: here for a moment, suspended between the familiar light behind
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Speaker 1: you and the distant lights ahead. There is nothing to reach,
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Speaker 1: nothing to solve. The stars can remain where they are,
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Speaker 1: and you can simply drift with the darkness between them,
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Speaker 1: because beyond this first quiet neighbourhood there are worlds unlike
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Speaker 1: anything you have ever known. You remain there for a
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Speaker 1: little while, floating between the stars. Behind you the sun
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Speaker 1: has already become just another point of light, and ahead
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Speaker 1: of you Proxima Centauri glows faintly in the darkness. The
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Speaker 1: star is small, its light is reddish, and somewhere around
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Speaker 1: it a world continues its quiet journey through space. You
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Speaker 1: cannot see the planet with your eyes. You cannot stand
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Speaker 1: on its surface. You cannot hear its winds or feel
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Speaker 1: whatever atmosphere may surround it. And yet, from the tiny
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Speaker 1: changes in the light of its star, from the delicate
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Speaker 1: movement of the star itself, astronomers can begin to sense
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Speaker 1: that another world is there. This is one of the
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Speaker 1: quiet wonders of modern astronomy. A planet does not have
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Speaker 1: to shine brightly for us to know that it exists. Sometimes,
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Speaker 1: all it needs to do is pass between us and
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Speaker 1: its star. For a brief moment, the star becomes just
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Speaker 1: a little dimmer. Then the planet continues along its path
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Speaker 1: and the light returns, again and again. A tiny change
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Speaker 1: in brightness, repeated over time, can become a clue. Another
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Speaker 1: method is even more subtle. a planet pulls gently on
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Speaker 1: its star with gravity, causing the star to move by
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Speaker 1: an almost imperceptible amount. The star does not seem to
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Speaker 1: wander dramatically, it simply shifts back and forth, almost as
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Speaker 1: if something invisible were quietly holding its hand. From that
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Speaker 1: tiny movement, astronomers can infer the presence of a planet.
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Speaker 1: You can imagine how strange that is. A distant world
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Speaker 1: may be millions or billions of kilometres away, completely hidden
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Speaker 1: in the darkness, and yet its gravity can leave a
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Speaker 1: tiny signature in the light of its star. The planet
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Speaker 1: itself remains unseen, but its presence can still be felt,
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Speaker 1: and once astronomers begin looking in this way, the night
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Speaker 1: sky becomes filled with possibilities. One nearby example is the
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Speaker 1: red dragon, known as Wolf 1061. It is a small,
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Speaker 1: faint star, much less massive and much dimmer than our sun.
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Speaker 1: To your eyes, from Earth, it would not be a
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Speaker 1: bright familiar point like Sirius. It would be hidden among
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Speaker 1: the many faint lights of the night. And yet, This
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Speaker 1: quiet little star has its own planetary neighbourhood. Several planets
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Speaker 1: have been found around it. The closest one, Wolf 1061b,
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Speaker 1: moves very near its star. It completes an orbit in
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Speaker 1: only a few Earth days, so close is its path
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Speaker 1: that the planet may be tidally locked, meaning that one
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Speaker 1: side could remain facing the star while the other side
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Speaker 1: turns away. You can imagine the difference. On one side,
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Speaker 1: the red star would remain fixed in the sky. There
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Speaker 1: would be no ordinary sunrise in the way you know it,
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Speaker 1: no familiar morning slowly brightening into afternoon. The star would
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Speaker 1: simply remain there, shining over one side of the world,
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Speaker 1: and on the opposite side, the sky would remain turned
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Speaker 1: toward darkness. Between these two regions there might be a
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Speaker 1: narrow boundary where day and night meet, a world of
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Speaker 1: permanent light on one side, permanent darkness on the other,
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Speaker 1: and an endless planetary rotation that has settled into a
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Speaker 1: very unusual rhythm. But this first planet is probably not
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Speaker 1: a gentle place. Its closeness to its star exposes it
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Speaker 1: to intense radiation and powerful streams of particles from the star.
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Speaker 1: Any atmosphere it once possessed may have had difficulty surviving
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Speaker 1: for long. So we move outward, just a little, to
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Speaker 1: another world. Wolf 1061c. This planet is farther from its star.
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Speaker 1: Its orbit takes roughly 18 Earth days, and it receives
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Speaker 1: less energy from its star than Earth receives from the Sun.
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Speaker 1: It is considered a potentially habitable world, but those words
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Speaker 1: need to be held very gently. Potentially habitable does not
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Speaker 1: mean inhabited. It does not mean that forests grow beneath
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Speaker 1: an alien sky. It does not mean that oceans shimmer
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Speaker 1: beneath clouds. It simply means that, under certain possible conditions,
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Speaker 1: temperatures and other factors might allow liquid water to exist,
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Speaker 1: and even that remains uncertain. Perhaps the atmosphere is thick
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Speaker 1: enough to hold warmth. Perhaps it is thin. Perhaps the
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Speaker 1: surface is very different from what we imagine. We do
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Speaker 1: not know. And there is something peaceful about leaving that
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Speaker 1: question open. You do not need to decide what the
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Speaker 1: planet looks like. You can simply imagine it turning quietly
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Speaker 1: around its small red star. A world several times more
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Speaker 1: massive than Earth. A world receiving a softer amount of energy.
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Speaker 1: A world moving through the darkness while no human being
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Speaker 1: watches from its surface. And perhaps somewhere beneath its distant sky,
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Speaker 1: if conditions are suitable, water could exist in liquid form.
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Speaker 1: Not necessarily an ocean like ours. Not necessarily rain falling
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Speaker 1: on continents. Just water, in whatever quiet form the physics
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Speaker 1: of that world allows. This is where the idea of
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Speaker 1: an exoplanet becomes more beautiful than a simple list of
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Speaker 1: distant planets. Each one is a different experiment performed by nature.
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Speaker 1: Change the size of the world. Change the distance from
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Speaker 1: the star. Change the atmosphere. change the amount of sunlight,
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Speaker 1: change the pressure, change the temperature. And the same basic
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Speaker 1: ingredients can become something completely unfamiliar. Some worlds may be
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Speaker 1: dry and rocky, some may be wrapped in thick atmospheres,
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Speaker 1: some may be covered by oceans deeper than anything on Earth,
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Speaker 1: and some may wander through darkness without a star at all.
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Speaker 1: Imagine one of those ocean worlds now, far away, beneath
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Speaker 1: a distant sun. you might have a planet where water
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Speaker 1: covers much of the surface. There may be no familiar
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Speaker 1: continents stretching toward the horizon. Instead, there may be an
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Speaker 1: immense blue darkness extending for hundreds of kilometres downward. As
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Speaker 1: you descend through such an ocean, sunlight would gradually disappear,
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Speaker 1: the water would become colder in some regions, warmer in others,
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Speaker 1: and the pressure would slowly increase. Far below, the water
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Speaker 1: could become compressed into unfamiliar forms of ice. Not because
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Speaker 1: the world is simply freezing, but because pressure itself can
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Speaker 1: change the way water behaves. You would not need to
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Speaker 1: understand the chemistry to imagine it. Just imagine descending. The
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Speaker 1: surface becomes distant, the light becomes faint, the blue becomes darker,
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Speaker 1: and eventually the ocean becomes a quiet boundary between the
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Speaker 1: atmosphere above and the hidden interior of the planet below.
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Speaker 1: Somewhere beneath all that water, a rocky world may remain. Perhaps.
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Speaker 1: We do not know, and that uncertainty is part of
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Speaker 1: the beauty. Another world, another set of conditions, another quiet possibility.
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Speaker 1: Even a planet that seems similar to Earth can be
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Speaker 1: deeply different. Even a world in a region where liquid
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Speaker 1: water might exist may have no life at all, And
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Speaker 1: a world that looks strange to us may still possess
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Speaker 1: an atmosphere, an ocean, a hidden interior, and a history
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Speaker 1: stretching back billions of years. The universe does not seem
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Speaker 1: to have one preferred way of making planets. It makes
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Speaker 1: rocky worlds beside red stars. It makes frozen worlds far
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Speaker 1: from warmth. It makes ocean worlds beneath distant suns. It
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Speaker 1: makes worlds that show one face to their star forever.
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Speaker 1: and it makes worlds that wander alone through the darkness.
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Speaker 1: You remain suspended among them for a moment. There is
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Speaker 1: no need to visit everyone. There is no need to
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Speaker 1: memorise their names. It is enough to know that beyond
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Speaker 1: the solar system, the darkness is not empty. There are
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Speaker 1: worlds moving silently through it. Worlds too distant to touch,
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Speaker 1: too faint to see directly, yet close enough for their
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Speaker 1: gravity and their passing shadows to to leave tiny traces
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Speaker 1: in the light of their stars. And as you drift
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Speaker 1: a little farther, you begin to notice that the planets
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Speaker 1: are only part of the story. Because before a world
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Speaker 1: can have an ocean, an atmosphere, or a quiet surface
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Speaker 1: beneath distant light, there must first be a star to
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Speaker 1: warm it. And stars too have their own long lives,
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Speaker 1: their own slow changes, and their own gentle endings. so
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Speaker 1: you can let the distant planets continue turning behind you,
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Speaker 1: while ahead, the stars begin to grow larger in your imagination,
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Speaker 1: glowing softly in the dark. You can let the distant
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Speaker 1: planets continue their quiet orbits for a while. Somewhere behind you,
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Speaker 1: a small world circles a faint red star. Perhaps it
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Speaker 1: is rocky. Perhaps it is covered by an ocean. Perhaps
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Speaker 1: it has an atmosphere. or perhaps most of that atmosphere,
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Speaker 1: has slowly drifted away into space. There is no need
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Speaker 1: to decide. The planet can remain mysterious, and as you
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Speaker 1: drift a little farther away, your attention slowly moves from
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Speaker 1: the worlds themselves to the stars that hold them in
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Speaker 1: their gentle gravitational embrace. A star can look so simple
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Speaker 1: from a distance, just a point of light, but every
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Speaker 1: point of light you see at night is the visible
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Speaker 1: surface of a long slow process. Deep inside a star
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Speaker 1: enormous pressure and heat allow very small particles to come
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Speaker 1: together and release energy. That energy begins deep within the
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Speaker 1: star and slowly works its way outward. Eventually some of
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Speaker 1: it escapes as light and that light crosses the darkness
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Speaker 1: until it reaches another world, you. A star can therefore
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Speaker 1: be thought of as a very slow source of warmth
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Speaker 1: and light. shining for millions or billions of years. Some
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Speaker 1: stars are small and quiet, some are enormous and restless,
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Speaker 1: some are young, some are ancient, and some have already
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Speaker 1: reached the end of the part of their lives when
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Speaker 1: they shine in the familiar way. The smallest stars can
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Speaker 1: be among the longest lived. Red dwarfs, for example, are
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Speaker 1: much smaller and cooler than the sun. They shine with
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Speaker 1: a soft reddish light, and because they use their fuel
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Speaker 1: very slowly, they can remain active for extraordinarily long periods
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Speaker 1: of time. Proxima Centauri, the small red star you visited earlier,
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Speaker 1: belongs to this family. From a distance, it seems almost delicate,
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Speaker 1: but its life may stretch far beyond the lifetime of larger,
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Speaker 1: brighter stars. You can imagine a red dwarf glowing quietly
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Speaker 1: in the darkness, not rushing anywhere. simply continuing to shine.
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Speaker 1: Its planets circle it, its light travels outward, and the
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Speaker 1: star continues through time. Some stars, however, begin their lives
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Speaker 1: with much greater energy. They are larger, hotter, and brighter.
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Speaker 1: They can live much shorter lives because they spend their
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Speaker 1: internal fuel more quickly. One such contrast can be found
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Speaker 1: in the Sirius system. Sirius A is a bright blue-white
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Speaker 1: star easily visible from Earth, but beside it is something
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Speaker 1: very different. Sirius B is a white dwarf. It is
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Speaker 1: the compact remnant of a star that has already passed
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Speaker 1: through the major stages of its life. Imagine taking a
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Speaker 1: star and allowing its outer layers to drift away. What
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Speaker 1: remains can become extraordinarily dense. with much of the original
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Speaker 1: stellar mass compressed into an object only a little larger
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Speaker 1: than Earth. It no longer shines because of the same
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Speaker 1: deep process that powers an ordinary active star. Instead, it
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Speaker 1: gradually cools over immense stretches of time. You can picture
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Speaker 1: Sirius A shining brightly beside its much smaller companion, one
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Speaker 1: star still in the familiar part of stellar life. the
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Speaker 1: other carrying the quiet memory of a star that has
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Speaker 1: already changed. And there is something beautiful in that contrast.
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Speaker 1: The universe does not erase everything that ends. Sometimes it
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Speaker 1: leaves behind a smaller, quieter version of what once was.
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Speaker 1: Farther away, another kind of transformation is taking place. A
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Speaker 1: star can become so large that its outer layers expand enormously.
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Speaker 1: The star cools at its surface, and takes on a
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Speaker 1: reddish appearance. It becomes a red giant. Imagine the star
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Speaker 1: slowly swelling outward, its outer atmosphere becoming vast and thin,
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Speaker 1: extending far beyond the compact shape it had during its
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Speaker 1: younger life. Inside, the star continues changing. Outside, its atmosphere
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Speaker 1: can rise and fall in slow pulsations. The entire star
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Speaker 1: may seem to breathe, expand, contract, expand again, not quickly,
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Speaker 1: not dramatically, but over long periods, in a rhythm so
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Speaker 1: slow that a human lifetime would capture only a tiny
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Speaker 1: part of the movement. Some red giants also lose material
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Speaker 1: through powerful stellar winds. Gas and dust drift away from
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Speaker 1: the surface and spread into space. A star can therefore
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Speaker 1: become both an ending and a beginning. The material it
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Speaker 1: releases does not simply disappear, It becomes part of the
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Speaker 1: surrounding space. Later, that material may contribute to clouds from
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Speaker 1: which other stars and planets form. In this way, the
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Speaker 1: life of one star can quietly become part of the
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Speaker 1: birth of another. You can imagine a vast cloud of
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Speaker 1: gas and dust drifting through darkness, carrying elements that were
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Speaker 1: once inside an older star. There is no sharp boundary
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Speaker 1: between one generation of stars and the next. The galaxy
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Speaker 1: is constantly recycling its material. Stars are born. Stars shine.
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Speaker 1: Stars change. Stars release their outer layers. And sometimes, when
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Speaker 1: a massive star reaches the end of its life, something
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Speaker 1: much more compact remains. A neutron star. Here, the scale
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Speaker 1: becomes almost difficult to imagine. A neutron star can contain
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Speaker 1: more mass than the Sun, while being only around the
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Speaker 1: size of a city. The material has been compressed so
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Speaker 1: intensely that ordinary ideas of density begin to feel almost meaningless.
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Speaker 1: You can imagine the mass of a star gathered into
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Speaker 1: a space small enough to cross in a short drive.
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Speaker 1: And some of these objects rotate very rapidly. As they turn,
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Speaker 1: beams of radiation can sweep through space. If one of
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Speaker 1: those beams happens to pass toward Earth again and again,
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Speaker 1: telescopes can detect a regular pulse. This is a pulsar.
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Speaker 1: A pulsar can therefore appear to us like a distant
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Speaker 1: cosmic lighthouse. Not because there is an actual lamp turning
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Speaker 1: in space, but because the rotating neutron star sends its
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Speaker 1: radiation outward in a repeating pattern. Pulse. Pause. Pulse. Pause.
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Speaker 1: The rhythm can be remarkably regular. And so, after drifting
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Speaker 1: among distant planets... you have now moved through several different
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Speaker 1: lives of stars. The quiet red dwarf, glowing for an
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Speaker 1: almost unimaginable length of time. The bright star like Sirius A.
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Speaker 1: The white dwarf, carrying the memory of an earlier star.
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Speaker 1: The enormous red giant, slowly expanding and releasing material into space.
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Speaker 1: And the tiny neutron star, turning through darkness and sending
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Speaker 1: its pulses outward. Each one is made from the same
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Speaker 1: broad cosmic ingredients, yet their lives are shaped by differences
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Speaker 1: in mass, temperature, internal pressure and time. A star is
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Speaker 1: not simply a light in the sky. It is a process,
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Speaker 1: a long transformation, a changing balance between gravity pulling inward
24:48
Speaker 1: and the energy created deep within pushing outward. For a while,
24:53
Speaker 1: the balance can remain remarkably stable. Then, slowly, the balance changes,
24:59
Speaker 1: and the star changes with it. You can let that
25:01
Speaker 1: thought settle quietly. Nothing in the universe needs to remain
25:05
Speaker 1: exactly as it is. Not planets, not stars, not even
25:10
Speaker 1: the great structures of galaxies. Everything moves through time. Everything
25:15
Speaker 1: changes slowly. And sometimes, the remains of one object become
25:20
Speaker 1: the beginning of another. Far behind you, the small red
25:24
Speaker 1: stars continue shining. The distant planets continue turning. The white
25:29
Speaker 1: dwarfs continue cooling. The red giants continue breathing. The pulsars
25:35
Speaker 1: continue their quiet rhythm. Pulse. Pause. Pulse. And somewhere ahead,
25:41
Speaker 1: among all these distant lights, there are objects where gravity
25:45
Speaker 1: has become deeper still, places where matter has been compressed
25:49
Speaker 1: beyond ordinary intuition, and where even light itself can find
25:54
Speaker 1: no easy path outward. You do not need to hurry
25:57
Speaker 1: toward them. For now, you can simply remain suspended among
26:01
Speaker 1: the stars, listening to their quiet rhythms, while the darkness
26:06
Speaker 1: around you grows a little deeper. You can remain among
26:12
Speaker 1: the stars for a little longer. The pulsar behind you
26:15
Speaker 1: continues its quiet rhythm. Pulse, pause, pulse. Its light travels
26:22
Speaker 1: outward through the darkness. crossing distances so large that the
26:26
Speaker 1: meaning of distance begins to feel different, and somewhere ahead,
26:31
Speaker 1: gravity becomes deeper. Not suddenly, not violently, just gradually, as
26:37
Speaker 1: you drift toward one of the strangest kinds of object
26:40
Speaker 1: known to exist, a black hole. The name can sound dramatic,
26:44
Speaker 1: but there is something much quieter about a black hole
26:47
Speaker 1: when you begin to understand it. It is not a
26:50
Speaker 1: hole in space like an empty tunnel. It is not
26:53
Speaker 1: a cosmic vacuum cleaner, reaching out and pulling everything indiscriminately
26:57
Speaker 1: toward itself. From far enough away, a black hole's gravity
27:02
Speaker 1: behaves much like the gravity of any other object with
27:05
Speaker 1: the same amount of mass. A planet can orbit it.
27:08
Speaker 1: A star can orbit it. Even a planetary system could,
27:12
Speaker 1: in principle, remain in stable motion around it if the
27:16
Speaker 1: distances and conditions were suitable. So imagine yourself far away
27:21
Speaker 1: from one. You are not falling. You are simply drifting
27:25
Speaker 1: through its neighbourhood. Around you, the stars continue along their paths.
27:30
Speaker 1: The black hole itself may be invisible. There is no
27:33
Speaker 1: bright surface, no solid world to see. Only the effect
27:38
Speaker 1: of its gravity, quietly shaping the paths of things around it.
27:42
Speaker 1: And then, perhaps, you begin to notice something glowing, a
27:46
Speaker 1: faint ring of light. Then another layer. Gas and dust
27:51
Speaker 1: are spiralling inward. This material is called an accretion disk,
27:56
Speaker 1: but you can simply imagine it as a vast, slowly
27:59
Speaker 1: turning river of matter, circling the darkness. The material does
28:04
Speaker 1: not immediately disappear, it moves, it circles, it rubs against itself.
28:10
Speaker 1: As it falls deeper into the gravitational field, it can
28:14
Speaker 1: become extraordinarily hot. The darkness at the centre remains invisible,
28:20
Speaker 1: while the material around it can shine brightly. This is
28:23
Speaker 1: one of the quiet paradoxes of a black hole. The
28:27
Speaker 1: black hole itself does not need to glow for its
28:30
Speaker 1: surroundings to become luminous. The darkness can reveal itself through
28:34
Speaker 1: the light moving around it. And if you were watching
28:37
Speaker 1: from a safe distance, there would be no sound, no roar,
28:41
Speaker 1: no cosmic scream, only light moving in curved paths around
28:46
Speaker 1: something you cannot directly see. At the centre of a
28:49
Speaker 1: black hole is a boundary called the event horizon. You
28:53
Speaker 1: can think of it as the point beyond which there
28:55
Speaker 1: is no ordinary path back outward. If something crosses that boundary,
29:00
Speaker 1: light itself can no longer escape to the outside universe.
29:04
Speaker 1: This does not mean the boundary is a physical wall.
29:07
Speaker 1: There is no surface to touch. It is simply a
29:10
Speaker 1: boundary in space and time, marking a region from which
29:14
Speaker 1: the outside universe can no longer receive information in the
29:17
Speaker 1: usual way. You can let that idea remain simple. A boundary.
29:23
Speaker 1: a one-way edge, a place beyond which the darkness becomes complete.
29:28
Speaker 1: And yet, from far away, the universe around a black
29:32
Speaker 1: hole can remain surprisingly orderly. Stars can orbit. Gas can circle. Planets,
29:40
Speaker 1: at suitable distances, could theoretically continue their paths. This is
29:46
Speaker 1: why the idea of a planet around a black hole
29:49
Speaker 1: is not as impossible as it first sounds. Imagine a
29:53
Speaker 1: cold world moving through darkness, following an invisible gravitational centre.
29:59
Speaker 1: There may be no familiar sunrise. There may be no
30:02
Speaker 1: bright star warming its surface. But if there were enough
30:06
Speaker 1: surrounding material and enough energy from the region around the
30:09
Speaker 1: black hole, the planet's environment could become surprisingly complex. Such
30:15
Speaker 1: planetary systems have not been directly confirmed around black holes,
30:19
Speaker 1: so this remains a possibility rather than an established observation.
30:24
Speaker 1: And it is perhaps better to leave it that way
30:26
Speaker 1: for tonight. A Possibility A quiet world moving around an
30:31
Speaker 1: invisible centre. Farther away, some black holes are born from
30:35
Speaker 1: the deaths of massive stars. When a sufficiently massive star
30:39
Speaker 1: reaches the end of its life, its core can collapse
30:43
Speaker 1: under gravity. The outer layers may be expelled, while the
30:47
Speaker 1: remaining core becomes extraordinarily compact. If enough mass remains concentrated
30:53
Speaker 1: in that small region, a black hole can form. The
30:57
Speaker 1: star that once filled an enormous volume of space is
31:00
Speaker 1: reduced to something far more compact. Its history remains in
31:04
Speaker 1: the objects around it. Its gravity remains. But the familiar
31:09
Speaker 1: star is gone. Other black holes are vastly larger. At
31:13
Speaker 1: the centres of many galaxies are supermassive black holes, objects
31:17
Speaker 1: containing millions or even billions of times the mass of
31:20
Speaker 1: the Sun. And now you can begin to imagine the
31:23
Speaker 1: centre of our own galaxy. The Milky Way is enormous,
31:27
Speaker 1: but at its heart is an object known as Sagittarius Astar.
31:32
Speaker 1: It is the supermassive black hole at the centre of
31:34
Speaker 1: our galaxy. Its mass is about four million times that
31:38
Speaker 1: of the Sun. From our distant position within the Milky Way,
31:42
Speaker 1: it is far away. Yet thousands of stars move through
31:46
Speaker 1: the region around it, tracing their quiet paths through a
31:50
Speaker 1: crowded galactic centre. Some of those stars move so quickly
31:54
Speaker 1: and so close to the black hole that their paths
31:57
Speaker 1: provide valuable evidence about the invisible object at the centre.
32:02
Speaker 1: Astronomers have watched these stars move for years. They do
32:06
Speaker 1: not see the black hole itself as an ordinary bright object. Instead,
32:11
Speaker 1: they watch the stars. They measure their positions. They follow
32:15
Speaker 1: their orbits. And from those slow movements, they can infer
32:19
Speaker 1: the presence and enormous mass of the hidden object. Eventually,
32:24
Speaker 1: in the year 2022, astronomers were able to produce an
32:28
Speaker 1: image of the immediate environment around Sagittarius A-star using a
32:33
Speaker 1: worldwide network of radio telescopes. The image did not show
32:38
Speaker 1: a simple photograph of a black sphere. Instead, it revealed
32:42
Speaker 1: a glowing ring-like structure surrounding a darker central region. Light
32:48
Speaker 1: from hot material curved around the invisible centre. The galaxy had,
32:53
Speaker 1: in a sense, shown us its heart. You can imagine
32:56
Speaker 1: drifting farther away now. The black hole becomes smaller. The
33:00
Speaker 1: glowing material around it becomes a faint point. The stars
33:04
Speaker 1: of the galactic centre begin to blend into the wider
33:07
Speaker 1: glow of the Milky Way. And the strange thing is
33:11
Speaker 1: that the black hole does not seem to have consumed
33:14
Speaker 1: the galaxy around it. The galaxy remains enormous. Stars continue
33:19
Speaker 1: to form. Clouds of gas continue to move. New worlds
33:24
Speaker 1: continue to appear. The black hole simply remains there, one
33:29
Speaker 1: deeply compact part of a much larger gravitational system. You
33:33
Speaker 1: can let that image become quiet, a dark centre, a
33:37
Speaker 1: slow ring of light, stars moving through the distance, and
33:41
Speaker 1: beyond them, billions upon billions of other stars. The darkness
33:46
Speaker 1: around you is no longer empty. It is filled with
33:49
Speaker 1: motion that is simply too slow, too distant, and too
33:54
Speaker 1: quiet for your senses to notice. You have now seen
33:57
Speaker 1: planets circling stars, stars changing their forms, remnants compressed into
34:03
Speaker 1: tiny objects, and gravity becoming deep enough to hide light itself.
34:09
Speaker 1: But all of these things belong somewhere. They belong to
34:12
Speaker 1: something much larger than And as you drift away from
34:15
Speaker 1: the dark heart of the galaxy, the faint structure surrounding
34:19
Speaker 1: you begins to take shape. The stars are no longer
34:22
Speaker 1: scattered points. They are part of a vast home, a galaxy.
34:28
Speaker 1: And slowly, gently, you begin to see the Milky Way,
34:32
Speaker 1: not from within it, but from farther away. You can
34:38
Speaker 1: let the darkness of Sagittarius, a star, remain behind you.
34:42
Speaker 1: the glowing ring of material around the galactic centre, becomes
34:47
Speaker 1: smaller as you slowly drift away. The stars around it
34:51
Speaker 1: begin to spread apart, the crowded centre becomes softer, and then,
34:56
Speaker 1: almost without noticing, you begin to see that the black
35:00
Speaker 1: hole was never the whole story. It was only the
35:03
Speaker 1: quiet centre of something much larger, the Milky Way. From
35:07
Speaker 1: Earth you know it as a pale band of light
35:10
Speaker 1: stretching across a dark night sky. For thousands of years,
35:14
Speaker 1: people could see that faint band without knowing what it
35:17
Speaker 1: really was. Some imagined it as a path across the heavens.
35:22
Speaker 1: Others gave it names connected with rivers, gods, or distant
35:26
Speaker 1: roads through the sky. Only much later did astronomy reveal
35:30
Speaker 1: the simpler explanation. The Milky Way is the combined light
35:35
Speaker 1: of enormous numbers of stars, most of them so distant
35:39
Speaker 1: that your eyes cannot separate them individually. You are now
35:43
Speaker 1: far enough away to see the shape behind that familiar band,
35:47
Speaker 1: and the shape is beautiful. The Milky Way is a
35:49
Speaker 1: barred spiral galaxy. At its centre is a dense, elongated
35:54
Speaker 1: region of stars, surrounded by a much larger disk. From
35:58
Speaker 1: that disk, spiral arms curve outward. The whole structure turns
36:03
Speaker 1: slowly through space, not like a solid wheel. The stars
36:08
Speaker 1: are not attached to one another. Instead, They follow their
36:11
Speaker 1: own paths under the influence of gravity, moving through an
36:15
Speaker 1: immense collection of stars, gas, dust and invisible matter. You
36:21
Speaker 1: can imagine the galaxy as a vast, slowly turning pattern
36:24
Speaker 1: of light. At the centre, the stars are crowded together.
36:28
Speaker 1: Farther outward, they become more widely separated. And between them
36:33
Speaker 1: are enormous regions of gas and dust. Some of these
36:37
Speaker 1: clouds are cold and dark. Others are warmed by nearby
36:40
Speaker 1: stars and begin to glow. Some are places where new
36:44
Speaker 1: stars are being born. So the Milky Way is not
36:47
Speaker 1: simply an ancient collection of old stars. It is also
36:50
Speaker 1: a place of continuing change. New stars appear. Older stars fade.
36:56
Speaker 1: Gas drifts between them. Dust gathers. Gravity slowly reshapes everything.
37:03
Speaker 1: And the whole galaxy continues its quiet movement. As you
37:06
Speaker 1: move away from the centre... the central bulge becomes easier
37:10
Speaker 1: to understand. It is a thick, bright concentration of stars
37:15
Speaker 1: surrounding the galactic heart. Many of these stars are old,
37:19
Speaker 1: some formed billions of years ago, long before Earth existed.
37:24
Speaker 1: And passing through this crowded region is the galactic bar,
37:28
Speaker 1: an elongated structure of stars stretching through the central part
37:32
Speaker 1: of the galaxy. From our position on Earth, Much of
37:36
Speaker 1: this region is hidden behind enormous clouds of gas and dust.
37:40
Speaker 1: The dust blocks visible light, but it does not block
37:44
Speaker 1: every kind of signal. Radio waves and infrared light can
37:48
Speaker 1: travel through some of these obscuring clouds, allowing astronomers to
37:52
Speaker 1: study parts of the galactic centre that ordinary visible light
37:55
Speaker 1: cannot reveal. You can imagine this as looking through a
37:59
Speaker 1: very thick mist. The landscape is hidden, But occasionally another
38:04
Speaker 1: kind of light finds a way through. And then, slowly,
38:08
Speaker 1: the central region begins to fade behind you. The galaxy
38:12
Speaker 1: opens outward. The bright concentration of stars becomes less intense.
38:17
Speaker 1: The spiral arms begin to appear. These arms are not
38:21
Speaker 1: rigid roads through space. They are broad regions where stars,
38:25
Speaker 1: gas and dust become more concentrated. They are also places
38:30
Speaker 1: where new stars are often born. Imagine drifting through one
38:34
Speaker 1: of these regions. The darkness around you is no longer
38:38
Speaker 1: completely dark. There are faint clouds, softly glowing nebulae, young
38:43
Speaker 1: stars shining with blue-white light, tiny particles of dust drifting
38:49
Speaker 1: through enormous spaces. The clouds can remain for millions of years,
38:54
Speaker 1: slowly changing shape as gravity gathers some regions together and
38:58
Speaker 1: disperses others. and from these clouds, stars can begin to form.
39:03
Speaker 1: A star may spend its earliest years inside a cluster
39:07
Speaker 1: of other young stars. But these clusters are not always
39:11
Speaker 1: permanent families. Over time, the stars slowly separate. Gravity from
39:16
Speaker 1: the rest of the galaxy gradually changes their paths. The
39:20
Speaker 1: young stars drift outward, becoming part of the larger galactic population.
39:25
Speaker 1: The Milky Way therefore carries the remains of countless generations
39:29
Speaker 1: of stars mixed together in a slow, complicated pattern. And
39:34
Speaker 1: somewhere within one of these spiral structures is your own
39:37
Speaker 1: small neighbourhood, the Orion Arm. It is sometimes called the
39:42
Speaker 1: Orion Spur, a smaller spiral feature located between larger structures
39:47
Speaker 1: of the Milky Way. This is where the solar system lives.
39:51
Speaker 1: You can imagine yourself drifting toward it now. The galaxy
39:55
Speaker 1: is still enormous. The sun is still only one star.
39:59
Speaker 1: But you can begin to recognize the neighborhood. There is
40:02
Speaker 1: the sun. There is Earth. There are the planets. And
40:06
Speaker 1: all around them, stars continue along their own paths. The
40:10
Speaker 1: solar system is not at the center of the galaxy.
40:12
Speaker 1: It is not close to the crowded galactic core. Instead,
40:16
Speaker 1: it occupies a relatively quieter region. tens of thousands of
40:20
Speaker 1: light years from the centre, within the broad disc of
40:23
Speaker 1: the Milky Way. That distance is difficult to imagine. Light
40:28
Speaker 1: itself would need many thousands of years to cross the
40:31
Speaker 1: space between our neighbourhood and the galactic centre. And yet,
40:35
Speaker 1: from Earth, the entire galaxy can appear as one soft
40:39
Speaker 1: band of light across the night. This is one of
40:42
Speaker 1: the strange gifts of distance. Things that are unimaginably large
40:47
Speaker 1: can become visually delicate when seen from far enough away.
40:50
Speaker 1: A galaxy containing hundreds of billions of stars can look
40:54
Speaker 1: like a faint ribbon. A star can become a point.
40:58
Speaker 1: A planet can disappear completely. And the spaces between everything
41:02
Speaker 1: remain almost invisible, even though they occupy so much of
41:06
Speaker 1: the universe. You can remain here for a moment, somewhere
41:10
Speaker 1: above the Orion Arm. The sun is behind you. The
41:14
Speaker 1: galactic centre is far away. Around you, stars continue to
41:18
Speaker 1: form and fade. Somewhere, in a cold cloud, gravity is
41:23
Speaker 1: gathering matter. Somewhere else, a young star is beginning to shine.
41:28
Speaker 1: Somewhere far beyond your view, an old star is releasing
41:31
Speaker 1: the last of its outer layers, and all of it
41:34
Speaker 1: belongs to the same enormous structure, the Milky Way. But
41:38
Speaker 1: there is another quiet mystery here. When astronomers watch the
41:42
Speaker 1: stars move around the galaxy, they find that the visible
41:45
Speaker 1: stars and gas are not enough to explain all of
41:48
Speaker 1: that motion. Something unseen appears to be contributing additional gravity.
41:54
Speaker 1: We call this dark matter. You cannot see it directly.
41:59
Speaker 1: It does not shine like a star. It does not
42:02
Speaker 1: form familiar clouds of visible light. But its gravitational influence
42:06
Speaker 1: can be inferred from the way stars and galaxies move.
42:10
Speaker 1: You can think of it as an invisible presence surrounding
42:14
Speaker 1: the visible galaxy, something that cannot be seen, but whose
42:18
Speaker 1: gravity helps shape the motion of everything you can see.
42:22
Speaker 1: And perhaps that is a fitting thought to carry with
42:24
Speaker 1: you as you drift farther outward. The Milky Way is
42:27
Speaker 1: not only the glowing spiral you can see, there is
42:31
Speaker 1: much more around it than the stars reveal. The visible
42:34
Speaker 1: disk is only one part of a much larger structure,
42:38
Speaker 1: so you begin to rise slowly above the spiral arms.
42:41
Speaker 1: The bright disk stretches beneath you. The stars become thinner,
42:46
Speaker 1: the glowing band begins to flatten into a distant shape,
42:50
Speaker 1: and unbeyond the familiar spiral, the Milky Way continues into
42:54
Speaker 1: regions that are much older, much darker, and much more
42:58
Speaker 1: sparsely populated. The galaxy you thought you knew from the
43:02
Speaker 1: night sky is about to become larger than you imagined.
43:09
Speaker 1: You are now floating above the Milky Way. Below you,
43:13
Speaker 1: the spiral arms stretch outward in a wide, luminous pattern.
43:17
Speaker 1: The Orion arm is somewhere beneath you. Far away, the
43:20
Speaker 1: galactic centre glows faintly through the enormous distances between stars.
43:25
Speaker 1: And behind all of this, the familiar disk of the
43:28
Speaker 1: galaxy begins to look thinner. The farther you rise, the
43:32
Speaker 1: less crowded the stars become. The bright spiral structure slowly fades.
43:37
Speaker 1: there are fewer glowing clouds, less visible gas, more darkness.
43:43
Speaker 1: And yet, the Milky Way does not simply end where
43:46
Speaker 1: its bright disk becomes difficult to see. There is another
43:50
Speaker 1: region surrounding it, a much older, much thinner, and much
43:54
Speaker 1: quieter part of the galaxy, the thick disk. You can
43:58
Speaker 1: imagine it as a faint layer above and below the
44:00
Speaker 1: brighter spiral disk, populated mainly by older stars. These stars
44:06
Speaker 1: have been travelling through the galaxy for enormous stretches of time.
44:10
Speaker 1: Some formed billions of years ago, when the Milky Way
44:14
Speaker 1: looked different from the galaxy you see today. There was
44:17
Speaker 1: less of the familiar spiral structure. The galaxy was still growing.
44:22
Speaker 1: Smaller collections of stars were being gathered together. Gas was moving.
44:27
Speaker 1: Gravity was slowly shaping everything into the large structure that
44:32
Speaker 1: exists now. And some of the stars you see in
44:35
Speaker 1: the thick disk may carry memories of those ancient stages.
44:39
Speaker 1: They are old lights, quiet lights. Stars that have remained
44:44
Speaker 1: long after many of the clouds and clusters from which
44:47
Speaker 1: they were born have changed beyond recognition. As you drift
44:51
Speaker 1: farther upward, the stars become more scattered. Now you begin
44:56
Speaker 1: to notice small, rounded groups of light. These are globular clusters.
45:01
Speaker 1: They contain many stars gathered together by gravity, often in dense,
45:06
Speaker 1: almost spherical arrangements. Some of their stars are among the
45:10
Speaker 1: oldest in the Milky Way. Imagine looking toward one of
45:13
Speaker 1: these clusters from a great distance. Instead of one star,
45:17
Speaker 1: you see a faint glowing cloud. But within that cloud
45:21
Speaker 1: are countless individual suns, bound together in a slow gravitational dance.
45:27
Speaker 1: Many of them have existed for more than 12 billion years.
45:31
Speaker 1: That means some of their light began its journey before
45:34
Speaker 1: the solar system existed. Before Earth, before the sun had formed,
45:39
Speaker 1: before the familiar planets had taken their present shapes, the
45:43
Speaker 1: stars simply continued shining. And now, billions of years later,
45:48
Speaker 1: their light is still crossing the darkness. You can let
45:52
Speaker 1: that thought move slowly through your mind. A very old star.
45:56
Speaker 1: Still shining, still moving, still part of the galaxy. But
46:01
Speaker 1: the halo around the Milky Way contained something else. Something
46:05
Speaker 1: that looks less like a cluster and more like a trail.
46:09
Speaker 1: Long streams of stars can stretch through the space around
46:13
Speaker 1: the galaxy. These are stellar streams. Some are the remains
46:17
Speaker 1: of smaller galaxies that once passed close to the Milky Way.
46:21
Speaker 1: Gravity slowly pulled at them. Their stars were separated from
46:25
Speaker 1: one another and stretched into long, delicate structures. Instead of
46:30
Speaker 1: remaining together as a small galaxy, the stars became a
46:34
Speaker 1: thin stream winding around the larger one. One well-known example
46:39
Speaker 1: is the Sagittarius Stream. It contains stars that are thought
46:43
Speaker 1: to have once belonged to a smaller companion galaxy. Over
46:47
Speaker 1: enormous periods of time, the Milky Way's gravity gradually drew
46:52
Speaker 1: those stars outward, stretching them into a long trail around
46:56
Speaker 1: our own galaxy. You can imagine it as a faint
46:59
Speaker 1: ribbon of stars, almost invisible against the darkness. Not a
47:04
Speaker 1: straight line, not a rigid structure, a slow, curved stream,
47:09
Speaker 1: following the gravitational shape of the galaxy. And this tells
47:13
Speaker 1: you something important. The Milky Way did not simply appear
47:16
Speaker 1: as one enormous galaxy and remain unchanged. It has grown.
47:22
Speaker 1: It has absorbed smaller systems. It has rearranged their stars.
47:27
Speaker 1: Some of the stars around you may have been born
47:29
Speaker 1: somewhere else. They may have belonged to another small galaxy
47:33
Speaker 1: long before becoming part of the Milky Way. In this way,
47:37
Speaker 1: a galaxy can carry pieces of its past within its
47:40
Speaker 1: present form. Somewhere else in the halo, another small galaxy
47:44
Speaker 1: is being pulled apart. Its stars are gradually being stretched
47:48
Speaker 1: away from one another. Gravity does not need to act quickly.
47:52
Speaker 1: Given enough time, even a small difference in gravitational pull
47:57
Speaker 1: can reshape an entire collection of stars. Millions of stars
48:01
Speaker 1: can become a stream. A small galaxy can become part
48:05
Speaker 1: of a larger one. And a galaxy can quietly grow
48:08
Speaker 1: without anything happening that a human observer would notice in
48:12
Speaker 1: a single lifetime. You continue drifting upward. The bright Milky
48:17
Speaker 1: Way is now far below you. The stars of the
48:20
Speaker 1: disk form a thin luminous plane. Around you, there is
48:24
Speaker 1: much more darkness. But even here, the space is not
48:27
Speaker 1: completely empty. The halo contains extremely hot gas. It is
48:32
Speaker 1: so hot that it can reach temperatures of millions of degrees,
48:36
Speaker 1: although the gas is spread very thinly through space. You
48:40
Speaker 1: would not feel a warm breeze. There is not enough
48:43
Speaker 1: material for anything like that. Instead, this hot gas forms
48:48
Speaker 1: a vast, faint environment surrounding the galaxy. It is almost
48:52
Speaker 1: impossible to see with ordinary eyes. yet it is there,
48:56
Speaker 1: a huge thin atmosphere around a galaxy. And beyond the
49:01
Speaker 1: visible stars and the faint gas, there is an even
49:04
Speaker 1: larger component, dark matter. You cannot see it directly, there
49:09
Speaker 1: is no soft glow to follow, no cloud to photograph,
49:14
Speaker 1: no ordinary light reflected from its surface. Instead, astronomers in
49:20
Speaker 1: its presence from gravity. Stars orbit the galaxy. Galaxies move
49:26
Speaker 1: within larger groups. Light bends as it passes through massive
49:30
Speaker 1: regions of space. Again and again, the motion of visible
49:34
Speaker 1: matter suggests that something unseen contributes to the overall gravitational structure.
49:40
Speaker 1: So you can imagine the Milky Way surrounded by an
49:43
Speaker 1: enormous invisible halo. The bright spiral disc is only the
49:47
Speaker 1: part you can easily see. The invisible structure extends much farther.
49:52
Speaker 1: You cannot look at it directly. But without something providing
49:56
Speaker 1: that additional gravitational influence, the motions of stars and galaxies
50:00
Speaker 1: would be difficult to explain. It is one of those
50:03
Speaker 1: places where astronomy asks you to imagine something you cannot see.
50:08
Speaker 1: Not because the idea is magical, but because sometimes the
50:11
Speaker 1: effects of an invisible thing are easier to observe than
50:15
Speaker 1: the thing itself. You can remain here for a while,
50:18
Speaker 1: the galaxy below you, the old globular clusters around you,
50:23
Speaker 1: the faint stellar streams slowly wrapping through the darkness, the
50:27
Speaker 1: hot gas spreading thinly through the halo, and the invisible
50:31
Speaker 1: gravitational structure extending farther still, the Milky Way that once
50:36
Speaker 1: seemed so enormous from Earth now appears almost delicate. A
50:41
Speaker 1: bright disk, a faint surrounding halo, and beyond it more darkness.
50:47
Speaker 1: You have travelled far enough that the galaxy is beginning
50:50
Speaker 1: to look less like a place and more like a
50:52
Speaker 1: single object floating in space. And now, as you drift
50:56
Speaker 1: still farther away, the Milky Way slowly becomes smaller. The
51:01
Speaker 1: spiral arms become difficult to distinguish. The central glow fades.
51:06
Speaker 1: The halo disappears into the surrounding darkness. And eventually, there
51:11
Speaker 1: is only one small galaxy of light behind you. Not
51:14
Speaker 1: the universe. Not even the largest structure. Just one galaxy
51:19
Speaker 1: among many. Somewhere ahead, another galaxy is waiting. The Milky
51:26
Speaker 1: Way is becoming smaller behind you. The spiral arms are
51:30
Speaker 1: no longer easy to distinguish. The bright centre has faded
51:34
Speaker 1: into a single soft glow. and the enormous halo that
51:38
Speaker 1: once seemed to surround the entire galaxy has almost disappeared
51:42
Speaker 1: into the darkness. For a moment, you might think that
51:45
Speaker 1: you have finally reached empty space, but then another point
51:49
Speaker 1: of light appears, and then another, and another. The darkness
51:53
Speaker 1: around you is not completely empty. There are other galaxies here.
51:58
Speaker 1: One of the largest and most familiar is Andromeda. From Earth,
52:03
Speaker 1: Andromeda appears as a faint, soft patch of light in
52:06
Speaker 1: the night sky. It is so far away that its
52:10
Speaker 1: individual stars cannot normally be seen with the unaided eye.
52:14
Speaker 1: Yet from here, farther outside the Milky Way, you can
52:18
Speaker 1: begin to imagine its enormous shape, a galaxy, another island
52:23
Speaker 1: of stars. Another vast collection of gas, dust, planets, stellar remnants,
52:30
Speaker 1: and invisible matter. The distance between the Milky Way and
52:33
Speaker 1: Andromeda is about two and a half million light years.
52:37
Speaker 1: You can let that number become an image rather than
52:40
Speaker 1: a calculation. Imagine light leaving the Milky Way. It travels
52:45
Speaker 1: for one year, then another, then another, and continues onward
52:50
Speaker 1: for more than two million years before reaching the other galaxy.
52:54
Speaker 1: During all that time, there is no bridge between the galaxies,
52:58
Speaker 1: no road, no cloud of matter filling the space, only
53:03
Speaker 1: a very thin sprinkling of particles spread through an enormous darkness.
53:08
Speaker 1: The space between galaxies is far emptier than the space
53:11
Speaker 1: between the planets, and even the space between stars, which
53:15
Speaker 1: seemed so vast earlier, now feels almost crowded by comparison.
53:20
Speaker 1: You drift across this distance slowly in your imagination. The
53:25
Speaker 1: Milky Way becomes smaller behind you, Andromeda becomes slightly larger ahead.
53:30
Speaker 1: Neither galaxy is rushing toward the other in any human sense.
53:35
Speaker 1: Their motion unfolds across immense stretches of time. Gravity is
53:39
Speaker 1: quietly shaping their paths. And neither galaxy exists completely alone.
53:44
Speaker 1: The Milky Way, Andromeda, the Triangulum Galaxy, and many smaller
53:49
Speaker 1: galaxies are bound together by gravity. Together, they form what
53:54
Speaker 1: astronomers call the Local Group. It is our small neighbourhood
53:58
Speaker 1: within the much larger universe. There are many galaxies here,
54:02
Speaker 1: but only a few large ones. The rest are mostly
54:05
Speaker 1: smaller companions, including faint dwarf galaxies that can be difficult
54:10
Speaker 1: to observe. Some circle the larger galaxies. Some have been
54:15
Speaker 1: pulled apart, some may have been absorbed over time, and
54:19
Speaker 1: some remain quietly orbiting at great distances. You can imagine
54:24
Speaker 1: the local group as a loose gathering of islands in
54:26
Speaker 1: an immense dark sea. The islands are galaxies, the sea
54:31
Speaker 1: is space, and most of the sea contains almost nothing.
54:36
Speaker 1: As you continue drifting outward, even the local group begins
54:39
Speaker 1: to look small. The Milky Way and Andromeda become two
54:43
Speaker 1: points of light among many others. The Triangulum Galaxy becomes
54:47
Speaker 1: another faint point. The smaller galaxies become increasingly difficult to distinguish.
54:53
Speaker 1: and then something even larger begins to appear. The galaxies
54:58
Speaker 1: are not scattered completely at random. They gather into broad arrangements,
55:02
Speaker 1: long structures, and flattened regions. Our local group is part
55:07
Speaker 1: of one of these larger arrangements, known as the local sheet.
55:11
Speaker 1: It contains many galaxies spread across an enormous region of space.
55:16
Speaker 1: From this distance, the structure can be imagined as a
55:18
Speaker 1: very thin layer of galaxies, floating within a much larger darkness.
55:24
Speaker 1: The galaxies are separated by vast distances, yet gravity still
55:28
Speaker 1: connects their motions. The universe, it seems, is not simply
55:33
Speaker 1: a box filled with randomly scattered stars. It has a pattern.
55:37
Speaker 1: Galaxies gather. Galaxies stretch into broad structures, and between those
55:42
Speaker 1: structures are enormous regions where galaxies become much less common.
55:47
Speaker 1: You are beginning to approach one of these regions now.
55:50
Speaker 1: The light around you becomes thinner. The galaxies become fewer.
55:55
Speaker 1: The darkness becomes larger. This is the local void. The
55:59
Speaker 1: name sounds simple, but the scale is difficult to imagine.
56:03
Speaker 1: It is an enormous region of space with far fewer
56:06
Speaker 1: galaxies than the surrounding areas. It is not perfectly empty.
56:11
Speaker 1: There are still galaxies within and around it. There is
56:14
Speaker 1: still thin gas. There is still gravity. There is still
56:18
Speaker 1: the slow expansion of the universe, but compared with the
56:22
Speaker 1: richer regions around it, the local void is unusually sparse.
56:26
Speaker 1: You can imagine standing inside an enormous room where only
56:30
Speaker 1: a few tiny lights are scattered across the walls. The
56:34
Speaker 1: room is not completely dark, it is simply mostly empty.
56:38
Speaker 1: And now the strange thing becomes clear. The void is
56:42
Speaker 1: not merely a blank space left over after everything else
56:46
Speaker 1: was created. It is part of the cosmic structure itself.
56:50
Speaker 1: Long ago, when the universe was much younger, matter was
56:54
Speaker 1: not distributed perfectly evenly. Some regions contained slightly more material.
57:00
Speaker 1: Others contained slightly less. Over enormous stretches of time, gravity
57:05
Speaker 1: pulled matter toward the denser regions. The richer regions became richer.
57:11
Speaker 1: The emptier regions became emptier. Galaxies gathered into groups and filaments.
57:17
Speaker 1: The spaces between them expanded and became increasingly sparse. The
57:22
Speaker 1: emptiness was slowly shaped alongside the galaxies, so the universe
57:26
Speaker 1: grew into a pattern of light and darkness. Matter gathered
57:30
Speaker 1: into threads. Galaxies gathered along those threads. And between them,
57:36
Speaker 1: great spaces opened. You can remain inside this idea for
57:40
Speaker 1: a moment. A universe that creates both things and spaces.
57:44
Speaker 1: Stars and darkness. galaxies and voids, light and distance. Nearby,
57:51
Speaker 1: within the broad region of the local void, there is
57:54
Speaker 1: a small spiral galaxy known as NGC 6503. It is
58:00
Speaker 1: much smaller than the Milky Way. Its spiral arms contain
58:04
Speaker 1: areas of bright gas and young stars. Some regions glow
58:08
Speaker 1: with the light of newly formed stars. Other regions are
58:12
Speaker 1: darkened by cooler gas and dust. and yet this small
58:15
Speaker 1: galaxy exists within a region that is otherwise unusually sparse.
58:20
Speaker 1: You can imagine it as one little island of light,
58:23
Speaker 1: surrounded by a vast, quiet sea. It is a useful
58:27
Speaker 1: reminder that even the emptiest regions of the universe are
58:31
Speaker 1: not perfectly empty. There are still exceptions, still isolated galaxies,
58:37
Speaker 1: still stars being born, still light moving outward, still time passing.
58:44
Speaker 1: And beyond this small galaxy, the local void continues. Its
58:49
Speaker 1: scale is so large that the idea of crossing it
58:51
Speaker 1: becomes almost meaningless. You would need millions of years for
58:55
Speaker 1: light to travel across some of its greatest dimensions. And
58:59
Speaker 1: even then, the journey would be mostly through darkness. No
59:03
Speaker 1: long chain of stars would guide you. No bright galaxy
59:07
Speaker 1: would appear at every step. For a very long time,
59:10
Speaker 1: there might be almost nothing to see. Perhaps only a
59:13
Speaker 1: faint point of light somewhere far ahead. Then another. Then
59:18
Speaker 1: nothing again. This is where the meaning of emptiness begins
59:22
Speaker 1: to change. Earlier, when you left Earth, emptiness meant the
59:27
Speaker 1: space between planets. Then it became the space between stars. Now,
59:32
Speaker 1: emptiness exists between galaxies. And the scale keeps growing. You
59:38
Speaker 1: are no longer moving between worlds. You are moving between
59:41
Speaker 1: entire galaxies. The Milky Way is behind you. Andromeda is
59:46
Speaker 1: somewhere within the larger group. The local sheet stretches across
59:50
Speaker 1: the darkness. And the local void opens quietly beside it.
59:55
Speaker 1: The universe seems less like a collection of objects now,
59:58
Speaker 1: and more like a vast arrangement of light, separated by
1:00:02
Speaker 1: enormous spaces. You can let yourself drift here. There is
1:00:06
Speaker 1: nowhere you need to reach. There is no destination waiting
1:00:10
Speaker 1: urgently ahead. Only more darkness. More distant galaxies. More faint structures.
1:00:17
Speaker 1: And beyond the local group, beyond the familiar neighbourhood of
1:00:20
Speaker 1: the Milky Way, the pattern becomes larger still. The threads
1:00:25
Speaker 1: become longer. The spaces become wider. and the quiet emptiness
1:00:30
Speaker 1: begins to occupy more and more of what you can imagine.
1:00:36
Speaker 1: The darkness around you continues. The local void stretches quietly
1:00:40
Speaker 1: behind you now, a vast region where galaxies are separated
1:00:44
Speaker 1: by enormous distances. The Milky Way is far away. Andromeda
1:00:50
Speaker 1: is far away. The small galaxies of the local group
1:00:53
Speaker 1: have become tiny points of light, and then even those
1:00:57
Speaker 1: points begin to fade. For a while there seems to
1:01:00
Speaker 1: be almost nothing ahead, just darkness. But as you drift
1:01:04
Speaker 1: farther outward, something begins to appear. Not a single galaxy,
1:01:09
Speaker 1: not even a single group. A pattern, very faint at first.
1:01:13
Speaker 1: Small concentrations of galaxies gather together, separated by broad spaces.
1:01:19
Speaker 1: Then those concentrations seem to connect. Long structures begin to emerge,
1:01:25
Speaker 1: threads of galaxies stretching across the darkness. These are part
1:01:29
Speaker 1: of what astronomers call the cosmic web. You can imagine
1:01:33
Speaker 1: it as an enormous, almost invisible network spread throughout the universe.
1:01:38
Speaker 1: But it is not a web made of threads you
1:01:40
Speaker 1: could touch. Its threads are made of galaxies, gas, and
1:01:44
Speaker 1: the gravitational structure that has gathered matter together over immense
1:01:48
Speaker 1: stretches of time. Where matter became slightly more concentrated in
1:01:53
Speaker 1: the young universe, gravity slowly pulled more matter toward it.
1:01:57
Speaker 1: Those regions became richer, galaxies formed, groups of galaxies formed,
1:02:03
Speaker 1: clusters formed, and between them the universe remained more empty.
1:02:08
Speaker 1: Over billions of years this difference became larger, the rich
1:02:13
Speaker 1: regions grew richer, the sparse regions became more spacious, and
1:02:18
Speaker 1: the universe gradually developed the pattern you are seeing now.
1:02:22
Speaker 1: Threads of galaxies, broad sheets, dense clusters, and enormous voids.
1:02:28
Speaker 1: The strange thing is that the empty spaces can be
1:02:31
Speaker 1: much larger than the galaxies themselves. A galaxy may be
1:02:35
Speaker 1: tens or hundreds of thousands of light years across. A
1:02:39
Speaker 1: cluster may contain thousands of galaxies. And yet the distances
1:02:43
Speaker 1: between these structures can stretch across millions or even hundreds
1:02:48
Speaker 1: of millions of light years. You can let that scale
1:02:51
Speaker 1: become softer rather than trying to calculate it. Imagine a
1:02:55
Speaker 1: handful of tiny lights scattered across a dark room. Now
1:03:00
Speaker 1: imagine that the room is so enormous that the lights
1:03:02
Speaker 1: themselves are separated by distances that light needs millions of
1:03:06
Speaker 1: years to cross. That is the kind of emptiness you
1:03:09
Speaker 1: are moving through. And still, there is structure. The galaxies
1:03:14
Speaker 1: are not simply scattered randomly. They trace the invisible architecture
1:03:19
Speaker 1: of gravity. Some gather along long filaments. Some collect where
1:03:24
Speaker 1: several filaments meet. Some remain near the edges of enormous voids,
1:03:29
Speaker 1: and some appear almost alone. You can imagine one small
1:03:33
Speaker 1: galaxy sitting quietly at the edge of a great empty region.
1:03:37
Speaker 1: Around it, there may be almost nothing. Yet far away,
1:03:42
Speaker 1: perhaps millions of light-years beyond, another galaxy shines. Its light
1:03:47
Speaker 1: begins its journey. It crosses the darkness. And eventually, perhaps,
1:03:53
Speaker 1: it reaches another galaxy. The universe is connected by light
1:03:57
Speaker 1: that travels through almost nothing. This is one of the
1:04:01
Speaker 1: quietest ideas in astronomy. The farther apart things are, the
1:04:05
Speaker 1: more time their light needs to travel between them. So,
1:04:09
Speaker 1: when you look outward, you are also looking backward. A
1:04:12
Speaker 1: nearby galaxy is seen relatively close to its present time.
1:04:16
Speaker 1: A galaxy billions of light years away is seen as
1:04:20
Speaker 1: it existed billions of years ago. The farther you look
1:04:23
Speaker 1: the younger the universe appears. You are not simply travelling
1:04:27
Speaker 1: across space. You are drifting backward through time. And eventually
1:04:32
Speaker 1: there is a limit to how far that view can reach.
1:04:35
Speaker 1: The universe has been expanding for billions of years. Light
1:04:40
Speaker 1: has been travelling through it. And because light moves at
1:04:43
Speaker 1: a finite speed, there are regions whose light has not
1:04:46
Speaker 1: had enough time to reach us. The part of the
1:04:49
Speaker 1: universe whose light can, in principle, reach us, is called
1:04:53
Speaker 1: the observable universe. The word observable is important. It does
1:04:58
Speaker 1: not necessarily mean that the universe ends there. It simply
1:05:02
Speaker 1: marks the region from which information has had time to
1:05:05
Speaker 1: reach us since the early universe. Beyond that boundary there
1:05:09
Speaker 1: may be much more, perhaps vastly more, but we cannot
1:05:13
Speaker 1: see it directly. You can let that boundary remain distant
1:05:17
Speaker 1: and gentle. There is no wall, no edge you could touch.
1:05:22
Speaker 1: only a limit created by the travel of light and
1:05:25
Speaker 1: the age of the universe. And as you drift toward
1:05:28
Speaker 1: the farthest light we can observe, something remarkable begins to
1:05:33
Speaker 1: surround you. A very faint glow. It does not come
1:05:37
Speaker 1: from a nearby star. It does not come from a galaxy.
1:05:41
Speaker 1: It is older than the galaxies around you. It is
1:05:44
Speaker 1: the cosmic microwave background, a faint remnant of the early universe.
1:05:50
Speaker 1: You can think of it as an ancient afterglow, a
1:05:53
Speaker 1: very old light that has been travelling across expanding space
1:05:56
Speaker 1: for billions of years. By the time it reaches us,
1:06:01
Speaker 1: it has become extremely faint and shifted into a form
1:06:04
Speaker 1: of radiation that cannot be seen with human eyes. But
1:06:08
Speaker 1: instruments can detect it, and within that faint background are
1:06:12
Speaker 1: tiny variations, very small differences in temperature and density. Those
1:06:17
Speaker 1: differences are important, because they represent the early seeds from
1:06:21
Speaker 1: which the later universe grew, the galaxies, the clusters, the
1:06:26
Speaker 1: great filaments, the enormous voids. All of these structures developed
1:06:31
Speaker 1: from a universe that was once much younger, much hotter,
1:06:35
Speaker 1: and much more uniform. You can imagine the early universe
1:06:38
Speaker 1: as a nearly smooth field with tiny irregularities. Almost nothing
1:06:43
Speaker 1: seems different. And yet, over immense stretches of time, Gravity
1:06:48
Speaker 1: slowly amplifies those small differences. A little more matter here,
1:06:53
Speaker 1: a little less matter there. A slightly denser region begins
1:06:57
Speaker 1: to gather more material. A slightly emptier region loses more
1:07:01
Speaker 1: material to its surroundings. And billions of years later, the
1:07:05
Speaker 1: cosmic web appears. Galaxies shine along its threads. Clusters gather
1:07:11
Speaker 1: at its intersections. Voids open between them. And the entire
1:07:15
Speaker 1: pattern continues to change. You have now drifted farther than
1:07:20
Speaker 1: any human body could ever travel. You have passed beyond Earth,
1:07:24
Speaker 1: beyond the planets, beyond the nearby stars, beyond the Milky Way,
1:07:30
Speaker 1: beyond the local group, beyond the great empty spaces around
1:07:34
Speaker 1: our galactic neighbourhood. And yet, after all that distance, the
1:07:39
Speaker 1: universe does not feel louder, it feels quieter. The farther
1:07:43
Speaker 1: you go, the more darkness surrounds the light. The more
1:07:47
Speaker 1: enormous the distances become, the more gently everything seems to move.
1:07:53
Speaker 1: And now, you can begin to drift back. Not because
1:07:57
Speaker 1: the universe has changed, but because you no longer need
1:08:00
Speaker 1: to travel farther. Let the distant cosmic web remain behind you.
1:08:05
Speaker 1: Let the ancient afterglow continue crossing space. Let galaxies continue turning.
1:08:11
Speaker 1: Let the voids remain vast and quiet. You can slowly
1:08:15
Speaker 1: return your attention to the Milky Way. It becomes a
1:08:19
Speaker 1: faint shape again. Then a smaller glow. The spiral arms return.
1:08:24
Speaker 1: Somewhere within them is the Orion Arm. Somewhere inside that
1:08:28
Speaker 1: arm is the Solar System. And somewhere within the Solar
1:08:32
Speaker 1: System is Earth. Your home. You do not need to
1:08:35
Speaker 1: imagine the entire journey back. You can simply let the
1:08:38
Speaker 1: distance disappear. The Milky Way becomes the night sky. The
1:08:43
Speaker 1: night sky becomes the window, the window becomes the room
1:08:47
Speaker 1: around you, and now there is no need to think
1:08:50
Speaker 1: about galaxies or black holes or distant planets. They can
1:08:54
Speaker 1: continue without you. The stars can continue shining. The planets continue.
1:09:00
Speaker 1: can continue turning. The galaxies can continue drifting through the
1:09:04
Speaker 1: cosmic web. The enormous empty spaces can remain empty. Everything
1:09:09
Speaker 1: can continue moving slowly, quietly, patiently, and you can become still.
1:09:15
Speaker 1: There is nothing you need to solve, nothing you need
1:09:18
Speaker 1: to remember. You have already travelled far enough. So perhaps
1:09:23
Speaker 1: you can simply listen to the quiet around you. Feel
1:09:26
Speaker 1: the weight of your body resting where it is. Notice
1:09:30
Speaker 1: the darkness behind your closed eyes. Let your breathing become
1:09:34
Speaker 1: slow and natural. And somewhere far beyond your room, ancient
1:09:38
Speaker 1: light is still travelling. A faint message from the early universe,
1:09:43
Speaker 1: crossing an almost unimaginable distance, arriving quietly. You do not
1:09:49
Speaker 1: need to follow it anymore. You can let it continue
1:09:52
Speaker 1: on its way. The universe is large enough to carry
1:09:55
Speaker 1: its own stories. And tonight, you can simply rest beneath them.
1:10:01
Speaker 1: And so, our journey drifts quietly to its end. Whether
1:10:06
Speaker 1: you are still listening beneath the soft glow of your screen,
1:10:09
Speaker 1: or already floating somewhere between dreams and distant stars, thank
1:10:14
Speaker 1: you for travelling here tonight, through silent worlds, ancient stars,
1:10:19
Speaker 1: dark galaxies, and the immense spaces that lie between everything.
1:10:24
Speaker 1: If this wandering brought you a little calm, you are
1:10:27
Speaker 1: welcome to subscribe or share it with someone who could
1:10:30
Speaker 1: use a gentler night beneath the cosmos. For now, rest well.
1:10:35
Speaker 1: The stars will keep burning, the planets will keep turning,
1:10:39
Speaker 1: the galaxies will keep drifting, and the universe will carry
1:10:43
Speaker 1: on softly around you while you sleep.