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What Actually Is Light_ Where Does It Come From · Night Science Stories

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  Speaker 1: Good evening, and welcome to the Sleepy Scientist. I'm very
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  Speaker 1: help the channel keep growing. And to everyone who already
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  Speaker 1: comes back night after night, thank you. It really does
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  Speaker 1: mean a lot to have you here. Tonight, we're following
• 1:13
  Speaker 1: something that is almost impossible to avoid. It streams through
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  Speaker 1: your windows in the morning, leaves distant stars millions of
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  Speaker 1: years before reaching your eyes, escapes from atoms, erupts from
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  Speaker 1: dying stars, races through empty space and quietly carries enormous
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  Speaker 1: amounts of information about places we could never hope to visit.
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  Speaker 1: And the strange thing is that what we casually call
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  Speaker 1: light is far larger than the small band of colours
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  Speaker 1: we can actually see. The same basic phenomenon stretches from
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  Speaker 1: radio waves longer than buildings to gamma rays carrying extraordinary
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  Speaker 1: amounts of energy. It can be produced by hot matter,
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  Speaker 1: excited atoms, magnetic fields, nuclear reactions, violent collisions, living organisms,
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  Speaker 1: electrical currents, and some of the most extreme objects in
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  Speaker 1: the universe. So tonight, we're going to follow light from
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  Speaker 1: the moment it is created, through everything that can happen
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  Speaker 1: to it along the way, and finally to the moment
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  Speaker 1: it reaches an eye, a telescope, or a detector here
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  Speaker 1: on Earth. So, whenever you're ready, Settle in. What actually
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  Speaker 1: is light? And where does it come from? Light is
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  Speaker 1: so familiar that it is very easy to forget how
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  Speaker 1: strange it really is. We wake to sunlight creeping through
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  Speaker 1: a window. We notice the blue of the sky, the
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  Speaker 1: warm colour of a lamp, the faint glow of a
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  Speaker 1: distant star, or the light from a screen resting a
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  Speaker 1: little too close to our face late at night. In
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  Speaker 1: everyday life, light seems to be simply the thing that
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  Speaker 1: allows us to see, but visible light is only a
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  Speaker 1: very small part of a much larger phenomenon. Human eyes
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  Speaker 1: are sensitive to only a narrow range of electromagnetic radiation,
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  Speaker 1: roughly from wavelengths of about 700 nanometres at the red
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  Speaker 1: end of vision to around 400 nanometres at the violet end.
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  Speaker 1: Beyond red lies infrared radiation, then microwaves and radio waves.
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  Speaker 1: Beyond violet lie ultraviolet radiation, X-rays and gamma rays. These
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  Speaker 1: forms of radiation can behave very differently when they interact
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  Speaker 1: with matter, but fundamentally they belong to the same family.
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  Speaker 1: A radio signal passing through the atmosphere, the warmth felt
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  Speaker 1: from infrared radiation, visible starlight, an ultraviolet photon from the sun,
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  Speaker 1: and an X-ray produced near a black hole are all
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  Speaker 1: examples of electromagnetic radiation. What changes is mainly their wavelength, frequency,
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  Speaker 1: and therefore the amount of energy associated with their photons.
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  Speaker 1: The realisation that visible light was part of something much
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  Speaker 1: larger emerged gradually during the 19th century. One of the
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  Speaker 1: most important figures in that story was the Scottish physicist
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  Speaker 1: James Clerk Maxwell. By bringing together earlier discoveries about electricity
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  Speaker 1: and magnetism, Maxwell developed a mathematical description showing that electric
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  Speaker 1: and magnetic phenomena were deeply connected. A changing electric field
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  Speaker 1: can be associated with a changing magnetic field, and a
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  Speaker 1: changing magnetic field can in turn be associated with an
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  Speaker 1: electric field. Under the right conditions, these disturbances do not
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  Speaker 1: remain confined to one location. They can propagate through space.
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  Speaker 1: When Maxwell calculated how quickly such an electromagnetic disturbance should
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  Speaker 1: travel through empty space, the result was extraordinary. The predicted
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  Speaker 1: speed was essentially the same as the measured speed of light.
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  Speaker 1: That was not a coincidence. Light itself was an electromagnetic wave.
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  Speaker 1: In empty space, light travels at approximately 300,000 km per second.
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  Speaker 1: At that speed, it could travel around Earth more than
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  Speaker 1: seven times in a single second. Light from the moon
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  Speaker 1: requires only a little more than one second to reach us.
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  Speaker 1: Light from the sun takes roughly eight minutes and twenty seconds.
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  Speaker 1: Even at such an enormous speed, however, the universe is
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  Speaker 1: so large that light can spend years, millions of years
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  Speaker 1: or billions of years travelling before reaching an observer. The
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  Speaker 1: electromagnetic wave picture explains an enormous amount about light. It
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  Speaker 1: explains interference, where overlapping waves can reinforce or cancel one another.
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  Speaker 1: It explains diffraction, where light spreads after passing through a
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  Speaker 1: narrow opening or around an obstacle. It explains polarisation, which
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  Speaker 1: concerns the orientation of the electromagnetic field. propagates through space
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  Speaker 1: and interacts with many forms of matter. But by the
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  Speaker 1: beginning of the 20th century, experiments were revealing behaviour that
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  Speaker 1: could not be understood fully by treating light only as
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  Speaker 1: a continuous classical wave. One of the crucial clues came
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  Speaker 1: from the photoelectric effect. When light shines onto certain materials,
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  Speaker 1: it can cause electrons to be released from their surfaces.
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  Speaker 1: Classical reasoning suggested that making the light more intense should
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  Speaker 1: eventually provide enough energy to remove electrons, regardless of the
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  Speaker 1: light's frequency. Experiments did not behave that way. Below a
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  Speaker 1: certain frequency, increasing the intensity did not solve the problem.
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  Speaker 1: Above that frequency, electrons could be emitted, and the energy
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  Speaker 1: they received depended strongly on the frequency of the incoming light.
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  Speaker 1: In 1905, Albert Einstein explained this behaviour by building upon
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  Speaker 1: the earlier quantum ideas of Max Planck. Einstein proposed that
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  Speaker 1: the energy carried by light could be exchanged in discrete amounts.
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  Speaker 1: In the modern description, these discrete excitations are called photons.
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  Speaker 1: The energy of an individual photon depends on the frequency
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  Speaker 1: of the light. Photon energy equals Planck's constant multiplied by frequency.
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  Speaker 1: Higher frequency radiation therefore corresponds to higher energy photons. A
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  Speaker 1: gamma ray photon can carry vastly more energy than a
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  Speaker 1: visible light photon, while a radio photon carries far less.
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  Speaker 1: This does not mean that light is secretly made from
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  Speaker 1: tiny classical balls racing through space while the wave description
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  Speaker 1: was simply mistaken. Quantum physics is subtler than that. light
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  Speaker 1: displays behavior that cannot be captured completely by either the
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  Speaker 1: everyday idea of a particle or the everyday idea of
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  Speaker 1: a wave. In quantum electrodynamics, light is associated with the
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  Speaker 1: electromagnetic field and photons are quantum excitations of that field.
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  Speaker 1: In some situations, the wave properties are the most useful
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  Speaker 1: way to describe what happens. In other situations, Particularly when
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  Speaker 1: energy is exchanged with matter in discrete events, the photon
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  Speaker 1: description becomes essential. Both descriptions arise from a deeper quantum framework.
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  Speaker 1: Photons also have an unusual combination of properties. A photon
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  Speaker 1: has no rest mass. There is no reference frame in
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  Speaker 1: which an ordinary photon can simply sit still. In vacuum,
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  Speaker 1: it propagates at the speed of light. Yet having no
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  Speaker 1: rest mass does not mean having no energy. A photon
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  Speaker 1: carries both energy and momentum. That allows light to do
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  Speaker 1: physical work in the broad sense of transferring energy and
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  Speaker 1: momentum to matter. Sunlight warms Earth's surface because electromagnetic radiation
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  Speaker 1: deposits energy. Light can exert a tiny pressure when it
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  Speaker 1: is absorbed or reflected. Powerful lasers can move microscopic objects.
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  Speaker 1: Radiation pressure can influence dust around stars and can even
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  Speaker 1: be used, in principle and increasingly in practice, to propel
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  Speaker 1: spacecraft using solar sails. Light is therefore not an insubstantial
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  Speaker 1: decoration laid over the physical universe. It participates in it.
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  Speaker 1: Electromagnetic radiation can be created when matter changes energy state,
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  Speaker 1: absorbed when matter gains energy, scattered into a new direction,
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  Speaker 1: shifted to a different frequency, reflected from surfaces, and stretched
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  Speaker 1: while crossing an expanding universe. Most importantly for astronomy, light
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  Speaker 1: carries information. A photon arriving from a star may contain
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  Speaker 1: evidence about the temperature of the material from which it escaped.
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  Speaker 1: A pattern of wavelengths can reveal which chemical elements were present.
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  Speaker 1: A shift in those wavelengths can reveal motion. Polarisation can
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  Speaker 1: reveal magnetic fields. Light bent by gravity can reveal matter
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  Speaker 1: that may otherwise be invisible. Almost everything we know about
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  Speaker 1: distant stars and galaxies has been learned without travelling anywhere
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  Speaker 1: near them. Instead, the universe sends information to us. Some
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  Speaker 1: of it arrives as radio waves, some as infrared radiation,
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  Speaker 1: some as visible light, some as X-rays or gamma rays.
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  Speaker 1: And although our eyes experience only a tiny slice of
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  Speaker 1: this enormous spectrum, All of it belongs to the same
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  Speaker 1: remarkable phenomenon. Light is not merely what makes the universe visible.
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  Speaker 1: It is one of the principal ways the universe communicates
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  Speaker 1: across distance, carrying energy and information from one place to
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  Speaker 1: another at the fastest speed that nature allows. Visible light
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  Speaker 1: is only a very small part of what light can be.
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  Speaker 1: The colours our eyes recognise from red through to violet,
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  Speaker 1: occupy a narrow band inside a much larger range known
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  Speaker 1: as the electromagnetic spectrum. Beyond red lies infrared radiation, then
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  Speaker 1: microwaves and radio waves. Beyond violet lie ultraviolet radiation, X-rays
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  Speaker 1: and gamma rays. These are not separate substances. They are
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  Speaker 1: all electromagnetic radiation. What distinguishes one part of the spectrum
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  Speaker 1: from another is mainly wavelength and frequency. Wavelength is the
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  Speaker 1: distance between repeating points in a wave. Frequency describes how
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  Speaker 1: rapidly the electromagnetic field oscillates. For light travelling through empty space,
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  Speaker 1: the two are linked. Longer wavelengths correspond to lower frequencies,
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  Speaker 1: while shorter wavelengths correspond to higher frequencies. The relationship can
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  Speaker 1: be written in words as the speed of light equals
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  Speaker 1: wavelength multiplied by frequency. Because the speed of light in
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  Speaker 1: vacuum remains constant, increasing the wavelength means lowering the frequency,
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  Speaker 1: and shortening the wavelength means raising it. The scale covered
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  Speaker 1: by the electromagnetic spectrum is enormous. Some radio waves have
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  Speaker 1: wavelengths measured in metres, tens of metres, or even kilometres.
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  Speaker 1: At the opposite extreme, gamma rays can have wavelengths far
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  Speaker 1: smaller than an atom. Visible light occupies only a tiny
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  Speaker 1: interval between those extremes. Frequency also determines the energy carried
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  Speaker 1: by an individual photon. Photon energy equals Planck's constant multiplied
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  Speaker 1: by frequency. A radio photon therefore carries far less energy
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  Speaker 1: than a visible photon. An X-ray photon carries much more,
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  Speaker 1: and a gamma-ray photon can carry vastly more energy again.
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  Speaker 1: This difference helps explain why different parts of the electromagnetic
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  Speaker 1: spectrum interact with matter in very different ways. Radio waves
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  Speaker 1: can pass through some materials and travel enormous distances without
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  Speaker 1: causing much disruption at the atomic level. Infrared radiation is
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  Speaker 1: strongly associated with thermal emission from warm objects. Ultraviolet light
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  Speaker 1: can drive chemical reactions and damage biological molecules. X-rays can
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  Speaker 1: pass through soft tissue more easily than through dense bone.
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  Speaker 1: Gamma rays can be energetic enough to take part in
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  Speaker 1: interactions involving atomic nuclei. For most of human history, none
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  Speaker 1: of this invisible radiation was known to exist. A major
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  Speaker 1: clue appeared in 1800 when the astronomer William Herschel passed
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  Speaker 1: sunlight through a prism. Herschel was measuring the temperature associated
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  Speaker 1: with different colours in the visible spectrum. As he moved
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  Speaker 1: a thermometer from violet toward red, the temperature increased. Then
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  Speaker 1: he placed the thermometer beyond the red end, in a
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  Speaker 1: region where no visible light could be seen. The temperature
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  Speaker 1: rose even higher. Herschel had detected infrared radiation. Only a
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  Speaker 1: year later, Johann Wilhelm Ritter investigated the other end of
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  Speaker 1: the visible spectrum. Ritter studied chemical reactions caused by sunlight
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  Speaker 1: and found that the strongest effect occurred just beyond violet,
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  Speaker 1: in a region invisible to the eye. That radiation would
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  Speaker 1: become known as ultraviolet. The visible rainbow now had invisible
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  Speaker 1: neighbours on both sides. Later in the 19th century, James
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  Speaker 1: Clerk Maxwell's theory of electromagnetism predicted that electromagnetic waves should
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  Speaker 1: exist across a far broader range of wavelengths. Heinrich Hertz
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  Speaker 1: then demonstrated this experimentally. During the 1880s, Hertz generated electromagnetic
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  Speaker 1: waves using electrical apparatus and detected them elsewhere in his laboratory.
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  Speaker 1: These waves had wavelengths far longer than visible light. They
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  Speaker 1: were radio waves. Hertz's experiments helped confirm that Maxwell had
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  Speaker 1: been correct. Visible light was not a special isolated phenomenon.
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  Speaker 1: It was simply one narrow portion of a vast electromagnetic spectrum.
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  Speaker 1: That raises an obvious question. Why do human eyes detect
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  Speaker 1: only this small section? The answer is not that visible
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  Speaker 1: light is fundamentally more important than the rest. human vision
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  Speaker 1: developed under very particular environmental conditions. The sun emits strongly
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  Speaker 1: across a broad range of wavelengths, but a large amount
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  Speaker 1: of the radiation reaching Earth's surface lies within and around
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  Speaker 1: the visible range. Earth's atmosphere is also relatively transparent to
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  Speaker 1: visible wavelengths. For organisms living on the surface, detecting this
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  Speaker 1: radiation became extremely useful. Our eyes are therefore adapted to
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  Speaker 1: the light that was readily available here. Other animals detect
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  Speaker 1: somewhat different ranges. Many insects can see into the ultraviolet.
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  Speaker 1: Some snakes possess sensory organs capable of detecting infrared radiation
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  Speaker 1: from warm objects. The universe does not restrict itself to
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  Speaker 1: the wavelength that happen to be useful to humans. Astronomy
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  Speaker 1: makes that especially clear. A galaxy can look completely different
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  Speaker 1: depending on which part of its electromagnetic radiation is observed.
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  Speaker 1: Visible light reveals many of its stars. Infrared observations can
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  Speaker 1: reveal warm dust and objects hidden behind clouds that visible
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  Speaker 1: light struggles to penetrate. Radio observations can expose cold gas
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  Speaker 1: and enormous jets of energetic particles. Ultraviolet light highlights very
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  Speaker 1: hot young stars. X-rays can reveal gas heated to millions
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  Speaker 1: of degrees, supernova remnants, neutron stars, and matter falling toward
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  Speaker 1: black holes. Gamma rays reveal some of the most energetic
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  Speaker 1: processes known, including powerful stellar explosions and extreme particle acceleration.
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  Speaker 1: The object itself has not changed, only the wavelength being
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  Speaker 1: observed has changed. The sky we see with our eyes
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  Speaker 1: is therefore only one version of the universe. There is
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  Speaker 1: also a radio sky, an infrared sky, an ultraviolet sky,
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  Speaker 1: an x-ray sky and a gamma ray sky, each revealing
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  Speaker 1: physical processes that can be faint or completely invisible in
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  Speaker 1: ordinary vision. This is why the question of where light
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  Speaker 1: comes from becomes much larger than simply asking why stars shine.
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  Speaker 1: Different environments produce different kinds of electromagnetic radiation through different
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  Speaker 1: physical mechanisms. Cool dust, hot stars, ionised gas, magnetic fields,
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  Speaker 1: black hole environments, neutron stars and violent explosions can all
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  Speaker 1: leave distinctive signatures across the spectrum. We experience light as
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  Speaker 1: a narrow band of colour. The universe produces it across
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  Speaker 1: an almost unimaginable range. Light does not have a single birthplace.
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  Speaker 1: There is no universal machine inside nature that simply manufactures
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  Speaker 1: photons in the same way every time. Instead, light appears
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  Speaker 1: whenever physical systems lose energy through electromagnetic interactions, and the
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  Speaker 1: exact process can vary enormously from one environment to another.
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  Speaker 1: Sometimes that energy begins as heat. Sometimes it comes from electricity, motion, gravity, chemistry,
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  Speaker 1: nuclear reactions or magnetic fields. What matters is that some
• 20:06
  Speaker 1: of the energy stored in a physical system is transferred
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  Speaker 1: into the electromagnetic field. One of the most important ingredients
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  Speaker 1: in this process is electric charge. Charged particles interact with
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  Speaker 1: electric and magnetic fields, and when those particles accelerate, they
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  Speaker 1: can emit electromagnetic radiation. Acceleration here does not only mean
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  Speaker 1: getting faster. A particle travelling at nearly constant speed but
• 20:37
  Speaker 1: continually changing direction is also accelerating. This simple principle helps
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  Speaker 1: explain an astonishing range of light sources. Electrons moving through
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  Speaker 1: antennas can produce radio waves. Charged particles twisting through magnetic
• 20:53
  Speaker 1: fields can create synchrotron radiation. Electrons rapidly changing their motion
• 20:59
  Speaker 1: near atomic nuclei can emit X-rays. The details differ, but
• 21:05
  Speaker 1: the connection between accelerating charge and electromagnetic radiation appears again
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  Speaker 1: and again. At the scale of atoms and molecules, however,
• 21:15
  Speaker 1: another part of the story becomes essential. Quantum physics tells
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  Speaker 1: us that many physical systems cannot possess just any arbitrary
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  Speaker 1: amount of internal energy. Atoms, for example, have allowed energy states.
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  Speaker 1: An electron bound within an atom can occupy one of
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  Speaker 1: these states, and if the atom gains energy... perhaps through
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  Speaker 1: a collision or by absorbing a photon, the electron may
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  Speaker 1: move into a higher energy configuration. That excited state is
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  Speaker 1: often temporary. Eventually the system can return toward a lower
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  Speaker 1: energy state. When it does, the lost energy can be
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  Speaker 1: released as a photon. The energy of that photon corresponds
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  Speaker 1: to the difference between the two states. The energy of
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  Speaker 1: the emitted photon equals the difference between the initial energy
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  Speaker 1: state and the final energy state. This is one of
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  Speaker 1: the reasons different atoms can produce distinctive colours and spectral lines.
• 22:15
  Speaker 1: Their internal energy structures are different, so the photons they
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  Speaker 1: emit can have characteristic energies. The idea that energy exchange
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  Speaker 1: might occur in discrete amounts became unavoidable around the beginning
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  Speaker 1: of the 20th century. In 1900, Max Planck was studying
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  Speaker 1: thermal radiations and found that the observed spectrum could be
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  Speaker 1: explained if energy was exchanged in discrete portions rather than
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  Speaker 1: varying completely continuously. Planck initially introduced this idea as part
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  Speaker 1: of a mathematical solution, but it became one of the
• 22:51
  Speaker 1: foundations of quantum physics. A few years later, Albert Einstein
• 22:56
  Speaker 1: extended the concept to light itself when explaining the photoelectric effect.
• 23:03
  Speaker 1: The broader lesson was profound. At microscopic scales, matter and
• 23:08
  Speaker 1: radiation do not always trade energy as a perfectly smooth flow.
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  Speaker 1: Sometimes the exchange occurs in distinct quantum amounts. But atomic
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  Speaker 1: transitions are only one way of producing photons. Hot matter
• 23:23
  Speaker 1: emits electromagnetic radiation because charged particles within it are in
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  Speaker 1: constant motion. The hotter the material becomes, the more energy
• 23:33
  Speaker 1: it can radiate and the characteristic wavelengths of that radiation
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  Speaker 1: change with temperature. Free electrons moving near ions can emit
• 23:42
  Speaker 1: radiation when their paths are deflected. Molecules can emit infrared
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  Speaker 1: light when they change rotational or vibrational states. Atomic nuclei
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  Speaker 1: can emit gamma rays when they move between nuclear energy states.
• 23:57
  Speaker 1: Particle and antiparticle annihilation can convert their mass and energy
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  Speaker 1: into photons and other particles. Strong magnetic environments can force
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  Speaker 1: charged particles into curved trajectories, generating powerful electromagnetic radiation. Even
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  Speaker 1: gravity can act as the original energy source. Matter falling
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  Speaker 1: toward a neutron star or black hole can accelerate to
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  Speaker 1: extraordinary speeds. Collisions and friction within that infalling material convert
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  Speaker 1: gravitational potential energy into heat and particle motion. and some
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  Speaker 1: of that energy ultimately emerges as light. So when an
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  Speaker 1: astronomer sees radiation from a distant object, one of the
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  Speaker 1: first questions is not simply how bright it is, it
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  Speaker 1: is what physical process produced that light. A cloud glowing
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  Speaker 1: in a narrow set of wavelengths may be dominated by
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  Speaker 1: atomic transitions. A hot star can produce a broad thermal spectrum,
• 25:01
  Speaker 1: A supernova remnant may combine thermal emission, atomic lines and
• 25:06
  Speaker 1: radiation from relativistic charged particles. Matter around a black hole
• 25:12
  Speaker 1: may produce intense ultraviolet and X-ray radiation as gravitational energy
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  Speaker 1: is converted into heat. The photons themselves therefore act as
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  Speaker 1: clues to the conditions in which they were born. It
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  Speaker 1: is also important to distinguish between producing light and producing energy.
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  Speaker 1: A glowing object usually does not create energy from nothing.
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  Speaker 1: It converts energy that already exists in another form. A
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  Speaker 1: filament converts electrical energy into heat and then radiation. A
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  Speaker 1: chemical flame releases stored chemical energy. A star converts nuclear
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  Speaker 1: binding energy through fusion. An accretion disk converts gravitational potential energy.
• 25:59
  Speaker 1: A laser converts supplied electrical or optical energy into a
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  Speaker 1: highly organised beam of photons. Energy conservation ties all of
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  Speaker 1: these cases together. The form may change, but the total
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  Speaker 1: energy is accounted for. Light is one of the ways
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  Speaker 1: that energy can leave a system. And because different mechanisms
• 26:22
  Speaker 1: produce different wavelengths, patterns, intensities and degrees of polarisation, that
• 26:28
  Speaker 1: escaping radiation carries information about how it was created. This
• 26:34
  Speaker 1: is one of the great advantages of astronomy. We cannot
• 26:37
  Speaker 1: place a thermometer inside a distant star or collect a
• 26:41
  Speaker 1: sample from an active galactic nucleus millions of light-years away.
• 26:46
  Speaker 1: But we can collect the light they produce. By studying
• 26:50
  Speaker 1: that light carefully, we can often work backwards from the
• 26:53
  Speaker 1: photons to the conditions that created them. So light is
• 26:57
  Speaker 1: not born in one particular way. It can emerge from
• 27:01
  Speaker 1: atoms dropping between energy states, particles accelerating through fields, matter
• 27:07
  Speaker 1: glowing because of heat, nuclei rearranging themselves, particles annihilating, or
• 27:14
  Speaker 1: material surrendering gravitational energy as it falls. Different Processes Different
• 27:22
  Speaker 1: Environments The Same Electromagnetic Universe One of the most useful
• 27:29
  Speaker 1: things about light is that atoms do not simply emit
• 27:32
  Speaker 1: it at random. They tend to produce very specific wavelengths.
• 27:37
  Speaker 1: That means an atom can leave behind something surprisingly close
• 27:41
  Speaker 1: to a signature. The reason lies in the way electrons
• 27:45
  Speaker 1: are arranged inside atoms. An electron bound to an atom
• 27:49
  Speaker 1: cannot possess just any amount of energy. Quantum mechanics allows
• 27:54
  Speaker 1: only particular energy states. These states are often described as
• 27:58
  Speaker 1: energy levels, although the modern picture is more subtle than
• 28:02
  Speaker 1: imagining electrons sitting on neat little circular shelves around the nucleus.
• 28:08
  Speaker 1: When an atom gains energy, perhaps through a collision with
• 28:11
  Speaker 1: another particle or by absorbing a photon, one of its
• 28:15
  Speaker 1: electrons can move into a higher energy state. That excited
• 28:18
  Speaker 1: configuration usually does not last forever. The electron can eventually
• 28:24
  Speaker 1: make a transition into a lower energy state and when
• 28:27
  Speaker 1: it does, the atom can release the energy difference as
• 28:30
  Speaker 1: a photon. The energy of that photon corresponds precisely to
• 28:36
  Speaker 1: the gap between the two states. The energy of the
• 28:39
  Speaker 1: emitted photon equals the difference between the higher energy state
• 28:43
  Speaker 1: and the lower energy state. Because photon energy is linked
• 28:47
  Speaker 1: directly to frequency and therefore to wavelength, Each possible transition
• 28:52
  Speaker 1: can produce a particular wavelength of light. This is where
• 28:56
  Speaker 1: spectral lines come from. Instead of producing an entirely smooth
• 29:01
  Speaker 1: rainbow containing every possible colour equally, an excited gas can
• 29:06
  Speaker 1: emit bright lines at selected wavelengths. Those lines are determined
• 29:11
  Speaker 1: by the internal quantum structure of the atoms and ions
• 29:15
  Speaker 1: in the gas. Different elements have different numbers of protons
• 29:19
  Speaker 1: and electrons, and therefore different arrangements of allowed energy states.
• 29:25
  Speaker 1: So hydrogen does not produce exactly the same pattern as helium.
• 29:29
  Speaker 1: Helium does not produce the same pattern as sodium. Sodium
• 29:33
  Speaker 1: differs from calcium, iron, oxygen, neon and every other element.
• 29:39
  Speaker 1: The patterns become remarkably distinctive. Scientists began seeing these patterns
• 29:45
  Speaker 1: long before they understood their quantum origin. In the early
• 29:49
  Speaker 1: 19th century, the German physicist Joseph von Fraunhofer examined sunlight
• 29:55
  Speaker 1: that had been spread into a spectrum. At first glance,
• 29:59
  Speaker 1: sunlight appears to produce a continuous rainbow. But Fraunhofer noticed
• 30:04
  Speaker 1: something unusual. Dark lines crossed the spectrum at particular wavelengths.
• 30:10
  Speaker 1: He carefully mapped hundreds of them. These became known as
• 30:13
  Speaker 1: Fraunhofer lines. At the time, their full meaning was not understood.
• 30:19
  Speaker 1: They were simply an intriguing feature hidden inside sunlight. Several
• 30:24
  Speaker 1: decades later, Gustav Kirchhoff and Robert Bunsen made the crucial connection.
• 30:30
  Speaker 1: Working with heated chemical elements in the laboratory, they found
• 30:34
  Speaker 1: that each element produced its own pattern of bright spectral lines. Sodium,
• 30:39
  Speaker 1: for example, produced particularly strong yellow lines. Other elements produced
• 30:44
  Speaker 1: different combinations. Kirchhoff recognised that the dark lines in sunlight
• 30:50
  Speaker 1: were closely related to these bright laboratory lines. The same
• 30:54
  Speaker 1: atoms capable of emitting particular wavelengths could also absorb those
• 30:58
  Speaker 1: wavelengths from light passing through them. This gives us two
• 31:03
  Speaker 1: complementary phenomena. Emission lines appear when excited atoms or ions
• 31:10
  Speaker 1: release photons at characteristic wavelengths. Absorption lines appear when light
• 31:15
  Speaker 1: containing many wavelengths passes through cooler material and atoms remove
• 31:20
  Speaker 1: photons corresponding to particular allowed transitions. The result in the
• 31:26
  Speaker 1: second case is a generally continuous spectrum crossed by dark lines,
• 31:31
  Speaker 1: where some wavelengths have been absorbed. Those missing wavelengths are
• 31:36
  Speaker 1: not necessarily gone forever. The atoms may later release the
• 31:40
  Speaker 1: energy again, often by emitting photons in different directions. But
• 31:45
  Speaker 1: from the point of view of an observer looking along
• 31:48
  Speaker 1: the original beam, those particular wavelengths had been weakened. This
• 31:53
  Speaker 1: simple principle completely changed astronomy. Suddenly, astronomers did not need
• 31:59
  Speaker 1: to collect a physical sample from the Sun in order
• 32:02
  Speaker 1: to learn what it contained. The light itself carried the evidence.
• 32:08
  Speaker 1: Hydrogen provides one of the clearest examples. A hydrogen atom
• 32:12
  Speaker 1: contains just one proton and one electron, making it the
• 32:16
  Speaker 1: simplest ordinary atom in the universe. Yet even hydrogen produces
• 32:22
  Speaker 1: a structured set of spectral lines. One especially important group
• 32:27
  Speaker 1: is the Balmer series, which contains lines produced when electrons
• 32:31
  Speaker 1: make transitions involving the second principal energy level. Some of
• 32:36
  Speaker 1: these lines fall within visible wavelengths. One particularly prominent hydrogen
• 32:43
  Speaker 1: line appears in the red part of the spectrum, and
• 32:46
  Speaker 1: is commonly associated with glowing clouds of ionised hydrogen in space.
• 32:51
  Speaker 1: Hydrogen can also produce spectral lines in ultraviolet and infrared
• 32:56
  Speaker 1: wavelengths through other sets of transitions. The fact that such
• 33:00
  Speaker 1: a simple atom generates a detailed spectrum was an important
• 33:04
  Speaker 1: challenge for early physics. In 1913, Niels Bohr proposed a
• 33:10
  Speaker 1: model of the hydrogen atom, in which electrons could occupy
• 33:13
  Speaker 1: only specific allowed orbits with specific energies. Transitions between these
• 33:19
  Speaker 1: allowed states could then produce or absorb photons with matching energies.
• 33:25
  Speaker 1: The Bohr model successfully explained important features of the hydrogen spectrum,
• 33:31
  Speaker 1: including why the observed wavelengths followed regular mathematical patterns. It
• 33:36
  Speaker 1: was an enormous advance. But it was not the final picture.
• 33:40
  Speaker 1: Electrons do not literally travel around atomic nuclei like tiny
• 33:44
  Speaker 1: planets following fixed classical tracks. The development of quantum mechanics
• 33:50
  Speaker 1: replaced that image with a description based on wavefunctions, quantum
• 33:54
  Speaker 1: states and probability distributions. The allowed energies remain real and measurable,
• 34:00
  Speaker 1: but their origin is understood through the quantum behaviour of
• 34:03
  Speaker 1: electrons bound by electromagnetic forces. The hydrogen spectrum therefore became
• 34:10
  Speaker 1: more than just a collection of colourful lines. It became
• 34:14
  Speaker 1: evidence that atoms themselves obey quantum rules, and those same
• 34:19
  Speaker 1: rules operate everywhere. A hydrogen atom in a laboratory on
• 34:23
  Speaker 1: Earth behaves according to the same physics as hydrogen in
• 34:27
  Speaker 1: a nebula thousands of light-years away. An iron atom in
• 34:32
  Speaker 1: the atmosphere of the Sun has the same underlying atomic
• 34:36
  Speaker 1: structure as iron on Earth. That universality is what makes
• 34:41
  Speaker 1: spectroscopy so powerful. Astronomers can collect light from a star,
• 34:47
  Speaker 1: spread it into a spectrum, and compare the observed lines
• 34:51
  Speaker 1: with measurements made in laboratories. From that pattern, they can
• 34:56
  Speaker 1: identify the elements present. The same method works with glowing nebulae,
• 35:02
  Speaker 1: stellar atmospheres, supernova remnants, galaxies and many other environments. The
• 35:09
  Speaker 1: strength of individual lines can reveal even more. It can
• 35:13
  Speaker 1: provide information about temperature, density, ionisation, motion, and other physical conditions.
• 35:21
  Speaker 1: A few dark or bright marks in a spectrum can
• 35:24
  Speaker 1: therefore contain an extraordinary amount of information. This is one
• 35:29
  Speaker 1: of the quiet miracles of astronomy. We may never touch
• 35:33
  Speaker 1: the surface of a distant star. We may never bring
• 35:36
  Speaker 1: back a sample from a nebula. but atoms advertise their
• 35:41
  Speaker 1: presence through the light they emit and absorb. Their internal
• 35:45
  Speaker 1: quantum structure is written into that light, and after travelling
• 35:49
  Speaker 1: across enormous distances, those photons can arrive at a telescope
• 35:54
  Speaker 1: carrying a record of exactly what kind of matter they
• 35:57
  Speaker 1: encountered along the way. There is a surprisingly simple reason
• 36:02
  Speaker 1: that warm objects glow. Anything with a temperature above absolute
• 36:07
  Speaker 1: zero contains particles with thermal energy. Atoms vibrate. Molecules rotate
• 36:14
  Speaker 1: and move. Electrons shift and jostle within matter. Because charged
• 36:20
  Speaker 1: particles are involved in all of this motion, thermal energy
• 36:23
  Speaker 1: can be converted into electromagnetic radiation. That means almost everything
• 36:29
  Speaker 1: around you is emitting light right now. Most of it
• 36:32
  Speaker 1: is simply invisible to human eyes. A table at room
• 36:37
  Speaker 1: temperature radiates mainly in infrared wavelengths. So does a wall,
• 36:43
  Speaker 1: a chair, a cup of tea and the human body.
• 36:47
  Speaker 1: We do not normally see this radiation because our eyes
• 36:50
  Speaker 1: are not sensitive to infrared light, but a thermal camera
• 36:53
  Speaker 1: can detect it quite easily. The hotter an object becomes,
• 36:58
  Speaker 1: the more dramatically its radiation changes. One of the most
• 37:02
  Speaker 1: useful idealisations for understanding this behaviour is the blackbody. A
• 37:07
  Speaker 1: perfect blackbody is an imaginary object that absorbs all electromagnetic
• 37:12
  Speaker 1: radiation falling onto it. It reflects none of that radiation
• 37:16
  Speaker 1: and transmits none of it. If it has a temperature
• 37:19
  Speaker 1: above absolute zero, it also emits its own characteristic spectrum
• 37:24
  Speaker 1: of thermal radiation. The remarkable part is that the overall
• 37:28
  Speaker 1: shape of that spectrum depends primarily on temperature. The exact
• 37:34
  Speaker 1: chemical composition of a perfect blackbody does not determine the
• 37:38
  Speaker 1: basic shape. Temperature does. Real objects are not perfect blackbodies,
• 37:45
  Speaker 1: but many stars, planets and other astronomical objects behave closely
• 37:50
  Speaker 1: enough to this ideal that blackbody physics becomes extremely useful.
• 37:55
  Speaker 1: As temperature increases, two major things happen. First, the object
• 38:01
  Speaker 1: emits more total energy. In the 19th century, Joseph Stefan
• 38:07
  Speaker 1: found experimentally that the total energy radiated from a surface
• 38:11
  Speaker 1: increases extremely rapidly with temperature. Ludwig Boltzmann later provided the
• 38:17
  Speaker 1: theoretical foundation for the relationship. The result is known as
• 38:21
  Speaker 1: the Stefan-Boltzmann law. The energy radiated per unit area increases
• 38:27
  Speaker 1: in proportion to the fourth power of absolute temperature. That
• 38:32
  Speaker 1: fourth power is important. If the absolute temperature of an
• 38:36
  Speaker 1: ideal blackbody doubles, the amount of energy emitted from each
• 38:40
  Speaker 1: unit of surface area does not merely double, it increases
• 38:44
  Speaker 1: by a factor of 16. Hot objects become much more
• 38:48
  Speaker 1: powerful radiators very quickly. The second important change concerns wavelength.
• 38:55
  Speaker 1: As an object becomes hotter, the wavelength at which its
• 38:58
  Speaker 1: thermal emission is strongest moves towards shorter values. Wilhelm Wien
• 39:03
  Speaker 1: described this relationship in what became known as Wien's displacement law.
• 39:09
  Speaker 1: The wavelength of maximum emission is inversely proportional to absolute temperature.
• 39:15
  Speaker 1: Cool objects therefore tend to radiate most strongly at relatively
• 39:20
  Speaker 1: long wavelengths. The human body has a temperature of roughly
• 39:25
  Speaker 1: 310 Kelvin and emits most strongly in the infrared part.
• 39:30
  Speaker 1: of the spectrum. Earth, with an effective radiating temperature of
• 39:34
  Speaker 1: roughly 250 Kelvin, also releases most of its thermal energy
• 39:39
  Speaker 1: as infrared radiation. Make an object much hotter and its
• 39:44
  Speaker 1: strongest emission shifts progressively towards shorter wavelengths. a metal object
• 39:49
  Speaker 1: heated to several hundred degrees Celsius may still emit mostly
• 39:53
  Speaker 1: infrared radiation. But eventually a faint visible red glow begins
• 39:58
  Speaker 1: to appear. Raise the temperature further and the glow becomes brighter,
• 40:03
  Speaker 1: moving through red, orange and yellowish shades. At sufficiently high temperatures,
• 40:11
  Speaker 1: a broad mixture of visible wavelengths is emitted strongly enough
• 40:15
  Speaker 1: that the object can appear white. This progression is familiar
• 40:20
  Speaker 1: from heated metal, molten material, incandescent lamp filaments, and stars.
• 40:28
  Speaker 1: Stars provide some of the grandest examples of thermal radiation
• 40:31
  Speaker 1: in the universe. The visible surface of the Sun has
• 40:35
  Speaker 1: a temperature of roughly 5,800 Kelvin. At that temperature, its
• 40:42
  Speaker 1: thermal spectrum is spread strongly across visible wavelengths. while also
• 40:47
  Speaker 1: extending well into infrared and ultraviolet radiation. The Sun does
• 40:52
  Speaker 1: not emit visible light because visible wavelengths possess some privileged
• 40:56
  Speaker 1: status in nature. Its surface temperature simply happens to place
• 41:01
  Speaker 1: a large part of its thermal output in that region
• 41:04
  Speaker 1: of the electromagnetic spectrum. This also helps explain the colours
• 41:09
  Speaker 1: of different stars. Cooler stars emit proportionally more radiation at
• 41:14
  Speaker 1: longer wavelengths. and often appear reddish. Hotter stars shift more
• 41:19
  Speaker 1: strongly towards shorter wavelengths, and can appear white or blue-white.
• 41:25
  Speaker 1: The colour of a star, therefore, provides an immediate clue
• 41:28
  Speaker 1: to its surface temperature. But the connection is not as
• 41:32
  Speaker 1: simple as assigning one wavelength to one temperature. A blackbody
• 41:36
  Speaker 1: emits a broad spectrum. A red-looking star still produces shorter
• 41:41
  Speaker 1: wavelength light, and a blue-looking star still emits red and
• 41:45
  Speaker 1: infrared radiation. The colour we perceive depends on the relative
• 41:50
  Speaker 1: strength of different parts of the spectrum. This broad thermal
• 41:54
  Speaker 1: spectrum presented physicists with a serious problem at the end
• 41:58
  Speaker 1: of the 19th century. Classical physics could describe parts of
• 42:03
  Speaker 1: blackbody radiation reasonably well, but when the theory was extended
• 42:08
  Speaker 1: towards shorter wavelengths, it predicted an absurd result. It suggested
• 42:13
  Speaker 1: that a hot object should emit an ever-increasing amount of
• 42:16
  Speaker 1: energy at high frequencies. The predicted total energy would become unbounded.
• 42:23
  Speaker 1: Nature fortunately declined to cooperate with this mathematical disaster. Experiments
• 42:29
  Speaker 1: showed a smooth spectrum that rose to a maximum and
• 42:33
  Speaker 1: then fell away again at shorter wavelengths. In 1900, Max
• 42:38
  Speaker 1: Planck found a way to reproduce the observed spectrum mathematically.
• 42:43
  Speaker 1: His solution required the energy exchanged between matter and electromagnetic
• 42:49
  Speaker 1: radiation to come in discrete amounts related to frequency. This
• 42:54
  Speaker 1: was the beginning of energy quantization. Planck did not initially
• 42:59
  Speaker 1: set out to revolutionize physics, but his solution to blackbody
• 43:03
  Speaker 1: radiations became one of the foundations from which quantum theory developed.
• 43:09
  Speaker 1: Something as ordinary as the glow of a warm object
• 43:12
  Speaker 1: had exposed a limit in classical physics. Thermal radiation is
• 43:18
  Speaker 1: also tremendously important in astronomy, because not everything in space
• 43:22
  Speaker 1: is hot enough to shine visibly. Planets emit infrared radiation.
• 43:28
  Speaker 1: Dust grains warmed by nearby stars glow strongly in infrared wavelengths.
• 43:34
  Speaker 1: Cold interstellar clouds can radiate at far infrared and microwave wavelengths.
• 43:41
  Speaker 1: Stars radiate across broad thermal spectra. Extremely hot plasma can
• 43:47
  Speaker 1: extend significant thermal emission into ultraviolet and X-ray wavelengths. Astronomers
• 43:54
  Speaker 1: can measure the shape of this radiation and estimate temperature
• 43:58
  Speaker 1: from it. The approximation is not always perfect. A real
• 44:02
  Speaker 1: star has an atmosphere filled with atoms and ions that
• 44:06
  Speaker 1: absorb and emit particular wavelengths. A planet has a surface
• 44:11
  Speaker 1: and atmosphere with complicated chemical properties. Dust grains emit differently
• 44:17
  Speaker 1: depending on their composition and size. So real astronomical spectra
• 44:22
  Speaker 1: often depart from an ideal blackbody. But the blackbody remains
• 44:27
  Speaker 1: one of the most powerful starting points in physics. It
• 44:30
  Speaker 1: provides a baseline against which those additional features can be understood.
• 44:35
  Speaker 1: And it reveals something wonderfully universal. A cold cloud between
• 44:39
  Speaker 1: the stars, a planet orbiting a distant sun, the surface
• 44:43
  Speaker 1: of our own sun, and your hand resting beneath a
• 44:47
  Speaker 1: blanket all obey the same basic principle. If matter has
• 44:51
  Speaker 1: thermal energy, it can emit electromagnetic radiation. Sometimes that radiation
• 44:58
  Speaker 1: arrives as visible light. Much more often, it is glowing
• 45:02
  Speaker 1: quietly in wavelengths our eyes were simply never built to see.
• 45:08
  Speaker 1: When we look at the sun, it is tempting to
• 45:10
  Speaker 1: imagine that the light reaching our eyes was created somewhere
• 45:15
  Speaker 1: deep in its core and then simply travelled outward until
• 45:19
  Speaker 1: it escaped. The real journey is much more complicated. The
• 45:24
  Speaker 1: sun shines because nuclear fusion in its central regions continually
• 45:29
  Speaker 1: releases energy. But the photons eventually leaving the solar surface
• 45:34
  Speaker 1: are not, in any simple sense, the same photons originally
• 45:38
  Speaker 1: produced during those nuclear reactions. The transformation from nuclear energy
• 45:44
  Speaker 1: into sunlight involves fusion, particle collisions, radiation transport, convection, absorption,
• 45:52
  Speaker 1: ray emission, and the behaviour of an enormous sphere of
• 45:55
  Speaker 1: hot plasma. At the centre of the Sun, the temperature
• 46:00
  Speaker 1: is roughly 15 million Kelvin. The pressure is immense, and
• 46:05
  Speaker 1: hydrogen nuclei are packed closely enough that nuclear reactions can
• 46:09
  Speaker 1: occur despite the electrical repulsion between positively charged protons. The
• 46:15
  Speaker 1: dominant process in a star like the Sun is known
• 46:18
  Speaker 1: as the proton-proton chain. Through a sequence of reactions, hydrogen
• 46:23
  Speaker 1: nuclei are ultimately converted into helium. The final helium nucleus
• 46:28
  Speaker 1: has slightly less mass than the original particles that went
• 46:32
  Speaker 1: into producing it. That missing mass has not disappeared. It
• 46:36
  Speaker 1: has been converted into energy. The relationship is expressed by
• 46:41
  Speaker 1: Albert Einstein's famous mass-energy equivalence. Energy equals mass multiplied by
• 46:48
  Speaker 1: the speed of light squared. Because the speed of light
• 46:52
  Speaker 1: is so enormous, even a very small amount of mass
• 46:55
  Speaker 1: corresponds to a tremendous amount of energy. Every second, the
• 47:01
  Speaker 1: Sun converts roughly 600 million tonnes of hydrogen into helium.
• 47:07
  Speaker 1: The resulting helium has about 4 million tonnes less mass
• 47:11
  Speaker 1: than the original material, with that difference appearing as energy
• 47:16
  Speaker 1: carried by radiation, particle motion and neutrinos. This process allows
• 47:22
  Speaker 1: the Sun to radiate roughly 400 trillion trillion watts of power.
• 47:27
  Speaker 1: That enormous output presented a serious mystery before nuclear physics
• 47:32
  Speaker 1: was understood. Chemical reactions could not possibly keep the Sun
• 47:37
  Speaker 1: shining for billions of years. Gravitational contraction could provide energy
• 47:43
  Speaker 1: for a while, but not for anything close to the
• 47:46
  Speaker 1: age suggested by geology and later astronomy. In the early
• 47:51
  Speaker 1: 20th century, Arthur Stanley Eddington argued that the source of
• 47:55
  Speaker 1: stellar energy must lie in transformations involving atomic nuclei. He
• 48:01
  Speaker 1: recognised that the conversion of hydrogen into helium could potentially
• 48:05
  Speaker 1: provide the enormous reservoir of energy required to sustain stars.
• 48:10
  Speaker 1: The detailed nuclear mechanisms were worked out later. During the 1930s,
• 48:16
  Speaker 1: Hans Bethe helped establish how nuclear reactions could power stars,
• 48:21
  Speaker 1: describing the processes through which hydrogen fusion operates under stellar conditions.
• 48:28
  Speaker 1: For the Sun, the proton-proton chain is the dominant pathway.
• 48:33
  Speaker 1: The energy released by these reactions initially appears in several forms.
• 48:38
  Speaker 1: Some is carried by neutrinos, which interact so weakly with
• 48:42
  Speaker 1: matter that that they can escape from the solar interior
• 48:45
  Speaker 1: remarkably quickly. Some appears as kinetic energy in particles. Some
• 48:51
  Speaker 1: appears as very energetic electromagnetic radiation. But the dense interior
• 48:57
  Speaker 1: of the Sun is not remotely transparent to those photons.
• 49:01
  Speaker 1: A high energy photon travelling through the solar core does
• 49:05
  Speaker 1: not simply fly outward in a straight line for eight
• 49:07
  Speaker 1: minutes and emerge at the surface. It may travel only
• 49:11
  Speaker 1: a short distance before interacting with matter. It can be scattered,
• 49:16
  Speaker 1: absorbed and later replaced by newly emitted photons. Energy is
• 49:21
  Speaker 1: repeatedly redistributed among particles and radiation. The path outward therefore
• 49:27
  Speaker 1: resembles an extraordinarily complicated random walk. The energy can take
• 49:33
  Speaker 1: tens of thousands to hundreds of thousands of years depending
• 49:37
  Speaker 1: on how the transport is estimated. to move from the
• 49:40
  Speaker 1: deep interior toward the outer layers. This does not mean
• 49:45
  Speaker 1: that one individual photon spends hundreds of thousands of years
• 49:49
  Speaker 1: patiently bouncing its way to freedom. The identity of individual
• 49:53
  Speaker 1: photons is continually lost through absorption and ray emission. It
• 49:58
  Speaker 1: is the energy that slowly works its way outward. Deep
• 50:03
  Speaker 1: inside the Sun, radiation is an important means of carrying
• 50:07
  Speaker 1: that energy. The region extending outward from the core is
• 50:11
  Speaker 1: therefore known as the radiative zone. Photons and charged particles
• 50:17
  Speaker 1: constantly interact, while energy gradually migrates through the dense plasma.
• 50:24
  Speaker 1: Farther outward, conditions change. The plasma becomes cooler and more
• 50:30
  Speaker 1: opaque in ways that make radiation less effective at transporting
• 50:34
  Speaker 1: the required energy. Convection takes over. In the Sun's convection zone,
• 50:40
  Speaker 1: hot plasma rises toward the surface, while cooler plasma sinks
• 50:44
  Speaker 1: back downward. Energy is now being transported substantially through the
• 50:49
  Speaker 1: bulk physical movement of matter, rather than solely through radiation.
• 50:55
  Speaker 1: This convection leaves visible traces at the surface. High-resolution observations
• 51:01
  Speaker 1: of the Sun reveal a constantly changing pattern known as granulation.
• 51:07
  Speaker 1: The bright centres of these granules correspond to hotter plasma
• 51:11
  Speaker 1: rising from below, while darker boundaries mark somewhat cooler material
• 51:16
  Speaker 1: sinking again. Eventually, energy reaches the region we commonly think
• 51:22
  Speaker 1: of as the visible surface of the Sun. This is
• 51:25
  Speaker 1: the photosphere. The photosphere is not a solid surface. The
• 51:31
  Speaker 1: Sun is made primarily of plasma, so there is no
• 51:33
  Speaker 1: ground on which someone could stand. Instead, the photosphere is
• 51:38
  Speaker 1: the relatively thin layer from which visible photons can finally
• 51:42
  Speaker 1: escape into space without being immediately scattered or absorbed again.
• 51:48
  Speaker 1: Its characteristic temperature is roughly 5,800 Kelvin. At that temperature,
• 51:55
  Speaker 1: the Sun emits a broad thermal spectrum with a great
• 51:58
  Speaker 1: deal of energy in visible wavelengths. This is the sunlight
• 52:03
  Speaker 1: that crosses the roughly 150 million kilometres between the Sun
• 52:07
  Speaker 1: and Earth in a little over eight minutes. But even here,
• 52:13
  Speaker 1: visible light is only part of the story. The Sun
• 52:17
  Speaker 1: emits electromagnetic radiation across a wide spectrum. Its photosphere produces
• 52:23
  Speaker 1: substantial infrared and visible radiation along with some ultraviolet light.
• 52:29
  Speaker 1: Higher regions of the solar atmosphere produce radiation through additional mechanisms.
• 52:35
  Speaker 1: The chromosphere and especially the corona contain very hot plasma.
• 52:40
  Speaker 1: The corona reaches temperatures of a million Kelvin or more
• 52:44
  Speaker 1: and can emit strongly in ultraviolet and X-ray wavelengths. Solar
• 52:49
  Speaker 1: magnetic activity also produces radio emission, ultraviolet radiation, X-rays and
• 52:57
  Speaker 1: and energetic bursts associated with solar flares. So even one
• 53:02
  Speaker 1: ordinary star can create different kinds of light in several
• 53:05
  Speaker 1: different regions and through several different mechanisms. And other stars
• 53:11
  Speaker 1: extend the range still further. A relatively cool red dwarf
• 53:15
  Speaker 1: has a lower surface temperature and emits a greater proportion
• 53:19
  Speaker 1: of its energy at longer wavelengths. Hot, massive stars can
• 53:23
  Speaker 1: possess surface temperatures of tens of thousands of Kelvin, and
• 53:27
  Speaker 1: emit enormous amounts of ultraviolet radiation. The mass of a
• 53:32
  Speaker 1: star also strongly influences its internal pressure, fusion rate, luminosity, lifetime,
• 53:40
  Speaker 1: and eventual fate. Massive stars consume their nuclear fuel far
• 53:44
  Speaker 1: more rapidly than stars like the Sun. They may shine
• 53:48
  Speaker 1: hundreds of thousands of times more brightly, while living only
• 53:51
  Speaker 1: a small fraction as long. The colour and brightness of
• 53:55
  Speaker 1: a star are therefore not merely decorative features. They are
• 53:59
  Speaker 1: outward signs of the physics occurring inside it. The light
• 54:03
  Speaker 1: carries information about surface temperature, chemical composition, motion, magnetic activity
• 54:11
  Speaker 1: and the conditions within the stellar atmosphere. And beneath all
• 54:14
  Speaker 1: of that visible radiation lies a long chain of energy conversion.
• 54:20
  Speaker 1: Nuclear reactions release energy in the core. That energy is
• 54:24
  Speaker 1: redistributed through dense plasma. Radiation carries it through much of
• 54:29
  Speaker 1: the interior. Convection transports it through outer regions. Finally, electromagnetic
• 54:36
  Speaker 1: radiation escapes from the surface and begins its journey through space.
• 54:42
  Speaker 1: A star is therefore not simply a giant fire burning
• 54:46
  Speaker 1: in the darkness. It is a gravitationally confined nuclear plasma.
• 54:51
  Speaker 1: in which fusion, radiation, convection, electromagnetism and quantum physics work
• 54:57
  Speaker 1: together to turn microscopic changes in atomic nuclei into the
• 55:01
  Speaker 1: light of an entire star. Not all cosmic light comes
• 55:07
  Speaker 1: from the relatively calm surfaces of stars. Some of the
• 55:11
  Speaker 1: brightest and most revealing radiation in the universe comes from
• 55:15
  Speaker 1: gas that has been heated, shocked, ionised or torn apart
• 55:20
  Speaker 1: so thoroughly that ordinary atoms no longer remain intact. In
• 55:25
  Speaker 1: these environments, matter often exists as plasma. A plasma is
• 55:30
  Speaker 1: a gas in which a significant fraction of the atoms
• 55:33
  Speaker 1: have been ionised. Electrons are no longer tightly bound to
• 55:38
  Speaker 1: individual nuclei, leaving a mixture of free electrons and positively
• 55:43
  Speaker 1: charged ions. Because these particles carry electric charge, Their motions
• 55:49
  Speaker 1: can produce electromagnetic radiation in ways that are very different
• 55:53
  Speaker 1: from the light emitted by a comparatively cool neutral gas.
• 55:57
  Speaker 1: One important mechanism is called Bremsstrahlung. The word comes from
• 56:02
  Speaker 1: German and means breaking radiation. Despite the name, the charged
• 56:07
  Speaker 1: particle does not necessarily have to come to a complete stop.
• 56:11
  Speaker 1: What matters is that its velocity changes. An electron passing
• 56:16
  Speaker 1: near a positively charged ion feels an electric force. Its
• 56:20
  Speaker 1: path bends, so the electron accelerates in the physical sense
• 56:24
  Speaker 1: of changing velocity. An accelerating electric charge can emit electromagnetic radiation.
• 56:32
  Speaker 1: That emitted light is bremsstrahlung. In an extremely hot plasma,
• 56:38
  Speaker 1: electrons are moving rapidly in every direction and repeatedly passing
• 56:42
  Speaker 1: close to ions. Each encounter can alter an electron's motion slightly,
• 56:48
  Speaker 1: and produce radiation. The combined effect of enormous numbers of
• 56:52
  Speaker 1: these interactions creates a broad spectrum. When the particles possess
• 56:57
  Speaker 1: a thermal distribution of energies, the process is known as
• 57:01
  Speaker 1: thermal bremsstrahlung. It becomes particularly important at very high temperatures.
• 57:08
  Speaker 1: Gas inside galaxy clusters, for example, can reach tens of
• 57:12
  Speaker 1: millions of Kelvin. At those temperatures, much of the thermal
• 57:16
  Speaker 1: radiation emerges in X-ray wavelengths. This creates a curious situation.
• 57:23
  Speaker 1: If we looked at a galaxy cluster only with human eyes,
• 57:27
  Speaker 1: most of the space between the galaxies would appear dark.
• 57:31
  Speaker 1: But X-ray telescopes reveal that the cluster is filled with
• 57:34
  Speaker 1: an enormous reservoir of incredibly hot gas. In fact, the
• 57:39
  Speaker 1: mass of this diffuse gas can exceed the combined mass
• 57:43
  Speaker 1: of the stars in all the cluster's galaxies. Supernova remnants
• 57:48
  Speaker 1: can contain similarly hot plasma. When a massive star explodes,
• 57:53
  Speaker 1: expanding debris drives shockwaves into surrounding material at speeds that
• 57:58
  Speaker 1: can reach thousands of kilometres per second. A shock does
• 58:02
  Speaker 1: not simply push material aside. It can compress and heat
• 58:07
  Speaker 1: the gas dramatically. Temperatures behind powerful astrophysical shocks may reach
• 58:13
  Speaker 1: millions or even tens of millions of Kelvin. At those temperatures,
• 58:19
  Speaker 1: atoms become highly ionised and substantial radiation moves into ultraviolet
• 58:25
  Speaker 1: and X-ray wavelengths. Brain stralung is only one way for
• 58:30
  Speaker 1: plasma to shine. Another important process begins when an electron
• 58:34
  Speaker 1: that has previously been stripped from an atom encounters a
• 58:38
  Speaker 1: positively charged ion and becomes bound again. This is called recombination.
• 58:44
  Speaker 1: The free electron initially possesses energy associated with its motion.
• 58:49
  Speaker 1: When it becomes captured into a bound atomic state, some
• 58:53
  Speaker 1: of that energy must be released. That energy can emerge
• 58:57
  Speaker 1: as electromagnetic radiation. The electron may initially enter a relatively
• 59:03
  Speaker 1: high energy state and then move through several lower states,
• 59:07
  Speaker 1: releasing additional photons along the way. Recombination can therefore produce
• 59:13
  Speaker 1: both broad emission features and characteristic spectral lines. This process
• 59:19
  Speaker 1: is common in glowing nebulae. The Orion Nebula provides a
• 59:23
  Speaker 1: beautiful nearby example. Located roughly 1,300 light-years from Earth, It
• 59:30
  Speaker 1: is a vast region of gas in which new stars
• 59:33
  Speaker 1: are forming. Some of the hot young stars inside the
• 59:37
  Speaker 1: Orion Nebula emit enormous quantities of ultraviolet radiation. Those ultraviolet
• 59:43
  Speaker 1: photons carry enough energy to ionise surrounding hydrogen. They remove
• 59:49
  Speaker 1: electrons from hydrogen atoms, creating a region filled with free
• 59:53
  Speaker 1: electrons and protons. The electrons do not remain free forever.
• 59:59
  Speaker 1: some eventually recombine with the protons. As they become bound
• 1:00:04
  Speaker 1: again and move into lower energy atomic states, they emit photons.
• 1:00:09
  Speaker 1: One particularly important visible wavelength comes from hydrogen and contributes
• 1:00:15
  Speaker 1: strongly to the reddish appearance commonly associated with ionised hydrogen regions.
• 1:00:22
  Speaker 1: The nebula is therefore glowing partly because nearby stars are
• 1:00:26
  Speaker 1: continually supplying enough ultraviolet radiation to ionise the gas. If
• 1:00:33
  Speaker 1: those stars stopped producing that radiation, the ionisation state of
• 1:00:38
  Speaker 1: the cloud would gradually change as electrons recombined with ions.
• 1:00:43
  Speaker 1: Collisions provide another route to light. In a hot gas,
• 1:00:47
  Speaker 1: particles are moving rapidly enough to collide constantly. An electron
• 1:00:52
  Speaker 1: can collide with an atom or ion, and transfer some
• 1:00:55
  Speaker 1: of its kinetic energy to it. That collision may raise
• 1:00:59
  Speaker 1: one of the bound electrons within the atom into a
• 1:01:02
  Speaker 1: higher energy state. The atom is now collisionally excited. When
• 1:01:07
  Speaker 1: it later returns toward a lower energy configuration, it can
• 1:01:11
  Speaker 1: emit a photon at a characteristic wavelength. This produces bright
• 1:01:16
  Speaker 1: emission lines. The exact lines present depend on which atoms
• 1:01:20
  Speaker 1: and ions exist in the gas, how hot the gas is,
• 1:01:24
  Speaker 1: how dense it is, and how frequently collisions occur. That
• 1:01:30
  Speaker 1: makes plasma emission extraordinarily useful to astronomers. Different ions become
• 1:01:36
  Speaker 1: common under different physical conditions. If a spectrum contains strong
• 1:01:40
  Speaker 1: lines from highly ionised elements, that can indicate very high
• 1:01:44
  Speaker 1: temperatures or intense radiation fields. The ratios between certain spectral
• 1:01:51
  Speaker 1: lines can reveal electron density. Other line combinations can provide
• 1:01:56
  Speaker 1: temperature estimates. The width of a line can contain information
• 1:02:01
  Speaker 1: about particle motions. Its position can reveal motion toward or
• 1:02:06
  Speaker 1: away from Earth. Even the absence of an expected line
• 1:02:10
  Speaker 1: can sometimes constrain what is happening inside the gas. This
• 1:02:15
  Speaker 1: is why an apparently simple glowing cloud can become a
• 1:02:18
  Speaker 1: detailed physical laboratory when its light is separated into a spectrum.
• 1:02:24
  Speaker 1: There is also an important distinction between thermal and non-thermal radiation.
• 1:02:31
  Speaker 1: Thermal radiation is closely tied to the statistical motions of
• 1:02:35
  Speaker 1: particles and therefore to temperature. Blackbody radiation and thermal bremsstrahlung
• 1:02:41
  Speaker 1: are examples where the energy distribution of the particles plays
• 1:02:45
  Speaker 1: a central role. But not all astronomical radiation can be
• 1:02:49
  Speaker 1: explained by temperature alone. Some environments contain particles accelerated to
• 1:02:55
  Speaker 1: energies far above what would be expected from an ordinary
• 1:02:59
  Speaker 1: thermal population. Those particles can generate non-thermal radiation through mechanisms
• 1:03:06
  Speaker 1: connected to magnetic fields and relativistic motion. So when astronomers
• 1:03:11
  Speaker 1: detect bright X-rays from hot plasma, glowing spectral lines from
• 1:03:16
  Speaker 1: ionised gas, or radiation created by violent shocks, They are
• 1:03:22
  Speaker 1: not simply seeing light. They are seeing the consequences of
• 1:03:27
  Speaker 1: charged particles colliding, accelerating, recombining and exchanging energy. A spectrum
• 1:03:36
  Speaker 1: can reveal whether the gas is cool or millions of
• 1:03:39
  Speaker 1: degrees hot. It can reveal whether atoms are neutral or
• 1:03:43
  Speaker 1: highly ionised. It can identify the chemical elements present. and
• 1:03:49
  Speaker 1: it can expose the shocks, collisions and radiation fields shaping
• 1:03:53
  Speaker 1: material that may lie thousands or millions of light-years away.
• 1:03:58
  Speaker 1: The gas itself may be impossibly distant, but the light
• 1:04:03
  Speaker 1: produced within it carries the physics all the way to us.
• 1:04:08
  Speaker 1: Some of the most striking light in the universe is
• 1:04:11
  Speaker 1: produced by particles that are not merely hot. They are
• 1:04:15
  Speaker 1: moving at extraordinary speeds through magnetic fields. This kind of
• 1:04:20
  Speaker 1: radiation is known as synchrotron radiation, and unlike ordinary thermal emission,
• 1:04:26
  Speaker 1: it does not arise simply because matter has a particular temperature.
• 1:04:31
  Speaker 1: It is produced when electrically charged particles, usually electrons, travel
• 1:04:36
  Speaker 1: at relativistic speeds and are forced onto curved paths by
• 1:04:40
  Speaker 1: magnetic fields. The important word here is acceleration. In everyday language,
• 1:04:47
  Speaker 1: acceleration usually means speeding up. In physics, it means any
• 1:04:52
  Speaker 1: change in velocity, and velocity includes direction as well as speed.
• 1:04:57
  Speaker 1: So a particle can be moving at almost the same
• 1:05:00
  Speaker 1: speed from moment to moment and still be accelerating if
• 1:05:04
  Speaker 1: its direction is continually changing. That is exactly what happens
• 1:05:09
  Speaker 1: to a charged particle moving through a magnetic field. The
• 1:05:13
  Speaker 1: magnetic field exerts a force that bends the particle's path.
• 1:05:18
  Speaker 1: Instead of travelling in a perfectly straight line, the particle
• 1:05:22
  Speaker 1: can spiral around magnetic field lines. Because the direction of
• 1:05:26
  Speaker 1: motion is changing, the particle is accelerating. And an accelerating
• 1:05:32
  Speaker 1: electric charge can emit electromagnetic radiation. At relatively modest particle speeds,
• 1:05:40
  Speaker 1: this process is often described as cyclotron radiation. Cyclotron radiation
• 1:05:46
  Speaker 1: is produced when charged particles spiral around magnetic field lines
• 1:05:51
  Speaker 1: without moving at speeds close to the speed of light.
• 1:05:55
  Speaker 1: The motion occurs at a characteristic frequency that depends on
• 1:05:59
  Speaker 1: the particle's charge, mass, and the strength of the magnetic field.
• 1:06:04
  Speaker 1: But as the particle becomes relativistic, meaning its speed approaches
• 1:06:08
  Speaker 1: the speed of light, the nature of the radiation changes.
• 1:06:13
  Speaker 1: This higher energy form is synchrotron radiation. Relativistic effects concentrate
• 1:06:19
  Speaker 1: much of the emitted radiation into a narrow region around
• 1:06:23
  Speaker 1: the direction in which the particle is moving. As the
• 1:06:27
  Speaker 1: particle curves through the magnetic field, this beam sweeps across space.
• 1:06:32
  Speaker 1: An observer can therefore receive sharp bursts or a steady
• 1:06:36
  Speaker 1: stream of radiation depending on how enormous populations of particles
• 1:06:41
  Speaker 1: are arranged. The wavelengths produced depend strongly on both the
• 1:06:45
  Speaker 1: energy of the charged particles and the strength of the
• 1:06:48
  Speaker 1: magnetic field. Relatively low energy electrons in weaker magnetic environments
• 1:06:54
  Speaker 1: may produce mainly radio waves. More energetic particles can emit
• 1:07:00
  Speaker 1: visible light. Under sufficiently extreme conditions, synchrotron radiation can reach ultraviolet,
• 1:07:07
  Speaker 1: and X-ray wavelengths. The same basic mechanism can therefore appear
• 1:07:12
  Speaker 1: across a surprisingly large portion of the electromagnetic spectrum. This
• 1:07:17
  Speaker 1: immediately distinguishes synchrotron radiation from a simple blackbody spectrum. A
• 1:07:23
  Speaker 1: thermal spectrum is closely connected to the temperature of matter.
• 1:07:28
  Speaker 1: Synchrotron emission is non-thermal. The energetic particles responsible for it
• 1:07:33
  Speaker 1: do not necessarily follow the ordinary distribution of particle energies
• 1:07:38
  Speaker 1: expected for matter in thermal equilibrium. Instead, some mechanism may
• 1:07:43
  Speaker 1: have accelerated a population of particles to extremely high energies.
• 1:07:49
  Speaker 1: Shock waves, magnetic fields and violent astrophysical environments can all
• 1:07:54
  Speaker 1: provide ways of doing this. Supernova remnants offer some of
• 1:07:58
  Speaker 1: the best known examples. When a massive star explodes, its
• 1:08:03
  Speaker 1: expanding debris crashes into surrounding material. The resulting shockwave can
• 1:08:09
  Speaker 1: travel through space at thousands of kilometres per second. Within
• 1:08:14
  Speaker 1: and around these shocks, particles can be accelerated to enormous energies.
• 1:08:20
  Speaker 1: If energetic electrons then move through magnetic fields in the remnant,
• 1:08:24
  Speaker 1: they emit synchrotron radiation. This is one reason supernova remnants
• 1:08:30
  Speaker 1: can glow strongly in radio wavelengths, even long after the
• 1:08:34
  Speaker 1: original visible explosion has faded. Some remnants also produce synchrotron
• 1:08:40
  Speaker 1: emission at much higher energies. The Crab Nebula provides an
• 1:08:45
  Speaker 1: especially dramatic example. The Crab Nebula is the remains of
• 1:08:50
  Speaker 1: a stellar explosion observed from Earth in the year 1054.
• 1:08:55
  Speaker 1: At its centre lies a rapidly rotating neutron star known
• 1:08:59
  Speaker 1: as the Crabbe star. Pulsar. That compact object possesses a
• 1:09:03
  Speaker 1: powerful magnetic field and rotates about 30 times every second.
• 1:09:09
  Speaker 1: Its environment is filled with energetic particles. The rotating neutron
• 1:09:14
  Speaker 1: star drives a fast outflow known as a pulsar wind
• 1:09:18
  Speaker 1: Carrying particles and energy into the surrounding nebula, those particles
• 1:09:23
  Speaker 1: encounter magnetic fields and produce intense synchrotron radiation. As a result,
• 1:09:31
  Speaker 1: the Crab Nebula shines across a remarkably broad range of wavelengths,
• 1:09:36
  Speaker 1: from radio waves through visible light and into X-rays. A
• 1:09:40
  Speaker 1: great deal of the glowing structure seen around the pulsar
• 1:09:44
  Speaker 1: is therefore not simply hot gas radiating because of temperature.
• 1:09:48
  Speaker 1: It is a vast population of relativistic charged particles being
• 1:09:53
  Speaker 1: guided and deflected by magnetic fields. Synchrotron radiation also appears
• 1:09:58
  Speaker 1: on scales far larger than a supernova remnant. Some galaxies
• 1:10:03
  Speaker 1: contain active central regions powered by matter falling towards supermassive
• 1:10:09
  Speaker 1: black holes. In certain systems, enormous jets emerge from the
• 1:10:14
  Speaker 1: region surrounding the black hole. These jets can contain particles
• 1:10:18
  Speaker 1: moving at speeds extremely close to the speed of light.
• 1:10:22
  Speaker 1: They can stretch for thousands or even millions of light years.
• 1:10:27
  Speaker 1: The black hole itself is not firing material outward from
• 1:10:30
  Speaker 1: inside its event horizon. Instead, the jets are associated with
• 1:10:35
  Speaker 1: the magnetised plasma in the environment outside the horizon, particularly
• 1:10:40
  Speaker 1: around the accretion flow. Magnetic fields can help organise and
• 1:10:46
  Speaker 1: accelerate the outflow. As relativistic electrons race along these jets
• 1:10:50
  Speaker 1: and spiral through magnetic fields, they produce synchrotron radiation. This
• 1:10:56
  Speaker 1: can make the jets visible in radio, optical and X-ray observations,
• 1:11:01
  Speaker 1: even at immense distances. The giant elliptical galaxy Messier 87
• 1:11:07
  Speaker 1: provides a famous example. Its central supermassive black hole is
• 1:11:13
  Speaker 1: surrounded by an energetic environment that launches a relativistic jet
• 1:11:17
  Speaker 1: extending thousands of light years. Much of the jet's radiation
• 1:11:22
  Speaker 1: is synchrotron emission. This light gives astronomers information about something
• 1:11:28
  Speaker 1: they cannot see directly. Magnetic fields themselves are invisible. We
• 1:11:33
  Speaker 1: cannot simply photograph a magnetic field as though it were
• 1:11:37
  Speaker 1: a glowing strand running through space. synchrotron radiation carries clues
• 1:11:42
  Speaker 1: about a field that shaped the motion of the emitting particles.
• 1:11:48
  Speaker 1: One particularly valuable clue is polarisation. Electromagnetic waves have an
• 1:11:54
  Speaker 1: orientation associated with their electric field. Synchrotron radiation can be
• 1:11:59
  Speaker 1: strongly polarised because the particles producing it are moving in
• 1:12:03
  Speaker 1: an organised way through magnetic fields. By measuring that polarisation,
• 1:12:09
  Speaker 1: astronomers can infer information about the orientation and structure of
• 1:12:13
  Speaker 1: magnetic fields in distant objects. That makes synchrotron light a
• 1:12:17
  Speaker 1: kind of indirect map of otherwise invisible magnetism. The shape
• 1:12:22
  Speaker 1: of the spectrum also reveals information about the particle population.
• 1:12:28
  Speaker 1: As energetic electrons radiate, they lose energy. Higher energy electrons
• 1:12:33
  Speaker 1: can lose energy particularly quickly through synchrotron emission. This produces
• 1:12:39
  Speaker 1: changes in the observed spectrum over time and can help
• 1:12:43
  Speaker 1: astronomers estimate how particles are being accelerated, how long they
• 1:12:48
  Speaker 1: have been radiating, and how strong the surrounding magnetic field
• 1:12:52
  Speaker 1: may be. So when a radio telescope detects a supernova remnant,
• 1:12:58
  Speaker 1: or an X-ray telescope traces a thin filament of emission,
• 1:13:03
  Speaker 1: the light may be revealing far more than temperature. It
• 1:13:07
  Speaker 1: may be showing where charged particles have been pushed to
• 1:13:10
  Speaker 1: relativistic speeds. It may reveal the presence of magnetic fields
• 1:13:16
  Speaker 1: that cannot otherwise be seen. It may trace shockwaves, pulsar winds,
• 1:13:21
  Speaker 1: or jets stretching far beyond entire galaxies. Magnetic fields do
• 1:13:27
  Speaker 1: not normally shine by themselves, but they shape the motion
• 1:13:30
  Speaker 1: of charged particles. Those particles accelerate. and through synchrotron radiation,
• 1:13:38
  Speaker 1: invisible magnetic structures can leave bright signatures across the universe.
• 1:13:44
  Speaker 1: Some light is so energetic that it immediately tells us
• 1:13:48
  Speaker 1: we are looking at a place where ordinary conditions have
• 1:13:51
  Speaker 1: broken down completely. X-rays and gamma rays sit at the
• 1:13:56
  Speaker 1: short wavelength high frequency end of the electromagnetic spectrum. Because
• 1:14:01
  Speaker 1: photon energy rises with frequency, These photons can carry vastly
• 1:14:06
  Speaker 1: more energy than visible light photons. That makes them particularly
• 1:14:10
  Speaker 1: useful for finding the hottest, fastest, most strongly magnetised and
• 1:14:16
  Speaker 1: most violently changing regions of the universe. One major source
• 1:14:22
  Speaker 1: of X-rays is accretion. Accretion occurs when matter falls toward
• 1:14:27
  Speaker 1: a massive object under gravity. Around an ordinary star, that
• 1:14:32
  Speaker 1: process may be fairly gentle. Around a neutron star or
• 1:14:37
  Speaker 1: black hole, it can become extreme. Matter falling inward loses
• 1:14:42
  Speaker 1: gravitational potential energy. If that material forms a disk, neighbouring
• 1:14:48
  Speaker 1: regions of the disk move at different speeds, creating intense
• 1:14:52
  Speaker 1: internal stresses, turbulence and heating. The inner parts of such
• 1:14:59
  Speaker 1: an accretion disk can become extraordinarily hot. Around stellar mass
• 1:15:04
  Speaker 1: black holes and neutron stars, temperatures can reach millions of Kelvin.
• 1:15:10
  Speaker 1: At those temperatures, a great deal of the radiation is
• 1:15:13
  Speaker 1: emitted in X-ray wavelengths. This is one reason X-ray astronomy
• 1:15:18
  Speaker 1: is so effective at finding compact objects. A black hole
• 1:15:22
  Speaker 1: itself does not shine from inside its event horizon. Once
• 1:15:27
  Speaker 1: light crosses that boundary, it cannot return to the outside universe.
• 1:15:32
  Speaker 1: The bright X-rays associated with an accreting black hole come
• 1:15:37
  Speaker 1: from material outside the event horizon, particularly from the hot
• 1:15:41
  Speaker 1: accretion flow and energetic plasma surrounding it. In some systems,
• 1:15:46
  Speaker 1: that material can convert gravitational energy into radiation with remarkable efficiency.
• 1:15:53
  Speaker 1: Matter falling toward a black hole can, depending on the
• 1:15:56
  Speaker 1: flow and the black hole's spin, release a substantial fraction
• 1:16:00
  Speaker 1: of its rest mass energy before crossing the event horizon.
• 1:16:04
  Speaker 1: That can make accretion more efficient at producing radiation than
• 1:16:08
  Speaker 1: nuclear fusion inside ordinary stars. Neutron stars can be even
• 1:16:14
  Speaker 1: more dramatic in another way. They possess solid surfaces, so
• 1:16:19
  Speaker 1: infalling matter does not simply disappear across an event horizon.
• 1:16:23
  Speaker 1: It can strike the neutron star at enormous speed. The
• 1:16:27
  Speaker 1: resulting impact and heating can generate intense X-ray emission. If
• 1:16:32
  Speaker 1: the neutron star also has a strong magnetic field, material
• 1:16:37
  Speaker 1: may be channelled toward its magnetic poles, creating compact hot
• 1:16:41
  Speaker 1: regions that rotate in and out of view. Supernovae create
• 1:16:46
  Speaker 1: high-energy light through several different mechanisms. When a massive star explodes,
• 1:16:53
  Speaker 1: a shockwave drives outward through the star and eventually into
• 1:16:57
  Speaker 1: surrounding space. The expanding debris can be heated enormously. Atoms
• 1:17:03
  Speaker 1: and ions collide. Electrons are stripped away. Shock-heated plasma can
• 1:17:09
  Speaker 1: emit ultraviolet light, X-rays and other radiation. But the visible
• 1:17:14
  Speaker 1: glow of many supernovae is also powered by radioactive decay.
• 1:17:19
  Speaker 1: In many explosions, large quantities of nickel-56 are created. Nickel-56
• 1:17:25
  Speaker 1: is unstable. It decays into cobalt-56, and cobalt-56 later decays
• 1:17:33
  Speaker 1: into iron-56. These decays release energy, including gamma rays and
• 1:17:39
  Speaker 1: energetic particles. As that energy is absorbed by the expanding
• 1:17:44
  Speaker 1: supernova debris, it heats the material. The heated ejector then
• 1:17:50
  Speaker 1: re-radiate much of that energy at longer wavelengths, including visible light.
• 1:17:56
  Speaker 1: This radioactive energy supply is one reason some supernovae remain bright,
• 1:18:02
  Speaker 1: long after the initial explosion. The shape of a supernova's
• 1:18:06
  Speaker 1: light curve can therefore contain information about how much radioactive
• 1:18:11
  Speaker 1: material was created and how the ejector are expanding. Gamma
• 1:18:16
  Speaker 1: rays take us into even more energetic territory. A gamma
• 1:18:21
  Speaker 1: ray photon can carry thousands, millions, or even billions of
• 1:18:25
  Speaker 1: times more energy than an ordinary visible light photon, depending
• 1:18:30
  Speaker 1: on the source. and some astronomical events release staggering quantities
• 1:18:35
  Speaker 1: of them in very short periods. Gamma-ray bursts are among
• 1:18:41
  Speaker 1: the most luminous electromagnetic events known. Some last less than
• 1:18:45
  Speaker 1: two seconds, others continue for tens of seconds or longer.
• 1:18:51
  Speaker 1: These two broad groups are commonly described as short duration
• 1:18:55
  Speaker 1: and long duration gamma ray bursts. Long duration gamma ray
• 1:19:00
  Speaker 1: bursts are strongly associated with the collapse of certain massive stars.
• 1:19:05
  Speaker 1: In these events, the core of a star collapses to
• 1:19:08
  Speaker 1: form a compact object, likely a black hole, or in
• 1:19:13
  Speaker 1: some cases a rapidly rotating neutron star. Under the right conditions,
• 1:19:19
  Speaker 1: narrow relativistic jets can emerge along the rotation axis. If
• 1:19:25
  Speaker 1: one of those jets points roughly toward Earth, it can
• 1:19:28
  Speaker 1: produce an intense burst of gamma rays. Short-duration gamma-ray bursts
• 1:19:34
  Speaker 1: are strongly associated with mergers involving compact objects, especially pairs
• 1:19:40
  Speaker 1: of neutron stars. This connection became particularly clear in 2017,
• 1:19:46
  Speaker 1: when gravitational waves from a neutron star merger were detected
• 1:19:51
  Speaker 1: together with a short gamma-ray burst. The event showed directly
• 1:19:56
  Speaker 1: that at least some short gamma-ray bursts come from merging
• 1:20:00
  Speaker 1: neutron stars. These bursts are so luminous that they can
• 1:20:05
  Speaker 1: be detected across enormous cosmological distances. For a brief moment,
• 1:20:10
  Speaker 1: a single event can outshine entire galaxies in high-energy radiation.
• 1:20:16
  Speaker 1: Then there are magnetars. A magnetar is a neutron star
• 1:20:20
  Speaker 1: with an extraordinarily intense magnetic field. Neutron stars are already
• 1:20:27
  Speaker 1: extreme objects. They can pack roughly the mass of the
• 1:20:31
  Speaker 1: Sun into a sphere only a few tens of kilometres across.
• 1:20:35
  Speaker 1: Magnetars add magnetic fields so strong that they can dominate
• 1:20:39
  Speaker 1: the behaviour of matter around them. Their surface magnetic fields
• 1:20:43
  Speaker 1: can reach around 100 million to 100 billion Tesla. For comparison,
• 1:20:49
  Speaker 1: the magnetic field at Earth's surface is only a few
• 1:20:52
  Speaker 1: tens of millions of a Tesla. Magnetar fields are therefore
• 1:20:56
  Speaker 1: stronger by factors of trillions. Changes in those magnetic fields
• 1:21:01
  Speaker 1: can release enormous amounts of energy. The crust of the
• 1:21:05
  Speaker 1: neutron star may shift or fracture. The external magnetic field
• 1:21:10
  Speaker 1: may rearrange itself. These disturbances can produce bursts of X-rays
• 1:21:15
  Speaker 1: and gamma rays. Occasionally, magnetars generate giant flares. A giant
• 1:21:22
  Speaker 1: flare can release an extraordinary amount of high-energy radiation in
• 1:21:26
  Speaker 1: a fraction of a second. briefly making an object tens
• 1:21:30
  Speaker 1: of thousands of light-years away one of the brightest gamma-ray
• 1:21:34
  Speaker 1: sources in the sky. Observing events this brief requires telescopes
• 1:21:39
  Speaker 1: that can monitor large areas of the sky and react quickly.
• 1:21:44
  Speaker 1: The National Aeronautics and Space Administration's Swift mission was designed
• 1:21:49
  Speaker 1: partly for this purpose. Swift can detect gamma-ray bursts and
• 1:21:54
  Speaker 1: rapidly reposition itself, so that the fading aftermath can be
• 1:21:58
  Speaker 1: observed in X-rays, ultraviolet radiation and visible light. The Fermi
• 1:22:05
  Speaker 1: Gamma-Ray Space Telescope surveys the gamma-ray sky and has detected
• 1:22:10
  Speaker 1: thousands of energetic transient events and persistent sources. These observatories
• 1:22:16
  Speaker 1: are valuable because high-energy events often do not politely announce
• 1:22:20
  Speaker 1: themselves in advance. A burst may last only milliseconds, A
• 1:22:25
  Speaker 1: flare may fade within seconds. An afterglow may evolve over
• 1:22:29
  Speaker 1: hours or days. The detectors have to be watching when
• 1:22:33
  Speaker 1: it happens. X-rays and gamma rays therefore act almost like
• 1:22:38
  Speaker 1: warning lights for extreme physics. They can signal gas heated
• 1:22:42
  Speaker 1: to millions of degrees. They can reveal matter falling into
• 1:22:47
  Speaker 1: deep gravitational wells. They can trace radioactive nuclei inside exploding stars.
• 1:22:54
  Speaker 1: They can expose relativistic jets, compact object mergers and magnetic
• 1:23:00
  Speaker 1: fields of almost unimaginable strength. The highest energy regions of
• 1:23:05
  Speaker 1: the electromagnetic spectrum are not simply more energetic versions of
• 1:23:10
  Speaker 1: visible light. They are windows into environments where gravity, magnetism,
• 1:23:16
  Speaker 1: nuclear physics and particle acceleration are being pushed toward their
• 1:23:20
  Speaker 1: most extreme known limits. And when those photons finally reach us,
• 1:23:25
  Speaker 1: They carry evidence of events that may have lasted only
• 1:23:28
  Speaker 1: a fraction of a second, yet released more energy than
• 1:23:31
  Speaker 1: a star like the Sun will produce across vast stretches
• 1:23:35
  Speaker 1: of its lifetime. Light does not always need a collapsing star,
• 1:23:40
  Speaker 1: a black hole, or a cloud of plasma heated to
• 1:23:43
  Speaker 1: millions of degrees. Some of the most interesting examples are
• 1:23:48
  Speaker 1: much closer to home. A flash of lightning, the green
• 1:23:52
  Speaker 1: glow of an aurora, the faint shimmer of airglow, or
• 1:23:56
  Speaker 1: the cold blue light produced by a living organism all
• 1:24:00
  Speaker 1: arise from the same broad principle. Energy is being transferred
• 1:24:04
  Speaker 1: into matter, and some of that energy is leaving again
• 1:24:07
  Speaker 1: as photons. Lightning is one of the most dramatic examples.
• 1:24:13
  Speaker 1: Inside a thunderstorm, collisions between ice particles, droplets and hail
• 1:24:18
  Speaker 1: can help separate electric charge into different regions of the cloud.
• 1:24:23
  Speaker 1: As that separation grows, the electric potential between different parts
• 1:24:27
  Speaker 1: of the storm can become enormous. Eventually, the electric field
• 1:24:32
  Speaker 1: becomes strong enough to break down the insulating properties of
• 1:24:35
  Speaker 1: the surrounding air. A conductive channel forms and a powerful
• 1:24:40
  Speaker 1: electrical discharge races through the atmosphere. The current can be immense.
• 1:24:46
  Speaker 1: The narrow channel of air through which the discharge passes
• 1:24:49
  Speaker 1: can be heated to around 30,000 Kelvin. that is several
• 1:24:53
  Speaker 1: times hotter than the visible surface of the Sun. The
• 1:24:57
  Speaker 1: hot gas expands violently, producing the pressure wave that we
• 1:25:01
  Speaker 1: hear as thunder. At the same time, the excited and
• 1:25:05
  Speaker 1: ionised atoms and molecules in the channel emit electromagnetic radiation.
• 1:25:12
  Speaker 1: Some of that radiation lies in visible wavelengths, creating the
• 1:25:16
  Speaker 1: brilliant flash we call lightning. The process is therefore not
• 1:25:20
  Speaker 1: simply electricity somehow turning directly into brightness. Electrical energy accelerates
• 1:25:27
  Speaker 1: charged particles and heats the gas. Atoms and molecules become
• 1:25:31
  Speaker 1: excited and ionised. As those particles interact and return toward
• 1:25:37
  Speaker 1: lower energy states, photons are produced across a range of wavelengths.
• 1:25:42
  Speaker 1: Lightning can also emit radio waves, ultraviolet radiation, X-rays, and
• 1:25:47
  Speaker 1: under some conditions even gamma rays. The atmosphere is capable
• 1:25:52
  Speaker 1: of producing far more energetic light than its ordinary appearance
• 1:25:56
  Speaker 1: might suggest. A much calmer example can be seen near
• 1:26:00
  Speaker 1: the polar regions. Auroras occur when charged particles from space
• 1:26:05
  Speaker 1: enter a planet's upper atmosphere and transfer energy to atoms
• 1:26:09
  Speaker 1: and molecules there. Around Earth, many of these particles originate
• 1:26:14
  Speaker 1: in the solar wind, and in disturbances within the magnetosphere.
• 1:26:19
  Speaker 1: Earth's magnetic field helps guide charged particles along paths that
• 1:26:23
  Speaker 1: lead toward high latitudes. When energetic electrons and other particles
• 1:26:28
  Speaker 1: collide with atmospheric gases, they can excite those atoms and
• 1:26:32
  Speaker 1: molecules into higher energy states. The excited particles do not
• 1:26:38
  Speaker 1: remain in those states indefinitely. When they return toward lower
• 1:26:43
  Speaker 1: energy configurations, they release photons. Different atmospheric species produce different colours.
• 1:26:52
  Speaker 1: Atomic oxygen is responsible for some of the most familiar
• 1:26:55
  Speaker 1: auroral emissions. A strong green line is commonly produced at
• 1:27:00
  Speaker 1: altitudes of roughly 100 to 150 kilometres. At greater heights,
• 1:27:06
  Speaker 1: oxygen can also produce deep red auroral light. Nitrogen and
• 1:27:11
  Speaker 1: ionised nitrogen contribute blue, violet and reddish colours depending on
• 1:27:16
  Speaker 1: the energy of the incoming particles and the altitude of
• 1:27:20
  Speaker 1: the interaction. This is why an aurora can contain several
• 1:27:25
  Speaker 1: colours at once. The colours are not chosen randomly. They
• 1:27:29
  Speaker 1: reflect the composition of the atmosphere, the energies of the
• 1:27:33
  Speaker 1: particles arriving from space, and the densities of the gases
• 1:27:38
  Speaker 1: at different heights. Other planets can produce auroras too, Jupiter
• 1:27:43
  Speaker 1: and Saturn possess powerful magnetic fields and active magnetospheres. Auroral
• 1:27:49
  Speaker 1: emissions have also been observed there, including strongly in ultraviolet wavelengths,
• 1:27:54
  Speaker 1: so even planetary atmospheres can act as enormous light-producing laboratories.
• 1:28:00
  Speaker 1: There is also a much fainter atmospheric glow that does
• 1:28:03
  Speaker 1: not require a dramatic auroral display. This is known as airglow.
• 1:28:10
  Speaker 1: On a truly dark night, Earth's upper atmosphere is not
• 1:28:14
  Speaker 1: completely dark. Atoms and molecules can emit weak light through
• 1:28:19
  Speaker 1: several mechanisms, including chemical reactions, recombination, and the relaxation of
• 1:28:24
  Speaker 1: particles that were excited by sunlight earlier in the day.
• 1:28:29
  Speaker 1: Air glow can occur across visible and invisible wavelengths. It
• 1:28:33
  Speaker 1: is faint enough that most people never notice it directly,
• 1:28:37
  Speaker 1: but sensitive astronomical instruments must account for it because it
• 1:28:41
  Speaker 1: can add a soft background glow to observations of the
• 1:28:44
  Speaker 1: night sky. Light can also be produced when matter absorbs
• 1:28:48
  Speaker 1: photons and then re-emits some of that energy. One familiar
• 1:28:53
  Speaker 1: example is fluorescence. A fluorescent material absorbs higher energy radiation
• 1:29:01
  Speaker 1: and excites electrons into higher energy states. The system then
• 1:29:06
  Speaker 1: loses some of that energy through internal processes before emitting
• 1:29:11
  Speaker 1: a new photon. Because some energy has usually been lost
• 1:29:15
  Speaker 1: before the photon is released, the emitted light often has
• 1:29:19
  Speaker 1: a lower frequency and a longer wavelength than the original
• 1:29:23
  Speaker 1: absorbed radiation. This is why ultraviolet radiation can cause certain
• 1:29:28
  Speaker 1: materials to glow visibly. The ultraviolet photons themselves are invisible
• 1:29:34
  Speaker 1: to human eyes, but the material converts part of their
• 1:29:38
  Speaker 1: energy into visible light. Phosphorescence follows a related principle, but
• 1:29:45
  Speaker 1: the excited state can persist for much longer. Certain materials
• 1:29:49
  Speaker 1: continue glowing after the original light source has been removed
• 1:29:52
  Speaker 1: and because the electrons become trapped in states from which
• 1:29:56
  Speaker 1: they return only slowly. The result can be a glow
• 1:30:00
  Speaker 1: lasting seconds, minutes or even hours. Not every light-producing process
• 1:30:05
  Speaker 1: needs an incoming photon. Chemical reactions can produce light directly.
• 1:30:11
  Speaker 1: This is known as chemiluminescence. In a chemiluminescent reaction, chemical
• 1:30:17
  Speaker 1: energy raises molecules or reaction products into excited electronic states.
• 1:30:23
  Speaker 1: When those excited states relax, photons can be emitted. Because
• 1:30:29
  Speaker 1: the energy does not first need to become large amounts
• 1:30:32
  Speaker 1: of heat, chemiluminescent light can be produced very efficiently. A
• 1:30:38
  Speaker 1: glow stick is a familiar example. The chemical reaction inside
• 1:30:43
  Speaker 1: it generates excited molecules that release visible photons without making
• 1:30:47
  Speaker 1: the container particularly hot. Nature has developed its own extremely
• 1:30:53
  Speaker 1: refined version of the same idea. Bioluminescence is light produced
• 1:30:59
  Speaker 1: by living organisms through controlled chemical reactions. It appears in bacteria, fungi, insects, fish, jellyfish, squid,
• 1:31:11
  Speaker 1: and many other forms of life. In many cases, a
• 1:31:14
  Speaker 1: light-producing molecule known as luciferin reacts with oxygen. An enzyme
• 1:31:20
  Speaker 1: known as luciferase helps control the reaction. The chemical energy
• 1:31:25
  Speaker 1: released places the molecules into excited states, and when they
• 1:31:29
  Speaker 1: return to lower energy states, photons are emitted. The exact
• 1:31:34
  Speaker 1: chemistry varies between species. There is no single universal luciferin
• 1:31:40
  Speaker 1: used by all bioluminescent organisms. Different evolutionary lineages have developed
• 1:31:46
  Speaker 1: their own versions of the process. The efficiency can be impressive.
• 1:31:51
  Speaker 1: Bioluminescence systems convert a large fraction of their released chemical
• 1:31:55
  Speaker 1: energy into visible light rather than wasting most of it
• 1:31:59
  Speaker 1: as heat. That is why bioluminescence is sometimes described as
• 1:32:03
  Speaker 1: cold light. The phrase does not mean that the organisms
• 1:32:07
  Speaker 1: are literally producing light without any thermal motion at all.
• 1:32:12
  Speaker 1: It means that the light is generated without heating the
• 1:32:15
  Speaker 1: material to the high temperatures required for incandescence. A firefly
• 1:32:21
  Speaker 1: does not need to become red-hot in order to glow.
• 1:32:25
  Speaker 1: That would rather spoil the evening for the firefly. Instead,
• 1:32:30
  Speaker 1: chemistry provides the necessary energy directly. The purposes of bioluminescence
• 1:32:35
  Speaker 1: are equally varied. Some organisms use it to attract mates.
• 1:32:40
  Speaker 1: Others use it to lure prey, communicate, startle predators, or
• 1:32:44
  Speaker 1: camouflage themselves against faint light filtering down from the ocean surface.
• 1:32:50
  Speaker 1: In the deep sea, where sunlight cannot penetrate, living organisms
• 1:32:54
  Speaker 1: can become some of the most important local sources of
• 1:32:57
  Speaker 1: visible light. These examples make one important point very clear.
• 1:33:03
  Speaker 1: Light does not require fire. It does not require a star.
• 1:33:08
  Speaker 1: It does not even require something to be particularly hot.
• 1:33:12
  Speaker 1: A photon can emerge whenever a physical system has a
• 1:33:15
  Speaker 1: suitable way to release energy electromagnetically. Electrical discharges can produce light.
• 1:33:23
  Speaker 1: Atoms in planetary atmospheres can glow after being struck by
• 1:33:26
  Speaker 1: charged particles. Molecules can absorb one photon and later release another.
• 1:33:33
  Speaker 1: Chemical reactions can leave molecules in excited states. Living organisms
• 1:33:38
  Speaker 1: can control those reactions with remarkable precision. The physical details differ,
• 1:33:44
  Speaker 1: but the central idea remains the same. Energy enters a
• 1:33:48
  Speaker 1: system in one form. Matter responds, and sometimes part of
• 1:33:54
  Speaker 1: that energy leaves again as light. Humans did not invent light.
• 1:34:01
  Speaker 1: What we learned to do was control the circumstances that
• 1:34:05
  Speaker 1: produce it. Every artificial light source, from a candle to
• 1:34:10
  Speaker 1: a laser, relies on the same underlying physics that operates
• 1:34:14
  Speaker 1: in stars, glowing gas, lightning and atoms throughout the universe.
• 1:34:20
  Speaker 1: The difference is that we arrange the energy source deliberately.
• 1:34:25
  Speaker 1: Sometimes we use heat. Sometimes we use electricity. Sometimes we
• 1:34:31
  Speaker 1: use carefully engineered quantum states inside materials. And in every case,
• 1:34:37
  Speaker 1: the result is energy leaving a system in the form
• 1:34:40
  Speaker 1: of electromagnetic radiation. One of the simplest methods is incandescence.
• 1:34:48
  Speaker 1: An incandescent object shines because it is hot. As its
• 1:34:52
  Speaker 1: temperature rises, its thermal radiation becomes more intense and shifts
• 1:34:57
  Speaker 1: towards shorter wavelengths. At first, much of that radiation may
• 1:35:01
  Speaker 1: lie in the infrared. Heat the material enough and visible
• 1:35:05
  Speaker 1: wavelengths begin to appear strongly. This is the principle behind
• 1:35:10
  Speaker 1: the traditional incandescent light bulb. Inside the bulb, an electric
• 1:35:16
  Speaker 1: current passes through a thin filament, usually made from tungsten.
• 1:35:23
  Speaker 1: The filament resists the current and becomes extremely hot. At
• 1:35:27
  Speaker 1: temperatures of a few thousand Kelvin, it emits a broad
• 1:35:31
  Speaker 1: thermal spectrum that includes visible light. The bulb works, but
• 1:35:36
  Speaker 1: it is not especially efficient. A large fraction of the
• 1:35:40
  Speaker 1: energy is emitted as infrared radiation rather than visible light.
• 1:35:46
  Speaker 1: In other words, the filament is very good at becoming
• 1:35:49
  Speaker 1: hot and only moderately good at becoming useful illumination. That
• 1:35:54
  Speaker 1: limitation encouraged engineers to find other ways of producing light.
• 1:35:59
  Speaker 1: Gas discharge lamps take a very different approach. Instead of
• 1:36:04
  Speaker 1: heating a solid filament until it glows, an electric current
• 1:36:08
  Speaker 1: passes through a gas. The moving electrons collide with atoms
• 1:36:12
  Speaker 1: or ions in the gas and transfer energy to them.
• 1:36:16
  Speaker 1: Those atoms become excited. When they return toward lower energy states,
• 1:36:21
  Speaker 1: they emit photons at characteristic wavelengths. This is closely related
• 1:36:26
  Speaker 1: to the same atomic emission processes seen in glowing nebulae.
• 1:36:31
  Speaker 1: Neon signs are a familiar example. Neon atoms emit strongly
• 1:36:36
  Speaker 1: in reddish-orange wavelengths when excited by an electrical discharge. Different
• 1:36:42
  Speaker 1: gases produce different spectral patterns. Mercury vapor can emit strongly
• 1:36:47
  Speaker 1: in ultraviolet and visible wavelengths. Sodium vapor lamps produce intense
• 1:36:53
  Speaker 1: yellow-orange light. The colour depends on the quantum structure of
• 1:36:57
  Speaker 1: the atoms involved. Fluorescent lamps add another step. Inside the tube,
• 1:37:03
  Speaker 1: mercury vapor produces ultraviolet radiation. That ultraviolet light strikes a
• 1:37:09
  Speaker 1: coating on the inside surface of the lamp. The coating
• 1:37:13
  Speaker 1: fluoresces and re-emits part of the energy as visible light.
• 1:37:18
  Speaker 1: So a fluorescent lamp is effectively using one type of
• 1:37:22
  Speaker 1: light to create another. Modern lighting often relies on an
• 1:37:26
  Speaker 1: even more direct quantum process. The light-emitting diode uses semiconductor materials.
• 1:37:34
  Speaker 1: A semiconductor sits somewhere between a good electrical conductor and
• 1:37:38
  Speaker 1: a good insulator. Its electrical properties can be carefully controlled
• 1:37:43
  Speaker 1: by adding tiny amounts of other elements and by designing
• 1:37:46
  Speaker 1: interfaces between different semiconductor regions. When electrons move through these materials,
• 1:37:53
  Speaker 1: they can encounter what are often described as electron vacancies
• 1:37:56
  Speaker 1: or holes. A hole is not a literal empty particle
• 1:38:00
  Speaker 1: floating around inside the material. It represents the absence of
• 1:38:05
  Speaker 1: an electron in a state that could otherwise be occupied.
• 1:38:09
  Speaker 1: When an electron falls into such an available state, energy
• 1:38:13
  Speaker 1: is released. In suitable semiconductor materials, that energy can emerge
• 1:38:19
  Speaker 1: as a photon. The energy difference within the material determines
• 1:38:23
  Speaker 1: the approximate energy, and therefore the wavelength. of the emitted light.
• 1:38:29
  Speaker 1: That means engineers can alter the materials to produce different colours.
• 1:38:35
  Speaker 1: Red light emitting diodes became practical relatively early. Green followed.
• 1:38:41
  Speaker 1: Blue proved much more difficult. The development of efficient blue
• 1:38:47
  Speaker 1: light emitting diodes became one of the most important advances
• 1:38:50
  Speaker 1: in modern lighting. Isamu Akasaki, Hiroshi Amano and Shuji Nakamura
• 1:38:57
  Speaker 1: played major roles in developing practical blue light emitting diodes
• 1:39:01
  Speaker 1: using gallium nitride based materials. Their work made it possible
• 1:39:06
  Speaker 1: to combine blue light with phosphor materials to produce efficient
• 1:39:10
  Speaker 1: white lighting. In 2014, The three researchers received the Nobel
• 1:39:16
  Speaker 1: Prize in Physics for this achievement. The importance of blue
• 1:39:20
  Speaker 1: light may sound slightly disproportionate until one remembers that efficient
• 1:39:26
  Speaker 1: white solid state lighting depends heavily on it. A modern
• 1:39:32
  Speaker 1: white light emitting diode can produce useful illumination while wasting
• 1:39:36
  Speaker 1: far less energy as heat than a traditional incandescent bulb.
• 1:39:41
  Speaker 1: But perhaps the most controlled form of artificial light is
• 1:39:45
  Speaker 1: the laser. Laser light is fundamentally different from the broad,
• 1:39:50
  Speaker 1: disorganised emission produced by most everyday light sources. The key
• 1:39:55
  Speaker 1: process is stimulated emission. Albert Einstein described stimulated emission theoretically
• 1:40:03
  Speaker 1: in 1917. Imagine an atom or molecule in an excited
• 1:40:08
  Speaker 1: energy state. Normally, it might eventually return to a lower
• 1:40:12
  Speaker 1: state by spontaneous emission, releasing a photon at an unpredictable moment.
• 1:40:18
  Speaker 1: But if a photon with the right energy interacts with
• 1:40:21
  Speaker 1: that excited system, it can stimulate the system to release
• 1:40:24
  Speaker 1: another photon. The remarkable part is that the new photon
• 1:40:29
  Speaker 1: is closely matched to the incoming one. It has the
• 1:40:32
  Speaker 1: same frequency, travels in the same direction, and maintains a
• 1:40:36
  Speaker 1: related phase. If this process is repeated through a large
• 1:40:41
  Speaker 1: population of excited atoms or molecules, light can be amplified.
• 1:40:46
  Speaker 1: A laser therefore requires a material known as the gain medium.
• 1:40:50
  Speaker 1: Energy is supplied to that medium through a process called pumping.
• 1:40:54
  Speaker 1: The goal is to create a population inversion, meaning that
• 1:40:58
  Speaker 1: more particles occupy suitable excited states than lower states. Photons
• 1:41:04
  Speaker 1: then stimulate further emissions. Mirrors placed around the gain medium
• 1:41:09
  Speaker 1: can make light pass through it repeatedly, encouraging additional amplification.
• 1:41:14
  Speaker 1: One mirror is partly transparent, allowing some of the amplified
• 1:41:18
  Speaker 1: light to escape as the laser beam. Theodor Maimon produced
• 1:41:23
  Speaker 1: the first operating laser in 1960 using a synthetic ruby crystal.
• 1:41:30
  Speaker 1: Since then, Lasers have been built using gases, semiconductors, optical fibers, crystals,
• 1:41:37
  Speaker 1: and many other materials. Their wavelengths can range from infrared
• 1:41:41
  Speaker 1: through visible light and into ultraviolet regions. Laser light is
• 1:41:46
  Speaker 1: often described as coherent. This means the electromagnetic waves maintain
• 1:41:52
  Speaker 1: strong phase relationships with one another. The light is also
• 1:41:56
  Speaker 1: typically concentrated into a narrow range of frequencies and a
• 1:42:00
  Speaker 1: tightly directed beam. Ordinary light sources emit photons in many directions,
• 1:42:06
  Speaker 1: with many wavelengths and largely unrelated phases. Laser emission is
• 1:42:12
  Speaker 1: much more organized. That organization allows lasers to be focused
• 1:42:17
  Speaker 1: intensely onto tiny regions. It also makes them useful in communications,
• 1:42:23
  Speaker 1: precision measurement, surgery, manufacturing, scientific experiments, barcode scanners, optical storage
• 1:42:34
  Speaker 1: and countless other technologies. Yet despite all this engineering sophistication,
• 1:42:41
  Speaker 1: the photons created inside a laser are not a special
• 1:42:45
  Speaker 1: technological form of light. A red photon from a laboratory
• 1:42:50
  Speaker 1: laser and a red photon emitted by a distant star
• 1:42:54
  Speaker 1: obey the same fundamental laws. Once produced, There is no
• 1:42:59
  Speaker 1: label attached to a photon explaining whether it came from
• 1:43:02
  Speaker 1: a tungsten filament, a semiconductor, a nebula, or the atmosphere
• 1:43:08
  Speaker 1: of a star. Its behaviour depends on its physical properties,
• 1:43:13
  Speaker 1: not on its biography. Human technology therefore does not create
• 1:43:17
  Speaker 1: a separate category of light. It simply gives us increasing
• 1:43:21
  Speaker 1: control over the processes that produce it. We can heat
• 1:43:25
  Speaker 1: matter until it glows. We can excite gases electrically. We
• 1:43:30
  Speaker 1: can engineer semiconductor energy states. We can stimulate atoms to
• 1:43:35
  Speaker 1: emit highly organised radiation. The same electromagnetic rules apply throughout.
• 1:43:43
  Speaker 1: What humans have learned to manufacture is not light itself
• 1:43:47
  Speaker 1: in some uniquely artificial sense. It is the environment in
• 1:43:51
  Speaker 1: which energy is persuaded, very efficiently, to leave as photons.
• 1:43:58
  Speaker 1: Once light has been created, its story is far from over.
• 1:44:02
  Speaker 1: The photons leaving a star, a lamp, a glowing cloud
• 1:44:06
  Speaker 1: of gas, or a distant galaxy may still have a
• 1:44:09
  Speaker 1: great deal to go through before they ever reach an observer.
• 1:44:13
  Speaker 1: They can be absorbed. They can be reflected. They can
• 1:44:17
  Speaker 1: pass through matter. They can change direction. They can be
• 1:44:21
  Speaker 1: scattered into new paths. and every one of those interactions
• 1:44:26
  Speaker 1: can alter what eventually arrives at an eye, a camera,
• 1:44:30
  Speaker 1: or a telescope. Absorption is one of the simplest places
• 1:44:34
  Speaker 1: to begin. When a photon encounters matter, its energy can
• 1:44:38
  Speaker 1: be transferred into that material. Exactly what happens next depends
• 1:44:43
  Speaker 1: on both the photon and the substance it strikes. A
• 1:44:47
  Speaker 1: visible photon might excite an electron into a higher energy state.
• 1:44:52
  Speaker 1: An infrared photon may increase molecular vibrations. Ultraviolet radiation can
• 1:44:58
  Speaker 1: trigger chemical reactions. An X-ray photon can eject electrons from atoms.
• 1:45:05
  Speaker 1: The incoming light is not simply swallowed in some vague sense.
• 1:45:10
  Speaker 1: Its energy is transferred into the physical system. That energy
• 1:45:15
  Speaker 1: may later reappear as heat, as chemical change, as motion,
• 1:45:20
  Speaker 1: or as new photons emitted at different wavelengths. This is
• 1:45:24
  Speaker 1: why a dark surface exposed to sunlight becomes warm. The
• 1:45:29
  Speaker 1: surface absorbs a large fraction of the incoming visible and
• 1:45:33
  Speaker 1: infrared radiation, and that absorbed energy becomes part of the
• 1:45:37
  Speaker 1: internal thermal motion of the material. Reflection seems more straightforward,
• 1:45:43
  Speaker 1: but even here the microscopic picture is subtler than the
• 1:45:47
  Speaker 1: everyday image of tiny particles bouncing off a wall. When
• 1:45:51
  Speaker 1: light reaches a material, its electromagnetic field interacts with charged
• 1:45:56
  Speaker 1: particles inside that material, especially electrons. Those charges respond to
• 1:46:03
  Speaker 1: the incoming field. Their collective response can generate electromagnetic radiation
• 1:46:10
  Speaker 1: that travels away from the surface. Under the right conditions,
• 1:46:14
  Speaker 1: the outgoing waves combine in a direction that produces what
• 1:46:18
  Speaker 1: we recognise as reflection. so the light is not behaving
• 1:46:22
  Speaker 1: exactly like a miniature rubber ball striking a perfectly hard surface.
• 1:46:27
  Speaker 1: Reflection emerges from the interaction between electromagnetic radiation and the
• 1:46:32
  Speaker 1: charged structure of matter. Smooth surfaces can produce organised, mirror-like reflection.
• 1:46:40
  Speaker 1: Rough surfaces scatter the reflected light in many directions. That
• 1:46:45
  Speaker 1: is why you can see a matte wall from many positions.
• 1:46:48
  Speaker 1: while a mirror forms a much more structured image. Some
• 1:46:52
  Speaker 1: light is neither absorbed nor reflected strongly. It passes through.
• 1:46:57
  Speaker 1: Glass is an obvious example. Visible light can travel through
• 1:47:02
  Speaker 1: many forms of glass because those photons do not interact
• 1:47:05
  Speaker 1: strongly enough with the material to be absorbed over short distances.
• 1:47:10
  Speaker 1: But transmission does not mean the material has no effect.
• 1:47:14
  Speaker 1: Light generally travels through a transparent medium differently than it
• 1:47:18
  Speaker 1: travels through vacuum. This leads to refraction. When light crosses
• 1:47:25
  Speaker 1: from one medium into another, its speed of propagation changes.
• 1:47:30
  Speaker 1: If it enters the new material at an angle, its
• 1:47:33
  Speaker 1: direction can also change. This bending is refraction. A straw
• 1:47:39
  Speaker 1: partly submerged in water appears displaced. because the light travelling
• 1:47:44
  Speaker 1: from the straw through water and then into air changes
• 1:47:48
  Speaker 1: direction before reaching the eye. The same principle allows lenses
• 1:47:53
  Speaker 1: to focus light. It is why eyeglasses can correct vision.
• 1:47:58
  Speaker 1: It is why cameras form images. And it is why
• 1:48:02
  Speaker 1: telescopes using lenses can bring distant objects into focus. The
• 1:48:07
  Speaker 1: large-scale relationship governing refraction is commonly described by Snell's Law.
• 1:48:12
  Speaker 1: named after Willibrod Snell. In words, it connects the angle
• 1:48:16
  Speaker 1: at which light enters a material with the angle at
• 1:48:19
  Speaker 1: which it travels inside that material and with the refractive
• 1:48:23
  Speaker 1: properties of the two media. But the underlying physics is
• 1:48:28
  Speaker 1: again electromagnetic. The oscillating electromagnetic field of the incoming light
• 1:48:34
  Speaker 1: interacts with charged particles in the material. those charges respond
• 1:48:39
  Speaker 1: and contribute their own electromagnetic fields. The combined effect is
• 1:48:44
  Speaker 1: a wave that propagates through the substance with a different
• 1:48:47
  Speaker 1: effective speed and phase relationship than it would have in vacuum.
• 1:48:53
  Speaker 1: Another important interaction is scattering. Scattering occurs when light is
• 1:48:59
  Speaker 1: redirected by matter. The photon may continue to exist at
• 1:49:04
  Speaker 1: roughly the same energy. or its energy may change depending
• 1:49:08
  Speaker 1: on the particular type of scattering. The details depend strongly
• 1:49:13
  Speaker 1: on the size and properties of the particles involved and
• 1:49:16
  Speaker 1: on the wavelength of the incoming radiation. One of the
• 1:49:20
  Speaker 1: most familiar examples is Rayleigh scattering. John William Strutt, better
• 1:49:27
  Speaker 1: known as Lord Rayleigh, studied how light scatters from particles
• 1:49:31
  Speaker 1: much smaller than the wavelength of the radiation. For visible
• 1:49:36
  Speaker 1: light in Earth's atmosphere the molecules of nitrogen and oxygen
• 1:49:40
  Speaker 1: are small enough for this type of scattering to be important.
• 1:49:44
  Speaker 1: Shorter visible wavelengths are scattered much more strongly than longer ones.
• 1:49:49
  Speaker 1: Blue light is therefore redirected far more efficiently than red light.
• 1:49:54
  Speaker 1: When sunlight enters Earth's atmosphere during the day some of
• 1:49:58
  Speaker 1: the blue portion is scattered in all directions that scattered
• 1:50:02
  Speaker 1: blue light reaches our eyes from across the sky. The
• 1:50:06
  Speaker 1: result is the familiar blue appearance of a clear daytime sky.
• 1:50:12
  Speaker 1: Violet light is scattered even more strongly than blue, but
• 1:50:16
  Speaker 1: the sun emits somewhat less violet light, human eyes are
• 1:50:20
  Speaker 1: less sensitive to it, and some is absorbed in the
• 1:50:23
  Speaker 1: upper atmosphere. So the combined effect produces a sky that
• 1:50:28
  Speaker 1: generally appears blue rather than violet. Sunsets reveal the same
• 1:50:34
  Speaker 1: physics from a different angle. When the sun is high
• 1:50:37
  Speaker 1: in the sky, its light travels through a relatively short
• 1:50:41
  Speaker 1: path in the atmosphere before reaching us. Near sunset, sunlight
• 1:50:46
  Speaker 1: travels through a much longer path. Along that extended route,
• 1:50:50
  Speaker 1: a great deal of the shorter wavelength blue and violet
• 1:50:53
  Speaker 1: light is scattered away from the direct beam. What remains
• 1:50:57
  Speaker 1: travelling more directly toward the observer is richer in longer
• 1:51:01
  Speaker 1: visible wavelengths. The sun and surrounding sky can therefore appear orange,
• 1:51:06
  Speaker 1: red or gold. The sun itself has not suddenly changed
• 1:51:10
  Speaker 1: its surface temperature or begun producing mainly red light. The
• 1:51:14
  Speaker 1: atmosphere has altered the mixture of wavelengths that reaches the observer.
• 1:51:19
  Speaker 1: A similar process occurs on vastly larger scales in space.
• 1:51:25
  Speaker 1: Interstellar space is not perfectly empty. It contains gas and
• 1:51:29
  Speaker 1: tiny solid particles known collectively as interstellar dust. These grains
• 1:51:34
  Speaker 1: can absorb and scatter starlight. Shorter visible wavelengths are often
• 1:51:39
  Speaker 1: affected more strongly than longer wavelengths. As a result, light
• 1:51:44
  Speaker 1: travelling through dusty regions can become reddened. A star viewed
• 1:51:49
  Speaker 1: through enough interstellar dust may appear redder than it truly is.
• 1:51:54
  Speaker 1: Its apparent brightness can also decrease. This reduction in observed
• 1:51:59
  Speaker 1: light is called extinction. Astronomers therefore have to distinguish between
• 1:52:04
  Speaker 1: the intrinsic properties of a source and changes imposed on
• 1:52:08
  Speaker 1: its light during the journey to Earth. A red-looking star
• 1:52:13
  Speaker 1: may genuinely have a relatively cool surface. Or it may
• 1:52:18
  Speaker 1: be a hotter star whose light has passed through significant
• 1:52:22
  Speaker 1: amounts of dust. Those two possibilities imply very different physical conditions.
• 1:52:29
  Speaker 1: By comparing observations at multiple wavelengths and studying the detailed spectrum,
• 1:52:34
  Speaker 1: astronomers can estimate how much reddening and extinction have occurred.
• 1:52:38
  Speaker 1: Dust can also hide regions almost completely in visible light,
• 1:52:42
  Speaker 1: while allowing longer wavelength infrared radiation to escape more easily.
• 1:52:47
  Speaker 1: This is one reason infrared telescopes are so useful for
• 1:52:50
  Speaker 1: studying stars forming inside dense clouds. The visible radiation may
• 1:52:56
  Speaker 1: never make it through the dust. The infrared radiation has
• 1:53:00
  Speaker 1: a better chance. All of this means that light rarely
• 1:53:04
  Speaker 1: arrives with a perfectly untouched record of its source. Before
• 1:53:09
  Speaker 1: reaching us, it may have crossed stellar atmospheres, clouds of gas,
• 1:53:14
  Speaker 1: dusty regions, planetary atmospheres, lenses, mirrors, detectors, or countless other
• 1:53:21
  Speaker 1: forms of matter. each interaction leaves its mark. Some photons
• 1:53:26
  Speaker 1: are removed, some are redirected, some change wavelength, some are delayed,
• 1:53:33
  Speaker 1: some are replaced by newly emitted photons carrying the redistributed energy.
• 1:53:40
  Speaker 1: So when astronomers observe light, they are rarely seeing only
• 1:53:44
  Speaker 1: the object that created it. They are also seeing evidence
• 1:53:48
  Speaker 1: of everything that happened along the way. the journey matters
• 1:53:52
  Speaker 1: almost as much as the source. And learning how light
• 1:53:56
  Speaker 1: interacts with matter is what allows us to separate those
• 1:53:59
  Speaker 1: two stories. One of the most remarkable things about astronomy
• 1:54:05
  Speaker 1: is that we can learn what distant objects are made
• 1:54:07
  Speaker 1: of without ever going anywhere near them. We cannot scoop
• 1:54:12
  Speaker 1: material from the surface of a star several thousand light
• 1:54:15
  Speaker 1: years away. We cannot bring a piece of a distant
• 1:54:19
  Speaker 1: galaxy into a laboratory. and yet astronomers can identify the
• 1:54:23
  Speaker 1: elements inside those objects, estimate their temperatures, measure how quickly
• 1:54:29
  Speaker 1: they are moving, and even detect magnetic fields surrounding them.
• 1:54:35
  Speaker 1: The reason is spectroscopy. Spectroscopy is the study of how
• 1:54:40
  Speaker 1: light is distributed across wavelength or frequency. Instead of treating
• 1:54:46
  Speaker 1: incoming light as a single glow, astronomers spread it out
• 1:54:50
  Speaker 1: into a spectrum, and examine the detailed pattern. That pattern
• 1:54:55
  Speaker 1: can contain a great deal of information. Some spectra appear
• 1:54:59
  Speaker 1: as a smooth continuum. Others contain dark absorption lines. Some
• 1:55:05
  Speaker 1: contain bright emission lines. Many astronomical objects show combinations of
• 1:55:11
  Speaker 1: all three. A hot, dense object can produce a broad
• 1:55:15
  Speaker 1: thermal continuum. If that light then passes through a cooler
• 1:55:20
  Speaker 1: layer of gas, Particular wavelengths may be absorbed. If a
• 1:55:25
  Speaker 1: thin cloud of excited gas is viewed on its own,
• 1:55:28
  Speaker 1: it may produce bright emission lines. The resulting spectrum becomes
• 1:55:32
  Speaker 1: something like a record of every physical process the light
• 1:55:35
  Speaker 1: has experienced. The positions of the spectral lines are especially valuable.
• 1:55:41
  Speaker 1: Atoms and ions possess specific quantum energy levels. When electrons
• 1:55:47
  Speaker 1: move between those levels … They absorb or emit photons
• 1:55:51
  Speaker 1: with particular energies. Because photon energy is linked to wavelength,
• 1:55:57
  Speaker 1: each element produces a characteristic pattern of spectral lines. Hydrogen
• 1:56:03
  Speaker 1: has one pattern. Helium has another. Iron produces an enormous
• 1:56:08
  Speaker 1: collection of lines. Calcium, sodium, oxygen, carbon, magnesium and every
• 1:56:16
  Speaker 1: other element possess their own combinations. These patterns are repeatable.
• 1:56:22
  Speaker 1: A hydrogen atom in a laboratory on Earth follows the
• 1:56:25
  Speaker 1: same quantum rules as a hydrogen atom in the atmosphere
• 1:56:29
  Speaker 1: of a star on the far side of the Milky Way.
• 1:56:32
  Speaker 1: That consistency allows laboratory measurements to become a reference library
• 1:56:37
  Speaker 1: for the universe. Astronomers compare the wavelengths observed in starlight
• 1:56:42
  Speaker 1: with wavelengths measured from known elements on Earth. When the
• 1:56:46
  Speaker 1: patterns match, the element can be identified. This technique completely
• 1:56:52
  Speaker 1: transformed our understanding of stars. One of the most important
• 1:56:56
  Speaker 1: advances came from Cecilia Payne-Gaposchkin. In 1925, Payne-Gaposchkin analysed stellar
• 1:57:06
  Speaker 1: spectra in her doctoral work and reached a conclusion that
• 1:57:09
  Speaker 1: was initially surprising. Stars were not composed of roughly the
• 1:57:13
  Speaker 1: same mixture of elements as Earth. They were overwhelmingly made
• 1:57:18
  Speaker 1: of hydrogen and helium. Hydrogen, in particular, was vastly more
• 1:57:23
  Speaker 1: abundant than many astronomers had previously assumed. The confusion arose
• 1:57:29
  Speaker 1: partly because the strength of a spectral line does not
• 1:57:32
  Speaker 1: depend only on how much of an element is present.
• 1:57:35
  Speaker 1: It also depends on the physical state of that element.
• 1:57:39
  Speaker 1: Temperature can determine how many atoms occupy the energy states
• 1:57:43
  Speaker 1: required to produce a particular line. Ionization can remove electrons
• 1:57:48
  Speaker 1: entirely from atoms, changing which transitions are possible. So a
• 1:57:53
  Speaker 1: weak hydrogen line does not necessarily mean that there is
• 1:57:56
  Speaker 1: very little hydrogen present. It may mean that the temperature
• 1:58:00
  Speaker 1: is causing hydrogen atoms to exist in states that do
• 1:58:04
  Speaker 1: not produce that particular line strongly. This was one of
• 1:58:08
  Speaker 1: the insights that allowed Payne-Gapishkin to interpret stellar spectra correctly. Today,
• 1:58:15
  Speaker 1: hydrogen and helium are known to account for the overwhelming
• 1:58:18
  Speaker 1: majority of ordinary matter in stars. But spectroscopy reveals far
• 1:58:23
  Speaker 1: more than composition. The broad shape of a star's spectrum–
• 1:58:28
  Speaker 1: provides information about temperature. Stars approximately resemble thermal emitters, so
• 1:58:35
  Speaker 1: their continua can often be compared with blackbody-like spectra. Cooler
• 1:58:40
  Speaker 1: stars emit proportionally more energy at longer visible and infrared
• 1:58:45
  Speaker 1: wavelengths and can appear reddish. Hotter stars produce a greater
• 1:58:49
  Speaker 1: proportion of short wavelength radiation and can appear blue-white. but
• 1:58:55
  Speaker 1: detailed spectral analysis provides much more precision than colour alone.
• 1:59:01
  Speaker 1: The relative strengths of different absorption lines can reveal temperature
• 1:59:06
  Speaker 1: because different atomic and ionic states become more common under
• 1:59:10
  Speaker 1: different conditions. Density and pressure also affect spectra. In dense environments,
• 1:59:18
  Speaker 1: frequent interactions between particles can broaden spectral lines. Different broadening
• 1:59:25
  Speaker 1: mechanisms can therefore reveal something about the physical conditions inside
• 1:59:30
  Speaker 1: stellar atmospheres or hot gas clouds. Motion leaves another unmistakable signature.
• 1:59:37
  Speaker 1: The Doppler effect shifts the observed wavelengths of light when
• 1:59:41
  Speaker 1: a source moves toward or away from an observer. If
• 1:59:45
  Speaker 1: a star is moving toward Earth, its spectral lines shift
• 1:59:49
  Speaker 1: slightly toward shorter wavelengths. If it is moving away, the
• 1:59:54
  Speaker 1: lines shift toward longer wavelengths. By measuring the amount of
• 1:59:58
  Speaker 1: this shift, astronomers can determine the radial velocity of the star,
• 2:00:03
  Speaker 1: meaning the component of its motion directly toward or away
• 2:00:07
  Speaker 1: from us. The changes can be extremely small. Modern instruments
• 2:00:13
  Speaker 1: can detect wavelength shifts corresponding to velocities of only a
• 2:00:17
  Speaker 1: few metres per second in favourable cases. That sensitivity is
• 2:00:21
  Speaker 1: one reason spectroscopy has become so important in the search
• 2:00:25
  Speaker 1: for exoplanets. A planet orbiting a star pulls gravitationally on
• 2:00:30
  Speaker 1: the star as well. The star therefore moves slightly back
• 2:00:35
  Speaker 1: and forth. That motion produces small, repeating Doppler shifts in
• 2:00:41
  Speaker 1: its spectral lines. By measuring them, astronomers can infer the
• 2:00:46
  Speaker 1: presence of an unseen planet. Rotation can also reveal itself.
• 2:00:52
  Speaker 1: One side of a rotating star moves slightly toward us,
• 2:00:55
  Speaker 1: while the opposite side moves slightly away. Light from the
• 2:01:00
  Speaker 1: approaching side is shifted toward shorter wavelengths. Light from the
• 2:01:05
  Speaker 1: receding side is shifted toward longer ones. When the light
• 2:01:09
  Speaker 1: from the entire star is combined, its spectral lines become broader.
• 2:01:14
  Speaker 1: This rotational broadening can be used to estimate how quickly
• 2:01:18
  Speaker 1: a star is spinning. Magnetic fields can leave fingerprints as well.
• 2:01:23
  Speaker 1: The Zeeman effect, discovered by Peter Zeeman in the 19th century,
• 2:01:28
  Speaker 1: occurs when magnetic fields alter atomic energy levels. A spectral
• 2:01:33
  Speaker 1: line that would normally appear at one wavelength can split
• 2:01:36
  Speaker 1: into several closely spaced components. The amount of splitting depends
• 2:01:41
  Speaker 1: on the strength of the magnetic field. By measuring this effect,
• 2:01:45
  Speaker 1: astronomers can estimate magnetic conditions on stars and in other
• 2:01:50
  Speaker 1: astronomical environments. Spectroscopy is even beginning to reveal the atmospheres
• 2:01:55
  Speaker 1: of planets orbiting other stars. When an exoplanet passes in
• 2:02:00
  Speaker 1: front of its host star, a small fraction of the
• 2:02:03
  Speaker 1: starlight can travel through the planet's atmosphere before reaching us.
• 2:02:08
  Speaker 1: Atoms and molecules in that atmosphere absorb characteristic wavelengths. By
• 2:02:13
  Speaker 1: comparing the spectrum observed during the transit with the spectrum
• 2:02:16
  Speaker 1: of the star by itself, astronomers can search for those
• 2:02:20
  Speaker 1: subtle absorption features. Water vapour, carbon dioxide, sodium, methane and
• 2:02:26
  Speaker 1: other substances can leave identifiable signatures under suitable conditions. Other
• 2:02:32
  Speaker 1: methods analyse light emitted directly by the planet or reflected
• 2:02:36
  Speaker 1: from its surface and atmosphere. These measurements are extraordinarily difficult.
• 2:02:43
  Speaker 1: because the planetary signal can be tiny compared with the
• 2:02:46
  Speaker 1: brightness of the star. But the principle is the same
• 2:02:50
  Speaker 1: one used to identify elements in stellar atmospheres. Matter interacts
• 2:02:56
  Speaker 1: with light. That interaction leaves a pattern, and the pattern
• 2:03:01
  Speaker 1: can be read. This is why spectroscopy is so central
• 2:03:05
  Speaker 1: to modern astronomy. A spectrum is not merely a rainbow
• 2:03:10
  Speaker 1: with a few lines drawn across it. It can reveal
• 2:03:13
  Speaker 1: chemical composition. It can reveal temperature. It can reveal density
• 2:03:18
  Speaker 1: and pressure. It can expose ionisation, rotation, radial motion and
• 2:03:24
  Speaker 1: magnetic fields. It can reveal material around stars, gas between
• 2:03:30
  Speaker 1: galaxies and molecules in the atmospheres of worlds many light-years away.
• 2:03:36
  Speaker 1: Astronomers rarely get to touch the objects they study. They
• 2:03:40
  Speaker 1: do something almost as powerful. They let the light touch
• 2:03:44
  Speaker 1: those objects instead. Then they collect what arrives and read
• 2:03:49
  Speaker 1: the evidence written into it. Observing light is really the
• 2:03:54
  Speaker 1: art of turning arriving electromagnetic radiation into information. The simplest
• 2:04:01
  Speaker 1: detector available to us is the human eye. Visible light
• 2:04:05
  Speaker 1: enters through the cornea, passes through the pupil and is
• 2:04:09
  Speaker 1: focused by the lens onto the retina at the back
• 2:04:12
  Speaker 1: of the eye. There, specialised photoreceptor cells convert incoming light
• 2:04:18
  Speaker 1: into electrical and chemical signals that can be processed by
• 2:04:21
  Speaker 1: the nervous system. There are two main types of photoreceptor
• 2:04:25
  Speaker 1: involved in ordinary human vision. Rod cells are extremely sensitive
• 2:04:30
  Speaker 1: to low levels of light. They are especially useful at night,
• 2:04:35
  Speaker 1: when only a small number of photons may be arriving
• 2:04:37
  Speaker 1: from a faint source. Cone cells are less sensitive in darkness.
• 2:04:42
  Speaker 1: but they allow us to distinguish colour. Humans normally possess
• 2:04:47
  Speaker 1: three broad classes of cone cell, each responding differently across
• 2:04:52
  Speaker 1: overlapping regions of the visible spectrum. This means colour is
• 2:04:57
  Speaker 1: not a property that exists in quite the same way
• 2:04:59
  Speaker 1: outside the observer. Electromagnetic radiation has wavelength, frequency, intensity and
• 2:05:08
  Speaker 1: other measurable properties. What we experience as red, green, blue
• 2:05:13
  Speaker 1: or any other colour is the brain's interpretation of how
• 2:05:17
  Speaker 1: different wavelengths stimulate the cone cells. The eye is therefore
• 2:05:22
  Speaker 1: not simply a window through which light passes. It is
• 2:05:26
  Speaker 1: already a biological measuring instrument. But it is a limited one.
• 2:05:31
  Speaker 1: The pupil can collect only a modest amount of light.
• 2:05:35
  Speaker 1: The retina cannot accumulate photons for hours and then add
• 2:05:39
  Speaker 1: them together. and the angular resolution of the eye limits
• 2:05:43
  Speaker 1: how finely distant objects can be separated. The telescope solves
• 2:05:48
  Speaker 1: several of these problems. The first great advantage is collecting area.
• 2:05:54
  Speaker 1: A large telescope mirror can gather far more light than
• 2:05:57
  Speaker 1: a human pupil. That makes faint objects detectable. If twice
• 2:06:02
  Speaker 1: as much collecting area is available, roughly twice as much
• 2:06:06
  Speaker 1: light can be gathered from a distant source during the
• 2:06:09
  Speaker 1: same exposure. Modern observatories use mirrors metres across, allowing them
• 2:06:15
  Speaker 1: to collect photons that would be hopelessly sparse to unaided vision.
• 2:06:19
  Speaker 1: A telescope also improves angular resolution. Resolution describes the ability
• 2:06:27
  Speaker 1: to distinguish two nearby points as separate objects rather than
• 2:06:31
  Speaker 1: blending them together. For an ideal telescope, resolution depends strongly
• 2:06:36
  Speaker 1: on the wavelength being observed and the diameter of the aperture.
• 2:06:41
  Speaker 1: In general, a larger aperture allows finer detail to be resolved.
• 2:06:46
  Speaker 1: This is one reason astronomers build ever larger telescopes. Galileo
• 2:06:51
  Speaker 1: Galilei demonstrated the power of even a small improvement in
• 2:06:55
  Speaker 1: light gathering and resolution in the early 17th century. His
• 2:07:00
  Speaker 1: telescope was crude by modern standards, but it revealed mountains
• 2:07:04
  Speaker 1: and craters on the moon, the four large moons now
• 2:07:07
  Speaker 1: known to orbit Jupiter, phases of Venus, and countless stars
• 2:07:12
  Speaker 1: invisible to the naked eye. The important change was not
• 2:07:16
  Speaker 1: that the universe suddenly became different. The detector became better.
• 2:07:22
  Speaker 1: For centuries, astronomers still had to rely mainly on their
• 2:07:25
  Speaker 1: own eyes to record what telescopes revealed. Photography changed that.
• 2:07:32
  Speaker 1: A photographic plate could accumulate light over a long exposure.
• 2:07:36
  Speaker 1: A faint galaxy that was almost invisible in real time
• 2:07:40
  Speaker 1: could gradually leave a measurable image if enough photons arrived
• 2:07:44
  Speaker 1: over minutes or hours. This was a profound improvement. The
• 2:07:49
  Speaker 1: eye sees only the light arriving now. A photographic plate
• 2:07:53
  Speaker 1: can remember. Electronic detectors eventually pushed sensitivity much further. In 1969,
• 2:08:02
  Speaker 1: Willard Boyle and George Smith developed the charge-coupled device at
• 2:08:06
  Speaker 1: Bell Laboratories. A charge-coupled device contains an array of light-sensitive elements.
• 2:08:13
  Speaker 1: When photons strike the material, they can generate electrical charge.
• 2:08:19
  Speaker 1: That charge is collected, transferred and measured, allowing the number
• 2:08:23
  Speaker 1: and position of arriving photons to be reconstructed electronically. These
• 2:08:29
  Speaker 1: detectors became extraordinarily important in astronomy. Compared with photographic plates,
• 2:08:35
  Speaker 1: they can be much more efficient at recording incoming light,
• 2:08:39
  Speaker 1: offer a wide dynamic range and produce data that can
• 2:08:42
  Speaker 1: be processed directly by computers. Modern astronomical cameras therefore do
• 2:08:48
  Speaker 1: not usually create images in the same way a human
• 2:08:51
  Speaker 1: observer experiences a scene. They count and measure incoming radiation
• 2:08:55
  Speaker 1: across thousands or millions of detector elements. The result can
• 2:09:00
  Speaker 1: then be calibrated, combined, filtered and converted into an image.
• 2:09:05
  Speaker 1: Visible light is only one part of the problem. Different
• 2:09:09
  Speaker 1: wavelengths require different detection methods. Radio astronomy provides a clear example.
• 2:09:18
  Speaker 1: Radio waves have much longer wavelengths than visible light, so
• 2:09:23
  Speaker 1: astronomers often use large dish-shaped antennas or networks of antennas.
• 2:09:29
  Speaker 1: The incoming electromagnetic wave induces tiny electrical signals in the
• 2:09:34
  Speaker 1: receiving system. Those signals can be amplified and analysed. In
• 2:09:41
  Speaker 1: the 1930s, Karlyansky detected radio emission coming from the direction
• 2:09:46
  Speaker 1: of the Milky Way while investigating interference in radio communications.
• 2:09:52
  Speaker 1: He had discovered that the galaxy itself was a radio source.
• 2:09:56
  Speaker 1: That result opened an entirely new window on the universe.
• 2:10:00
  Speaker 1: Radio telescopes can detect cold gas, pulsars, supernova remnants, relativistic jets,
• 2:10:09
  Speaker 1: and many other phenomena that may look very different in
• 2:10:12
  Speaker 1: visible light. Infrared astronomy requires another approach. Infrared detectors are
• 2:10:19
  Speaker 1: often cooled to very low temperatures because warm instruments emit
• 2:10:23
  Speaker 1: infrared radiation themselves. A detector trying to observe faint cosmic
• 2:10:28
  Speaker 1: infrared light while glowing strongly in infrared would be rather
• 2:10:33
  Speaker 1: like trying to see a candle while holding it next
• 2:10:36
  Speaker 1: to a floodlight. Cooling reduces this unwanted background. Ultraviolet radiation
• 2:10:44
  Speaker 1: presents a different challenge because Earth's atmosphere absorbs much of it.
• 2:10:49
  Speaker 1: That absorption is extremely useful for life on the surface,
• 2:10:53
  Speaker 1: since high-energy ultraviolet radiation can damage biological tissue. But it
• 2:11:00
  Speaker 1: makes ultraviolet astronomy difficult from the ground. X-rays are absorbed
• 2:11:06
  Speaker 1: even more strongly by the atmosphere. Gamma rays are also
• 2:11:11
  Speaker 1: largely blocked before reaching Earth's surface. For these wavelengths, astronomers
• 2:11:18
  Speaker 1: often have to place their detectors above the atmosphere. This
• 2:11:22
  Speaker 1: is why space astronomy is so important. The Hubble Space Telescope,
• 2:11:26
  Speaker 1: operated through the National Aeronautics and Space Administration and the
• 2:11:30
  Speaker 1: European Space Agency, observes mainly visible and ultraviolet wavelengths, with
• 2:11:36
  Speaker 1: some sensitivity into the near-infrared. The Chandra X-ray Observatory was
• 2:11:42
  Speaker 1: designed to study high-energy X-ray sources such as supernova remnants,
• 2:11:48
  Speaker 1: hot gas, neutron stars and black hole environments. The Fermi
• 2:11:54
  Speaker 1: Gamma Ray Space Telescope surveys some of the most energetic
• 2:11:57
  Speaker 1: electromagnetic radiation reaching Earth. The James Webb Space Telescope, developed
• 2:12:04
  Speaker 1: through the National Aeronautics and Space Administration, European Space Agency,
• 2:12:09
  Speaker 1: and Canadian Space Agency, is primarily an infrared observatory. Its
• 2:12:15
  Speaker 1: sensitivity allows it to study cool objects, dust-obscured regions, planetary atmospheres,
• 2:12:22
  Speaker 1: and extremely distant galaxies whose original visible and ultraviolet light
• 2:12:28
  Speaker 1: has been stretched into infrared wavelengths by cosmic expansion. Ground-based
• 2:12:34
  Speaker 1: observatories remain equally important where Earth's atmosphere is transparent enough.
• 2:12:41
  Speaker 1: The European Southern Observatory operates major facilities in Chile, where
• 2:12:46
  Speaker 1: dry air, high altitude and dark skies provide excellent observing conditions.
• 2:12:53
  Speaker 1: Large optical telescopes can collect enormous amounts of visible and
• 2:12:57
  Speaker 1: infrared light. Radio observatories can combine signals from several antennas
• 2:13:03
  Speaker 1: using interferometry, effectively creating a telescope with a resolving power
• 2:13:08
  Speaker 1: related to the distance between the antennas. This allows astronomers
• 2:13:12
  Speaker 1: to achieve extraordinarily fine angular resolution without building one physically
• 2:13:18
  Speaker 1: solid dish thousands of kilometres wide. Different detectors therefore reveal
• 2:13:24
  Speaker 1: different universes. A supernova remnant viewed in visible light may
• 2:13:29
  Speaker 1: show glowing filaments of gas. In radio wavelengths, synchrotron radiation
• 2:13:36
  Speaker 1: can reveal energetic electrons moving through magnetic fields. In X-rays,
• 2:13:42
  Speaker 1: shock-heated plasma at millions of Kelvin may dominate the view.
• 2:13:47
  Speaker 1: The object is the same. What changes is the type
• 2:13:50
  Speaker 1: of light being collected. Even the beautiful astronomical images commonly
• 2:13:56
  Speaker 1: shown to the public often combine information in ways that
• 2:13:59
  Speaker 1: human eyes could never experience directly. Invisible infrared, ultraviolet, X-ray
• 2:14:08
  Speaker 1: or radio measurements may be assigned visible colours so that
• 2:14:12
  Speaker 1: structures and differences in intensity can be represented clearly. These
• 2:14:18
  Speaker 1: are not necessarily attempts to reproduce what an astronaut would
• 2:14:22
  Speaker 1: see with the naked eye. They are visual translations of
• 2:14:26
  Speaker 1: measured electromagnetic information. At the deepest level, every telescope and
• 2:14:33
  Speaker 1: detector is doing something quite simple. Radiation arrives. The instrument
• 2:14:40
  Speaker 1: interacts with it. That interaction is converted into a measurable signal.
• 2:14:47
  Speaker 1: The signal becomes data. And from that data, astronomers reconstruct
• 2:14:53
  Speaker 1: an image, a spectrum, a light curve, or some other
• 2:14:56
  Speaker 1: description of the source. A photon may have travelled for
• 2:15:01
  Speaker 1: millions or billions of years before reaching a detector. Its
• 2:15:06
  Speaker 1: final journey through a telescope might last only a fraction
• 2:15:09
  Speaker 1: of a second, but in that brief encounter, the energy
• 2:15:14
  Speaker 1: it carries is transformed into a measurement, and from enough
• 2:15:18
  Speaker 1: of those measurements, the invisible universe becomes something we can study.
• 2:15:24
  Speaker 1: Light can spend almost its entire existence travelling. A photon
• 2:15:29
  Speaker 1: may be created inside the atmosphere of a star, escape
• 2:15:32
  Speaker 1: into space, cross an entire planetary system, pass through clouds
• 2:15:37
  Speaker 1: of gas and dust, travel between galaxies, skim past massive objects,
• 2:15:43
  Speaker 1: and continue onward for millions or billions of years before
• 2:15:47
  Speaker 1: finally reaching a detector. By the time that happens, the
• 2:15:51
  Speaker 1: journey itself may have become part of the information the
• 2:15:54
  Speaker 1: light carries. In an ideal vacuum, light travels locally at
• 2:15:59
  Speaker 1: the speed of light, That speed is roughly 300,000 kilometres
• 2:16:03
  Speaker 1: per second. For everyday distances, this is almost unimaginably fast.
• 2:16:10
  Speaker 1: Light can cross the diameter of Earth in a fraction
• 2:16:13
  Speaker 1: of a second. It can travel from Earth to the
• 2:16:15
  Speaker 1: Moon in a little more than one second. Sunlight takes
• 2:16:20
  Speaker 1: roughly 8 minutes and 20 seconds to cover the approximately
• 2:16:23
  Speaker 1: 150 million kilometres separating the Sun from Earth. But astronomy
• 2:16:30
  Speaker 1: changes our sense of scale. Once distances become large enough,
• 2:16:35
  Speaker 1: even the speed of light begins to feel slow. Light
• 2:16:38
  Speaker 1: from Proxima Centauri, the nearest known star to the Sun,
• 2:16:43
  Speaker 1: takes a little over four years to reach us. That
• 2:16:46
  Speaker 1: means every time we observe Proxima Centauri, we are seeing
• 2:16:50
  Speaker 1: it not as it is now, but as it was
• 2:16:52
  Speaker 1: a little more than four years ago. The Andromeda Galaxy
• 2:16:56
  Speaker 1: is much farther away. its light takes roughly 2.5 million
• 2:17:01
  Speaker 1: years to reach Earth. So the Andromeda galaxy we see
• 2:17:05
  Speaker 1: tonight is really Andromeda as it existed around 2.5 million
• 2:17:09
  Speaker 1: years in the past. Nothing about the telescope can remove
• 2:17:14
  Speaker 1: this delay. It is built into the structure of spacetime.
• 2:17:20
  Speaker 1: Light has a finite speed, so distance automatically becomes a
• 2:17:24
  Speaker 1: kind of time machine. The farther away we look, The
• 2:17:28
  Speaker 1: farther into the past we see, but while travelling, light
• 2:17:32
  Speaker 1: may also be altered. Interstellar space is not perfectly empty.
• 2:17:38
  Speaker 1: It contains thinly spread gas, dust, magnetic fields, charged particles
• 2:17:45
  Speaker 1: and other forms of matter. Intergalactic space is even more rarefied.
• 2:17:50
  Speaker 1: but it too contains material. A photon crossing these environments
• 2:17:56
  Speaker 1: can be absorbed, scattered or redirected. It may pass through
• 2:18:00
  Speaker 1: completely unchanged, or the journey may leave a measurable imprint.
• 2:18:06
  Speaker 1: Dust is particularly important. Tiny dust grains between the stars
• 2:18:11
  Speaker 1: can absorb and scatter shorter wavelengths more efficiently than longer ones.
• 2:18:16
  Speaker 1: This causes extinction, making a distant object appear dimmer. It
• 2:18:22
  Speaker 1: can also cause reddening, shifting the observed balance of visible
• 2:18:26
  Speaker 1: light toward longer wavelengths. The star itself may not be
• 2:18:32
  Speaker 1: unusually red. Its light may simply have travelled through enough
• 2:18:36
  Speaker 1: dust to lose more of its blue light along the way.
• 2:18:40
  Speaker 1: Astronomers therefore have to separate the properties of the source
• 2:18:43
  Speaker 1: from the effects of everything between the source and the observer.
• 2:18:47
  Speaker 1: Gas can leave its own signatures. If starlight passes through
• 2:18:51
  Speaker 1: a cloud containing hydrogen, sodium, calcium or other elements, those
• 2:18:58
  Speaker 1: atoms can absorb particular wavelengths. The resulting absorption lines reveal
• 2:19:03
  Speaker 1: the presence of material that may otherwise be completely invisible.
• 2:19:08
  Speaker 1: In this way, the light from a distant object can
• 2:19:11
  Speaker 1: tell us about matter encountered during the journey, not just
• 2:19:14
  Speaker 1: about the object that created the light. Gravity can alter
• 2:19:19
  Speaker 1: the path as well. Albert Einstein's general theory of relativity
• 2:19:23
  Speaker 1: describes gravity not simply as a force pulling objects through space,
• 2:19:28
  Speaker 1: but as the curvature of spacetime produced by mass and energy.
• 2:19:33
  Speaker 1: Light follows the geometry of that curved spacetime. This means
• 2:19:38
  Speaker 1: a sufficiently massive object can bend the path of light
• 2:19:42
  Speaker 1: passing nearby. The effect is known as gravitational lensing. A
• 2:19:47
  Speaker 1: galaxy located between Earth and a more distant galaxy can
• 2:19:52
  Speaker 1: distort the background galaxy's light. Sometimes the result is a
• 2:19:56
  Speaker 1: stretched arc. Sometimes several images of the same object appear.
• 2:20:02
  Speaker 1: Under particularly symmetrical conditions, the light can form a ring
• 2:20:06
  Speaker 1: around the foreground mass, known as an Einstein ring. Entire
• 2:20:12
  Speaker 1: clusters of galaxies can act as enormous gravitational lenses. Their
• 2:20:17
  Speaker 1: combined mass bends and magnifies light from objects much farther away.
• 2:20:23
  Speaker 1: This can actually help astronomers observe galaxies that would otherwise
• 2:20:26
  Speaker 1: be too faint to detect. Gravity, in that sense, sometimes
• 2:20:32
  Speaker 1: provides the universe with its own enormous telescope lens. But
• 2:20:37
  Speaker 1: gravitational lensing also reveals something deeper. The amount of bending
• 2:20:42
  Speaker 1: depends on total mass, including mass that does not emit light.
• 2:20:48
  Speaker 1: Astronomers can therefore use lensing to map concentrations of dark
• 2:20:52
  Speaker 1: matter by observing how background light has been distorted. The
• 2:20:57
  Speaker 1: path of a photon can reveal the presence of something invisible.
• 2:21:01
  Speaker 1: Gravity can affect light in another way as well. When
• 2:21:05
  Speaker 1: electromagnetic radiation moves between regions of different gravitational potential, its
• 2:21:11
  Speaker 1: frequency can change. This is gravitational redshift. Light climbing away
• 2:21:18
  Speaker 1: from a strong gravitational field can be observed at a
• 2:21:21
  Speaker 1: lower frequency and therefore a longer wavelength by a distant observer.
• 2:21:26
  Speaker 1: In general relativity, this is connected to the different rates
• 2:21:30
  Speaker 1: at which time passes in different gravitational environments. Light emitted
• 2:21:35
  Speaker 1: close to a very compact object can therefore reach a
• 2:21:38
  Speaker 1: distant observer shifted toward longer wavelengths. There is another kind
• 2:21:43
  Speaker 1: of redshift that becomes dominant on the largest scales. This
• 2:21:48
  Speaker 1: one is caused by the expansion of the universe. As
• 2:21:52
  Speaker 1: light travels through expanding space, the wavelength of sufficiently distant
• 2:21:57
  Speaker 1: radiation is stretched. The result is cosmological redshift. The light
• 2:22:03
  Speaker 1: does not simply become tired and lose energy because it
• 2:22:06
  Speaker 1: has been travelling for a long time. The change is
• 2:22:10
  Speaker 1: tied to the expansion of the universe itself. A photon
• 2:22:14
  Speaker 1: may begin its journey as ultraviolet radiation and arrive billions
• 2:22:18
  Speaker 1: of years later with its wavelength stretched into visible or
• 2:22:22
  Speaker 1: infrared regions. This effect is central to modern cosmology. In
• 2:22:28
  Speaker 1: the early 20th century, Georges LaMartre developed a physical picture
• 2:22:33
  Speaker 1: of an expanding universe based on general relativity and connected
• 2:22:37
  Speaker 1: that expansion with observations of distant galaxies. Edwin Hubble later
• 2:22:42
  Speaker 1: published measurements showing a relationship between the distances of galaxies
• 2:22:47
  Speaker 1: and their apparent recession speeds. The farther away many galaxies were,
• 2:22:52
  Speaker 1: the more strongly their light tended to be redshifted. This
• 2:22:56
  Speaker 1: became one of the major observational foundations of the expanding
• 2:23:00
  Speaker 1: universe model. At relatively modest cosmological distances, the relationship can
• 2:23:07
  Speaker 1: be approximately described through what is often called the Hubble-Lemartre law.
• 2:23:12
  Speaker 1: More distant objects require a fuller cosmological treatment because the
• 2:23:17
  Speaker 1: expansion rate has changed over cosmic history. The practical effect
• 2:23:22
  Speaker 1: for astronomy is enormous. Radiation that began its journey in
• 2:23:26
  Speaker 1: ultraviolet wavelengths in the early universe can now arrive at
• 2:23:30
  Speaker 1: Earth in infrared wavelengths. Visible light can also be shifted
• 2:23:35
  Speaker 1: deep into the infrared. This is one of the reasons
• 2:23:39
  Speaker 1: infrared astronomy is so important for studying very distant galaxies.
• 2:23:45
  Speaker 1: The James Webb Space Telescope was designed primarily to observe
• 2:23:49
  Speaker 1: infrared radiation. allowing it to detect light that left some
• 2:23:53
  Speaker 1: galaxies when the universe was only a small fraction of
• 2:23:57
  Speaker 1: its current age. That light may have crossed more than
• 2:24:01
  Speaker 1: 13 billion years of cosmic history before reaching the telescope.
• 2:24:06
  Speaker 1: During that journey, the universe itself expanded around it. Its
• 2:24:11
  Speaker 1: wavelength stretched, its frequency fell, and by the time it
• 2:24:16
  Speaker 1: reached us, the radiation could occupy a very different part
• 2:24:20
  Speaker 1: of the electromagnetic spectrum from the one in which it
• 2:24:23
  Speaker 1: was originally emitted. This creates a challenge. Astronomers cannot simply
• 2:24:29
  Speaker 1: observe a wavelength and assume that the source produced that
• 2:24:33
  Speaker 1: exact wavelength in its own local environment. They have to
• 2:24:37
  Speaker 1: reconstruct what happened. Was a feature created by the source itself?
• 2:24:43
  Speaker 1: Was it altered by dust? Was a spectral line shifted
• 2:24:47
  Speaker 1: because the object was moving? Was gravitational redshift involved? Has
• 2:24:53
  Speaker 1: cosmic expansion stretched the entire spectrum? Did intervening gas absorb
• 2:24:58
  Speaker 1: part of the light? Was the apparent shape of the
• 2:25:01
  Speaker 1: object distorted by gravitational lensing? Every observation can therefore contain
• 2:25:07
  Speaker 1: two intertwined stories. One story belongs to the source, the
• 2:25:11
  Speaker 1: other belongs to the journey. The farther the light has travelled,
• 2:25:16
  Speaker 1: the more carefully those stories may need to be separated.
• 2:25:20
  Speaker 1: And yet that complexity is useful. A photon passing through
• 2:25:24
  Speaker 1: gas can reveal the gas. A photon bent by gravity
• 2:25:28
  Speaker 1: can reveal mass. A photon stretched by expansion can reveal
• 2:25:33
  Speaker 1: cosmic history. Light does not merely survive the journey. It
• 2:25:39
  Speaker 1: records parts of it. This is why astronomical observation is
• 2:25:44
  Speaker 1: inherently historical. When we look at the Sun, we see
• 2:25:49
  Speaker 1: roughly 8 minutes into the past. When we look at
• 2:25:52
  Speaker 1: Proxima Centauri, we see more than 4 years into the past.
• 2:25:57
  Speaker 1: When we look at Andromeda, we see roughly 2.5 million
• 2:26:01
  Speaker 1: years into the past. When we observe some of the
• 2:26:05
  Speaker 1: most distant galaxies known, we are receiving light that began
• 2:26:09
  Speaker 1: travelling when the universe was still very young. There is
• 2:26:13
  Speaker 1: no universal present view of the cosmos available to us.
• 2:26:18
  Speaker 1: The sky is a layered collection of different moments. Nearby
• 2:26:22
  Speaker 1: objects appear almost current. Distant ones appear increasingly ancient. The
• 2:26:28
  Speaker 1: farther we look, the older the light becomes. And after
• 2:26:32
  Speaker 1: travelling across the universe, passing through matter, bending through curved space-time,
• 2:26:38
  Speaker 1: and stretching with cosmic expansion … that light finally reaches
• 2:26:43
  Speaker 1: us carrying not only an image of where it came from,
• 2:26:46
  Speaker 1: but traces of the entire journey in between. The farther
• 2:26:51
  Speaker 1: we look into space, the farther back in time we see.
• 2:26:56
  Speaker 1: That is not a metaphor. It is a direct consequence
• 2:26:59
  Speaker 1: of the fact that light takes time to travel. A
• 2:27:03
  Speaker 1: nearby star may show us what happened only a few
• 2:27:06
  Speaker 1: years ago, A distant galaxy may reveal conditions from billions
• 2:27:11
  Speaker 1: of years in the past. And if we keep looking farther,
• 2:27:15
  Speaker 1: eventually we reach a remarkable limit. There is a point
• 2:27:19
  Speaker 1: beyond which ordinary light can no longer show us an
• 2:27:23
  Speaker 1: earlier universe in the same way. The reason lies in
• 2:27:28
  Speaker 1: what the cosmos used to be like. Today, most of
• 2:27:33
  Speaker 1: space is transparent to visible light. a photon can travel
• 2:27:38
  Speaker 1: enormous distances without hitting anything. But the early universe was
• 2:27:44
  Speaker 1: very different. Soon after the Big Bang, matter was extremely
• 2:27:50
  Speaker 1: hot and dense. Temperatures were so high that electrons could
• 2:27:54
  Speaker 1: not remain bound to atomic nuclei for long. Instead of
• 2:27:58
  Speaker 1: neutral atoms, the universe contained an ionised plasma filled with
• 2:28:03
  Speaker 1: free electrons and positively charged nuclei. This made the young
• 2:28:09
  Speaker 1: cosmos difficult for light to cross. Photons constantly interacted with
• 2:28:14
  Speaker 1: the free electrons. They scattered again and again. Instead of
• 2:28:19
  Speaker 1: travelling freely across vast distances, radiation was trapped in a dense,
• 2:28:24
  Speaker 1: glowing mixture of matter and light. The universe was therefore opaque,
• 2:28:30
  Speaker 1: not dark. Quite the opposite. It was filled with radiation.
• 2:28:36
  Speaker 1: But that radiation could not travel very far in a
• 2:28:39
  Speaker 1: straight line before being scattered. If you could somehow have
• 2:28:43
  Speaker 1: existed inside this early plasma, looking outward would have been
• 2:28:47
  Speaker 1: something like trying to see through an extremely dense luminous fog.
• 2:28:51
  Speaker 1: There would be light everywhere, but no clear distant view.
• 2:28:56
  Speaker 1: As the universe expanded, however, it also cooled. After roughly 380,000 years,
• 2:29:05
  Speaker 1: the temperature had fallen to around 3,000 Kelvin. At that point,
• 2:29:10
  Speaker 1: conditions changed dramatically. Electrons could begin remaining bound to atomic nuclei,
• 2:29:16
  Speaker 1: especially protons. Hydrogen atoms formed in large numbers, helium atoms
• 2:29:22
  Speaker 1: formed as well. This transition is known as recombination. The
• 2:29:27
  Speaker 1: name is slightly misleading because, cosmologically speaking, These electrons and
• 2:29:32
  Speaker 1: nuclei were combining in this stable way for the first time.
• 2:29:37
  Speaker 1: But the term has remained. Once free electrons became locked
• 2:29:41
  Speaker 1: into neutral atoms, the way light interacted with matter changed.
• 2:29:46
  Speaker 1: There were suddenly far fewer free electrons available to scatter photons.
• 2:29:52
  Speaker 1: The universe became transparent. Radiation that had previously been trapped
• 2:29:58
  Speaker 1: could now travel for enormous distances. This transition is often
• 2:30:03
  Speaker 1: called photon decoupling. The region from which those photons last
• 2:30:08
  Speaker 1: scattered is described as the surface of last scattering. It
• 2:30:13
  Speaker 1: is not a solid surface. There is no cosmic wall
• 2:30:17
  Speaker 1: sitting somewhere at the edge of space. Instead, when we
• 2:30:21
  Speaker 1: look outward in any direction, we eventually look back to
• 2:30:24
  Speaker 1: the time when the universe became transparent. The photons released
• 2:30:29
  Speaker 1: then have been travelling ever since. Today, we detect that
• 2:30:33
  Speaker 1: ancient radiation as the cosmic microwave background. When those photons
• 2:30:39
  Speaker 1: began travelling freely, the universe was much hotter than it
• 2:30:43
  Speaker 1: is now. The radiation had a thermal spectrum corresponding to
• 2:30:47
  Speaker 1: a temperature of around 3000 Kelvin. Since then, cosmic expansion
• 2:30:53
  Speaker 1: has stretched the wavelengths of the photons enormously. Radiation that
• 2:30:58
  Speaker 1: began at much shorter wavelengths has been shifted into the
• 2:31:02
  Speaker 1: microwave region of the electromagnetic spectrum. Its present average temperature
• 2:31:07
  Speaker 1: is only about 2.7 Kelvin. That tiny temperature is one
• 2:31:13
  Speaker 1: of the clearest examples of how deeply cosmic expansion has
• 2:31:17
  Speaker 1: altered ancient light. The cosmic microwave background surrounds us from
• 2:31:23
  Speaker 1: every direction. Wherever we look deeply enough into space, this
• 2:31:27
  Speaker 1: ancient radiation is there. The photons reaching us today have
• 2:31:31
  Speaker 1: travelled for more than 13 billion years. Because the universe
• 2:31:36
  Speaker 1: expanded while they were travelling, the regions that originally emitted
• 2:31:40
  Speaker 1: them are now much farther away than a simple calculation
• 2:31:44
  Speaker 1: based on travel time alone might suggest. The cosmic microwave
• 2:31:48
  Speaker 1: background was not discovered because astronomers were deliberately trying to
• 2:31:53
  Speaker 1: photograph the infant universe. its discovery arrived in a much
• 2:31:57
  Speaker 1: more accidental way. In the 1960s, Arno Penzias and Robert
• 2:32:03
  Speaker 1: Wilson were working with a sensitive radio antenna at Bell Laboratories.
• 2:32:09
  Speaker 1: They encountered a persistent background noise that seemed to come
• 2:32:12
  Speaker 1: from every direction. They carefully investigated possible sources. Equipment problems
• 2:32:19
  Speaker 1: were considered. Local radio interference was considered. Even material left
• 2:32:26
  Speaker 1: by pigeons inside the antenna had to be cleaned away.
• 2:32:30
  Speaker 1: The unwanted signal remained. In 1965, the connection became clear.
• 2:32:38
  Speaker 1: Penzias and Wilson had detected the relic radiation of the
• 2:32:42
  Speaker 1: early universe. At roughly the same time, other researchers were
• 2:32:47
  Speaker 1: preparing to search for exactly this kind of cosmic signal.
• 2:32:51
  Speaker 1: the discovery became one of the strongest pieces of observational
• 2:32:54
  Speaker 1: evidence for the Hot Big Bang model. If the universe
• 2:32:58
  Speaker 1: had once been much hotter and denser, a cooled remnant
• 2:33:01
  Speaker 1: of that early radiation was expected. And there it was.
• 2:33:07
  Speaker 1: Later missions measured the cosmic microwave background with far greater precision.
• 2:33:12
  Speaker 1: The Cosmic Background Explorer satellite demonstrated that its spectrum matches
• 2:33:17
  Speaker 1: that of a nearly perfect blackbody with extraordinary accuracy. That
• 2:33:22
  Speaker 1: result showed that the early universe had once been in
• 2:33:25
  Speaker 1: an extremely well-defined thermal state. The Wilkinson Microwave Anisotropy Probe
• 2:33:32
  Speaker 1: later mapped tiny variations in the temperature of the cosmic
• 2:33:35
  Speaker 1: microwave background across the sky. The European Space Agency Planck
• 2:33:40
  Speaker 1: mission improved those measurements further. The variations are extremely small
• 2:33:47
  Speaker 1: Typical temperature differences are only around one part in 100,000,
• 2:33:53
  Speaker 1: yet those tiny fluctuations are enormously important. They reveal slight
• 2:33:58
  Speaker 1: differences in the density of the early universe. Regions that
• 2:34:02
  Speaker 1: were slightly denser than their surroundings eventually helped seed the
• 2:34:06
  Speaker 1: formation of galaxies, galaxy clusters and the vast cosmic web.
• 2:34:12
  Speaker 1: In that sense, the cosmic microwave background is not merely
• 2:34:16
  Speaker 1: old light. It is a kind of infant portrait of
• 2:34:19
  Speaker 1: the observable universe. The stars, galaxies, planets and eventually living
• 2:34:25
  Speaker 1: creatures that appeared much later grew within a universe whose
• 2:34:29
  Speaker 1: earliest large-scale structure can still be traced in those faint
• 2:34:33
  Speaker 1: microwave variations. But the cosmic microwave background also marks a limit.
• 2:34:40
  Speaker 1: Ordinary electromagnetic radiation cannot directly show us earlier times in
• 2:34:44
  Speaker 1: the same straightforward way. Before recombination, the universe was opaque.
• 2:34:50
  Speaker 1: Photons were repeatedly scattered by free electrons. Any electromagnetic information
• 2:34:56
  Speaker 1: from much earlier epochs was effectively mixed and reprocessed before
• 2:35:01
  Speaker 1: the universe became transparent. We cannot simply build a larger
• 2:35:05
  Speaker 1: visible light telescope and expect to see through that plasma.
• 2:35:09
  Speaker 1: The obstacle is not insufficient telescope power. It is the
• 2:35:14
  Speaker 1: physics of the early universe itself. That does not mean
• 2:35:18
  Speaker 1: earlier cosmic history is completely inaccessible. Other messengers may allow
• 2:35:23
  Speaker 1: us to probe further back. Neutrinos interact extremely weakly with matter.
• 2:35:30
  Speaker 1: A relic background of neutrinos is expected to have decoupled
• 2:35:34
  Speaker 1: when the universe was only about one second old. Detecting
• 2:35:38
  Speaker 1: that cosmic neutrino background directly is extraordinarily difficult, but in
• 2:35:44
  Speaker 1: principle it carries information from much earlier, than the cosmic
• 2:35:48
  Speaker 1: microwave background. Gravitational waves offer another possibility. Very early events
• 2:35:55
  Speaker 1: in the universe may have produced gravitational radiation capable of
• 2:35:59
  Speaker 1: travelling through matter with relatively little interference. A primordial gravitational
• 2:36:05
  Speaker 1: wave background, if detected, could provide information about conditions far
• 2:36:11
  Speaker 1: earlier than recombination. So light is not the only possible
• 2:36:16
  Speaker 1: messenger from the past. but it remains the one that
• 2:36:19
  Speaker 1: has given us by far the richest picture of the
• 2:36:22
  Speaker 1: observable universe. From sunlight only minutes old to starlight thousands
• 2:36:28
  Speaker 1: of years old, from distant galaxies seen billions of years
• 2:36:32
  Speaker 1: in the past to microwaves released when the cosmos was
• 2:36:36
  Speaker 1: just 380,000 years old, electromagnetic radiation carries history across space.
• 2:36:44
  Speaker 1: Every photon has a story. Some began in an atom.
• 2:36:49
  Speaker 1: Some escaped from the surface of a star. Some were
• 2:36:53
  Speaker 1: produced by violent shocks, magnetic fields or matter falling toward
• 2:36:58
  Speaker 1: a black hole. And some have been travelling since the
• 2:37:02
  Speaker 1: universe first became transparent. The oldest light we can freely
• 2:37:06
  Speaker 1: observe does not show us the very beginning. But it
• 2:37:10
  Speaker 1: takes us extraordinarily close to the earliest visible moment. A
• 2:37:15
  Speaker 1: faint microwave glow, stretched and cooled across more than 13
• 2:37:19
  Speaker 1: billion years, still surrounds us in every direction. And hidden
• 2:37:24
  Speaker 1: inside that glow is a record of the young universe
• 2:37:28
  Speaker 1: from which everything we see today eventually emerged. So what
• 2:37:34
  Speaker 1: actually is light, and where does it come from? At
• 2:37:38
  Speaker 1: the deepest level, light is electromagnetic radiation. In classical physics
• 2:37:44
  Speaker 1: it can be described as changing electric and magnetic fields
• 2:37:48
  Speaker 1: travelling through space. In quantum physics, light is described in
• 2:37:53
  Speaker 1: terms of photons, the discrete quanta of the electromagnetic field.
• 2:37:59
  Speaker 1: Visible light is only a narrow part of that much
• 2:38:02
  Speaker 1: larger picture. The electromagnetic spectrum stretches from long-wavelength radio waves
• 2:38:07
  Speaker 1: through microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and
• 2:38:16
  Speaker 1: finally gamma rays at the highest frequencies and energies. and
• 2:38:21
  Speaker 1: there is no single process responsible for creating all of it.
• 2:38:27
  Speaker 1: Atoms can emit photons when electrons move between quantum states.
• 2:38:31
  Speaker 1: Warm matter produces thermal radiation. Accelerating charged particles can create
• 2:38:37
  Speaker 1: electromagnetic waves. Hot plasma can shine through Bremsstrahlung and recombination.
• 2:38:44
  Speaker 1: Magnetic fields can guide relativistic particles into producing synchrotron radiation.
• 2:38:51
  Speaker 1: Chemical reactions can create light. Semiconductors can turn electrical energy
• 2:38:56
  Speaker 1: into photons. Lasers can produce highly organised beams through stimulated emission.
• 2:39:03
  Speaker 1: And some of the most violent events in the universe
• 2:39:05
  Speaker 1: can generate intense X-rays and gamma rays. Stars remain the
• 2:39:10
  Speaker 1: most familiar cosmic light sources, but even there the story
• 2:39:14
  Speaker 1: is more complicated than it first appears. Nuclear fusion in
• 2:39:19
  Speaker 1: a stellar core provides the energy. That energy is then transported, absorbed, scattered, re-emitted,
• 2:39:28
  Speaker 1: and carried through plasma before radiation finally escapes from the surface.
• 2:39:34
  Speaker 1: The photons reaching us are therefore part of a much
• 2:39:37
  Speaker 1: longer chain of energy conversion. The same basic electromagnetic physics
• 2:39:43
  Speaker 1: then appears in very different settings. It appears in supernovae,
• 2:39:49
  Speaker 1: in accretion flows around black holes, in neutron stars and
• 2:39:53
  Speaker 1: relativistic jets, in glowing nebulae, auroras, lightning, living organisms, lamps,
• 2:40:02
  Speaker 1: light-emitting diodes and lasers. Different environments, different energy sources, the
• 2:40:10
  Speaker 1: same underlying rules. And once light is created, It does
• 2:40:15
  Speaker 1: not simply travel untouched. It can be absorbed, reflected, refracted, scattered, reddened,
• 2:40:25
  Speaker 1: shifted in frequency or re-emitted. Those interactions change the light,
• 2:40:29
  Speaker 1: but they also give it information. That is why spectroscopy
• 2:40:33
  Speaker 1: is so powerful. A spectrum can reveal chemical composition, temperature, density, ionization, motion,
• 2:40:42
  Speaker 1: rotation and magnetic fields. Light can tell us what distant
• 2:40:47
  Speaker 1: stars are made of without us ever touching them. It
• 2:40:51
  Speaker 1: can reveal the atmosphere of a planet orbiting another star.
• 2:40:55
  Speaker 1: It can trace gas between galaxies. It can even expose
• 2:40:59
  Speaker 1: matter that does not shine at all by showing us
• 2:41:02
  Speaker 1: how gravity bends the light passing nearby. Our eyes, cameras,
• 2:41:07
  Speaker 1: radio antennas, spectrographs and space telescopes are all variations on
• 2:41:13
  Speaker 1: the same basic idea. Radiation arrives. A detector interacts with it.
• 2:41:20
  Speaker 1: And that interaction becomes information. Because light travels at a
• 2:41:25
  Speaker 1: finite speed, that information is also historical. Sunlight shows us
• 2:41:31
  Speaker 1: the sun as it was a little over eight minutes ago.
• 2:41:34
  Speaker 1: Light from nearby stars can be years old. Light from
• 2:41:37
  Speaker 1: distant galaxies can be billions of years old. And the
• 2:41:41
  Speaker 1: cosmic microwave background carries radiation that began travelling, when the
• 2:41:46
  Speaker 1: universe was only about 380,000 years old. That faint microwave
• 2:41:52
  Speaker 1: glow is the oldest freely travelling electromagnetic radiation we can
• 2:41:57
  Speaker 1: observe directly. It connects the physics of light on the
• 2:42:01
  Speaker 1: smallest quantum scales with the history of the observable universe itself.
• 2:42:07
  Speaker 1: In that sense, much of astronomy is simply the patient
• 2:42:10
  Speaker 1: study of very old light. Photons leave atoms, stars, galaxies
• 2:42:16
  Speaker 1: and explosions. They cross space. They interact with matter. They
• 2:42:23
  Speaker 1: bend through gravity. They stretch with cosmic expansion. And eventually,
• 2:42:29
  Speaker 1: some tiny fraction of them reaches us. Then we try
• 2:42:34
  Speaker 1: to read the story they carried all that way.