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How Can a Black Hole Lose Mass If Nothing Can Escape · Night Science Stories

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2:03
Speaker 2: of your day. Now, tonight's subject appears to break one
2:07
Speaker 2: of the universe's firmest rules. Black holes are famous for
2:11
Speaker 2: one thing above all else. Nothing gets out. Not matter.
2:15
Speaker 2: Not radiation. Not even light. Once something crosses the edge,
2:19
Speaker 2: there's no known way back. And yet in the 1970s,
2:23
Speaker 2: Stephen Hawking realised something remarkable. When you add the strange
2:27
Speaker 2: rules of the very small to the picture... Black holes
2:30
Speaker 2: ought to give off a faint, steady trickle of energy
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Speaker 2: into the space around them, and as that energy leaks
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Speaker 2: away they very slowly lose weight. Give it an almost
2:39
Speaker 2: unimaginable stretch of time and a black hole could eventually
2:43
Speaker 2: vanish altogether is a peculiar thought. The one object famous
2:47
Speaker 2: for never letting anything go might not be quite as
2:50
Speaker 2: permanent as it looks, which raises a rather lovely puzzle.
2:54
Speaker 2: If nothing can escape from inside a black hole, where
2:58
Speaker 2: on earth is that energy coming from? So make yourself comfortable,
3:02
Speaker 2: let the evening settle around you, and we'll unpick it
3:05
Speaker 2: at a gentle pace. Shall we begin? Because even the
3:09
Speaker 2: darkest objects in the universe may not last forever. A
3:12
Speaker 2: black hole can lose mass, and the truly remarkable part
3:16
Speaker 2: is that nothing ever has to escape from inside it.
3:19
Speaker 2: For most of the history of modern physics, black holes
3:22
Speaker 2: appeared to possess one rather remarkable property. They could gain mass,
3:27
Speaker 2: but there seemed to be no way for them to
3:29
Speaker 2: lose it. Anything that crossed their event horizon was permanently
3:33
Speaker 2: removed from the observable universe. Matter could fall inward. Light
3:39
Speaker 2: could disappear. Entire stars could be swallowed. But nothing that
3:43
Speaker 2: entered could ever make the journey back out. And for
3:47
Speaker 2: a long time, this seemed to be the end of
3:49
Speaker 2: the matter. A black hole might continue growing as more
3:53
Speaker 2: material fell into it. It might remain isolated in a
3:57
Speaker 2: quiet region of space. with very little happening around it.
4:01
Speaker 2: But according to the classical laws of gravity, there was
4:04
Speaker 2: no obvious reason for an ordinary, isolated black hole to
4:08
Speaker 2: gradually become smaller. Then, in the 1970s, physicists discovered something
4:14
Speaker 2: that completely changed this picture. A black hole, it turned out,
4:18
Speaker 2: might not be entirely black. To understand why this was
4:22
Speaker 2: so surprising, it helps to consider what makes a black
4:25
Speaker 2: hole different from almost everything else in the universe. An
4:29
Speaker 2: ordinary star can release energy through light, heat, and streams
4:33
Speaker 2: of particles. Even a star that has exhausted its nuclear
4:38
Speaker 2: fuel can continue radiating away some of its remaining thermal energy.
4:42
Speaker 2: A black hole is different. Its defining feature is the
4:46
Speaker 2: event horizon, an invisible boundary in spacetime beyond which no
4:50
Speaker 2: outward path leads back to the distant universe. Once light
4:54
Speaker 2: crosses that boundary, it cannot return. And since nothing carrying
4:59
Speaker 2: information can travel faster than light, neither matter nor any
5:03
Speaker 2: ordinary signal can escape from inside it. This is not
5:06
Speaker 2: simply because the black hole pulls unusually hard. The explanation
5:11
Speaker 2: goes considerably deeper. In 1915, Albert Einstein completed his general
5:16
Speaker 2: theory of relativity, which transformed our understanding of gravity. Rather
5:22
Speaker 2: than describing gravity merely as a force pulling objects towards
5:25
Speaker 2: one another, Einstein showed that matter and energy influence the
5:29
Speaker 2: geometry of spacetime itself. Objects moving under gravity follow paths
5:34
Speaker 2: determined by that geometry. Planets orbit stars, light bends around
5:40
Speaker 2: massive objects, and clocks run at different rates depending on
5:44
Speaker 2: the gravitational conditions surrounding them. Near a black hole, these
5:48
Speaker 2: effects become extraordinary. The curvature of spacetime becomes so extreme that,
5:54
Speaker 2: inside the event horizon, Every possible future path taken by
5:58
Speaker 2: light remains within the black hole. There is no direction
6:01
Speaker 2: that leads back outside and that presents a peculiar difficulty.
6:06
Speaker 2: If nothing can escape from a black hole, how could
6:08
Speaker 2: the black hole possibly release energy? And if it cannot
6:12
Speaker 2: release energy, how could its mass ever decrease? The answer
6:16
Speaker 2: emerged when a young British theoretical physicist named Stephen Hawking
6:20
Speaker 2: began investigating the relationship between gravity and quantum physics at
6:24
Speaker 2: the University of Cambridge. At the time, physicists already possessed
6:29
Speaker 2: two extraordinarily successful descriptions of nature. Einstein's general relativity explained
6:35
Speaker 2: the large-scale behaviour of gravity in spacetime. Quantum theory explained
6:40
Speaker 2: the behaviour of matter and energy at extremely small scales.
6:44
Speaker 2: Both worked remarkably well within their respective domains. but combining
6:49
Speaker 2: their ideas was far from straightforward. Hawking wanted to understand
6:53
Speaker 2: what happened when quantum fields were placed in the unusual
6:56
Speaker 2: spacetime surrounding a black hole, and the result was astonishing.
7:01
Speaker 2: His calculations predicted that black holes should emit a faint
7:04
Speaker 2: form of radiation, now known as Hawking radiation. An isolated
7:09
Speaker 2: black hole would therefore gradually lose energy, and because mass
7:12
Speaker 2: and energy are fundamentally related, losing that energy would also
7:17
Speaker 2: mean losing mass. the black hole would slowly become smaller.
7:21
Speaker 2: There is an important distinction here, however. Hawking was not
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Speaker 2: suggesting that ordinary particles could suddenly escape from inside the
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Speaker 2: event horizon. Einstein's rule remained intact. Instead, the radiation arises
7:35
Speaker 2: from the quantum behaviour of fields in the curved spacetime
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Speaker 2: associated with the black hole. The process does not require
7:42
Speaker 2: a particle to begin its journey somewhere deep inside the
7:45
Speaker 2: horizon before somehow forcing its way out. Nothing needs to
7:50
Speaker 2: break the universal speed limit, and nothing needs to escape
7:53
Speaker 2: through a boundary that cannot be crossed outward. The radiation
7:57
Speaker 2: can be observed outside the black hole, while the event
8:00
Speaker 2: horizon continues to perform its familiar role. This also leads
8:05
Speaker 2: to another surprising prediction. Black holes have temperatures. Not temperatures
8:10
Speaker 2: in quite the same sense as a glowing piece of
8:12
Speaker 2: metal or the surface of a star, but characteristic temperatures
8:16
Speaker 2: associated with the radiation they emit. and their temperatures behave
8:20
Speaker 2: in a rather unexpected way. The larger a black hole is,
8:24
Speaker 2: the colder it becomes. A black hole with roughly the
8:27
Speaker 2: mass of the Sun would have a Hawking temperature of
8:29
Speaker 2: only about 60 billionths of a Kelvin above absolute zero.
8:33
Speaker 2: That is extraordinarily cold. A smaller black hole, however, would
8:38
Speaker 2: have a higher temperature and could radiate energy more rapidly.
8:41
Speaker 2: This means that as an isolated black hole loses mass,
8:45
Speaker 2: its radiation can become stronger. the process accelerates as the
8:49
Speaker 2: black hole becomes smaller. Given a sufficiently long time, the
8:54
Speaker 2: familiar semi-classical theory predicts that a black hole could evaporate
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Speaker 2: almost completely, although precisely what happens at the very end
9:02
Speaker 2: remains unknown. For ordinary stellar black holes, this would take
9:06
Speaker 2: a length of time almost impossible to imagine, far longer
9:10
Speaker 2: than the universe has existed so far. And there is
9:13
Speaker 2: another important qualification. Astronomers have never directly detected Hawking radiation
9:19
Speaker 2: coming from an actual astrophysical black hole. The prediction is
9:23
Speaker 2: supported by a substantial body of theoretical physics, but measuring
9:28
Speaker 2: such incredibly weak radiation remains beyond our present observational capabilities. Nevertheless,
9:35
Speaker 2: Hawking's discovery opened an extraordinary collection of questions. It connected
9:40
Speaker 2: gravity with thermodynamics, the science of heat and energy. It
9:45
Speaker 2: suggested that spacetime horizons possess physical properties that classical relativity
9:50
Speaker 2: alone does not fully explain, and it introduced a deeper
9:54
Speaker 2: mystery concerning information. If a black hole eventually disappears, what
9:59
Speaker 2: happens to the information associated with everything that once fell
10:02
Speaker 2: inside it? That question remains one of the great challenges
10:07
Speaker 2: of theoretical physics, but perhaps the most remarkable part of
10:11
Speaker 2: the original discovery is its simplicity. A black hole does
10:15
Speaker 2: not have to let anything escape from within its event horizon,
10:18
Speaker 2: in order to lose mass. And something once thought to
10:22
Speaker 2: be permanently black may, through the subtle laws of quantum physics,
10:26
Speaker 2: possess the faintest possible glow. When we picture a black hole,
10:31
Speaker 2: it is tempting to imagine an enormous dark object floating
10:35
Speaker 2: somewhere in space. Perhaps a perfectly black sphere, surrounded by
10:40
Speaker 2: swirling clouds of gas, with gravity so powerful that anything
10:45
Speaker 2: wandering too close is immediately pulled inside. But a black
10:49
Speaker 2: hole is something considerably stranger than an ordinary object with
10:53
Speaker 2: a particularly strong gravitational field. It is a region of
10:57
Speaker 2: space-time from which nothing can return to the distant universe.
11:02
Speaker 2: And that distinction matters, because understanding what a black hole
11:06
Speaker 2: actually is helps explain how something apparently capable of swallowing
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Speaker 2: everything can nevertheless lose mass. The mathematical foundations of black
11:17
Speaker 2: holes emerged remarkably soon after Albert Einstein completed his general
11:21
Speaker 2: theory of relativity. In 1916, a German physicist named Karl
11:27
Speaker 2: Schwarzschild discovered an exact solution to Einstein's gravitational field equations.
11:33
Speaker 2: Schwarzschild was serving in the German army during the First
11:36
Speaker 2: World War when he carried out this extraordinary piece of
11:39
Speaker 2: theoretical work. His solution described the geometry of spacetime outside
11:44
Speaker 2: an idealised, spherical, non-rotating, electrically uncharged mass. It revealed that
11:52
Speaker 2: Einstein's equations permitted a form of spacetime geometry far more
11:57
Speaker 2: extreme than anything familiar from ordinary experience. Today, that mathematical
12:02
Speaker 2: description is known as the Schwarzschild solution, and it remains
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Speaker 2: one of the most important foundations of black hole physics.
12:10
Speaker 2: One particularly significant feature of the solution is a distance
12:14
Speaker 2: now called the Schwarzschild radius. For an idealised non-rotating black hole,
12:20
Speaker 2: this radius marks the location of the event horizon. Its
12:23
Speaker 2: size depends directly on the black hole's mass. The greater
12:27
Speaker 2: the mass, the larger the radius. If the entire mass
12:31
Speaker 2: of our Sun were somehow compressed into a sufficiently small
12:35
Speaker 2: region to produce a non-rotating black hole, Its event horizon
12:40
Speaker 2: would have a radius of approximately 3 kilometres. That is
12:45
Speaker 2: an extraordinary comparison. The Sun currently stretches almost 700,000 kilometres
12:51
Speaker 2: from its centre to its visible surface. Yet the gravitational
12:55
Speaker 2: mass of that enormous star could, in principle, be associated
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Speaker 2: with an event horizon only a few kilometres across in radius.
13:05
Speaker 2: A black hole containing 10 times the mass of the
13:08
Speaker 2: Sun would have a Schwarzschild radius of approximately 30 kilometres,
13:12
Speaker 2: and a black hole containing 100 times the Sun's mass
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Speaker 2: would have a radius of roughly 300 kilometres. The relationship
13:20
Speaker 2: is straightforward. Double the mass, and the Schwarzschild radius doubles.
13:26
Speaker 2: Of course, the Sun is not going to become a
13:28
Speaker 2: black hole. It does not possess enough mass to undergo
13:31
Speaker 2: the particular kind of gravitational collapse required. Black holes formed
13:36
Speaker 2: from stars generally originate in substantially more massive stellar systems.
13:41
Speaker 2: Throughout most of a star's life, the inward pull of
13:44
Speaker 2: gravity is balanced by pressure within its interior. Nuclear fusion
13:49
Speaker 2: releases energy in the central regions, helping maintain the conditions
13:54
Speaker 2: that keep the star from collapsing under its own weight.
13:57
Speaker 2: But the situation changes when a massive star's core can
14:01
Speaker 2: no longer generate sufficient pressure to support itself. gravity begins
14:06
Speaker 2: to dominate. The core contracts, and under suitable conditions, it
14:11
Speaker 2: can collapse into an extraordinarily compact remnant. Some collapsing stellar
14:16
Speaker 2: cores become neutron stars, supported against further collapse by the
14:21
Speaker 2: remarkable properties of matter at extreme densities. Others form black holes.
14:27
Speaker 2: Once an event horizon develops, a region of spacetime exists
14:31
Speaker 2: from which future-directed light signals can no longer reach the
14:34
Speaker 2: distant exterior. This produces what astronomers call a stellar-mass black hole.
14:41
Speaker 2: These objects typically contain several to tens of times the
14:44
Speaker 2: mass of the Sun, although stellar evolution can produce a
14:48
Speaker 2: broader range of masses depending on the original star and
14:51
Speaker 2: its environment. But stellar-mass black holes are only part of
14:55
Speaker 2: the picture. At the centres of many galaxies, Astronomers have
14:59
Speaker 2: discovered black holes containing millions, or even billions of times
15:04
Speaker 2: the mass of our Sun. These are known as supermassive
15:08
Speaker 2: black holes. Our own Milky Way contains one. It is
15:12
Speaker 2: called Sagittarius A Star, and it lies approximately 26,000 light
15:17
Speaker 2: years from Earth, near the centre of the galaxy. Sagittarius
15:22
Speaker 2: A Star contains roughly 4 million solar masses. Despite possessing
15:27
Speaker 2: the mass of millions of suns, its event horizon is
15:31
Speaker 2: remarkably compact on astronomical scales. For a non-rotating black hole
15:36
Speaker 2: of that mass, the Schwarzschild radius would be approximately 12
15:40
Speaker 2: million kilometres. That is only a small fraction of the
15:44
Speaker 2: distance between Earth and the Sun. One of the most
15:47
Speaker 2: important things to understand about these objects is that the
15:50
Speaker 2: event horizon is not a physical surface. There is no
15:53
Speaker 2: solid outer shell. There is no wall made from compressed matter,
15:58
Speaker 2: and there is no invisible membrane that physically catches objects
16:01
Speaker 2: as they arrive. Instead, the event horizon is a boundary
16:06
Speaker 2: defined by the geometry and causal structure of spacetime. For
16:10
Speaker 2: a sufficiently large black hole, an observer falling through this
16:14
Speaker 2: boundary might experience nothing particularly unusual at the exact moment
16:19
Speaker 2: of crossing. There need not be a sudden impact or
16:22
Speaker 2: any locally detectable change announcing that the event horizon has
16:26
Speaker 2: been passed. The crucial difference concerns what becomes possible afterwards.
16:32
Speaker 2: From inside the horizon, every future-directed path available to light
16:36
Speaker 2: or ordinary matter remains within the black hole. The distant
16:41
Speaker 2: universe can no longer be reached. And this brings us
16:44
Speaker 2: to another frequently misunderstood feature of black holes– At the
16:49
Speaker 2: centre of the idealised Schwarzschild solution, classical general relativity predicts
16:55
Speaker 2: a singularity. Mathematically, this is a region where quantities describing
17:00
Speaker 2: spacetime curvature become unbounded. But physicists do not know whether
17:05
Speaker 2: an actual black hole contains a singularity in precisely this
17:09
Speaker 2: classical sense. The prediction may instead reveal a limitation of
17:14
Speaker 2: general relativity under conditions where quantum gravitational effects become important.
17:20
Speaker 2: A complete theory of quantum gravity may eventually provide a
17:23
Speaker 2: more satisfactory description of what happens in these extreme regions.
17:28
Speaker 2: For now, the physical nature of the deepest interior remains uncertain. Fortunately,
17:34
Speaker 2: we do not need to resolve that uncertainty to understand
17:37
Speaker 2: how a black hole influences its surroundings. And its influence
17:41
Speaker 2: is not quite as indiscriminately destructive As popular descriptions sometimes suggest,
17:48
Speaker 2: a black hole does not automatically swallow everything nearby. An
17:52
Speaker 2: object can orbit a black hole just as it can
17:55
Speaker 2: orbit a star, provided its motion and distance allow a
17:59
Speaker 2: stable orbit. If the Sun could somehow be replaced by
18:03
Speaker 2: a black hole possessing exactly the same mass, Earth's orbit
18:07
Speaker 2: would remain essentially unchanged, assuming the replacement introduced no other disturbance.
18:14
Speaker 2: our planet would not suddenly be pulled inward simply because
18:17
Speaker 2: the central mass had become a black hole. It would
18:20
Speaker 2: admittedly become rather dark and cold without sunlight, which presents
18:25
Speaker 2: an entirely different difficulty. The important point is that gravity
18:30
Speaker 2: outside a spherical body depends on its mass and the
18:33
Speaker 2: surrounding spacetime geometry, not simply on whether that mass happens
18:37
Speaker 2: to be shining, and here we encounter a subtle but
18:41
Speaker 2: essential idea. A black hole's mass is not merely a
18:45
Speaker 2: quantity hidden somewhere behind the event horizon. It is reflected
18:49
Speaker 2: in the curvature of spacetime outside that boundary. The gravitational
18:53
Speaker 2: geometry surrounding a black hole tells distant observers that mass
18:57
Speaker 2: is present. If the black hole's total mass changes, the
19:02
Speaker 2: geometry associated with it can change as well. And that
19:05
Speaker 2: means a shrinking black hole does not necessarily require an
19:09
Speaker 2: object to travel outward through the event horizon. Its mass
19:13
Speaker 2: can decrease through a physical process involving energy outside that boundary,
19:18
Speaker 2: while the event horizon remains a one-way limit on escape.
19:23
Speaker 2: This is the distinction that makes Hawking radiation possible. A
19:27
Speaker 2: black hole is not an ordinary container whose contents must
19:31
Speaker 2: somehow leak through its walls. It's a region defined by
19:35
Speaker 2: spacetime itself, and the spacetime surrounding it is not merely
19:39
Speaker 2: an empty stage on which gravity operates. It has a geometry.
19:43
Speaker 2: It interacts with quantum fields and under the right theoretical
19:47
Speaker 2: conditions it can be associated with the outward emission of energy.
19:53
Speaker 2: An apparently permanent darkness can therefore possess a much more
19:56
Speaker 2: complicated physical life than its appearance suggests. There is something
20:01
Speaker 2: rather unusual about the boundary of a black hole. Unlike
20:05
Speaker 2: the surface of a planet or the outer layers of
20:07
Speaker 2: a star, it is not made of anything that could
20:10
Speaker 2: be touched, measured with a ruler, or examined under a microscope.
20:15
Speaker 2: It is a boundary defined by what the universe will
20:18
Speaker 2: allow to happen. And once something crosses it, the possibilities
20:22
Speaker 2: for where that object can travel are permanently changed. This
20:26
Speaker 2: boundary is called the event horizon. In physics, the event
20:30
Speaker 2: horizon is described as a causal boundary. The word causal
20:34
Speaker 2: refers to cause and effect, and particularly to whether one
20:38
Speaker 2: event can influence another. A flash of light, for example,
20:42
Speaker 2: can carry information from one location to another. A radio
20:47
Speaker 2: transmission can communicate a message across space. Even the gravitational
20:51
Speaker 2: effects of a changing distribution of matter propagate according to
20:55
Speaker 2: the causal rules of general relativity. But all these influences
21:00
Speaker 2: are limited by the speed of a light. Nothing can
21:03
Speaker 2: transmit information faster than light travels through a vacuum, approximately 300,000
21:09
Speaker 2: kilometres every second. And it is this fundamental restriction, combined
21:15
Speaker 2: with the geometry of spacetime, that makes a black holes
21:18
Speaker 2: event horizon so extraordinary. Albert Einstein's general theory of relativity
21:24
Speaker 2: describes gravity as a consequence of curved spacetime. Matter and
21:29
Speaker 2: energy influence that curvature. and the resulting geometry determines the
21:34
Speaker 2: paths that freely moving objects follow. A planet orbiting a
21:38
Speaker 2: star follows a path through curved space-time. A beam of
21:43
Speaker 2: light passing near a massive object is deflected because the
21:46
Speaker 2: geometry through which it travels is curved. Even when light
21:50
Speaker 2: travels through an extremely strong gravitational field, its speed measured
21:54
Speaker 2: by a nearby freely falling observer remains the familiar speed
21:58
Speaker 2: of light. What changes is the geometry of the possible
22:03
Speaker 2: paths available to it, and near a black hole, those
22:07
Speaker 2: paths behave in a remarkable way. Physicists often describe this
22:11
Speaker 2: using something called a light cone. A light cone is
22:15
Speaker 2: a mathematical representation of the directions in spacetime that light
22:19
Speaker 2: can travel from a particular event. Imagine a brief flash
22:23
Speaker 2: of light occurring at a single point. As time passes,
22:26
Speaker 2: the light spreads outward. In an ordinary three-dimensional picture, It
22:30
Speaker 2: forms an expanding sphere. But when time is included as
22:35
Speaker 2: another dimension, the expanding possibilities form the shape of a cone.
22:40
Speaker 2: The future light cone represents all the events that a
22:43
Speaker 2: light signal could potentially reach. Anything travelling slower than light
22:48
Speaker 2: must follow a path inside that cone. Anything outside it
22:52
Speaker 2: cannot be reached by a physical signal originating from the event.
22:56
Speaker 2: This gives spacetime a very precise structure. It determines not
23:00
Speaker 2: merely where an object happens to be, but where that
23:03
Speaker 2: object can possibly go. Far from a black hole, light
23:07
Speaker 2: cones allow signals to travel outward in many directions. Light
23:12
Speaker 2: emitted from a distant star can spread across space, eventually
23:16
Speaker 2: reaching planets, telescopes and observers many light-years away. Near a
23:21
Speaker 2: black hole, however, the curvature of spacetime changes the relationship
23:25
Speaker 2: between these possible directions and the distant universe. In the
23:29
Speaker 2: familiar diagrams used to represent black holes, the light cones
23:33
Speaker 2: appear increasingly tilted towards the interior. This is a useful
23:38
Speaker 2: visual description, although the appearance of that tilt depends on
23:41
Speaker 2: the coordinates used to draw the spacetime diagram. The essential
23:45
Speaker 2: physical fact is that, once inside the event horizon of
23:49
Speaker 2: an idealised non-rotating black hole, every possible future-directed path leads
23:54
Speaker 2: towards smaller values of the radial coordinate Even a beam
23:58
Speaker 2: of light directed as far outward as possible cannot reach
24:02
Speaker 2: the exterior. Its future remains inside the black hole. This
24:07
Speaker 2: is rather different from the familiar explanation involving escape velocity.
24:11
Speaker 2: You may have heard that a black hole is an
24:13
Speaker 2: object whose escape velocity exceeds the speed of light. For
24:17
Speaker 2: ordinary objects, escape velocity is the speed needed to travel
24:21
Speaker 2: away from a gravitational body without subsequently falling back, assuming
24:26
Speaker 2: no additional propulsion. Earth's escape velocity is approximately 11 kilometers
24:31
Speaker 2: per second. The sun's escape velocity at its visible surface
24:36
Speaker 2: is approximately 618 kilometers per second. The comparison suggests that
24:41
Speaker 2: if an object becomes sufficiently compact, the speed required to
24:45
Speaker 2: escape could become greater than the speed of light. And
24:49
Speaker 2: although this gives a useful introduction, it is not the
24:52
Speaker 2: complete relativistic explanation. Inside a black hole, Light is not
24:57
Speaker 2: struggling against gravity because it lacks sufficient speed. It already
25:01
Speaker 2: travels at the maximum locally permitted speed. The problem is
25:06
Speaker 2: that the structure of spacetime provides no future directed route
25:10
Speaker 2: leading back outside. An outward directed photon cannot simply accelerate
25:15
Speaker 2: harder and overcome this restriction. There is no additional speed
25:19
Speaker 2: available and no outward escape path for it to follow.
25:22
Speaker 2: The distinction becomes even more interesting when we consider time.
25:27
Speaker 2: Suppose an astronaut were falling towards a sufficiently large black hole,
25:31
Speaker 2: while a distant observer watched through a powerful telescope. As
25:35
Speaker 2: the astronaut approached the event horizon, light travelling outward from
25:39
Speaker 2: the astronaut would become increasingly gravitationally redshifted, its wavelength would stretch,
25:45
Speaker 2: and the signals reaching the distant observer would become progressively
25:49
Speaker 2: weaker and more delayed. In the usual description using Schwarzschild coordinates,
25:56
Speaker 2: The astronaut appears to approach the event horizon without ever
25:59
Speaker 2: quite crossing it. But that does not mean the astronaut
26:02
Speaker 2: personally experiences an endless journey. According to the astronaut's own clock,
26:08
Speaker 2: the event horizon can be crossed after a perfectly finite
26:11
Speaker 2: amount of time. The distant observer simply cannot continue receiving
26:16
Speaker 2: an ordinary sequence of signals showing the astronaut crossing and
26:20
Speaker 2: travelling further inward. The astronaut's experience is quite different from
26:25
Speaker 2: the increasingly faint and delayed picture visible far away, and
26:29
Speaker 2: for a sufficiently massive black hole, the crossing itself might
26:32
Speaker 2: be surprisingly uneventful. The difference in gravitational attraction between the
26:37
Speaker 2: astronaut's head and feet produces tidal forces which can stretch
26:43
Speaker 2: objects along one direction while compressing them along another. For
26:47
Speaker 2: smaller black holes, these tidal forces can become destructive before
26:51
Speaker 2: the horizon is reached, but the event horizon of a
26:54
Speaker 2: supermassive black hole can have relatively gentle tidal forces. An
26:59
Speaker 2: astronaut falling freely through such a horizon might not encounter
27:03
Speaker 2: anything locally remarkable at the precise moment of crossing. There
27:07
Speaker 2: is no solid surface to strike. The crucial change concerns
27:11
Speaker 2: which distant places can still be reached. There is also
27:14
Speaker 2: another boundary associated with black holes that is sometimes confused
27:18
Speaker 2: with the event horizon. It is called the photon sphere.
27:22
Speaker 2: For an idealised non-rotating black hole, the photon sphere lies
27:27
Speaker 2: at a radius one and a half times the Schwarzschild radius.
27:31
Speaker 2: At this distance, light can follow circular paths around the
27:34
Speaker 2: black hole. But these orbits are unstable. Even a tiny
27:38
Speaker 2: disturbance can cause a photon to move away from its
27:41
Speaker 2: circular path potentially escaping outward or falling inward. The photon
27:47
Speaker 2: sphere is therefore not the same as the event horizon.
27:50
Speaker 2: Outside the horizon, outward escape remains possible along suitable paths.
27:55
Speaker 2: Inside it, no future-directed light signal can reach the distant exterior.
28:01
Speaker 2: And this distinction brings us back to the remarkable prediction
28:04
Speaker 2: of Hawking radiation. Nothing in the standard theory requires ordinary
28:09
Speaker 2: particles to travel outward from within the event horizon. The
28:13
Speaker 2: event horizon remains a genuine causal boundary. Instead, the surprising
28:18
Speaker 2: possibility of radiation arises from the behaviour of quantum fields
28:23
Speaker 2: in the curved space-time associated with the black hole. The
28:27
Speaker 2: rule that nothing can escape from inside remains intact. What
28:30
Speaker 2: changes is our understanding of what can happen outside, and
28:34
Speaker 2: how the energy of the black hole itself can gradually
28:37
Speaker 2: be reduced. The boundary remains one way. even when the
28:41
Speaker 2: black hole is slowly losing mass. And that is what
28:44
Speaker 2: makes the physics so remarkable. For a long time, the
28:48
Speaker 2: laws of gravity appeared to give black holes a rather
28:51
Speaker 2: remarkable advantage over almost everything else in the universe. They
28:55
Speaker 2: could become more massive, but there seemed to be no
28:57
Speaker 2: ordinary way for them to become smaller. A star can
29:01
Speaker 2: lose mass through radiation and stellar winds. A planet can
29:06
Speaker 2: gradually lose parts of its atmosphere. even an isolated, hot
29:10
Speaker 2: object will eventually cool as it releases energy into its surroundings.
29:16
Speaker 2: But a black hole appeared to behave differently. According to
29:20
Speaker 2: classical general relativity, its event horizon permits matter and radiation
29:25
Speaker 2: to enter, while preventing anything inside from returning to the
29:29
Speaker 2: exterior universe. And if a black hole were completely isolated,
29:34
Speaker 2: with no surrounding matter, no incoming radiation, and no significant
29:39
Speaker 2: changes in its gravitational environment, there was no obvious mechanism
29:44
Speaker 2: that would cause it to lose mass. It could simply
29:47
Speaker 2: remain there, not necessarily forever in every conceivable theory, but
29:52
Speaker 2: indefinitely within the idealised classical descriptions. This understanding emerged gradually
29:59
Speaker 2: as physicists investigated the remarkable predictions of Albert Einstein's gravitational equations.
30:06
Speaker 2: During the 1960s, the mathematical study of black holes developed considerably.
30:12
Speaker 2: One of the most influential figures was the British mathematical
30:15
Speaker 2: physicist Roger Penrose. In 1965, Penrose published an important theorem
30:21
Speaker 2: concerning gravitational collapse and spacetime singularities. His work demonstrated that,
30:27
Speaker 2: under certain physically reasonable assumptions, gravitational collapse could produce an
30:32
Speaker 2: incomplete spacetime, even without requiring the perfectly symmetrical conditions used
30:38
Speaker 2: in some earlier calculations. In mathematical terms, this is known
30:42
Speaker 2: as geodesic incompleteness. It means that certain paths through spacetime
30:47
Speaker 2: cannot be extended indefinitely within the classical description. The results
30:52
Speaker 2: strengthened the scientific case that black holes were not merely
30:55
Speaker 2: peculiar mathematical curiosities. They could be genuine consequences of gravitational collapse.
31:02
Speaker 2: But the same classical equations also suggested that once an
31:06
Speaker 2: event horizon had formed, the black hole would behave as
31:09
Speaker 2: a remarkably efficient absorber of whatever crossed it. Consider a
31:13
Speaker 2: black hole surrounded by a cloud of gas. Some of
31:17
Speaker 2: that gas might gradually spiral inward. losing energy and angular
31:21
Speaker 2: momentum as it moves closer to the event horizon. Eventually,
31:25
Speaker 2: part of the material may cross the horizon and contribute
31:29
Speaker 2: to the black hole's mass. This process is known as accretion,
31:33
Speaker 2: and accretion is one of the principal ways that black
31:36
Speaker 2: holes grow. In some environments, the incoming gas forms an
31:41
Speaker 2: accretion disk, a flattened, rotating structure surrounding the black hole.
31:46
Speaker 2: The material within the disk can become extraordinarily hot as
31:50
Speaker 2: gravitational energy is converted into other forms of energy through
31:54
Speaker 2: processes involving turbulence, magnetic fields and the movement of gas.
32:00
Speaker 2: The resulting radiation can be remarkably intense. Some accreting black
32:05
Speaker 2: holes produce more observable radiation from their surroundings than enormous
32:10
Speaker 2: populations of ordinary stars. The brightest examples are associated with
32:15
Speaker 2: active galactic nuclei and quasars, where matter falling towards supermassive
32:20
Speaker 2: black holes can generate extraordinary luminosities. But there is an
32:24
Speaker 2: important distinction. The radiation from an accretion disk is not light,
32:30
Speaker 2: escaping from inside the event horizon. It originates in material
32:34
Speaker 2: that remains outside that boundary. The black hole itself can
32:38
Speaker 2: remain perfectly dark in the classical description. even while its
32:42
Speaker 2: surroundings shine brilliantly, rather like an invisible object surrounded by
32:47
Speaker 2: an exceptionally enthusiastic collection of light bulbs. Black holes can
32:52
Speaker 2: also change their mass through collisions with other black holes.
32:56
Speaker 2: When two black holes orbit one another, their accelerating gravitational
33:00
Speaker 2: fields produce gravitational waves. These are propagating disturbances in spacetime
33:06
Speaker 2: predicted by Einstein's general theory of relativity. As the waves
33:11
Speaker 2: carry energy away, the orbiting black holes gradually lose orbital
33:15
Speaker 2: energy and move closer together. Eventually, they can merge into
33:20
Speaker 2: a single larger black hole. But the resulting black hole
33:24
Speaker 2: does not necessarily contain the full sum of the original masses.
33:29
Speaker 2: Some of the system's mass energy escapes as gravitational waves.
33:33
Speaker 2: On the 14th of September 2015, the Laser Interferometer Gravitational
33:39
Speaker 2: Wave Observatory made the first direct detection of gravitational waves
33:43
Speaker 2: from a black hole merger. The event involved two black
33:47
Speaker 2: holes with masses of approximately 36 and 29 times the
33:51
Speaker 2: mass of the Sun. After merging, the resulting black hole
33:54
Speaker 2: contained approximately 62 solar masses. The equivalent of roughly three
34:00
Speaker 2: solar masses had been carried away as gravitational wave energy.
34:04
Speaker 2: That is an almost unimaginable amount of energy released in
34:07
Speaker 2: a very short time. and yet the event did not
34:11
Speaker 2: involve anything escaping outward through an event horizon. The gravitational
34:16
Speaker 2: waves were generated by the changing gravitational geometry of the
34:19
Speaker 2: merging system. This provides a useful reminder that black holes
34:24
Speaker 2: can lose mass energy through classical processes without violating the
34:28
Speaker 2: one-way nature of their horizons. There is another possibility involving
34:33
Speaker 2: rotating black holes. Most astrophysical black holes are expected to
34:37
Speaker 2: possess at least some angular momentum inherited from the matter
34:42
Speaker 2: and systems that formed them. A rotating black hole is
34:45
Speaker 2: described by a spacetime geometry different from that of a
34:49
Speaker 2: non-rotating Schwarzschild black hole. In particular, it possesses a region
34:54
Speaker 2: outside the event horizon called the ergosphere. Within this region,
34:59
Speaker 2: spacetime exhibits a phenomenon known as frame-dragging. in which the
35:03
Speaker 2: rotation of the black hole influences the motion of nearby objects.
35:09
Speaker 2: In 1969, Roger Penrose proposed a theoretical method for extracting
35:13
Speaker 2: rotational energy from a black hole. The Penrose process involves
35:17
Speaker 2: the behaviour of particles within the ergosphere, allowing energy to
35:21
Speaker 2: be transferred away, while the black hole loses some of
35:24
Speaker 2: its rotational energy. Related electromagnetic processes can also extract energy
35:30
Speaker 2: from rotating black holes, through interactions involving magnetic fields and
35:34
Speaker 2: surrounding plasma. These ideas help explain how rotating black holes
35:39
Speaker 2: may power some of the enormous jets observed around active
35:42
Speaker 2: galactic nuclei. But once again, the energy is not being
35:47
Speaker 2: released by ordinary matter climbing outward from inside the event horizon.
35:52
Speaker 2: These are processes associated with the rotating black hole and
35:55
Speaker 2: its exterior environment, and they are different from the evaporation
35:59
Speaker 2: predicted by Stephen Hawking. A black hole, losing energy through
36:04
Speaker 2: a merger, requires another black hole and a changing gravitational system.
36:10
Speaker 2: Extracting rotational energy requires a spinning black hole and an
36:13
Speaker 2: appropriate physical mechanism. Accretion, meanwhile, generally increases a black hole's
36:19
Speaker 2: mass rather than reducing it. Hawking radiation presents a fundamentally
36:24
Speaker 2: different possibility. It predicts that even an isolated black hole,
36:29
Speaker 2: with no rotation, no electrical charge, and no surrounding material,
36:33
Speaker 2: can emit radiation because of the quantum behaviour of fields
36:37
Speaker 2: in curved spacetime. Classical general relativity alone does not produce
36:41
Speaker 2: that thermal emission. And this is why Hawking's discovery was
36:45
Speaker 2: so unexpected. The classical picture allowed black holes to absorb matter,
36:50
Speaker 2: participate in mergers, and exchange energy through particular gravitational processes.
36:57
Speaker 2: but an isolated, stationary black hole seemed to have no
37:00
Speaker 2: reason to radiate away its own mass. There was, however,
37:04
Speaker 2: a peculiar mathematical clue hidden within the classical theory. As
37:09
Speaker 2: physicists examined the behaviour of event horizons, they began finding
37:13
Speaker 2: relationships that looked surprisingly similar to the laws governing heat,
37:18
Speaker 2: energy and entropy. The geometry of a black hole appeared
37:22
Speaker 2: to possess something resembling the mathematics of thermodynamics. At first,
37:28
Speaker 2: this seemed like an interesting coincidence. After all, a black
37:31
Speaker 2: hole was not supposed to have an ordinary temperature, and
37:34
Speaker 2: it certainly was not expected to glow. But those mathematical
37:38
Speaker 2: similarities were pointing towards something much deeper. Even an object
37:43
Speaker 2: that appears perfectly dark can have properties that connect it
37:47
Speaker 2: to the physics of heat. An understanding that connection would
37:51
Speaker 2: eventually reveal why a black hole might lose mass without
37:54
Speaker 2: ever allowing anything to escape from within its horizon. There
37:58
Speaker 2: is a rather curious connection between black holes and something
38:02
Speaker 2: as ordinary as a warm cup of tea. At first glance,
38:05
Speaker 2: they seem to have very little in common. A cup
38:08
Speaker 2: of tea gradually cools as heat moves into the surrounding air.
38:12
Speaker 2: A black hole, according to classical physics, cannot release anything
38:16
Speaker 2: from within its event horizon. And yet, during the 1970s,
38:21
Speaker 2: physicists began discovering that the mathematical rules governing black holes
38:26
Speaker 2: looked remarkably similar to the laws governing heat. The connection
38:30
Speaker 2: emerged from thermodynamics. Thermodynamics is the branch of physics concerned
38:35
Speaker 2: with energy, heat, temperature, and the ways energy moves between
38:40
Speaker 2: physical systems. It also deals with a particularly important quantity
38:44
Speaker 2: called entropy. Entropy is often described as a measure of disorder,
38:50
Speaker 2: although that familiar explanation only captures part of its meaning.
38:55
Speaker 2: More precisely, entropy is related to the number of microscopic
38:59
Speaker 2: arrangements compatible with the overall physical condition of a system.
39:04
Speaker 2: Consider a container filled with gas. The gas contains an
39:08
Speaker 2: enormous number of molecules moving in different directions at different speeds.
39:13
Speaker 2: There are countless possible arrangements of those molecules that would
39:16
Speaker 2: produce approximately the same temperature, pressure and density. Entropy helps
39:21
Speaker 2: describe the statistical possibilities available to the system, and one
39:26
Speaker 2: of the most fundamental principles of thermodynamics concerns what happens
39:30
Speaker 2: to entropy over time. It is called the second law
39:33
Speaker 2: of thermodynamics. In its familiar statistical interpretation, the second law
39:39
Speaker 2: tells us that the total entropy of an isolated system
39:42
Speaker 2: does not decrease in ordinary macroscopic processes. A hot object
39:47
Speaker 2: placed in a cooler room will naturally transfer energy to
39:50
Speaker 2: its surroundings until thermal equilibrium is approached. The reverse process
39:55
Speaker 2: in which the cooler room spontaneously transfers enough energy to
39:59
Speaker 2: make the already hot object even hotter is extraordinarily improbable.
40:05
Speaker 2: This preferred direction of ordinary thermodynamic change is closely connected
40:09
Speaker 2: with entropy, but black holes appear to create a serious difficulty.
40:14
Speaker 2: Imagine a container of warm gas falling towards a black hole.
40:18
Speaker 2: The gas possesses energy, and it also possesses entropy. As
40:23
Speaker 2: the container crosses the event horizon, it disappears from the
40:26
Speaker 2: accessible exterior universe. Its energy contributes to the black hole's mass.
40:32
Speaker 2: But what happens to its entropy? If the black hole
40:35
Speaker 2: itself possesses no entropy, then the entropy carried by the
40:39
Speaker 2: gas appears to have vanished from the world available to
40:42
Speaker 2: outside observers. That would create an uncomfortable conflict with the
40:46
Speaker 2: second law of thermodynamics. The problem attracted the attention of
40:50
Speaker 2: a young physicist named Jacob Bekenstein. During the early 1970s,
40:56
Speaker 2: Bekenstein proposed an extraordinary solution. Perhaps black holes themselves possess entropy.
41:02
Speaker 2: Perhaps the entropy associated with falling matter is not simply
41:06
Speaker 2: lost from the thermodynamic description of the universe, but is
41:10
Speaker 2: accounted for by a change in the black hole. And
41:13
Speaker 2: Bekenstein suggested something even more surprising. The entropy of a
41:17
Speaker 2: black hole appeared to be related to the area of
41:20
Speaker 2: its event horizon. Not the volume enclosed by that horizon.
41:24
Speaker 2: The area. For an ordinary container, we might expect the
41:28
Speaker 2: number of possible microscopic arrangements to depend strongly on how
41:32
Speaker 2: much space is available inside it. A larger box can
41:35
Speaker 2: accommodate more particles and more possible arrangements. But Bekenstein's proposal
41:40
Speaker 2: suggested that black hole entropy increases in proportion to the
41:44
Speaker 2: surface area of the event horizon. For a spherical black hole,
41:48
Speaker 2: this is the area of the imaginary sphere marking the horizon,
41:52
Speaker 2: and it leads to a remarkable scaling relationship. If the
41:55
Speaker 2: mass of an idealised non-rotating black hole doubles, its Schwarzschild
42:00
Speaker 2: radius doubles. But because the area of a sphere depends
42:04
Speaker 2: on the square of its radius, the event horizon area
42:07
Speaker 2: becomes four times larger. The black hole's proposed entropy therefore
42:11
Speaker 2: becomes four times greater. This unusual relationship was connected to
42:16
Speaker 2: another discovery. Stephen Hawking had already established an important result
42:21
Speaker 2: in classical general relativity, known as the black hole area theorem.
42:26
Speaker 2: Under particular assumptions about the behaviour of matter and spacetime,
42:30
Speaker 2: the theorem states that the total area of black hole
42:33
Speaker 2: event horizons cannot decrease. Matter falling into a black hole
42:38
Speaker 2: can increase its horizon area. When black holes merge, the
42:42
Speaker 2: area of the resulting horizon must be at least as
42:45
Speaker 2: large as the combined areas of the original horizons within
42:48
Speaker 2: the classical assumptions of the theorem. The resemblance to the
42:52
Speaker 2: second law of thermodynamics was difficult to ignore. Ordinary entropy
42:57
Speaker 2: does not decrease in an isolated system, and classical black
43:01
Speaker 2: hole horizon area does not decrease under the conditions of
43:04
Speaker 2: Hawking's theorem. Perhaps these were not entirely separate ideas. In 1973,
43:11
Speaker 2: James Bardeen, Brandon Carter and Stephen Hawking published a detailed
43:16
Speaker 2: investigation of what became known as the Four Laws of
43:19
Speaker 2: Black Hole Mechanics. Their work identified mathematical relationships between black
43:24
Speaker 2: hole mass, horizon area, angular momentum, electrical charge and another
43:30
Speaker 2: quantity called surface gravity. These relationships closely resembled the familiar
43:35
Speaker 2: laws of thermodynamics. One of the most intriguing involves surface gravity.
43:41
Speaker 2: On Earth, surface gravity describes the gravitational acceleration experienced near
43:46
Speaker 2: the ground. For a black hole, however, the concept requires
43:50
Speaker 2: a more careful definition. There is no solid surface on
43:54
Speaker 2: which an observer can stand, and the acceleration required to
43:58
Speaker 2: hover stationary just outside a non-rotating black hole's event Horizon
44:03
Speaker 2: grows without bound as the horizon is approached. The surface
44:07
Speaker 2: gravity used in black hole mechanics is instead a particular
44:10
Speaker 2: relativistic quantity, defined with an appropriate normalisation for the stationary
44:16
Speaker 2: black hole spacetime. It characterises important properties of the horizon,
44:21
Speaker 2: and remains finite for an ordinary Schwarzschild black hole. The
44:25
Speaker 2: four laws revealed that surface gravity played a mathematical role
44:29
Speaker 2: resembling temperature. Meanwhile, horizon area played a role resembling entropy.
44:35
Speaker 2: Changes in black hole mass could be related to changes
44:38
Speaker 2: in area, rotation and electrical charge, much as thermodynamics relates
44:44
Speaker 2: energy changes to heat and other forms of work. There
44:48
Speaker 2: was even a counterpart to the thermodynamic principle that temperature
44:52
Speaker 2: becomes uniform throughout a system in equilibrium. For a stationary
44:56
Speaker 2: black hole, The appropriately defined surface gravity is constant across
45:01
Speaker 2: its event horizon. The mathematics fitted together remarkably well. But
45:06
Speaker 2: there was a substantial problem. If black holes genuinely possessed
45:10
Speaker 2: thermodynamic entropy, should they not also possess a genuine temperature?
45:16
Speaker 2: And if a black hole had a non-zero temperature, would
45:18
Speaker 2: it not be capable of emitting thermal radiation? Classical general
45:23
Speaker 2: relativity did not provide such a mechanism. an isolated stationary
45:27
Speaker 2: black hole was expected to remain dark. For this reason,
45:31
Speaker 2: the thermodynamic relationships initially appeared to be mathematical analogies rather
45:37
Speaker 2: than literal descriptions of heat and temperature. Nevertheless, Bekenstein's proposal
45:42
Speaker 2: offered a way to address the apparent disappearance of entropy
45:45
Speaker 2: behind the horizon. It led to the generalised second law
45:49
Speaker 2: of thermodynamics. According to this idea, The relevant entropy balance
45:54
Speaker 2: includes both the entropy of matter and radiation outside black
45:58
Speaker 2: holes and the entropy associated with the black holes themselves.
46:02
Speaker 2: Entropy carried inward by falling matter can be accompanied by
46:05
Speaker 2: an increase in black hole entropy. The apparent violation of
46:08
Speaker 2: the second law is thereby avoided. But the proposal raised
46:12
Speaker 2: another question. What physical processes could justify assigning an actual
46:17
Speaker 2: entropy and temperature to an object that was not supposed
46:20
Speaker 2: to radiate? The answer emerged when Hawking investigated quantum fields
46:26
Speaker 2: in black hole spacetime. His calculations showed that black holes
46:30
Speaker 2: should produce thermal radiation, giving physical meaning to the temperature
46:34
Speaker 2: that the mathematical analogy had suggested. And when that temperature
46:38
Speaker 2: was combined with the laws of black hole mechanics, it
46:41
Speaker 2: established the precise relationship between black hole entropy and horizon area.
46:47
Speaker 2: The connection was no longer merely an interesting resemblance. it
46:51
Speaker 2: had become a prediction about the physical behaviour of black holes,
46:55
Speaker 2: and there was an especially remarkable consequence. If a black
46:58
Speaker 2: hole emitted radiation, it could lose mass, its event horizon
47:02
Speaker 2: could become smaller, and its area could decrease. This does
47:06
Speaker 2: not contradict Hawking's classical area theorem because the quantum effects
47:10
Speaker 2: responsible for the radiation fall outside the assumptions on which
47:14
Speaker 2: that classical result depends. Instead, The more complete thermodynamic picture
47:20
Speaker 2: accounts for entropy carried away by radiation as well as
47:24
Speaker 2: entropy remaining in the black hole. An object once thought
47:27
Speaker 2: to possess no temperature and no ordinary means of cooling
47:31
Speaker 2: had acquired a place within the physics of heat and
47:34
Speaker 2: the mystery of black hole evaporation had become inseparable from
47:38
Speaker 2: one of the oldest questions in thermodynamics. What happens to
47:42
Speaker 2: energy and to the information about a physical system as
47:46
Speaker 2: nature carries that system from one state to another. One
47:50
Speaker 2: of the strangest discoveries of modern physics is that empty
47:53
Speaker 2: space is not quite as empty as it appears. Imagine
47:57
Speaker 2: travelling far beyond the Earth, beyond the planets, and eventually
48:01
Speaker 2: into an enormous region of space where there are no stars,
48:05
Speaker 2: no clouds of gas, and almost no particles of matter.
48:09
Speaker 2: There might be nothing visible for millions of kilometres in
48:12
Speaker 2: every direction, And yet, according to quantum physics, this seemingly
48:17
Speaker 2: empty region is not an absolute nothingness. It still contains
48:22
Speaker 2: quantum fields. These fields are fundamental to our modern understanding
48:27
Speaker 2: of matter and energy, and their behaviour provides an essential
48:31
Speaker 2: part of the explanation for why black holes can emit radiation.
48:36
Speaker 2: The theory describing them is called quantum field theory. In
48:40
Speaker 2: everyday life, we tend to think of particles as tiny
48:43
Speaker 2: individual objects. An electron seems like a very small piece
48:47
Speaker 2: of matter. A photon seems like a little packet of light.
48:52
Speaker 2: And an atom appears to be a collection of particles
48:54
Speaker 2: held together by electromagnetic forces. This picture is useful, but
48:59
Speaker 2: quantum field theory provides a more fundamental description. Rather than
49:04
Speaker 2: treating particles as completely independent objects moving through otherwise empty space,
49:09
Speaker 2: The theory describes them as excitations of underlying fields. An
49:14
Speaker 2: electron is an excitation of the electron field. A photon
49:18
Speaker 2: is an excitation of the electromagnetic field. And other fundamental
49:23
Speaker 2: particles are associated with their own quantum fields. These fields
49:27
Speaker 2: are not confined to the locations where particles happen to
49:30
Speaker 2: be detected. They extend throughout spacetime. Even in a region
49:35
Speaker 2: containing no electrons, the electron field remains part of the
49:38
Speaker 2: physical description. and even in complete darkness, the electromagnetic field
49:43
Speaker 2: still exists. An absence of photons does not mean the
49:48
Speaker 2: electromagnetic field has somehow disappeared. This distinction leads to a
49:52
Speaker 2: fascinating question. What does a quantum field do when no
49:56
Speaker 2: particles are present? The answer involves something called the quantum vacuum.
50:02
Speaker 2: In ordinary conversation, a vacuum means a region containing little
50:06
Speaker 2: or no matter. but in quantum field theory the vacuum
50:09
Speaker 2: has a more specific meaning. It is the lowest energy
50:12
Speaker 2: state available to a quantum field under the conditions being considered,
50:17
Speaker 2: and being in that lowest energy state does not mean
50:20
Speaker 2: that every measurable property of the field has a perfectly
50:23
Speaker 2: definite value. Quantum physics does not generally allow that kind
50:28
Speaker 2: of complete certainty. This was one of the remarkable insights
50:32
Speaker 2: associated with the German physicist Werner Heisenberg, In 1927, Heisenberg
50:38
Speaker 2: introduced the uncertainty principle, which established fundamental limits on how
50:43
Speaker 2: precisely certain pairs of physical quantities can be specified together.
50:49
Speaker 2: Position and momentum provide the most familiar example. The more
50:53
Speaker 2: precisely the position of a quantum particle is specified, the
50:57
Speaker 2: less precisely its momentum can generally be specified at the
51:00
Speaker 2: same time. This is not simply a problem caused by
51:04
Speaker 2: imperfect measuring equipment. it is a property of quantum systems themselves.
51:09
Speaker 2: A related mathematical structure appears in quantum fields. Even in
51:14
Speaker 2: their lowest energy states, field observables can possess unavoidable quantum uncertainties.
51:20
Speaker 2: These are commonly described as vacuum fluctuations. The phrase can
51:25
Speaker 2: sound rather dramatic, as though empty space is constantly producing
51:29
Speaker 2: tiny objects that appear and disappear without explanation. but that
51:34
Speaker 2: familiar picture needs some care. Vacuum fluctuations do not mean
51:38
Speaker 2: that ordinary particles are continually being manufactured from nothing in
51:43
Speaker 2: a way that violates the conservation of energy. And the
51:46
Speaker 2: uncertainty principle does not provide permission to borrow energy temporarily
51:51
Speaker 2: before returning it a moment later. Energy conservation remains an
51:55
Speaker 2: essential part of quantum field theory under the appropriate physical conditions.
52:01
Speaker 2: What the theory actually describes is the behaviour of quantum states,
52:05
Speaker 2: including the uncertainties and correlations associated with fields even when
52:10
Speaker 2: no real particles are present. A vacuum can therefore possess
52:14
Speaker 2: physical properties without containing a permanent population of freely travelling particles.
52:20
Speaker 2: This becomes particularly interesting when we consider what physicists mean
52:24
Speaker 2: by a particle. In ordinary circumstances, far from strong gravitational effects,
52:31
Speaker 2: It is often straightforward to identify the particle excitations of
52:34
Speaker 2: a quantum field. But the distinction between a vacuum state
52:40
Speaker 2: and a state containing particles can become more subtle when
52:44
Speaker 2: spacetime is changing or when observers are moving differently. And
52:49
Speaker 2: this is where gravity enters the picture. During the 1960s,
52:53
Speaker 2: the American physicist Leonard Parker began investigating the behaviour of
52:58
Speaker 2: quantum fields in an expanding universe. Parker discovered that the
53:02
Speaker 2: expansion of spacetime could lead to particle creation in certain circumstances.
53:08
Speaker 2: A quantum field that began in a state identified as
53:11
Speaker 2: a vacuum could evolve so that observers at a later
53:14
Speaker 2: time would describe the field as containing particles. The changing
53:18
Speaker 2: geometry of the universe affected the relationship between the field's
53:22
Speaker 2: earlier and later states. This was an extraordinary result. It
53:27
Speaker 2: suggested that particle creation could be connected not merely to
53:31
Speaker 2: familiar interactions between matter and energy, but also to the
53:34
Speaker 2: changing structure of spacetime itself. The gravitational background could influence
53:40
Speaker 2: how quantum fields behaved, and a related insight emerged from
53:44
Speaker 2: the work of the physicist William Unruh. In 1976, Unruh
53:49
Speaker 2: investigated what happens when an observer accelerates through a quantum
53:53
Speaker 2: field that an ordinary inertial observer regards as being in
53:56
Speaker 2: its vacuum state. An inertial observer, moving without acceleration through
54:01
Speaker 2: otherwise flat spacetime, would describe the field as containing no particles.
54:07
Speaker 2: But an observer undergoing constant acceleration can describe that same
54:12
Speaker 2: vacuum in terms of a thermal distribution of particles. This
54:16
Speaker 2: is known as the Unruh effect. The predicted temperature depends
54:19
Speaker 2: on the observer's acceleration. Greater acceleration corresponds to a higher
54:24
Speaker 2: unruh temperature. For accelerations familiar from everyday life, the temperature
54:29
Speaker 2: is so incredibly small that detecting the effect directly would
54:33
Speaker 2: be extraordinarily difficult. Nevertheless, the theoretical implication is remarkable. Two
54:41
Speaker 2: observers can describe the same quantum field state differently because
54:45
Speaker 2: of their motion. One may identify a vacuum, while the
54:49
Speaker 2: other encounters a thermal response. This does not mean that
54:53
Speaker 2: particles are imaginary, or that observers can choose whatever physical
54:57
Speaker 2: reality they prefer. It means that the particle description of
55:01
Speaker 2: a quantum field is connected to the observer's motion and
55:05
Speaker 2: to the structure of spacetime. In the accelerated observer's description,
55:10
Speaker 2: a horizon also appears, separating events that can communicate with
55:15
Speaker 2: the observer from events that cannot. This creates an intriguing
55:19
Speaker 2: connection between acceleration, horizons and temperature, and black holes possess
55:26
Speaker 2: horizons of their own. The Unruh effect and Hawking radiation
55:31
Speaker 2: are not identical phenomena, but they reveal closely related aspects
55:35
Speaker 2: of quantum field theory. In both cases, the apparently simple
55:41
Speaker 2: concept of empty space becomes more complicated when observers and
55:45
Speaker 2: horizons are considered. A gravitational field does not need to
55:49
Speaker 2: behave like a machine that manufactures particles out of nothing. Instead,
55:53
Speaker 2: quantum fields respond to their spacetime environment, and the distinction
55:58
Speaker 2: between vacuum and particle excitations can depend on that environment.
56:03
Speaker 2: Einstein's classical general relativity describes the geometry of spacetime with
56:08
Speaker 2: extraordinary precision. Quantum field theory describes the behaviour of matter
56:13
Speaker 2: and radiation at the smallest scales. By placing quantum fields
56:18
Speaker 2: within curved spacetime, physicists can investigate phenomena that neither description
56:24
Speaker 2: reveals quite so clearly on its own. And near a
56:27
Speaker 2: black hole, this combination produces one of the most unexpected
56:31
Speaker 2: predictions in all of theoretical physics. A region that classical
56:36
Speaker 2: gravity describes as perfectly dark can nevertheless be associated with radiation.
56:42
Speaker 2: Not because particles have somehow escaped from inside the event horizon,
56:46
Speaker 2: but because the quantum fields surrounding the black hole possess
56:50
Speaker 2: a richer physical behaviour than classical empty space would suggest.
56:55
Speaker 2: The vacuum, it turns out, is not simply an absence
56:58
Speaker 2: of things. An understanding that subtle distinction changes what we
57:02
Speaker 2: can reasonably expect even the darkest objects in the universe
57:06
Speaker 2: to do. During the early 1970s, an extraordinary collection of
57:11
Speaker 2: ideas was beginning to emerge from the study of black holes.
57:15
Speaker 2: Physicists had discovered that black holes appear to obey mathematical
57:18
Speaker 2: laws resembling the laws of thermodynamics. They had found connections
57:23
Speaker 2: between event horizon area, entropy, and something rather similar to temperature.
57:29
Speaker 2: But there was still a considerable problem. According to classical
57:33
Speaker 2: general relativity, an isolated black hole was not supposed to
57:37
Speaker 2: emit anything. It could absorb radiation, but it could not
57:41
Speaker 2: release ordinary radiation from within its event horizon. and if
57:45
Speaker 2: it could not radiate, the suggestion that it possessed a
57:48
Speaker 2: genuine temperature seemed rather difficult to explain. At the University
57:53
Speaker 2: of Cambridge, a young theoretical physicist named Stephen Hawking was
57:58
Speaker 2: investigating precisely these questions. Hawking had already made important contributions
58:03
Speaker 2: to the mathematical understanding of gravitational collapse and black hole singularities.
58:09
Speaker 2: His work had helped establish that black holes were not
58:11
Speaker 2: merely unusual solutions to Einstein's equations, but could arise naturally
58:16
Speaker 2: from the behaviour of gravity under extreme conditions. Now, however,
58:20
Speaker 2: he was interested in something different. What would happen if
58:24
Speaker 2: quantum physics were introduced into the curved spacetime surrounding a
58:27
Speaker 2: black hole? The question sounds relatively straightforward, but the answer
58:32
Speaker 2: would overturn one of the most widely accepted assumptions about
58:36
Speaker 2: these extraordinary objects. Hawking began by considering the formation of
58:41
Speaker 2: a black hole through gravitational collapse. Imagine a massive star
58:47
Speaker 2: reaching the end of its life. Its core can no
58:49
Speaker 2: longer maintain sufficient pressure to resist gravity, and the central
58:53
Speaker 2: regions begin collapsing inward. Under suitable conditions, the collapse continues
58:59
Speaker 2: until an event horizon forms. Classical general relativity describes the
59:04
Speaker 2: gravitational geometry associated with this process, but Hawking considered what
59:09
Speaker 2: happened to quantum fields as the geometry changed. These fields
59:13
Speaker 2: exist throughout spacetime, even in regions that contain no ordinary particles,
59:19
Speaker 2: and the behaviour of those fields depends on the spacetime
59:22
Speaker 2: in which they exist. Hawking realised that the process of
59:26
Speaker 2: gravitational collapse could affect the relationship between what observers regarded
59:30
Speaker 2: as particles before the black hole formed and what they
59:33
Speaker 2: would observe afterwards. To understand the significance of this, it
59:38
Speaker 2: helps to return briefly to the idea of a quantum vacuum.
59:42
Speaker 2: In quantum field theory, the vacuum is the lowest energy
59:45
Speaker 2: state of a field under the conditions being considered. In
59:49
Speaker 2: an approximately flat region of spacetime, far from strong gravitational disturbances,
59:54
Speaker 2: it is possible to describe a field state containing no
59:57
Speaker 2: incoming particles, but a field that begins in such a
1:00:01
Speaker 2: state does not necessarily remain a vacuum, according to the
1:00:04
Speaker 2: particle description used by observers in the distant future. The
1:00:09
Speaker 2: change in gravitational geometry can alter the relationship between the
1:00:12
Speaker 2: earlier and later descriptions, and this was the remarkable possibility
1:00:17
Speaker 2: that Hawking investigated. In 1974, he published a short paper
1:00:22
Speaker 2: in the scientific journal Nature. Its title was Black Hole Explosions.
1:00:29
Speaker 2: The question mark was rather appropriate because the prediction was extraordinary.
1:00:33
Speaker 2: Hawking suggested that black holes could emit particles and gradually
1:00:37
Speaker 2: lose mass. The following year, he published a much more
1:00:41
Speaker 2: detailed mathematical treatment in the journal Communications in Mathematical Physics.
1:00:46
Speaker 2: That paper was called Particle Creation by Black Holes. It
1:00:50
Speaker 2: provided the theoretical foundation for what we now call Hawking radiation.
1:00:55
Speaker 2: The calculation involved examining the mathematical behaviour of quantum field modes.
1:01:01
Speaker 2: A mode is a particular wave pattern that can contribute
1:01:03
Speaker 2: to the description of a field. Much as a musical
1:01:07
Speaker 2: instrument can produce different patterns of vibration, a quantum field
1:01:12
Speaker 2: can be described using different mathematical wave patterns. The comparison
1:01:16
Speaker 2: is not exact, but it provides a useful way to
1:01:20
Speaker 2: think about the underlying idea. Hawking followed the relationship between
1:01:24
Speaker 2: field modes in the distant past, before the black hole
1:01:28
Speaker 2: had formed, and those associated with radiation reaching distant observers
1:01:32
Speaker 2: in the future. And he discovered something surprising. A state
1:01:38
Speaker 2: described as containing no incoming particles did not correspond to
1:01:42
Speaker 2: a state containing no outgoing particles. Instead, The later description
1:01:47
Speaker 2: contained a population of outgoing particles. Even more remarkably, that
1:01:52
Speaker 2: population possessed a thermal character. It resembled the distribution of
1:01:56
Speaker 2: radiation associated with a particular temperature. This was not an
1:02:00
Speaker 2: ordinary star glowing because its interior was hot. It was
1:02:04
Speaker 2: a prediction emerging from the interaction between quantum fields and
1:02:08
Speaker 2: black hole spacetime. One essential part of the calculation involved
1:02:12
Speaker 2: gravitational redshift. Light travelling outward through a gravitational field can
1:02:18
Speaker 2: lose energy as measured by observers at increasing distances, causing
1:02:23
Speaker 2: its frequency to decrease and its wavelength to increase. Near
1:02:27
Speaker 2: a forming black hole's event horizon, this effect becomes particularly important.
1:02:33
Speaker 2: Outgoing wave patterns that eventually reach distant observers are related
1:02:37
Speaker 2: to earlier field modes through an enormous gravitational redshift. For
1:02:42
Speaker 2: radiation detected at progressively later times, tracing the relevant modes
1:02:47
Speaker 2: backwards reveals an increasingly extreme relationship between their frequencies. The
1:02:52
Speaker 2: mathematical connection between the early and late wave patterns is
1:02:56
Speaker 2: what produces the characteristic particle distribution. More specifically, quantum field
1:03:02
Speaker 2: modes that are identified as positive frequency waves in one
1:03:06
Speaker 2: description can contain a mixture of positive frequency and negative
1:03:10
Speaker 2: frequency components in another. That mixture is central to the
1:03:14
Speaker 2: calculation of particle creation. It means that the quantum vacuum
1:03:18
Speaker 2: defined in the distant past need not be regarded as
1:03:21
Speaker 2: empty of particles by observers examining outgoing radiation in the
1:03:26
Speaker 2: distant future. And the resulting spectrum was not arbitrary. Its
1:03:31
Speaker 2: temperature was determined by a property of the black hole
1:03:34
Speaker 2: called surface gravity. This provided a remarkable connection with the
1:03:38
Speaker 2: earlier work on black hole thermodynamics. The mathematical quantity that
1:03:43
Speaker 2: had appeared to play the role of temperature in classical
1:03:46
Speaker 2: black hole mechanics was now associated with a genuine physical temperature.
1:03:51
Speaker 2: Hawking had found a reason for black holes to radiate,
1:03:54
Speaker 2: and because radiation carries energy, he had also found a
1:03:57
Speaker 2: reason for black holes to lose mass. The result was
1:04:01
Speaker 2: surprising even to Hawking himself. Classical black holes had appeared
1:04:05
Speaker 2: to be perfect absorbers, objects that could receive energy without
1:04:09
Speaker 2: emitting their own thermal radiation. Yet the quantum calculation suggested
1:04:14
Speaker 2: that even an isolated black hole could release a faint
1:04:17
Speaker 2: outward flow of energy. The apparent permanence of black holes
1:04:20
Speaker 2: was no longer guaranteed. There is an important detail about
1:04:24
Speaker 2: how Hawking reached this conclusion. He did not begin with
1:04:28
Speaker 2: a complete theory of quantum gravity. Such a theory would
1:04:31
Speaker 2: need to describe the gravitational field itself according to quantum principles,
1:04:36
Speaker 2: something physicists still have not fully achieved. Instead, Hawking used
1:04:41
Speaker 2: an approach now called quantum field theory in curved spacetime.
1:04:46
Speaker 2: The gravitational geometry was treated classically, using Einstein's general relativity.
1:04:52
Speaker 2: Quantum fields were then studied against that curved spacetime background.
1:04:56
Speaker 2: This is known as a semi-classical approach. It combines essential
1:05:01
Speaker 2: ingredients from two successful theories without claiming to provide a
1:05:05
Speaker 2: complete quantum description of gravity. and within its domain of applicability,
1:05:10
Speaker 2: the calculation produces an extraordinarily important prediction. Black holes possess
1:05:16
Speaker 2: a characteristic temperature and can emit Hawking radiation. For an
1:05:20
Speaker 2: ordinary stellar-mass black hole, however, that temperature is incredibly small.
1:05:26
Speaker 2: The predicted radiation is so weak that detecting it against
1:05:29
Speaker 2: the surrounding cosmic environment would be exceptionally difficult. No genuine
1:05:34
Speaker 2: astrophysical black hole has yet been directly observed emitting Hawking radiation,
1:05:40
Speaker 2: so the distinction between theoretical prediction and experimental confirmation remains important. Nevertheless,
1:05:47
Speaker 2: Hawking's calculation has become one of the most influential results
1:05:52
Speaker 2: in modern theoretical physics. It revealed that the combination of
1:05:56
Speaker 2: quantum fields and curved space-time leads to phenomena that neither
1:05:59
Speaker 2: classical gravity nor an overly simple picture of particles would
1:06:03
Speaker 2: have suggested. And perhaps most importantly, it did not require
1:06:07
Speaker 2: the fundamental rule of the event horizon to be abandoned.
1:06:11
Speaker 2: The radiation detected by a distant observer does not consist
1:06:15
Speaker 2: of ordinary particles that have somehow travelled outward from deep
1:06:20
Speaker 2: inside the black hole. No signal needs to escape through
1:06:23
Speaker 2: the horizon. Instead, the radiation emerges from the quantum behaviour
1:06:28
Speaker 2: of fields in the spacetime associated with the black hole.
1:06:32
Speaker 2: This distinction is subtle, but it resolves the apparent contradiction.
1:06:36
Speaker 2: A black hole can remain a region from which nothing
1:06:39
Speaker 2: inside can escape, while still being associated with an outward
1:06:43
Speaker 2: flow of radiation that gradually reduces its mass. And something
1:06:48
Speaker 2: that appeared perfectly black under classical physics can possess the
1:06:52
Speaker 2: faintest of thermal glows. Not because Einstein's description of the
1:06:57
Speaker 2: event horizon was simply wrong, but because quantum physics revealed
1:07:01
Speaker 2: that there was more happening in the surrounding spacetime than
1:07:04
Speaker 2: the classical description alone could explain. There is something wonderfully
1:07:08
Speaker 2: peculiar about the idea of a black hole producing radiation.
1:07:12
Speaker 2: The event horizon is supposed to be a boundary from
1:07:15
Speaker 2: which nothing can escape. Once matter or light has crossed it,
1:07:19
Speaker 2: there is no future path leading back into the surrounding universe.
1:07:23
Speaker 2: And yet, according to Stephen Hawking's calculations, an isolated black
1:07:27
Speaker 2: hole can gradually release energy into space. The obvious question
1:07:32
Speaker 2: is where that energy actually comes from. One of the
1:07:35
Speaker 2: most familiar explanations involves something called virtual particle pairs. You
1:07:40
Speaker 2: may have encountered the idea that empty space is constantly
1:07:43
Speaker 2: producing pairs of particles, which appear briefly before disappearing again.
1:07:49
Speaker 2: According to the popular explanation of Hawking radiation, One of
1:07:53
Speaker 2: these pairs happens to form near a black hole's event horizon.
1:07:57
Speaker 2: One particle falls inward, while the other escapes into the
1:08:01
Speaker 2: surrounding universe. The escaping particle becomes Hawking radiation, and the
1:08:06
Speaker 2: particle falling inward somehow reduces the black hole's mass. It
1:08:10
Speaker 2: is an appealing picture, and it has helped introduce millions
1:08:14
Speaker 2: of people to one of the most extraordinary predictions of
1:08:17
Speaker 2: modern physics. But there is an important difficulty. This is
1:08:22
Speaker 2: not a literal description of the process Hawking calculated. The
1:08:26
Speaker 2: virtual particle pair explanation is a simplified illustration intended to
1:08:31
Speaker 2: capture certain features of a much more complicated quantum phenomenon.
1:08:35
Speaker 2: In quantum field theory, virtual particles are mathematical elements that
1:08:40
Speaker 2: can appear in calculations. They are not necessarily ordinary, short-lived
1:08:45
Speaker 2: particles physically popping into existence at particular locations. and the
1:08:50
Speaker 2: original derivation of Hawking radiation does not depend on a
1:08:54
Speaker 2: pair of ordinary particles suddenly appearing on opposite sides of
1:08:57
Speaker 2: the event horizon. The actual explanation is more subtle. It
1:09:02
Speaker 2: begins with the quantum fields that exist throughout spacetime. These
1:09:06
Speaker 2: fields can occupy different quantum states, including states that observers
1:09:11
Speaker 2: identify as containing no particles. But the description of a
1:09:15
Speaker 2: particle depends on how the field's mathematical wave patterns are
1:09:18
Speaker 2: separated into different modes. And in curved or changing spacetime,
1:09:23
Speaker 2: that separation is not always the same for observers considering
1:09:27
Speaker 2: different regions or periods. This becomes particularly important when a
1:09:32
Speaker 2: massive star collapses to form a black hole. Before the collapse,
1:09:36
Speaker 2: quantum fields can be described using wave patterns associated with
1:09:40
Speaker 2: the earlier spacetime. Afterwards, The geometry has changed considerably, an
1:09:45
Speaker 2: event horizon has formed, and the relationship between the original
1:09:49
Speaker 2: field modes and those reaching distant observers is no longer straightforward.
1:09:54
Speaker 2: A quantum state that initially contained no incoming particles can
1:09:58
Speaker 2: therefore produce a non-zero population of outgoing particles in the
1:10:01
Speaker 2: distant future. That is the essential mathematical result behind Hawking radiations.
1:10:07
Speaker 2: The radiation is not explained by ordinary particles climbing outward
1:10:11
Speaker 2: through the event horizon. It emerges from the relationship between
1:10:15
Speaker 2: quantum fields and the changing gravitational geometry associated with black
1:10:20
Speaker 2: hole formation. And one of the most important ingredients is
1:10:24
Speaker 2: gravitational redshift. Light travelling outward from a strong gravitational field
1:10:29
Speaker 2: has a lower frequency when measured by distant observers than
1:10:33
Speaker 2: when measured locally near its point of emission. Its wavelength
1:10:37
Speaker 2: becomes longer. and the energy associated with each photon decreases.
1:10:42
Speaker 2: Near a forming black hole's event horizon, the relevant relationship
1:10:46
Speaker 2: between outgoing field modes and their earlier history involves an
1:10:50
Speaker 2: extraordinarily large redshift. Wave patterns detected at very late times
1:10:56
Speaker 2: can be traced backwards through the changing spacetime geometry, and
1:11:00
Speaker 2: their mathematical relationship with the original vacuum state produces the
1:11:04
Speaker 2: characteristic distribution of outgoing radiation. Rather remarkably, this distribution has
1:11:10
Speaker 2: a thermal character. It corresponds to a particular temperature determined
1:11:14
Speaker 2: by the black hole's properties. This is why Hawking radiation
1:11:18
Speaker 2: resembles the emission associated with a warm object, even though
1:11:22
Speaker 2: the black hole does not possess an ordinary glowing surface.
1:11:26
Speaker 2: The prediction arises from the behaviour of quantum fields in spacetime.
1:11:31
Speaker 2: There is another essential ingredient, however, quantum correlations. In quantum physics,
1:11:36
Speaker 2: different parts of a system can possess relationships that cannot
1:11:39
Speaker 2: be understood by treating every part as completely independent. Two-field
1:11:44
Speaker 2: excitations may be correlated even when they are associated with
1:11:48
Speaker 2: different regions. And in Hawking's description, the outgoing radiation is
1:11:53
Speaker 2: connected through quantum correlations to field degrees of freedom that
1:11:57
Speaker 2: are not accessible to distant observers. These are sometimes described
1:12:02
Speaker 2: as the partners of the outgoing Hawking particles. The language
1:12:06
Speaker 2: of partners is useful, provided we remember that it does
1:12:09
Speaker 2: not require little objects to be physically created together at
1:12:13
Speaker 2: a precise point on the horizon. The underlying relationship concerns
1:12:18
Speaker 2: the quantum state of the fields. When an outside observer
1:12:21
Speaker 2: has access only to part of that state, the radiation
1:12:25
Speaker 2: can appear thermal. This also helps explain why identifying the
1:12:29
Speaker 2: exact location where a Hawking particle was created is not
1:12:33
Speaker 2: always a well-defined question. The particle description depends on the
1:12:37
Speaker 2: quantum field modes and the observer's circumstances. There is no
1:12:41
Speaker 2: universally agreed picture in which every outgoing particle must have
1:12:45
Speaker 2: emerged from a single microscopic location immediately outside the event horizon,
1:12:51
Speaker 2: but the energy carried away by the radiation is very
1:12:53
Speaker 2: real within the semi-classical prediction. And that presents an important question.
1:12:59
Speaker 2: If radiation carries positive energy out into the universe, Where
1:13:03
Speaker 2: does the corresponding reduction in the black hole's energy appear?
1:13:08
Speaker 2: One useful description involves what physicists call a negative energy flux.
1:13:13
Speaker 2: In an appropriate stationary black hole spacetime, energy can be
1:13:17
Speaker 2: defined relative to the time measured by observers far away.
1:13:21
Speaker 2: This is sometimes called the conserved energy at infinity. In
1:13:26
Speaker 2: the semi-classical description of an evaporating black hole, the outward
1:13:31
Speaker 2: flow of positive energy is accompanied by an inward flux
1:13:34
Speaker 2: that can be described as carrying negative energy, relative to
1:13:38
Speaker 2: this definition. The negative energy contribution reduces the mass energy
1:13:43
Speaker 2: associated with the black hole. Now the phrase negative energy
1:13:47
Speaker 2: can sound rather suspicious. It does not mean that ordinary
1:13:51
Speaker 2: particles with negative rest mass are travelling around inside the
1:13:54
Speaker 2: black hole. Nor does it mean that nature has discovered
1:13:57
Speaker 2: a convenient way to create unlimited energy from nothing. The
1:14:01
Speaker 2: relevant quantities arise from the mathematical description of quantum fields
1:14:05
Speaker 2: in curved spacetime, including the energy and momentum associated with
1:14:10
Speaker 2: their quantum states. The negative energy flux is a feature
1:14:13
Speaker 2: of that description, not a collection of familiar particles behaving
1:14:17
Speaker 2: like ordinary matter with a minus sign attached. The essential
1:14:22
Speaker 2: point is that the energy accounting remains consistent. Positive energy
1:14:26
Speaker 2: travels outward as Hawking radiation. the energy associated with the
1:14:30
Speaker 2: black hole decreases correspondingly, and because mass and energy are related,
1:14:35
Speaker 2: the black hole gradually loses mass. For an isolated black hole,
1:14:40
Speaker 2: this can be understood through the relationship between the energy
1:14:43
Speaker 2: carried to distant observers and the changing gravitational mass of
1:14:47
Speaker 2: the system. There is no need for the event horizon
1:14:50
Speaker 2: to become temporarily permeable. There is no need for light
1:14:53
Speaker 2: to exceed its universal speed limit. and there is no
1:14:57
Speaker 2: requirement for an ordinary particle to travel from deep inside
1:15:00
Speaker 2: the black hole to the outside universe. The causal boundary
1:15:04
Speaker 2: remains intact. What changes is the energy associated with the
1:15:08
Speaker 2: black hole and the surrounding quantum fields. This is a
1:15:12
Speaker 2: particularly beautiful distinction, because it shows why Hawking radiation does
1:15:17
Speaker 2: not simply contradict Einstein's general relativity. The event horizon still
1:15:22
Speaker 2: prevents anything inside from sending an ordinary signal outward. but
1:15:27
Speaker 2: the quantum fields inhabiting black hole spacetime possess physical behaviour
1:15:32
Speaker 2: that classical gravity alone does not describe, and under the
1:15:36
Speaker 2: conditions of Hawking's calculation, that behaviour produces an outward flow
1:15:40
Speaker 2: of radiation. The result is an object that can remain
1:15:44
Speaker 2: completely inaccessible from within its horizon while gradually losing energy
1:15:48
Speaker 2: to the universe around it. The black hole does not
1:15:52
Speaker 2: need to release anything that has fallen inside. It simply
1:15:56
Speaker 2: needs to exist within a universe governed by quantum physics
1:16:00
Speaker 2: as well as gravity. And that is enough according to
1:16:03
Speaker 2: Hawking's remarkable prediction. For even a black hole to possess
1:16:07
Speaker 2: a faint and persistent glow, temperature is something we usually
1:16:11
Speaker 2: associate with objects that contain enormous numbers of moving particles.
1:16:16
Speaker 2: A cup of tea is warm because its molecules possess
1:16:19
Speaker 2: thermal energy. The surface of the sun is hot because
1:16:23
Speaker 2: the matter within it exists at temperatures of several thousand degrees.
1:16:27
Speaker 2: Even the faint background radiation filling the universe has a
1:16:31
Speaker 2: measurable temperature. But a black hole presents a rather unusual difficulty.
1:16:36
Speaker 2: It has no ordinary surface made from particles that can vibrate,
1:16:40
Speaker 2: collide or exchange heat with their surroundings. And according to
1:16:44
Speaker 2: classical general relativity, it should not release thermal radiation from
1:16:48
Speaker 2: within its event horizon. So the suggestion that a black
1:16:52
Speaker 2: hole possesses a temperature sounds almost contradictory. Yet this was
1:16:57
Speaker 2: one of the most remarkable consequences of Stephen Hawking's work.
1:17:01
Speaker 2: His calculations showed that the quantum fields associated with a
1:17:04
Speaker 2: black hole produce radiation with a characteristic thermal distribution, and
1:17:10
Speaker 2: whenever radiation has this kind of distribution, physicists can associate
1:17:14
Speaker 2: it with a temperature. This became known as the Hawking temperature.
1:17:19
Speaker 2: It is important to understand exactly what that means. A
1:17:22
Speaker 2: black hole's Hawking temperature is not the temperature of a
1:17:25
Speaker 2: solid surface, hidden somewhere at the event horizon. There is
1:17:30
Speaker 2: no ordinary surface there. Instead, it describes the characteristic temperature
1:17:35
Speaker 2: of the radiation predicted to reach observers very far away
1:17:38
Speaker 2: from the black hole, where the effects of its gravitational
1:17:42
Speaker 2: field have become comparatively weak. And that distinction matters because
1:17:47
Speaker 2: measurements of energy and temperature near a black hole are
1:17:50
Speaker 2: influenced by gravitational redshift. A stationary observer close to the
1:17:54
Speaker 2: event horizon would describe the radiation differently from an observer
1:17:59
Speaker 2: far away. The Hawking temperature provides a particularly useful reference
1:18:04
Speaker 2: because it describes the radiation in terms of a distant
1:18:07
Speaker 2: observer's measurements. And when physicists calculate this temperature, they discover
1:18:13
Speaker 2: something extraordinary. It depends on the mass of the black hole.
1:18:18
Speaker 2: But not in the way we might initially expect. A
1:18:20
Speaker 2: more massive black hole is colder. A less massive black
1:18:24
Speaker 2: hole is hotter. For an idealised black hole that is
1:18:27
Speaker 2: not rotating and carries no electrical charge, the mathematical relationship
1:18:32
Speaker 2: is remarkably simple. Hawking temperature is inversely proportional to the
1:18:37
Speaker 2: mass of the black hole. This means that increasing the
1:18:40
Speaker 2: mass reduces the temperature by the same factor. If the
1:18:44
Speaker 2: mass doubles, the Hawking temperature becomes half as large. If
1:18:48
Speaker 2: the mass increases tenfold, the temperature becomes one tenth of
1:18:52
Speaker 2: its original value, and if the mass decreases the temperature rises.
1:18:57
Speaker 2: It is a very different relationship from the one we
1:18:59
Speaker 2: might associate with ordinary stars, where greater mass can often
1:19:03
Speaker 2: mean more extreme temperatures within the interior. For black holes,
1:19:08
Speaker 2: The temperature arises from quantum field behaviour associated with the
1:19:12
Speaker 2: event horizon and the surrounding spacetime. It is not determined
1:19:16
Speaker 2: by the thermal motion of matter inside the object. The
1:19:20
Speaker 2: relationship can also be understood through a property called surface gravity.
1:19:25
Speaker 2: For a non-rotating black hole, surface gravity is a particular
1:19:29
Speaker 2: quantity describing the strength of the gravitational geometry associated with
1:19:33
Speaker 2: its horizon. It is not simply the acceleration experienced by
1:19:37
Speaker 2: someone standing on a solid surface. Indeed, there is nowhere
1:19:41
Speaker 2: to stand. But surface gravity provides an important connection between
1:19:45
Speaker 2: the geometry of a black hole and the temperature of
1:19:48
Speaker 2: the radiation it produces. The greater the surface gravity in
1:19:52
Speaker 2: this idealised description, the higher the Hawking temperature. And for
1:19:57
Speaker 2: a Schwarzschild black hole, surface gravity decreases as the mass increases.
1:20:03
Speaker 2: These relationships explain why enormous black holes are predicted to
1:20:07
Speaker 2: be so extraordinarily cold. Consider a black hole with approximately
1:20:11
Speaker 2: the mass of our Sun. The Sun is an enormous object,
1:20:15
Speaker 2: containing almost 2,000 trillion trillion kilograms of matter. If that
1:20:21
Speaker 2: mass were concentrated into a non-rotating black hole, the event
1:20:25
Speaker 2: horizon would have a radius of approximately 3 kilometres. and
1:20:28
Speaker 2: its Hawking temperature would be roughly 60 billionths of a
1:20:32
Speaker 2: Kelvin above absolute zero. That is an astonishingly small temperature.
1:20:38
Speaker 2: For comparison, the cosmic microwave background radiation filling the present-day
1:20:43
Speaker 2: universe has a temperature of approximately 2.7 Kelvin. This means
1:20:48
Speaker 2: the surrounding cosmic radiation is vastly warmer than the Hawking
1:20:52
Speaker 2: temperature of a solar-mass black hole. A black hole ten
1:20:56
Speaker 2: times more massive than the Sun would be colder still.
1:20:59
Speaker 2: Its Hawking temperature would be approximately six billionths of a Kelvin,
1:21:04
Speaker 2: and a supermassive black hole containing millions of solar masses
1:21:07
Speaker 2: would have an even smaller temperature. The larger the black
1:21:11
Speaker 2: hole becomes, the more incredibly faint its predicted thermal emission becomes.
1:21:17
Speaker 2: But the relationship also works in the opposite direction. Imagine
1:21:21
Speaker 2: a hypothetical black hole far smaller than the ones formed
1:21:25
Speaker 2: through ordinary stellar collapse. As its mass decreases, its Hawking
1:21:29
Speaker 2: temperature rises. At sufficiently small masses, the radiation could become
1:21:33
Speaker 2: much more energetic. This introduces another important connection between temperature
1:21:39
Speaker 2: and the behaviour of radiation. Hotter objects generally emit radiation
1:21:44
Speaker 2: with greater characteristic photon energies. A familiar example is a
1:21:49
Speaker 2: piece of metal being heated. At first, it releases energy
1:21:53
Speaker 2: largely as infrared radiation. invisible to human eyes. As its
1:21:58
Speaker 2: temperature rises sufficiently, it begins glowing red, and eventually its
1:22:02
Speaker 2: visible emission shifts towards shorter wavelengths. The precise mechanism producing
1:22:08
Speaker 2: Hawking radiation is different, but the relationship between thermal temperature
1:22:13
Speaker 2: and characteristic radiation energy remains important. Very cold black holes
1:22:19
Speaker 2: produce radiation whose characteristic particle energies are extraordinarily small For
1:22:25
Speaker 2: stellar mass black holes, the associated photon frequencies are extremely low.
1:22:31
Speaker 2: Their Hawking radiation would be exceptionally difficult to distinguish from
1:22:35
Speaker 2: the much stronger radiation already present in the surrounding environment.
1:22:39
Speaker 2: This is one reason astronomers have not directly observed Hawking
1:22:43
Speaker 2: radiation from an astrophysical black hole. There is, however, another subtlety.
1:22:48
Speaker 2: Although Hawking radiation is commonly described as thermal radiation, The
1:22:52
Speaker 2: spectrum measured by a distant observer is not exactly the
1:22:56
Speaker 2: same as that of an ideal blackbody. A blackbody is
1:22:59
Speaker 2: an idealised physical system whose thermal radiation follows a particular
1:23:04
Speaker 2: mathematical distribution determined by its temperature, but radiation associated with
1:23:09
Speaker 2: a black hole must travel through curved spacetime before reaching
1:23:13
Speaker 2: distant observers, and that surrounding spacetime affects the radiation. Different
1:23:18
Speaker 2: frequencies and different types of field excitations have different probabilities
1:23:23
Speaker 2: of travelling outward successfully. Some wave modes encounter an effective
1:23:27
Speaker 2: gravitational potential barrier, which can reflect or scatter part of
1:23:32
Speaker 2: their outward propagation. The mathematical corrections describing these effects are
1:23:37
Speaker 2: called grey body factors. The name is rather appropriate. An
1:23:42
Speaker 2: ideal black body represents a perfect thermal emitter. while a
1:23:46
Speaker 2: greybody has an emission spectrum modified by the way it
1:23:48
Speaker 2: interacts with radiation. In black hole physics, greybody factors modify
1:23:53
Speaker 2: the spectrum predicted to reach distant observers. They depend on
1:23:58
Speaker 2: the frequency and characteristics of the radiation, as well as
1:24:01
Speaker 2: the properties of the black hole. The resulting spectrum retains
1:24:05
Speaker 2: its connection to the Hawking temperature, but its detailed shape
1:24:09
Speaker 2: and total energy output are influenced by these additional effects.
1:24:14
Speaker 2: And this brings us to perhaps the strangest consequence of all.
1:24:17
Speaker 2: When an ordinary hot object loses energy, it usually cools down.
1:24:21
Speaker 2: A cup of tea left on a table gradually approaches
1:24:25
Speaker 2: room temperature. A glowing piece of metal becomes cooler as
1:24:29
Speaker 2: it radiates energy into its surroundings. But an isolated black
1:24:34
Speaker 2: hole undergoing Hawking evaporation behaves in the opposite way. As
1:24:38
Speaker 2: it emits radiation, it loses mass. As its mass decreases,
1:24:43
Speaker 2: its Hawking temperature increases, and as its temperature rises it
1:24:47
Speaker 2: generally emits energy more rapidly. The black hole becomes hotter
1:24:51
Speaker 2: as it loses energy. In thermodynamics, this unusual behaviour is
1:24:56
Speaker 2: associated with a negative heat capacity. It means that removing
1:25:01
Speaker 2: energy from the system causes its temperature to rise rather
1:25:04
Speaker 2: than fall. For an idealised Schwarzschild black hole, This is
1:25:09
Speaker 2: a direct consequence of the inverse relationship between Hawking temperature
1:25:13
Speaker 2: and mass, and it creates a remarkable feedback process. A
1:25:17
Speaker 2: very large black hole can remain extraordinarily cold, losing mass
1:25:22
Speaker 2: at an almost imperceptible rate, but a sufficiently small black
1:25:26
Speaker 2: hole can radiate more intensely, becoming hotter, and losing its
1:25:31
Speaker 2: remaining mass progressively faster. So a black hole's temperature is
1:25:35
Speaker 2: not merely an interesting mathematical curiosity. It determines the characteristic
1:25:40
Speaker 2: energies of its Hawking radiation and helps govern the rate
1:25:43
Speaker 2: at which the black hole can evaporate. Something that classical
1:25:47
Speaker 2: physics once described as perfectly dark has acquired a measurable
1:25:52
Speaker 2: theoretical temperature. And unlike almost every familiar cooling object, it
1:25:57
Speaker 2: is predicted to grow hotter as its energy slowly disappears.
1:26:01
Speaker 2: One of the most important discoveries in physics is that
1:26:04
Speaker 2: mass and energy are not entirely separate things. They are
1:26:07
Speaker 2: different expressions of the same fundamental physical quantity, and this
1:26:12
Speaker 2: relationship provides the essential reason why a black hole can
1:26:16
Speaker 2: become smaller when it releases radiation. To understand the connection,
1:26:21
Speaker 2: consider something considerably more familiar than a black hole. Imagine
1:26:25
Speaker 2: a perfectly sealed container floating through space. Inside it is
1:26:29
Speaker 2: a small battery connected to a light bulb. When the
1:26:33
Speaker 2: bulb is switched on, the battery gradually releases its stored
1:26:36
Speaker 2: chemical energy. converting some of that energy into light and heat.
1:26:41
Speaker 2: If the container is perfectly sealed, the radiation remains inside,
1:26:45
Speaker 2: and the total energy of the complete system does not change.
1:26:49
Speaker 2: But suppose we allow the light to escape through a
1:26:51
Speaker 2: small opening. Energy now leaves the container, and although the
1:26:56
Speaker 2: change would be far too small to notice on an
1:26:58
Speaker 2: ordinary set of scales, the total mass of the container
1:27:01
Speaker 2: and everything remaining inside it would decrease. This might seem surprising.
1:27:06
Speaker 2: After all, no substantial piece of matter has been removed.
1:27:09
Speaker 2: The container is still there, the battery and bulb are
1:27:12
Speaker 2: still present, but energy has escaped, and energy contributes to mass.
1:27:18
Speaker 2: This relationship was established by Albert Einstein in his theory
1:27:22
Speaker 2: of special relativity, published in 1905. It is expressed through
1:27:27
Speaker 2: one of the most famous equations in science, energy equals
1:27:31
Speaker 2: mass multiplied by the speed of light squared. The equation
1:27:35
Speaker 2: tells us that a relatively small amount of mass corresponds
1:27:38
Speaker 2: to an enormous quantity of energy. It also tells us
1:27:41
Speaker 2: that the energy stored within a physical system contributes to
1:27:45
Speaker 2: its total mass. A fully charged battery, for example, possesses
1:27:50
Speaker 2: slightly more mass than the same battery after its stored
1:27:53
Speaker 2: energy has been released into the surroundings. The difference is
1:27:56
Speaker 2: extraordinarily small, but it is real. And the same fundamental
1:28:01
Speaker 2: relationship applies when we consider the radiation emitted by a
1:28:05
Speaker 2: black hole. According to Stephen Hawking's calculation, quantum fields in
1:28:10
Speaker 2: black hole spacetime can produce an outward flow of radiation.
1:28:14
Speaker 2: That radiation carries energy. A photon, for example, has no
1:28:18
Speaker 2: rest mass, but it possesses energy and momentum. And when
1:28:22
Speaker 2: photons travel away from a black hole, they transport energy
1:28:26
Speaker 2: into the surrounding universe. For an isolated black hole that
1:28:30
Speaker 2: energy must be accounted for. It cannot simply appear without
1:28:34
Speaker 2: any corresponding change in the system. In the semi-classical description
1:28:39
Speaker 2: of Hawking evaporation, the energy carried outward is accompanied by
1:28:43
Speaker 2: a decrease in the black hole's mass. The black hole
1:28:47
Speaker 2: becomes very slightly lighter, and this is the essential mechanism
1:28:51
Speaker 2: behind its gradual evaporation. There is no requirement for a
1:28:55
Speaker 2: lump of matter to emerge from inside the event horizon.
1:28:58
Speaker 2: The black hole loses mass because its gravitational mass energy
1:29:02
Speaker 2: decreases as radiation carries energy away. But there is another
1:29:06
Speaker 2: interesting question hidden within this explanation. What exactly do we
1:29:11
Speaker 2: mean by the mass of a black hole? For an
1:29:13
Speaker 2: ordinary object, we might imagine placing it on a set
1:29:16
Speaker 2: of scales. That is not particularly practical for a black hole.
1:29:21
Speaker 2: There is no solid surface to support it, and approaching
1:29:24
Speaker 2: too closely could make the measurement rather difficult to complete. Fortunately,
1:29:29
Speaker 2: general relativity provides another way of understanding mass. The mass
1:29:33
Speaker 2: of an isolated black hole is reflected in the geometry
1:29:36
Speaker 2: of spacetime surrounding it. A distant observer can investigate the
1:29:41
Speaker 2: black hole's gravitational influence by studying the motion of nearby objects,
1:29:46
Speaker 2: the paths taken by light, and the structure of the
1:29:49
Speaker 2: surrounding gravitational field. For an idealised non-rotating black hole, the
1:29:54
Speaker 2: Schwarzschild solution relates this geometry directly to the black hole's mass.
1:30:00
Speaker 2: And one of its most important features is the Schwarzschild radius.
1:30:04
Speaker 2: The Schwarzschild radius determines the location of the event horizon
1:30:07
Speaker 2: for a stationary, non-rotating black hole. Its relationship with mass
1:30:12
Speaker 2: is particularly simple. The Schwarzschild radius is directly proportional to
1:30:17
Speaker 2: the mass of the black hole. Double the mass and
1:30:19
Speaker 2: the radius doubles. Halve the mass and the radius is halved.
1:30:24
Speaker 2: A black hole with the mass of the Sun has
1:30:26
Speaker 2: a Schwarzschild radius of approximately 3 kilometres. If its mass
1:30:31
Speaker 2: were reduced to half the Sun's mass, the corresponding Schwarzschild
1:30:34
Speaker 2: radius would be approximately 1.5 kilometres. And this allows us
1:30:39
Speaker 2: to understand what happens as a black hole gradually evaporates.
1:30:43
Speaker 2: As Hawking radiation carries energy away, the black hole's mass decreases.
1:30:48
Speaker 2: The gravitational geometry associated with that mass changes. and for
1:30:52
Speaker 2: a slowly evaporating, approximately spherical and non-rotating black hole, the
1:30:57
Speaker 2: horizon gradually becomes smaller. Strictly speaking, an evaporating black hole
1:31:02
Speaker 2: is not perfectly described by the stationary Schwarzschild solution. Its
1:31:07
Speaker 2: spacetime changes over time, and the precise location of an
1:31:10
Speaker 2: event horizon depends on the wider causal structure of that
1:31:14
Speaker 2: evolving spacetime. Nevertheless, when evaporation is extremely slow, The familiar
1:31:20
Speaker 2: Schwarzschild relationship provides an excellent approximation. The black hole loses
1:31:25
Speaker 2: a little mass, and its characteristic horizon radius becomes a
1:31:29
Speaker 2: little smaller. Now, this raises an apparently troublesome possibility. If
1:31:35
Speaker 2: the event horizon is shrinking, does that mean some of
1:31:38
Speaker 2: the matter previously trapped inside it has somehow escaped? Not
1:31:42
Speaker 2: at all. The event horizon is not a material shell
1:31:46
Speaker 2: surrounding a collection of objects It is a causal boundary
1:31:49
Speaker 2: defined by spacetime geometry. And as the gravitational mass energy
1:31:55
Speaker 2: associated with the black hole changes, the geometry and its
1:31:58
Speaker 2: boundaries can evolve. A shrinking horizon does not require ordinary
1:32:03
Speaker 2: particles to cross outward through it, nor does it mean
1:32:06
Speaker 2: that an object already trapped inside has discovered a route
1:32:09
Speaker 2: back to the distant universe. The prohibition against outward escape
1:32:13
Speaker 2: remains intact. What changes is the gravitational structure of the
1:32:18
Speaker 2: black hole as a whole. There is, however, an important
1:32:21
Speaker 2: subtlety concerning energy conservation. In everyday physics, we often describe
1:32:26
Speaker 2: energy as a quantity that can be transferred between objects
1:32:29
Speaker 2: without being created or destroyed. This principle remains fundamental, but
1:32:35
Speaker 2: its mathematical expression becomes more complicated in general relativity. Einstein's
1:32:41
Speaker 2: theory describes the local conservation of energy and momentum through
1:32:44
Speaker 2: the behaviour of the energy-momentum tensor in curved spacetime. In
1:32:48
Speaker 2: a sufficiently small region, the usual conservation principles retain their
1:32:53
Speaker 2: familiar meaning, but defining one universal quantity called the total
1:32:57
Speaker 2: energy of the entire universe is not always possible in
1:33:01
Speaker 2: an arbitrary changing spacetime. The geometry itself can evolve, and
1:33:06
Speaker 2: different spacetimes possess different mathematical symmetries. For a suitably isolated
1:33:11
Speaker 2: black hole, however, useful definitions of mass and energy are
1:33:15
Speaker 2: available to distant observers. In particular, physicists can describe the
1:33:20
Speaker 2: energy carried away by outgoing radiation and relate it to
1:33:24
Speaker 2: the declining mass associated with the remaining black hole. The
1:33:28
Speaker 2: mass measured for the black hole decreases, while the escaping
1:33:31
Speaker 2: radiation contributes energy to the wider system. The energy has
1:33:35
Speaker 2: not emerged without explanation. It has been transferred. In the
1:33:40
Speaker 2: semi-classical description, this balance can also be represented through an
1:33:44
Speaker 2: inward negative energy flux, relative to the energy definition associated
1:33:49
Speaker 2: with distant observers. This does not mean ordinary particles with
1:33:53
Speaker 2: negative rest mass are falling into the black hole. It
1:33:56
Speaker 2: is a feature of the quantum field's energy distribution in
1:33:59
Speaker 2: curved spacetime. The important result is that the outgoing positive
1:34:03
Speaker 2: energy radiation corresponds to a reduction in the black hole's
1:34:06
Speaker 2: mass energy. And once that reduction begins, something rather unusual happens.
1:34:13
Speaker 2: A smaller black hole has a higher Hawking temperature. As
1:34:17
Speaker 2: the mass decreases, the characteristic temperature increases. And as the
1:34:21
Speaker 2: temperature rises, the predicted radiation generally becomes more energetic and
1:34:25
Speaker 2: more intense. The rate of mass loss can therefore increase
1:34:29
Speaker 2: as the black hole becomes smaller. For a stellar mass
1:34:32
Speaker 2: black hole, the process would initially be almost unimaginably slow.
1:34:36
Speaker 2: its Hawking radiation would carry away an extraordinarily tiny amount
1:34:41
Speaker 2: of energy compared with the total mass energy stored in
1:34:44
Speaker 2: the black hole. In realistic surroundings, incoming radiation and falling
1:34:49
Speaker 2: matter can also increase its mass, potentially overwhelming the energy
1:34:53
Speaker 2: lost through Hawking emission. But for a sufficiently isolated black hole,
1:34:59
Speaker 2: the basic theoretical relationship remains. Radiation carries energy outward. The
1:35:04
Speaker 2: black hole loses mass. Its gravitational geometry changes, its horizon
1:35:10
Speaker 2: becomes smaller and its Hawking temperature rises. And throughout this
1:35:15
Speaker 2: process nothing needs to escape from inside the event horizon.
1:35:19
Speaker 2: The black hole is not slowly emptying itself like a
1:35:22
Speaker 2: container with a leak. It is losing gravitational mass energy
1:35:26
Speaker 2: through the quantum behaviour of fields in the spacetime surrounding it.
1:35:31
Speaker 2: Which means that even an object defined by the impossibility
1:35:34
Speaker 2: of escape can escape. in principle, gradually surrender its mass
1:35:39
Speaker 2: to the wider universe. There is a rather extraordinary feature
1:35:42
Speaker 2: of black hole evaporation that makes it unlike almost any
1:35:46
Speaker 2: familiar process of cooling. The smaller a black hole becomes,
1:35:51
Speaker 2: the faster it loses the mass it has left. For
1:35:54
Speaker 2: an ordinary object, losing energy generally means becoming cooler. A
1:35:59
Speaker 2: hot stone gradually releases heat into its surroundings, and as
1:36:02
Speaker 2: its temperature falls, the rate of energy loss usually decreases.
1:36:07
Speaker 2: A black hole behaves in precisely the opposite way. According
1:36:11
Speaker 2: to Stephen Hawking's prediction, an isolated black hole becomes hotter
1:36:16
Speaker 2: as it loses mass, and that rising temperature means its
1:36:20
Speaker 2: radiation becomes progressively more energetic. The result is a curious
1:36:25
Speaker 2: feedback process. A black hole emits radiation. That radiation carries
1:36:30
Speaker 2: energy away. reducing the black hole's mass. The reduction in
1:36:35
Speaker 2: mass increases its Hawking temperature, and the higher temperature generally
1:36:39
Speaker 2: causes it to radiate energy more rapidly. The faster it radiates,
1:36:44
Speaker 2: the faster it loses mass, and the process continues. For
1:36:48
Speaker 2: an idealised black hole that is not rotating and has
1:36:51
Speaker 2: no electrical charge, the relationship between mass and radiation output
1:36:56
Speaker 2: can be expressed mathematically. Physicists refer to the total energy
1:37:01
Speaker 2: emitted each second as luminosity, or radiated power. And for
1:37:06
Speaker 2: a black hole emitting Hawking radiation, the luminosity depends approximately
1:37:10
Speaker 2: on the inverse square of its mass, provided the available
1:37:14
Speaker 2: particle species and other relevant conditions remain unchanged. Hawking luminosity
1:37:19
Speaker 2: is approximately inversely proportional to the square of the black
1:37:23
Speaker 2: hole's mass. This means that if a black hole's mass
1:37:26
Speaker 2: becomes half as large, its predicted radiation output becomes approximately
1:37:31
Speaker 2: four times greater. If its mass decreases to one-tenth of
1:37:35
Speaker 2: its original value, the output becomes roughly 100 times greater.
1:37:40
Speaker 2: And if its mass falls to one-hundredth, its radiation output
1:37:44
Speaker 2: increases by approximately 10,000 times. These relationships reveal something remarkable.
1:37:50
Speaker 2: A black hole containing an enormous amount of mass may
1:37:54
Speaker 2: produce almost no detectable Hawking radiations. but a much smaller
1:37:58
Speaker 2: black hole can release energy far more rapidly, despite containing
1:38:02
Speaker 2: considerably less mass. Consider a black hole with approximately the
1:38:07
Speaker 2: mass of our Sun. Its Hawking temperature would be only
1:38:10
Speaker 2: about 60 billionths of a Kelvin above absolute zero. The
1:38:14
Speaker 2: predicted power emitted through Hawking radiation would be extraordinarily small,
1:38:20
Speaker 2: vastly less than the energy required to illuminate even the
1:38:23
Speaker 2: faintest ordinary lightbulb. For such a massive object, evaporation would
1:38:28
Speaker 2: proceed at an almost unimaginable pace. Or rather, an almost
1:38:33
Speaker 2: unimaginable lack of pace. Even if the black hole were
1:38:37
Speaker 2: completely isolated, with no surrounding matter or radiation falling into it,
1:38:41
Speaker 2: the time required for it to lose its mass would
1:38:44
Speaker 2: be extraordinary. The usual idealised calculation gives an evaporation lifetime
1:38:50
Speaker 2: of roughly 10 to the power of 67 years for
1:38:54
Speaker 2: a black hole with the mass of the Sun. That
1:38:57
Speaker 2: is a 1 followed by 67 zeros. For comparison, the
1:39:02
Speaker 2: universe is approximately 13.8 billion years old. The predicted lifetime
1:39:08
Speaker 2: of a solar mass black hole is therefore enormously greater
1:39:11
Speaker 2: than the entire history of the universe so far. galaxies
1:39:16
Speaker 2: have evolved and countless generations of stellar systems have appeared
1:39:21
Speaker 2: and disappeared during a period that would represent an almost
1:39:25
Speaker 2: imperceptible fraction of such a black hole's evaporation time, and
1:39:30
Speaker 2: supermassive black holes are expected to survive for considerably longer.
1:39:35
Speaker 2: The relationship between black hole mass and evaporation lifetime is
1:39:38
Speaker 2: particularly striking for an idealised non-rotating black hole. The lifetime
1:39:44
Speaker 2: scales approximately with the cube of its initial mass. The
1:39:48
Speaker 2: evaporation lifetime is approximately proportional to the cube of the
1:39:52
Speaker 2: black hole's initial mass. This means that a black hole
1:39:56
Speaker 2: 10 times more massive would take approximately 1000 times longer
1:39:59
Speaker 2: to evaporate under the same simplifying assumptions. A black hole
1:40:05
Speaker 2: 100 times more massive would have a lifetime approximately 1
1:40:08
Speaker 2: million times greater. and a black hole 1,000 times more
1:40:12
Speaker 2: massive would take approximately 1 billion times longer. Now consider
1:40:18
Speaker 2: Sagittarius A star, the supermassive black hole at the centre
1:40:22
Speaker 2: of our Milky Way. It contains roughly 4 million times
1:40:26
Speaker 2: the mass of the Sun. In an idealised isolated evaporation calculation,
1:40:31
Speaker 2: its lifetime would be of the order of 10 to
1:40:33
Speaker 2: the power of 87 years. Such a timescale is so
1:40:38
Speaker 2: vast that the present age of the universe becomes almost
1:40:41
Speaker 2: insignificant by comparison. Of course, actual black holes do not
1:40:46
Speaker 2: exist in perfectly isolated conditions. They can absorb surrounding matter
1:40:51
Speaker 2: and radiation, and these processes can increase their mass. But
1:40:55
Speaker 2: the idealised calculation reveals just how slowly Hawking evaporation operates
1:41:01
Speaker 2: for enormous black holes. the situation changes dramatically as the
1:41:05
Speaker 2: mass becomes smaller. A hypothetical black hole containing far less
1:41:11
Speaker 2: mass than an ordinary stellar black hole would possess a
1:41:13
Speaker 2: much higher Hawking temperature. And as that temperature increases, the
1:41:18
Speaker 2: characteristic energies of the particles it can emit also increase.
1:41:22
Speaker 2: At very low temperatures, the radiation is dominated by extremely
1:41:26
Speaker 2: low energy particles, including photons. As the temperature rises sufficiently
1:41:31
Speaker 2: … Additional particle species become energetically accessible. The black hole
1:41:36
Speaker 2: may begin emitting electrons, positrons, neutrinos and other particles depending
1:41:41
Speaker 2: on the available energy and the relevant quantum field interactions.
1:41:46
Speaker 2: At still higher temperatures, more massive particles can contribute to
1:41:50
Speaker 2: the radiation. This matters because the black hole is no
1:41:54
Speaker 2: longer limited to emitting the same collection of low-energy particles
1:41:58
Speaker 2: throughout its lifetimes. Its changing temperature affects both the energy
1:42:02
Speaker 2: of the radiation and the kinds of particles that can appear.
1:42:07
Speaker 2: These additional emission channels can increase the rate at which
1:42:10
Speaker 2: energy is carried away. The exact calculation therefore becomes more
1:42:14
Speaker 2: complicated than the simplest relationship between luminosity and mass. The
1:42:19
Speaker 2: particle species available at different temperatures, their interactions with the
1:42:24
Speaker 2: surrounding gravitational geometry, and the black hole's rotation or electrical
1:42:29
Speaker 2: charge can all influence the result. The simple scaling laws
1:42:33
Speaker 2: remain extremely useful, but they are approximations based on particular
1:42:37
Speaker 2: physical assumptions, and near the final stages of evaporation, the
1:42:42
Speaker 2: limitations become especially important. As a black hole becomes smaller,
1:42:47
Speaker 2: its temperature increases and its remaining lifetime decreases. For much
1:42:52
Speaker 2: of its existence, an evaporating black hole loses mass extremely slowly,
1:42:57
Speaker 2: It may spend an overwhelming proportion of its lifetime changing
1:43:00
Speaker 2: by only a relatively modest fraction of its initial mass.
1:43:04
Speaker 2: But once it has become sufficiently small, the process accelerates.
1:43:08
Speaker 2: The remaining mass can disappear much more rapidly than the
1:43:12
Speaker 2: enormous amount of time required for the earlier stages. This
1:43:16
Speaker 2: does not mean the black hole is mechanically falling apart.
1:43:20
Speaker 2: There is no solid structure gradually crumbling, no outer shell
1:43:24
Speaker 2: wearing away. and no reservoir of ordinary fuel being consumed. Instead,
1:43:30
Speaker 2: the gravitational mass energy associated with the black hole is
1:43:34
Speaker 2: being transferred into the outward radiation predicted by quantum field theory.
1:43:39
Speaker 2: And as the mass decreases, the geometry and temperature associated
1:43:43
Speaker 2: with the black hole change accordingly. The familiar semi-classical calculation
1:43:48
Speaker 2: eventually reaches conditions where quantum gravity is expected to become important.
1:43:54
Speaker 2: so the precise final outcome remains uncertain. But the central
1:43:58
Speaker 2: prediction is already extraordinary. A large black hole can remain
1:44:03
Speaker 2: almost unchanged for lengths of time that dwarf the age
1:44:06
Speaker 2: of the universe. A much smaller one can radiate considerably
1:44:10
Speaker 2: more intensely, and as an isolated black hole loses mass,
1:44:15
Speaker 2: it becomes progressively hotter, making the loss of its remaining
1:44:19
Speaker 2: energy increasingly rapid. What begins as an almost imperceptible process
1:44:24
Speaker 2: can therefore develop into something far more energetic, not because
1:44:29
Speaker 2: anything has escaped from within the event horizon, but because
1:44:32
Speaker 2: the quantum fields associated with the black hole continue carrying
1:44:36
Speaker 2: energy into the surrounding universe, and in doing so they
1:44:40
Speaker 2: gradually reduce the mass of an object that classical physics
1:44:43
Speaker 2: once seemed to suggest could last indefinitely. If black holes
1:44:47
Speaker 2: really do emit Hawking radiation, and if that radiation gradually
1:44:51
Speaker 2: carries away their mass, then a rather obvious question presents itself.
1:44:57
Speaker 2: Why are the black holes around us not disappearing? Astronomers
1:45:01
Speaker 2: have discovered black holes throughout the universe, from relatively small
1:45:05
Speaker 2: stellar remnants to enormous objects containing billions of times the
1:45:09
Speaker 2: mass of the Sun. Some have existed for billions of years,
1:45:14
Speaker 2: And yet there is no evidence that ordinary astrophysical black
1:45:18
Speaker 2: holes are gradually shrinking away through Hawking evaporation. The explanation
1:45:23
Speaker 2: involves an important distinction between emitting radiation and actually losing
1:45:27
Speaker 2: mass overall. A black hole can release energy through Hawking
1:45:32
Speaker 2: radiation while simultaneously absorbing energy from its surroundings. And whether
1:45:37
Speaker 2: its mass increases or decreases depends on which process dominates.
1:45:41
Speaker 2: To understand this, it helps to consider the environment in
1:45:44
Speaker 2: which black holes actually exist. Space is often described as
1:45:49
Speaker 2: cold and empty. And compared with the atmosphere of Earth
1:45:53
Speaker 2: or the interior of a star, it certainly is. But
1:45:56
Speaker 2: even the most remote regions of the universe are not
1:45:59
Speaker 2: completely devoid of radiation. There is a faint electromagnetic glow
1:46:03
Speaker 2: extending throughout the observable cosmos. It is called the Cosmic
1:46:08
Speaker 2: Microwave Background. This radiation is a surviving remnant of the
1:46:13
Speaker 2: early universe, originating from a time when the expanding cosmos
1:46:17
Speaker 2: had cooled sufficiently for light to travel freely over enormous distances.
1:46:23
Speaker 2: That happened approximately 380,000 years after the Big Bang. Before then,
1:46:29
Speaker 2: the universe was filled with a hot, dense plasma of
1:46:32
Speaker 2: charged particles and radiation. Photons were repeatedly scattered by free electrons.
1:46:38
Speaker 2: preventing light from travelling very far without interacting with matter.
1:46:43
Speaker 2: As the universe expanded and cooled, electrons combined with atomic
1:46:47
Speaker 2: nuclei to form neutral atoms. This process, known as recombination,
1:46:53
Speaker 2: allowed the radiation to travel much more freely. And that
1:46:57
Speaker 2: ancient light is still travelling through space today. Over billions
1:47:01
Speaker 2: of years, the expansion of the universe has stretched its wavelengths,
1:47:05
Speaker 2: shifting the radiation into the microwave region, of the electromagnetic spectrum. Today,
1:47:12
Speaker 2: the cosmic microwave background has a temperature of approximately 2.7 Kelvin.
1:47:17
Speaker 2: That is only a few degrees above absolute zero. For
1:47:21
Speaker 2: most everyday purposes, this is extraordinarily cold. But compared with
1:47:26
Speaker 2: the Hawking temperature of a typical black hole, it is
1:47:29
Speaker 2: surprisingly warm. A black hole with approximately the mass of
1:47:33
Speaker 2: the sun would have a Hawking temperature of roughly 60
1:47:36
Speaker 2: billionths of a Kelvin. The cosmic microwave background is therefore
1:47:40
Speaker 2: tens of millions of times hotter than the characteristic temperature
1:47:44
Speaker 2: of such a black hole. And this makes an enormous difference.
1:47:48
Speaker 2: Imagine placing a small, cold object inside a room that
1:47:52
Speaker 2: is warmer than the object itself. The object may emit
1:47:56
Speaker 2: thermal radiation, but it also absorbs radiation from its surroundings.
1:48:01
Speaker 2: If the incoming energy exceeds the outgoing energy, the object
1:48:05
Speaker 2: gains energy overall. A black hole is not an ordinary
1:48:09
Speaker 2: material object, but a similar principle applies to its exchange
1:48:13
Speaker 2: of radiation with the surrounding universe. The black hole continues
1:48:17
Speaker 2: to emit Hawking radiation. That process does not simply stop
1:48:22
Speaker 2: because the surrounding environment is warmer, but the black hole
1:48:25
Speaker 2: also absorbs incoming photons, and for an ordinary stellar mass
1:48:29
Speaker 2: black hole in the present cosmic environment, the energy absorbed
1:48:33
Speaker 2: from the cosmic microwave background exceeds the extraordinarily tiny energy
1:48:39
Speaker 2: released through Hawking radiation. The result is that the black
1:48:43
Speaker 2: hole does not experience a net loss of mass through
1:48:46
Speaker 2: this exchange. Its Hawking radiation is real within the theoretical prediction,
1:48:52
Speaker 2: but it is overwhelmed by the energy arriving from outside,
1:48:56
Speaker 2: and the cosmic microwave background is only one possible source
1:48:59
Speaker 2: of incoming energy. Real black holes frequently exist in environments
1:49:04
Speaker 2: containing gas, dust, stars, and other forms of matter. A
1:49:09
Speaker 2: stellar-mass black hole may have a companion star, allowing material
1:49:13
Speaker 2: from the companion to move towards the black hole. In
1:49:16
Speaker 2: some systems, this material forms an accretion disk. As the
1:49:20
Speaker 2: gas spirals inward, it becomes extremely hot and can produce
1:49:24
Speaker 2: powerful electromagnetic radiation. Some of the material eventually crosses the
1:49:29
Speaker 2: event horizon, increasing the black hole's mass. In active galaxies,
1:49:35
Speaker 2: supermassive black holes can be surrounded by enormous reservoirs of gas.
1:49:39
Speaker 2: These environments may supply substantial amounts of matter over millions
1:49:44
Speaker 2: of years, and compared with the almost imperceptible energy loss
1:49:48
Speaker 2: predicted through Hawking radiation, the mass gained through accretion can
1:49:52
Speaker 2: be enormous. Even a relatively small amount of incoming matter
1:49:57
Speaker 2: can outweigh an immense period of Hawking emission. This is
1:50:00
Speaker 2: why black holes observed in galactic centres are not expected
1:50:04
Speaker 2: to be visibly evaporating in the present universe. Many are growing,
1:50:09
Speaker 2: others may remain relatively quiet, gaining little mass over long periods,
1:50:14
Speaker 2: but their Hawking radiation is so weak that its effect
1:50:18
Speaker 2: on their total mass is essentially negligible on ordinary astronomical timescales.
1:50:24
Speaker 2: There is, however, an interesting possibility hidden in the future
1:50:27
Speaker 2: of the universe. The cosmic microwave background will not remain
1:50:32
Speaker 2: at its present temperature forever. As the universe continues expanding,
1:50:36
Speaker 2: the wavelengths of this ancient radiation become progressively longer. And
1:50:41
Speaker 2: as those wavelengths stretch, the radiation's temperature decreases. In a
1:50:47
Speaker 2: continually expanding universe, the cosmic microwave background is expected to
1:50:52
Speaker 2: become colder and colder. Eventually, under suitable future conditions, the
1:50:58
Speaker 2: surrounding radiation environment could become colder than the Hawking temperature
1:51:02
Speaker 2: of an isolated black hole. At that point, the balance
1:51:06
Speaker 2: between incoming and outgoing energy would change. The black hole
1:51:10
Speaker 2: could begin losing more energy through Hawking radiation than it
1:51:14
Speaker 2: absorbs from the surrounding background, and its mass would begin
1:51:17
Speaker 2: decreasing overall. For ordinary stellar-mass black holes, this would occur
1:51:22
Speaker 2: on timescales far beyond the present age of the universe.
1:51:26
Speaker 2: assuming the appropriate conditions eventually arise. The situation would also
1:51:31
Speaker 2: depend on whether the black hole continues encountering matter, whether
1:51:34
Speaker 2: nearby stars remain active, and how the surrounding cosmic environment evolves.
1:51:40
Speaker 2: A black hole passing through a region containing gas, or
1:51:43
Speaker 2: interacting with another compact object, could continue gaining mass. An
1:51:48
Speaker 2: increasingly isolated black hole, however, might eventually experience conditions in
1:51:53
Speaker 2: which Hawking evaporation becomes its dominant long-term process, and the
1:51:58
Speaker 2: consequences would unfold very slowly. As the black hole radiates energy,
1:52:03
Speaker 2: its mass would gradually decrease, its Hawking temperature would rise,
1:52:08
Speaker 2: and that increase would eventually make its radiation more intense.
1:52:12
Speaker 2: But for a black hole containing several times the mass
1:52:15
Speaker 2: of the Sun, the timescale would still be almost unimaginably long.
1:52:20
Speaker 2: The universe could undergo enormous changes while the black hole
1:52:24
Speaker 2: lost only a tiny fraction of its mass. Stars would
1:52:28
Speaker 2: exhaust their nuclear fuel, galaxies would continue evolving, and the
1:52:33
Speaker 2: cosmic background radiation would become progressively colder as space expanded.
1:52:38
Speaker 2: This distant future is not something astronomers can observe directly.
1:52:43
Speaker 2: It is a prediction based on our current understanding of
1:52:46
Speaker 2: cosmic expansion, quantum field theory, and black hole thermodynamics, and
1:52:52
Speaker 2: precisely how individual black holes evolve will depend on their
1:52:55
Speaker 2: environments and the long-term behaviour of the universe. Nevertheless, the
1:53:00
Speaker 2: essential distinction is clear. The prediction of Hawking radiation does
1:53:05
Speaker 2: not mean that every black hole must be shrinking at
1:53:08
Speaker 2: this very moment. A black hole can emit radiation while
1:53:12
Speaker 2: gaining even more energy from its surroundings. The idealised black
1:53:17
Speaker 2: holes used in evaporation calculations are often assumed to be
1:53:20
Speaker 2: completely isolated, allowing the effect of Hawking radiation to be
1:53:25
Speaker 2: considered without incoming matter or radiation. Real astrophysical black holes
1:53:31
Speaker 2: exist within a much more complicated universe. They are surrounded
1:53:35
Speaker 2: by radiation, influenced by nearby objects, and sometimes supplied with
1:53:40
Speaker 2: enormous quantities of matter. And for the black holes astronomers
1:53:45
Speaker 2: observe today, those environmental effects generally overwhelm the extraordinarily weak
1:53:50
Speaker 2: radiation predicted by Hawking. So the black holes around us
1:53:54
Speaker 2: are not failing to evaporate because the underlying theory has
1:53:57
Speaker 2: stopped working. They simply exist in conditions where the predicted
1:54:02
Speaker 2: loss is far too small to overcome the energy they
1:54:04
Speaker 2: can absorb. But as the universe grows older, colder and
1:54:08
Speaker 2: more dilute, that balance may eventually change. And in the
1:54:12
Speaker 2: immense stretches of cosmic time ahead, black holes that once
1:54:16
Speaker 2: grew by swallowing matter may gradually begin surrendering their mass
1:54:20
Speaker 2: to the surrounding darkness. If black holes as massive as
1:54:23
Speaker 2: stars produce almost impossibly weak Hawking radiation, perhaps the best
1:54:28
Speaker 2: place to search for it is somewhere entirely different. Perhaps
1:54:32
Speaker 2: we should be looking for black holes that are much,
1:54:34
Speaker 2: much smaller. The problem is that ordinary black holes are
1:54:38
Speaker 2: formed through processes involving enormous amounts of matter. When a
1:54:43
Speaker 2: massive star collapses, The resulting black hole generally contains several
1:54:47
Speaker 2: times the mass of our Sun, and an object that
1:54:51
Speaker 2: massive has an extraordinarily low Hawking temperature, but physicists have
1:54:55
Speaker 2: proposed that some black holes might have formed through an
1:54:58
Speaker 2: entirely different process, long before the first stars existed. These
1:55:03
Speaker 2: hypothetical objects are called primordial black holes, and if they
1:55:07
Speaker 2: exist they could provide one of the most promising opportunities
1:55:10
Speaker 2: to investigate Hawking radiation directly. The idea begins in the
1:55:15
Speaker 2: very early universe. Shortly after the Big Bang, the universe
1:55:19
Speaker 2: was incredibly hot and dense. Matter and radiation were distributed
1:55:23
Speaker 2: throughout rapidly expanding space, but the density was not necessarily
1:55:28
Speaker 2: perfectly uniform. Small variations in density could exist, with some
1:55:33
Speaker 2: regions containing slightly more energy than their surroundings. Under ordinary circumstances,
1:55:38
Speaker 2: these variations would not be sufficient to produce black holes.
1:55:42
Speaker 2: But if a region was sufficiently overdense, gravity could potentially
1:55:46
Speaker 2: overwhelm the expansion and cause it to collapse. Under suitable conditions,
1:55:51
Speaker 2: the result could be a primordial black hole. Unlike black
1:55:54
Speaker 2: holes produced by dying stars, these objects would not need
1:55:58
Speaker 2: a massive stellar core. Their masses would depend on the
1:56:01
Speaker 2: conditions and scales involved in their formation. This opens the
1:56:05
Speaker 2: possibility of black holes far smaller than anything produced through
1:56:09
Speaker 2: ordinary stellar evolution. During the 1970s, the physicists Bernard Carr
1:56:14
Speaker 2: and Stephen Hawking investigated the possible formation and consequences of
1:56:19
Speaker 2: primordial black holes. Their research explored how density variations in
1:56:24
Speaker 2: the early universe might create black holes across a broad
1:56:27
Speaker 2: range of masses, and Hawking's prediction of evaporation made the
1:56:31
Speaker 2: idea particularly interesting. A primordial black hole formed with an
1:56:36
Speaker 2: extremely small mass would possess a relatively high Hawking temperature.
1:56:42
Speaker 2: It could therefore radiate energy much more rapidly than a
1:56:45
Speaker 2: stellar-mass black hole. Some hypothetical primordial black holes might have
1:56:49
Speaker 2: evaporated long ago. Others, if sufficiently massive when they formed,
1:56:55
Speaker 2: could potentially have survived until the present day. There is
1:56:59
Speaker 2: an especially interesting range of masses. According to standard evaporation calculations,
1:57:05
Speaker 2: a primordial black hole formed with a mass of roughly
1:57:08
Speaker 2: 500 billion kilograms would have a lifetime comparable to the
1:57:12
Speaker 2: present age of the universe. That is an enormous mass
1:57:16
Speaker 2: by everyday standards. But for a black hole, it is
1:57:19
Speaker 2: remarkably small. Its event horizon would be vastly smaller than
1:57:24
Speaker 2: an atomic nucleus, and despite containing the mass of a
1:57:28
Speaker 2: substantial mountain, it could have a Hawking temperature high enough
1:57:31
Speaker 2: to produce energetic particles. A black hole formed with substantially
1:57:36
Speaker 2: less mass would generally have evaporated much earlier. assuming the
1:57:40
Speaker 2: standard theory remains applicable. One formed with considerably greater mass
1:57:45
Speaker 2: could still exist. And a primordial black hole approaching the
1:57:49
Speaker 2: later stages of evaporation would become increasingly interesting to astronomers.
1:57:55
Speaker 2: As its remaining mass decreased, its Hawking temperature would rise,
1:57:59
Speaker 2: its radiation would become more energetic, and the rate at
1:58:03
Speaker 2: which energy escaped would increase. Eventually, the emitted particles could
1:58:07
Speaker 2: include high-energy photons known as galvanophores, rays. Gamma rays are
1:58:12
Speaker 2: a form of electromagnetic radiation, just like visible light, but
1:58:16
Speaker 2: their photons possess much greater energies. And unlike the extraordinarily weak,
1:58:22
Speaker 2: low-energy radiation predicted from ordinary stellar black holes, Energetic gamma
1:58:27
Speaker 2: rays can be detected by specialised astronomical instruments. One important
1:58:32
Speaker 2: example is the Fermi Gamma-Ray Space Telescope, operated by the
1:58:37
Speaker 2: National Aeronautics and Space Administration with international collaborators. Launched in 2008,
1:58:44
Speaker 2: Fermi observes high-energy radiation arriving from throughout the cosmos. Its
1:58:50
Speaker 2: instruments investigate phenomena including pulses, active galaxies and powerful gamma-ray bursts.
1:58:57
Speaker 2: Researchers have also used observations from Fermi to search for
1:59:00
Speaker 2: possible signatures of evaporating primordial black holes. A nearby primordial
1:59:06
Speaker 2: black hole approaching the end of its evaporation might appear
1:59:09
Speaker 2: as a gamma-ray source becoming progressively brighter and more energetic.
1:59:14
Speaker 2: Depending on its distance and motion, astronomers might also look
1:59:17
Speaker 2: for changes in its apparent position. Searches can investigate both
1:59:22
Speaker 2: individual candidate sources and the broader gamma-ray background produced by
1:59:26
Speaker 2: unresolved populations of objects. But there is an important qualification.
1:59:31
Speaker 2: No primordial black hole has been conclusively identified, and no
1:59:36
Speaker 2: confirmed detection of Hawking radiation from an actual astrophysical black
1:59:40
Speaker 2: hole has been achieved. The absence of a confirmed signal
1:59:44
Speaker 2: allows researchers to place limits on how common certain populations
1:59:48
Speaker 2: of primordial black holes could be. it does not necessarily
1:59:52
Speaker 2: establish that primordial black holes of every possible mass are absent.
1:59:56
Speaker 2: Some possibilities remain compatible with observational constraints, and physicists continue
2:00:02
Speaker 2: investigating whether primordial black holes could have contributed to the
2:00:06
Speaker 2: dark matter inferred from the gravitational behaviour of galaxies and
2:00:10
Speaker 2: larger cosmic structures. There is also another way to investigate
2:00:14
Speaker 2: the physics of Hawking radiation, and it does not require
2:00:18
Speaker 2: finding a tiny black hole somewhere in space. It involves
2:00:21
Speaker 2: creating an analogue of a black hole in a laboratory.
2:00:25
Speaker 2: The idea was explored by William Yunru, who recognised that
2:00:28
Speaker 2: certain moving fluids could produce horizons mathematically resembling aspects of
2:00:33
Speaker 2: gravitational event horizons. Imagine a fluid flowing through a narrow region.
2:00:39
Speaker 2: If the fluid moves faster than waves can travel against
2:00:42
Speaker 2: its flow, those waves cannot propagate upstream beyond a particular boundary.
2:00:47
Speaker 2: This creates an analogue horizon. it does not trap light
2:00:50
Speaker 2: through the curvature of spacetime, and it does not create
2:00:54
Speaker 2: a genuine gravitational black hole. But the mathematics describing certain
2:00:59
Speaker 2: wave behaviours can resemble the mathematics involved in Hawking radiation.
2:01:04
Speaker 2: The physicist Jeff Steinhauer and his colleagues at the Technion
2:01:08
Speaker 2: Israel Institute of Technology investigated this possibility using an exceptionally
2:01:14
Speaker 2: cold collection of atoms called a Bose-Einstein condensate. In this
2:01:18
Speaker 2: unusual quantum state, atoms behave collectively in ways that allow
2:01:22
Speaker 2: researchers to study delicate wave phenomena. By controlling the flow
2:01:26
Speaker 2: of the condensate, the researchers created an analogue horizon for
2:01:30
Speaker 2: sound waves. Their experiments reported Hawking-like radiation and correlations between
2:01:36
Speaker 2: outgoing excitations and their partners. These observations provide an important
2:01:41
Speaker 2: experimental opportunity to investigate aspects of horizon physics and quantum
2:01:46
Speaker 2: field behaviour. But an analogue black hole is not an
2:01:49
Speaker 2: actual black hole in spacetime. The laboratory experiments involve sound-like
2:01:54
Speaker 2: excitations in a specially prepared quantum system, rather than photons
2:01:59
Speaker 2: emitted by an astrophysical event horizon. They therefore cannot establish
2:02:04
Speaker 2: every aspect of Hawking evaporation, or confirm that real black
2:02:08
Speaker 2: holes lose mass in precisely the predicted way. Nevertheless, they
2:02:12
Speaker 2: offer valuable evidence that some of the mathematical mechanisms associated
2:02:16
Speaker 2: with Hawking radiation can produce observable effects. And together, these
2:02:22
Speaker 2: different approaches reveal how physicists are investigating the same extraordinary
2:02:26
Speaker 2: prediction from several directions. Astronomers search the universe for energetic
2:02:31
Speaker 2: radiation that might reveal evaporating primordial black holes. Experimental physicists
2:02:38
Speaker 2: create analogue horizons to investigate related quantum effects. and theoretical
2:02:43
Speaker 2: researchers continue refining the calculations that connect quantum fields with
2:02:47
Speaker 2: black hole spacetime. The ultimate goal remains a direct observation
2:02:52
Speaker 2: of Hawking radiation from a genuine black hole. Such a
2:02:56
Speaker 2: discovery would provide extraordinary evidence that black holes are not
2:02:59
Speaker 2: entirely black, and it would bring us closer to understanding
2:03:03
Speaker 2: how something defined by the impossibility of escape can nevertheless
2:03:06
Speaker 2: release energy into the universe. For now, the smallest black
2:03:10
Speaker 2: holes remain hypothetical, but if primordial black holes exist, one
2:03:15
Speaker 2: of them may eventually reveal a phenomenon that has remained
2:03:18
Speaker 2: hidden since Stephen Hawking first predicted it. The faint quantum
2:03:23
Speaker 2: glow of an object that classical physics once considered perfectly dark.
2:03:27
Speaker 2: There is something rather extraordinary about the final stages of
2:03:30
Speaker 2: a black hole's predicted existence. For almost its entire lifetime,
2:03:36
Speaker 2: a sufficiently massive black hole can remain remarkably quiet. It
2:03:40
Speaker 2: may release Hawking radiation so slowly that its mass changes
2:03:45
Speaker 2: by an almost imperceptible amount over billions upon billions of years.
2:03:50
Speaker 2: But if the black hole continues evaporating, its behaviour gradually changes.
2:03:54
Speaker 2: As its mass decreases, its temperature increases, and eventually, according
2:04:00
Speaker 2: to the familiar calculations of quantum field theory in curved spacetimes,
2:04:04
Speaker 2: the process can become dramatically more energetic. The black hole
2:04:09
Speaker 2: that once released only the faintest radiation could begin emitting
2:04:13
Speaker 2: particles with enormous energies. And this raises a fascinating question.
2:04:18
Speaker 2: What actually happens when a black hole has almost nothing left?
2:04:23
Speaker 2: To understand the difficulty, it helps to consider the relationship
2:04:26
Speaker 2: between a black hole's mass and its Hawking temperature. For
2:04:30
Speaker 2: an idealised non-rotating black hole, Temperature is inversely proportional to mass.
2:04:36
Speaker 2: A large black hole is extremely cold, a smaller black
2:04:40
Speaker 2: hole is hotter, and as the mass continues to decrease,
2:04:44
Speaker 2: the characteristic energy of the emitted radiation rises. For a
2:04:48
Speaker 2: black hole with the mass of the Sun, the Hawking
2:04:51
Speaker 2: temperature is only about 60 billionths of a Kelvin. Its
2:04:55
Speaker 2: radiation would be extraordinarily weak. But imagine a hypothetical primordial
2:05:01
Speaker 2: black hole that has already lost almost all its original mass.
2:05:05
Speaker 2: Its event horizon would be incredibly small, and its temperature
2:05:09
Speaker 2: could be far greater than anything associated with an ordinary
2:05:12
Speaker 2: stellar black hole. At sufficiently high temperatures, the radiation would
2:05:17
Speaker 2: no longer consist predominantly of extremely low-energy particles. The black
2:05:21
Speaker 2: hole could emit energetic photons, including gamma rays, along with
2:05:27
Speaker 2: various other particles. As the available energy increased, additional particle
2:05:32
Speaker 2: species could contribute to the emission. Electrons and positrons, for example,
2:05:38
Speaker 2: could be produced once the characteristic energies became sufficiently high.
2:05:43
Speaker 2: At still greater temperatures, heavier particles could become energetically accessible.
2:05:48
Speaker 2: The precise emission rates would depend on the quantum fields
2:05:51
Speaker 2: involved and the way different particle species interact with the
2:05:55
Speaker 2: black hole's gravitational environment But the general trend is clear.
2:06:00
Speaker 2: As the black hole becomes smaller, its radiation becomes increasingly energetic,
2:06:06
Speaker 2: and the rate at which it loses mass generally increases.
2:06:10
Speaker 2: This leads to one of the more remarkable predictions associated
2:06:13
Speaker 2: with Hawking evaporation. A sufficiently small black hole approaching the
2:06:18
Speaker 2: end of its semi-classical lifetime could release its remaining energy
2:06:22
Speaker 2: over a comparatively short period. The final stages might produce
2:06:27
Speaker 2: a powerful burst of high-energy particles and radiation. This is
2:06:31
Speaker 2: sometimes described as a black hole explosion. But it is
2:06:35
Speaker 2: important to understand what that description means. The black hole
2:06:39
Speaker 2: is not exploding because an enormous pressure is built up
2:06:42
Speaker 2: behind its event horizon. It is not behaving like a
2:06:46
Speaker 2: star undergoing a supernova, and it is not suddenly releasing
2:06:50
Speaker 2: all the ordinary matter that previously fell inside. Instead, The
2:06:54
Speaker 2: predicted increase in radiation follows from the relationship between decreasing mass,
2:06:59
Speaker 2: increasing temperature and the resulting acceleration of Hawking evaporation. A
2:07:05
Speaker 2: small black hole can radiate much more intensely than a
2:07:09
Speaker 2: large one, and as its remaining mass becomes smaller, the
2:07:13
Speaker 2: rate of energy loss can increase dramatically. However, there is
2:07:17
Speaker 2: a major uncertainty. The familiar equations describing Hawking radiation cannot
2:07:23
Speaker 2: necessarily be trusted all the way to the very end.
2:07:26
Speaker 2: The reason involves something called the Planck scale. The name
2:07:30
Speaker 2: comes from the German physicist Max Planck, whose work on
2:07:34
Speaker 2: thermal radiation helped establish the foundations of quantum theory at
2:07:38
Speaker 2: the beginning of the 20th century. Planck recognised the importance
2:07:43
Speaker 2: of a fundamental constant that now bears his name. Together
2:07:47
Speaker 2: with the speed of light and Newton's gravitational constant, Planck's
2:07:52
Speaker 2: constant allows physicists to construct natural scales of length, time
2:07:57
Speaker 2: and mass. These scales are known as Planck units, and
2:08:01
Speaker 2: they identify a regime where the relationship between gravity and
2:08:04
Speaker 2: quantum physics becomes especially important. Consider the Planck length. It
2:08:10
Speaker 2: is approximately 1.6 times 10 to the power of minus
2:08:14
Speaker 2: 35 metres. That is an almost unimaginably small distance. For comparison,
2:08:20
Speaker 2: the diameter of a proton is roughly 10 to the
2:08:23
Speaker 2: power of minus 15 metres. The Planck length is therefore
2:08:27
Speaker 2: approximately 20 orders of magnitude smaller than the scale of
2:08:31
Speaker 2: a proton. There is also the Planck mass. Its value
2:08:35
Speaker 2: is approximately 22 micrograms. That may sound rather ordinary compared
2:08:40
Speaker 2: with the enormous masses normally associated with black holes. But
2:08:44
Speaker 2: when considering the gravitational effects of quantum systems, The Planck
2:08:48
Speaker 2: mass represents an important natural scale. A black hole approaching
2:08:53
Speaker 2: this mass would possess a horizon radius comparable in order
2:08:57
Speaker 2: of magnitude to the Planck length. And at such scales,
2:09:01
Speaker 2: the assumptions used in the ordinary semi-classical description become increasingly questionable.
2:09:06
Speaker 2: Stephen Hawking's original calculation treated quantum fields as existing within
2:09:11
Speaker 2: a classical curved spacetime. The gravitational geometry was described using
2:09:16
Speaker 2: Einstein's general theory of relativity, while the fields themselves obeyed
2:09:21
Speaker 2: quantum principles. For a sufficiently large black hole, this separation
2:09:26
Speaker 2: can provide an extraordinarily useful approximation. But near the Planck scale,
2:09:32
Speaker 2: quantum effects associated with gravity itself are expected to become important.
2:09:38
Speaker 2: The geometry of spacetime may no longer be adequately described
2:09:42
Speaker 2: as a smooth classical background, and without a complete quantum
2:09:46
Speaker 2: theory of gravity, physicists cannot confidently determine every detail of
2:09:51
Speaker 2: what happens. This creates several possibilities. One is that evaporation
2:09:56
Speaker 2: continues until the black hole disappears completely, leaving its remaining
2:10:00
Speaker 2: energy in radiation or other quantum excitations. Another is that
2:10:06
Speaker 2: evaporation eventually produces some form of extremely small remnant. Such
2:10:11
Speaker 2: a remnant might possess properties that prevent further evaporation, although
2:10:15
Speaker 2: no experimentally established theory demonstrates that this must happen. Other
2:10:21
Speaker 2: theoretical proposals suggest more complicated outcomes involving quantum gravitational effects
2:10:27
Speaker 2: and changes to the familiar description of spacetime. These possibilities
2:10:31
Speaker 2: remain subjects of active investigation. and none has been conclusively
2:10:36
Speaker 2: confirmed by observations. This is why physicists must be careful
2:10:41
Speaker 2: when describing a black hole's final disappearance. The well-developed semi-classical
2:10:46
Speaker 2: calculation predicts that sufficiently isolated black holes lose mass through
2:10:52
Speaker 2: Hawking radiation. It also predicts that smaller black holes become
2:10:56
Speaker 2: hotter and generally evaporate more rapidly. But extending those same
2:11:01
Speaker 2: equations all the way to zero mass assumes that the
2:11:05
Speaker 2: underlying approximations remain valid even when the system reaches a
2:11:09
Speaker 2: regime where quantum gravity can no longer be neglected. There
2:11:13
Speaker 2: is no guarantee that this assumption is correct. The final burst,
2:11:18
Speaker 2: often associated with an evaporating black hole, is therefore a
2:11:22
Speaker 2: prediction obtained by extending the familiar theory, rather than an
2:11:26
Speaker 2: experimentally established description of the exact end point. And the
2:11:31
Speaker 2: difference matters. For most of a black hole's predicted lifetime,
2:11:36
Speaker 2: the semi-classical description can remain reliable. A stellar-mass black hole
2:11:41
Speaker 2: could emit extraordinarily weak Hawking radiation for timescales vastly greater
2:11:46
Speaker 2: than the present age of the universe. A hypothetical tiny
2:11:50
Speaker 2: black hole, by contrast, could become hot enough to release
2:11:53
Speaker 2: highly energetic particles. The same underlying relationship connects these enormously
2:11:59
Speaker 2: different behaviours, but at the smallest scales, the physics enters
2:12:04
Speaker 2: territory that remains incompletely understood. And there is one further difficulty.
2:12:10
Speaker 2: If a black hole eventually disappears, what happens to the
2:12:14
Speaker 2: information associated with everything that once fell inside it? The
2:12:18
Speaker 2: energy can be carried away by radiation. But quantum mechanics
2:12:22
Speaker 2: suggest that the complete physical state contains information that should
2:12:26
Speaker 2: not simply vanish. If the black hole is gone and
2:12:30
Speaker 2: only radiation remains, that radiation may need to preserve information
2:12:34
Speaker 2: about the original system through subtle quantum correlations. This is
2:12:39
Speaker 2: a much deeper problem than simply calculating how quickly mass
2:12:43
Speaker 2: is lost. It concerns the fundamental relationship between gravity, quantum mechanics,
2:12:49
Speaker 2: and the evolution of physical information. And it means that
2:12:53
Speaker 2: the final moments of an evaporating black hole may reveal
2:12:56
Speaker 2: something profound about the laws of nature. A black hole
2:13:00
Speaker 2: can spend an almost unimaginable length of time losing energy
2:13:03
Speaker 2: at an extraordinary gentle rate. Yet as it becomes smaller,
2:13:08
Speaker 2: its temperature rises, its radiation becomes more energetic, and the
2:13:12
Speaker 2: limitations of our current theories become increasingly important. The mathematics
2:13:18
Speaker 2: predicts a remarkable transformation, but exactly how that transformation ends
2:13:23
Speaker 2: remains one of the great unanswered questions of modern physics.
2:13:27
Speaker 2: Even after describing how a black hole can lose nearly
2:13:30
Speaker 2: all its mass, we have not completely explained what happens
2:13:34
Speaker 2: to everything that made it a black hole in the
2:13:36
Speaker 2: first place. There is a question about black holes that
2:13:39
Speaker 2: may be even more difficult than understanding how they lose mass.
2:13:43
Speaker 2: It concerns what happens to everything they once contained. According
2:13:49
Speaker 2: to Stephen Hawking's calculations, an isolated black hole can gradually
2:13:53
Speaker 2: emit radiation. becoming smaller as energy escapes into the surrounding universe. Eventually,
2:13:59
Speaker 2: if the familiar process continues, the black hole might disappear altogether.
2:14:05
Speaker 2: But this creates an extraordinary problem. If the black hole
2:14:08
Speaker 2: is gone, what happens to the information associated with everything
2:14:12
Speaker 2: that fell inside it? This question is known as the
2:14:15
Speaker 2: black hole information paradox, and it has become one of
2:14:19
Speaker 2: the most important unresolved problems in theoretical physics. To understand why,
2:14:24
Speaker 2: we need to consider what physicists actually mean by information.
2:14:29
Speaker 2: In everyday life, information might mean words in a book,
2:14:32
Speaker 2: photographs stored on a computer, or memories of something that
2:14:36
Speaker 2: happened long ago. But in quantum physics, information has a
2:14:40
Speaker 2: much broader meaning. It refers to the distinctions and correlations
2:14:45
Speaker 2: contained within the complete quantum state of a physical system.
2:14:49
Speaker 2: A quantum state describes the probabilities associated with possible measurement outcomes,
2:14:55
Speaker 2: together with the relationships between different parts of the system.
2:14:59
Speaker 2: Those relationships can contain information even when there is no
2:15:03
Speaker 2: obvious message or record to be found. Consider an ordinary
2:15:06
Speaker 2: piece of paper. Suppose it contains a carefully written description
2:15:10
Speaker 2: of something, and we place it into a fire. The
2:15:13
Speaker 2: paper burns, producing heat, light, smoke and ash. The original
2:15:18
Speaker 2: writing disappears. and recovering the words from everything produced by
2:15:22
Speaker 2: the fire would be fantastically difficult. But according to the
2:15:27
Speaker 2: usual laws of quantum mechanics, the complete microscopic evolution of
2:15:31
Speaker 2: an isolated system does not simply destroy the information encoded
2:15:35
Speaker 2: in its original quantum state. In principle, sufficiently detailed knowledge
2:15:40
Speaker 2: of the complete final state would preserve the distinctions needed
2:15:44
Speaker 2: to describe the earlier state. The information becomes extraordinarily dispersed
2:15:49
Speaker 2: and practically inaccessible, but it is not fundamentally erased. This
2:15:55
Speaker 2: principle is connected to something called quantum unitarity. In quantum mechanics,
2:16:00
Speaker 2: unitary evolution preserves the mathematical relationships between possible quantum states.
2:16:06
Speaker 2: It ensures that the complete evolution of an isolated system can,
2:16:11
Speaker 2: in principle, be reversed. A system may become increasingly complicated
2:16:16
Speaker 2: and its information may become distributed among enormous numbers of particles. Nevertheless,
2:16:22
Speaker 2: the information remains encoded within the complete quantum state, and
2:16:26
Speaker 2: this is where black holes become troublesome. Imagine an object
2:16:30
Speaker 2: falling through an event horizon. The object possesses a particular
2:16:34
Speaker 2: quantum state containing information about its physical properties. According to
2:16:40
Speaker 2: classical general relativity, once the object has crossed the horizon,
2:16:45
Speaker 2: No ordinary signal can carry that information back to the
2:16:48
Speaker 2: outside universe. That alone does not necessarily violate quantum mechanics.
2:16:53
Speaker 2: The information might simply remain inaccessible behind the horizon. But
2:16:58
Speaker 2: Hawking radiation introduces a new possibility. If the black hole
2:17:03
Speaker 2: gradually evaporates and eventually disappears completely, there may be no
2:17:07
Speaker 2: remaining interior in which the information could be stored. Only
2:17:11
Speaker 2: the emitted radiation would remain. and Hawking's original semi-classical calculation
2:17:17
Speaker 2: suggested that this radiation had a thermal character. Thermal radiation
2:17:22
Speaker 2: is largely characterised by quantities such as temperature, rather than
2:17:26
Speaker 2: by the detailed identity of the objects that originally formed
2:17:29
Speaker 2: the black hole. This raises the possibility that very different
2:17:33
Speaker 2: initial states could produce final radiation, described by the same
2:17:38
Speaker 2: thermal statistical state, and that would present a serious difficulty.
2:17:42
Speaker 2: To understand the problem more precisely, physicists distinguish between pure
2:17:47
Speaker 2: quantum states and mixed states. A pure state represents a
2:17:51
Speaker 2: complete quantum description of a system, even when individual measurement
2:17:55
Speaker 2: outcomes remain uncertain. A mixed state represents a statistical description
2:18:01
Speaker 2: in which the system is treated as occupying different possible
2:18:05
Speaker 2: quantum states with particular probabilities. Importantly, a mixed state can
2:18:10
Speaker 2: arise when we examine only part of a larger system.
2:18:14
Speaker 2: Two quantum systems may be entangled, meaning their quantum states
2:18:19
Speaker 2: possess correlations that cannot be described independently. If we have
2:18:24
Speaker 2: access to only one of those systems, its local state
2:18:27
Speaker 2: may appear mixed, even though the complete combined system remains
2:18:31
Speaker 2: in a pure state. This distinction is essential to the
2:18:34
Speaker 2: information paradox. In Hawking's semi-classical description, Outgoing radiation is correlated
2:18:41
Speaker 2: with quantum field degrees of freedom that are inaccessible to
2:18:44
Speaker 2: an observer remaining outside the black hole. When physicists calculate
2:18:49
Speaker 2: the state of the accessible radiation, they effectively exclude those
2:18:53
Speaker 2: inaccessible degrees of freedom. This mathematical procedure is known as
2:18:57
Speaker 2: taking a partial trace. It can produce a thermal-looking mixed
2:19:01
Speaker 2: state for the radiation observed outside, While the black hole
2:19:06
Speaker 2: still exists, this does not necessarily imply that the complete
2:19:09
Speaker 2: quantum state has lost information. The inaccessible correlations may still
2:19:13
Speaker 2: be relevant. But if the black hole disappears entirely, the
2:19:18
Speaker 2: question becomes much more difficult. If the only remaining system
2:19:22
Speaker 2: is radiation in a genuinely mixed state, despite the complete
2:19:26
Speaker 2: system beginning in a pure state, ordinary unitary quantum evolution
2:19:31
Speaker 2: would appear to have failed. In 1976, Stephen Hawking explored
2:19:36
Speaker 2: this possibility in a paper titled Breakdown of Predictability and
2:19:40
Speaker 2: Gravitational Collapse. His work suggested that black hole evaporation might
2:19:45
Speaker 2: involve fundamental information loss, and that conclusion challenged one of
2:19:49
Speaker 2: the central principles of quantum mechanics. Physicists began debating whether
2:19:54
Speaker 2: information could genuinely disappear, whether it might remain in some
2:19:59
Speaker 2: unknown remnant. or whether the outgoing Hawking radiation could preserve
2:20:03
Speaker 2: it through correlations not captured by the simplest calculation. One
2:20:08
Speaker 2: important contribution came from the physicist Don Page. Page investigated
2:20:13
Speaker 2: how the entropy associated with Hawking radiation should behave if
2:20:17
Speaker 2: the complete evaporation process preserves quantum information. The resulting pattern
2:20:22
Speaker 2: is known as the Page curve. At first, as the
2:20:25
Speaker 2: black hole emits radiation, the entanglement entropy of the accumulated
2:20:29
Speaker 2: radiation is expected to increase. The emitted radiation becomes increasingly
2:20:34
Speaker 2: entangled with the remaining black hole, but in an idealised
2:20:38
Speaker 2: information-preserving evaporation process, that increase cannot continue indefinitely. As the
2:20:45
Speaker 2: black hole becomes smaller, the radiation must eventually begin carrying
2:20:49
Speaker 2: the relevant quantum correlations in a way that reduces the
2:20:52
Speaker 2: entanglement entropy of the collected radiation. If the black hole
2:20:57
Speaker 2: evaporates completely and the total system began in a pure state,
2:21:02
Speaker 2: the final radiation must also form a pure state. The
2:21:06
Speaker 2: Page curve therefore rises during the earlier stages of evaporation,
2:21:10
Speaker 2: reaches a maximum and subsequently decreases. For complete evaporation without
2:21:16
Speaker 2: a remnant or other inaccessible system, it eventually returns to zero.
2:21:21
Speaker 2: This differs from the continued entropy increase suggested by the
2:21:24
Speaker 2: simplest semi-classical description of independent thermal emissions, and it reveals
2:21:30
Speaker 2: an important subtlety. Radiation can appear thermal in its individual
2:21:34
Speaker 2: properties while still containing information through correlations between different parts
2:21:39
Speaker 2: of the emitted radiation. Measuring the energy distribution of photons
2:21:44
Speaker 2: does not necessarily reveal the complete quantum state. Two collections
2:21:48
Speaker 2: of radiation can possess nearly identical thermal spectra while differing
2:21:53
Speaker 2: in their underlying correlations, so observing approximately thermal radiation would not,
2:21:58
Speaker 2: by itself, prove that information had been destroyed. The real
2:22:03
Speaker 2: question concerns the complete quantum state of everything remaining after evaporation,
2:22:08
Speaker 2: and this is different from asking where the energy comes from.
2:22:12
Speaker 2: Energy conservation concerns the accounting of physical energy as radiation escapes,
2:22:17
Speaker 2: and the black hole loses mass. Information preservation concerns whether
2:22:22
Speaker 2: the full quantum evolution retains the distinctions encoded in the
2:22:26
Speaker 2: original state. A theory might correctly account for every unit
2:22:30
Speaker 2: of energy, while still encountering difficulties with quantum unitarity. The
2:22:36
Speaker 2: information paradox therefore cannot be solved merely by showing that
2:22:40
Speaker 2: the black hole's lost mass corresponds to the energy of
2:22:43
Speaker 2: its radiation. It requires a deeper understanding of the quantum
2:22:47
Speaker 2: correlations involved. The event horizon must also be taken seriously.
2:22:52
Speaker 2: Ordinary information cannot simply travel outward from inside it. Yet
2:22:57
Speaker 2: if black hole evaporation preserves quantum information, the complete radiation
2:23:01
Speaker 2: must somehow encode the information associated with the original system.
2:23:06
Speaker 2: Understanding how these requirements fit together has motivated decades of
2:23:11
Speaker 2: research into the quantum nature of spacetime. Modern theoretical developments
2:23:16
Speaker 2: have provided important evidence that information-preserving evaporation is possible in
2:23:20
Speaker 2: certain models. But a complete, experimentally confirmed description of the
2:23:25
Speaker 2: process in real astrophysical black holes remains unavailable. And this
2:23:31
Speaker 2: is what makes the black hole information paradox so remarkable.
2:23:35
Speaker 2: A black hole can lose mass without anything escaping outward
2:23:38
Speaker 2: through its event horizon. The energy accounting can be understood
2:23:42
Speaker 2: through Hawking radiation. But the fate of the complete quantum
2:23:46
Speaker 2: state raises a much deeper question. If the black hole
2:23:50
Speaker 2: eventually disappears, the information associated with its entire history may
2:23:55
Speaker 2: need to survive in the radiation left behind. Not necessarily
2:23:59
Speaker 2: as anything we could read, reconstruct or recognise, but as
2:24:03
Speaker 2: correlations preserved within the fundamental quantum description of the universe.
2:24:07
Speaker 2: And understanding those correlations may reveal something extraordinary about the
2:24:12
Speaker 2: relationship between gravity, quantum mechanics and spacetime itself. Perhaps the
2:24:18
Speaker 2: most remarkable thing about Hawking radiation is not simply that
2:24:22
Speaker 2: black holes can lose mass, it is what this discovery
2:24:25
Speaker 2: suggests about the fundamental laws of the universe. For most
2:24:29
Speaker 2: of the 20th century, physicists developed two extraordinarily successful descriptions
2:24:35
Speaker 2: of nature. Albert Einstein's general theory of relativity explained gravity
2:24:40
Speaker 2: through the curvature of spacetime. Quantum physics explained the behaviour
2:24:45
Speaker 2: of matter and energy at microscopic scales. Both theories produced
2:24:50
Speaker 2: predictions of astonishing accuracy, yet bringing them together revealed questions
2:24:55
Speaker 2: that neither could completely answer on its own, and black
2:24:58
Speaker 2: holes became one of the most important places where those
2:25:01
Speaker 2: questions could be investigated. Stephen Hawking's discovery showed that quantum
2:25:06
Speaker 2: fields in curved spacetime could produce radiation associated with an
2:25:10
Speaker 2: event horizon. The result connected gravity with temperature, entropy, and
2:25:16
Speaker 2: the statistical behaviour of quantum systems. A black hole was
2:25:20
Speaker 2: no longer simply a region from which nothing could escape.
2:25:24
Speaker 2: It was also a thermodynamic object, possessing properties that seemed
2:25:28
Speaker 2: to require a deeper microscopic explanation. Consider the entropy of
2:25:32
Speaker 2: a black hole. In ordinary thermodynamics, entropy is connected to
2:25:37
Speaker 2: the number of microscopic arrangements compatible with a system's overall
2:25:41
Speaker 2: physical condition. But black hole entropy behaves in an unusual way.
2:25:46
Speaker 2: It is proportional to the area of the event horizon
2:25:49
Speaker 2: rather than to the volume enclosed by it. For an
2:25:52
Speaker 2: idealised black hole, doubling the horizon radius increases the horizon
2:25:56
Speaker 2: area and therefore its entropy by a factor of four.
2:26:01
Speaker 2: This relationship raises a fascinating possibility. Perhaps the information associated
2:26:06
Speaker 2: with a gravitational region is not organised in quite the
2:26:10
Speaker 2: way we would expect from our everyday experience of three-dimensional space.
2:26:15
Speaker 2: During the 1990s, the physicists Gerard Tuft and Leonard Susskind
2:26:19
Speaker 2: developed ideas that became known as the holographic principle. The
2:26:24
Speaker 2: central suggestion was extraordinary. Under suitable conditions, the information contained
2:26:29
Speaker 2: within a region of space might be represented by degrees
2:26:32
Speaker 2: of freedom associated with a boundary having fewer dimensions. It
2:26:38
Speaker 2: is rather like the way a hologram can contain information
2:26:41
Speaker 2: about a three-dimensional image while being recorded on a two-dimensional surface.
2:26:46
Speaker 2: The comparison is useful, although the actual physics is much
2:26:49
Speaker 2: more sophisticated. The holographic principle does not simply claim that
2:26:55
Speaker 2: the universe is an ordinary projection or that physical objects
2:26:58
Speaker 2: are illusions. Instead, it suggests that gravitational systems may possess
2:27:03
Speaker 2: descriptions in which information is organised according to principles very
2:27:09
Speaker 2: different from those suggested by classical geometry alone, and this
2:27:13
Speaker 2: possibility became considerably more concrete in 1997. That year, the
2:27:19
Speaker 2: theoretical physicist Juan Maldacena proposed a remarkable mathematical correspondence between
2:27:26
Speaker 2: certain theories containing gravity and quantum field theories defined on
2:27:30
Speaker 2: lower dimensional boundaries. It became known as the anti-de Sitter
2:27:35
Speaker 2: and conformal field theory correspondence. Anti-de Sitter space is a
2:27:40
Speaker 2: particular kind of curved spacetime with negative curvature. A conformal
2:27:46
Speaker 2: field theory is a quantum field theory possessing special symmetries,
2:27:50
Speaker 2: including invariance under certain changes of scale. Maldacena's proposal suggested that,
2:27:57
Speaker 2: in particular theoretical settings, A gravitational system in a higher
2:28:01
Speaker 2: dimensional space could be mathematically equivalent to a quantum theory
2:28:06
Speaker 2: without dynamical gravity living on its boundary. Two descriptions that
2:28:11
Speaker 2: appear very different could therefore represent the same underlying physics.
2:28:15
Speaker 2: This is called holographic duality, and its implications for black
2:28:20
Speaker 2: holes are profound. In suitable examples, a black hole described
2:28:25
Speaker 2: using gravity corresponds to a state in a quantum theory
2:28:29
Speaker 2: whose evolution preserves information. That provides important theoretical support for
2:28:34
Speaker 2: the idea that black hole evaporation can be compatible with
2:28:38
Speaker 2: quantum unitarity. The information need not be fundamentally destroyed simply
2:28:43
Speaker 2: because the semi-classical description appears to produce thermal radiation. However,
2:28:48
Speaker 2: the correspondence is most precisely understood in specialised mathematical settings.
2:28:54
Speaker 2: Our own expanding universe is not simply the ideal anti-de-sitter
2:28:58
Speaker 2: spacetime used in many of these calculations, so holographic duality
2:29:02
Speaker 2: does not automatically provide a complete description of every astrophysical
2:29:06
Speaker 2: black hole. Nevertheless, it has transformed how physicists think about
2:29:11
Speaker 2: the relationship between gravity, quantum mechanics and information. And more
2:29:17
Speaker 2: recent theoretical developments have produced another remarkable result. Researchers have
2:29:22
Speaker 2: found ways to reproduce the information-preserving page curve in certain
2:29:26
Speaker 2: models of evaporating black holes. The page curve describes how
2:29:30
Speaker 2: the entanglement entropy of collected Hawking radiation should change if
2:29:35
Speaker 2: the complete evaporation process preserves quantum information. Initially, that entropy rises,
2:29:41
Speaker 2: but after reaching a maximum, it begins decreasing as the
2:29:45
Speaker 2: radiation contains more of the correlations needed to describe the
2:29:49
Speaker 2: complete quantum state. In the simplest information-preserving model of complete evaporation,
2:29:56
Speaker 2: the final radiation forms a pure quantum state. For many years,
2:30:00
Speaker 2: explaining this behaviour directly through calculations involving black hole spacetime
2:30:05
Speaker 2: proved extremely difficult. But new methods involving quantum extremal surfaces,
2:30:10
Speaker 2: entanglement islands and replica wormholes have provided important progress. The
2:30:16
Speaker 2: idea of an entanglement island is particularly intriguing. When calculating
2:30:21
Speaker 2: the entropy associated with Hawking radiation, physicists ordinarily consider the
2:30:26
Speaker 2: radiation collected outside the black hole. But in certain gravitational calculations,
2:30:31
Speaker 2: the correct entropy prescription also includes a region that would
2:30:35
Speaker 2: usually be regarded as lying within the black hole interior.
2:30:39
Speaker 2: This region is called an island. The method involves identifying
2:30:43
Speaker 2: special surfaces, known as quantum extremal surfaces, which balance a
2:30:48
Speaker 2: geometric area contribution with the entropy associated with quantum fields.
2:30:53
Speaker 2: Under suitable conditions, this changes the result of the entropy calculation,
2:30:58
Speaker 2: and remarkably, it can produce the expected page curve. Research
2:31:03
Speaker 2: by Ahmed Almeyri, Raghu Mahajan, Juan Maldacena, and Ying Zhao
2:31:09
Speaker 2: helped establish this approach. Related work by Jeff Pennington and
2:31:13
Speaker 2: other researchers developed the connection between quantum extremal surfaces, entanglement,
2:31:19
Speaker 2: and the information carried by Hawking radiation. Another important ingredient
2:31:24
Speaker 2: involves replica wormholes. These are particular mathematical contributions that arise
2:31:30
Speaker 2: when gravitational path integral methods are used to calculate quantum
2:31:34
Speaker 2: entanglement entropy. Despite their rather mysterious name, they should not
2:31:39
Speaker 2: be imagined as ordinary tunnels through which particles escape from
2:31:43
Speaker 2: inside a black hole. They are features of the theoretical calculation,
2:31:48
Speaker 2: and in certain models, including these contributions, produces the information-preserving
2:31:52
Speaker 2: behaviour that simpler calculations had failed to recover. The significance
2:31:58
Speaker 2: is considerable. It suggests that the apparently lost information may
2:32:02
Speaker 2: remain accounted for when quantum gravity and the structure of
2:32:05
Speaker 2: entanglement are treated more carefully. But important questions remain. Physicists
2:32:10
Speaker 2: do not yet possess a complete experimentally verified account of
2:32:14
Speaker 2: precisely how information is encoded in Hawking radiation from a
2:32:18
Speaker 2: real astrophysical black hole, nor is there a settled description
2:32:22
Speaker 2: of the ultimate endpoint of evaporation. The relationship between quantum
2:32:27
Speaker 2: entanglement and the emergence of spacetime geometry is itself an
2:32:31
Speaker 2: active subject of investigation, and some of the most successful
2:32:35
Speaker 2: calculations concern idealised models rather than black holes existing in
2:32:41
Speaker 2: the complicated environment of our own universe. There is also
2:32:45
Speaker 2: a crucial distinction between theoretical understanding and direct observation. Hawking
2:32:51
Speaker 2: radiation remains a powerful prediction of quantum field theory in
2:32:54
Speaker 2: curved spacetime. Its mathematical foundations have been studied extensively, and
2:33:00
Speaker 2: related effects have been investigated in laboratory analogue systems. But
2:33:05
Speaker 2: Hawking radiation from a genuine astrophysical black hole has not
2:33:09
Speaker 2: yet been directly detected The detailed process by which complete
2:33:14
Speaker 2: quantum information might emerge during evaporation remains an open scientific problem.
2:33:19
Speaker 2: And this brings us back to the deceptively simple question
2:33:22
Speaker 2: that began the investigation. How can a black hole lose
2:33:26
Speaker 2: mass if nothing can escape? The answer does not require
2:33:29
Speaker 2: ordinary particles to travel outward from inside the event horizon.
2:33:33
Speaker 2: It does not require light to exceed its universal speed limit.
2:33:37
Speaker 2: and it does not require Einstein's description of the event
2:33:40
Speaker 2: horizon to be abandoned. Instead, Hawking radiation arises from the
2:33:45
Speaker 2: behaviour of quantum fields in black hole spacetime. That radiation
2:33:49
Speaker 2: carries energy away, and the black hole's gravitational mass decreases accordingly.
2:33:55
Speaker 2: What happens to the complete quantum information associated with the
2:33:59
Speaker 2: black hole is a deeper and more difficult question. but
2:34:03
Speaker 2: investigating it has revealed unexpected connections between horizon area, entropy,
2:34:09
Speaker 2: quantum entanglement, and the possible microscopic structure of spacetime. A
2:34:15
Speaker 2: black hole, once regarded as little more than a permanent
2:34:18
Speaker 2: region of darkness, has become one of the most remarkable
2:34:21
Speaker 2: laboratories for theoretical physics. It brings together the geometry of
2:34:26
Speaker 2: the largest objects, the quantum behaviour of the smallest. and
2:34:30
Speaker 2: the fundamental principles governing energy and information. And perhaps that
2:34:35
Speaker 2: is the most extraordinary consequence of Hawking's discovery. By investigating
2:34:41
Speaker 2: something from which nothing can escape, physicists have uncovered clues
2:34:45
Speaker 2: about how the laws of the entire universe may fit together.
2:34:49
Speaker 2: And so, we return to the question that makes black
2:34:52
Speaker 2: holes so extraordinary. How can something lose mass when absolutely
2:34:56
Speaker 2: nothing can escape from inside it? The answer lies in
2:34:59
Speaker 2: the remarkable relationship between gravity and quantum physics. Albert Einstein's
2:35:05
Speaker 2: general theory of relativity tells us that an event horizon
2:35:08
Speaker 2: is a genuine boundary in spacetime, beyond which no light,
2:35:12
Speaker 2: matter or ordinary signal can return to the outside universe.
2:35:16
Speaker 2: But Jacob Bekenstein's work on black hole entropy and Stephen
2:35:20
Speaker 2: Hawking's subsequent discovery of thermal radiation revealed that black holes
2:35:25
Speaker 2: possess physical properties extending beyond the classical description. Hawking radiation
2:35:31
Speaker 2: does not require particles to escape from inside the event horizon.
2:35:35
Speaker 2: The familiar picture of virtual particle pairs is only a
2:35:38
Speaker 2: simplified illustration. The deeper explanation involves quantum fields, gravitational redshift,
2:35:45
Speaker 2: and the unusual geometry of black hole spacetime. Radiation carries
2:35:49
Speaker 2: energy away, and an isolated black hole consequently loses mass.
2:35:54
Speaker 2: As it becomes smaller, its temperature rises, allowing evaporation to accelerate.
2:36:00
Speaker 2: Yet for a black hole with the mass of the Sun,
2:36:02
Speaker 2: complete evaporation would take roughly 10 to the power of
2:36:05
Speaker 2: 67 years. And in today's universe, ordinary black holes generally
2:36:11
Speaker 2: absorb more energy from their surroundings than they release through
2:36:14
Speaker 2: Hawking radiation. Scientists continue searching for possible primordial black holes
2:36:20
Speaker 2: and investigating analogous effects in laboratories. But Hawking radiation from
2:36:25
Speaker 2: an actual astrophysical black hole has never been directly observed.
2:36:30
Speaker 2: and profound questions remain. The final stages of evaporation are uncertain,
2:36:36
Speaker 2: while the black hole information paradox challenges our understanding of
2:36:40
Speaker 2: how quantum information is preserved. Research into holography and the
2:36:44
Speaker 2: page curve has provided important theoretical progress, although a complete
2:36:49
Speaker 2: explanation remains beyond our reach. Perhaps the most remarkable discovery
2:36:54
Speaker 2: is that Einstein's event horizon need not be violated at all.
2:36:58
Speaker 2: A black hole can remain a place from which nothing escapes,
2:37:02
Speaker 2: while gradually surrendering its energy to the surrounding universe. And
2:37:06
Speaker 2: in understanding that curious distinction, we have discovered connections between gravity, heat,
2:37:13
Speaker 2: quantum mechanics and space-time that are still reshaping our understanding
2:37:17
Speaker 2: of nature. Even the darkest objects in the universe may
2:37:20
Speaker 2: have a faint glow, and even black holes may not
2:37:23
Speaker 2: last forever.