Brian Cox - The Most Mysterious Facts About The Universe
Brian Cox walks through the questions that keep working physicists up at night. He opens on the Fermi paradox, a galaxy of 400 billion suns that has stayed silent for over 10 billion years, and runs every serious resolution: rare Earth, undetectable machines, the von Neumann probe argument, the dark forest, Frank Drake's rare orchid analogy, and the great filter ahead of us or behind us. His own guess is that complex life is so rare we may be alone, because the eukaryotic cell evolved only once after three billion years of single cells. He then explains why space is flat, why quantum mechanics is the probabilistic base layer of reality, the true scale of the universe seen through Andromeda, the live hunt for life on Mars, real quantum teleportation, and wormholes hiding in century old math. The back half is a full history of the black hole, from Michell and Laplace's dark stars to the event horizon where time stops, the singularity that is a moment rather than a place, Hawking radiation, and the unsolved information paradox.
Published Apr 15, 202656:36 video41 min readAdded Jun 17, 2026Open on YouTube →
At a glance
This is Brian Cox walking through the questions that genuinely keep working physicists up at night, compiled by Astral Curiosity into one continuous hour. It opens on the Fermi paradox, the simple and brutal fact that a galaxy of 400 billion suns and trillions of planets, given more than ten billion years to work with, has produced no visible sign of anyone else at all. Cox takes that silence apart resolution by resolution: maybe nobody ever evolved, maybe they are already here as machines the size of an iPhone, maybe the galaxy is just too big, maybe the smart ones hide, maybe civilizations rise and die like rare orchids that never bloom at the same time, and maybe there is a great filter waiting in our future or buried in our past. His own guess, stated as a guess, is bleak and specific: we may be alone.
From there the conversation widens into the deepest machinery of the cosmos. He explains why space measures perfectly flat and why that flatness is the strongest hint that the universe is vastly bigger than the part we can see. He explains quantum mechanics as the base layer of reality and why its probabilities shocked even Rutherford. He tries, and admits he fails, to convey the true scale of the universe using the light from Andromeda. He covers the live hunt for life on Mars, real quantum teleportation done in the lab, and wormholes hiding inside the century old mathematics of black holes.
The back half is a complete history of the black hole, from John Michell and Laplace imagining dark stars in the 1780s, through Schwarzschild solving Einstein's equations in 1916, to the event horizon where time stops, the singularity that turns out to be a moment in time rather than a place, and Stephen Hawking discovering that black holes glow, shrink, and eventually die. It ends on the black hole information paradox, the single unsolved problem that forces our two greatest theories into the same room. What follows is the whole hour, in order, with every name, date, and number kept.
The Fermi paradox: a galaxy this big should not be this quiet
Cox lays the setup carefully because the whole hour hangs on it. Confine the discussion to our own Milky Way galaxy. We now know it contains something like 400 billion suns, and we now know that most of those suns have planetary systems around them. That is trillions of planets. The galaxy has been around for pretty much the age of the universe, more than ten billion years. So there is a lot of real estate, and there has been plenty of time for civilizations to develop.
The Fermi paradox, named for Enrico Fermi, is at its heart the statement that notwithstanding billions of years on billions of worlds for civilizations to arise, we see no evidence of any of them in the galaxy at all. The paradox is simply: why? Cox calls it an extremely good question, and the intellectual value of it comes from accepting two things honestly. First, accept that we genuinely have not seen any. There is no UFO sitting in a warehouse in Roswell. Second, accept the picture of the Milky Way he just gave, because that part is measurement, not speculation. We know the count and we know the age. The only open question is how to resolve the apparent contradiction.
Astronomers have a name for the result of all this. They call it the great silence. The rest of this segment is Cox running through every serious way physicists have tried to break it.
Figure 1. The seven exits Cox walks through. Every branch is a different answer to the same question: a galaxy with 400 billion suns has been silent the whole time we have been listening. His own bet sits on the lower right, the filter in our past, the idea that complex life is so rare we may be the only ones who ever made it this far.
Resolution one: nobody ever evolved (the rare Earth idea)
The first answer is that life simply never got complex anywhere else. Why might that be true? Look at what it took to produce our civilization on this planet. One thing it needed was time. We have good evidence that life was present here 3.8 billion years ago, perhaps earlier. The planet is 4.5 billion years old. So as a matter of observation, it took roughly 4 billion years to go from the origin of life to a civilization, from cell to civilization. That is a full third of the age of the universe.
So a possible answer is that the Earth is close to unique in the Milky Way in being stable enough, for long enough, for that journey to finish. Cox stresses how big an ask that is. On this planet an unbroken chain of life persisted for almost four billion years despite living in a violent universe. The Sun had to stay stable enough for long enough. Its output has changed over those billions of years, but never so radically that it erased life. No star massive enough to detonate as a supernova and sterilize the Earth went off close enough to us in four billion years. There were impacts, including the famous one that wiped out the large dinosaurs, but none big enough to break the unbroken chain.
So even though billions of planets may have liquid water and oceans that could support life, it may be that none of them in the whole galaxy stayed stable enough for long enough. That is the rare Earth hypothesis. Cox sharpens it: it is really a property of the whole solar system, not just the planet, because it depends on the parent star. Many stars are binary systems. Can a planet hold a stable enough climate and orbit in a binary system to support four billion years of unbroken life? Perhaps not. So he would rather call it the rare solar system hypothesis.
Resolution two: it is not a paradox, they are already here
Maybe there are intelligent civilizations out there and they are present in the solar system right now. What might such an intelligence look like? Who knows. They could have sent nano machines to our solar system. There could be probes all over the place, but if they are the size of an iPhone, we would have no way of detecting them. The technology of a sufficiently advanced species could be so far beyond anything we can comprehend or detect that we have simply been fooled into thinking nobody is here. Entirely possible, Cox says.
Resolution three: the galaxy is just too big
Another resolution is that the galaxy is so large and the distances between stars so great that even a civilization on the far side, with the most powerful radio transmitters imaginable, would have its signals diluted below detection. Or it may just be very, very difficult in an engineering sense to build interstellar spacecraft. Perhaps you can build something that hops a few light years to a neighbor, four light years in our case to Alpha Centauri, but you cannot build something that crosses a whole galaxy.
Cox has a counter to this one that he finds persuasive: the space travel argument, framed in terms of self replicating machines, so called von Neumann probes after John von Neumann. Imagine a civilization builds a machine, some kind of AI, smart enough to fly to a nearby solar system, copy itself, and send the copy onward. Then you get exponential growth: one, then two, then four, then eight. You can show that even with our rocketry, you could cover a galaxy like the Milky Way in a reasonably short time, by which he means maybe 100 million years, which is short on galactic scales. The key point is that once a single successful replicator launches, it is inevitable the galaxy fills with them within tens of millions of years. We see no evidence of any. If we assume we could detect them, their absence lets us infer that no civilization has ever reached that point. He calls that a quite persuasive argument.
Resolution four: the dark forest, where the wise stay quiet
Maybe there are many civilizations, but advanced ones choose to remain hidden. That is the dark forest hypothesis, also called the quarantine hypothesis. It asks us to make a moral judgment about how an alien civilization would behave, which is of course impossible to do with confidence. But imagine civilizations that become technologically advanced also become morally advanced, and decide, perhaps for good reason, to stay hidden so as not to draw attention. If you think carefully and believe other advanced civilizations are out there, maybe the logical move is to stay silent and hide as best you can.
Cox finds this hard to believe given human history. We have made no attempt to stay hidden. We have broadcast radio signals to the stars, like the Arecibo message, albeit weak ones. We launched pulsar maps on probes like Voyager that show the location of our solar system to anyone who finds them. So far we have done the opposite of hiding, trying at every opportunity to broadcast our existence. He offers the counterpoint that Carl Sagan argued: a civilization advanced enough to build interstellar craft and communicate across interstellar distances may be wise enough to have overcome the primitive instincts to fight, to colonize, to walk over others. Perhaps technological advance inevitably brings wisdom. Or maybe anyone clever enough to build an interstellar ship is also clever enough to hide it. Or maybe it is simply immoral to interfere, like the prime directive in Star Trek, a law of nature for sufficiently intelligent beings never to introduce themselves to another civilization.
Resolution five: civilizations are rare orchids that never bloom together
Maybe civilizations rise and then fall, and because of the sheer timescales and the sheer size of the galaxy, no two ever overlap. Cox tells a personal story. He once had the great pleasure of meeting Frank Drake, the legend behind the Drake equation, at his house. Drake also grew orchids, and Cox arrived, coincidentally, on the single day a rare orchid flowered. It blooms for one or two days, then disappears for the year, then blooms again the next year for one or two days. Drake used it as an analogy: maybe civilizations are like that, flowering and dying, flowering and dying, and just because of the timescales, none of them ever overlap. There could be the wreckage, the ashes, the fossils of civilizations out there, and we would have no way of knowing until we explore the galaxy and stumble on the ruins. Will we exist in 10,000 years? Cox says it is largely in our hands, and we may be stupid enough not to last beyond the next century.
Resolution six and seven: the great filter, ahead or behind
There is an idea called the great filter, and the filter can lie in our future or our past.
A filter in our future would mean civilizations do arise, reach roughly where we are now (rocketry, nuclear power, nuclear weapons, industrialization), and then hit a wall that stops them from becoming true spacefaring, multi planetary species. Why would such a wall exist? Cox does not think it is technology. He sees nothing in the laws of nature that would in principle stop us becoming an interstellar species, whether that takes a thousand, ten thousand, or a hundred thousand years, and a hundred thousand years is a blink in the life of a galaxy. The only thing he can see stopping us is our own stupidity. It could be that our scientific prowess exceeds our wisdom and political skill. Once a civilization develops the means to destroy itself, through nuclear weapons, biological weapons, or some loss of control over AI, it may go ahead and destroy itself almost inexorably, because it is just too hard politically to run a civilization that holds the power to end itself. He points to real near misses: the Cuban missile crisis, documented events in the 1980s where nuclear launches could have happened and did not, and surely others we never heard about. He adds the challenge of climate change, which we seem completely incapable of addressing as a global civilization, plus biological weapons and the threat of unregulated AI. It might be almost a law of nature that things which can build an industrial civilization are inherently too stupid to reach the stars, and he would not put that past us.
But his favorite is the filter in the past, and this is where he states his own guess plainly. If he had to guess why we see no evidence of others, the great silence, it is because there aren't any and there never have been. The reason is biology. Look at the history of life on Earth: it began 3.8 billion years ago, but for the best part of 3 billion years there was nothing more complex than a single cell. Only in the last billion years or so has multicellular life existed. There may be a good biological reason. The eukaryotic cell, the cell with a nucleus and organelles and (in plants) chloroplasts, which forms every multicellular living thing, appears to have evolved exactly once on this planet. That is widely accepted and is sometimes called the fateful encounter, the moment one cell engulfed another in the event that produced mitochondria. It seems a very unusual evolutionary event a billion to two billion years ago laid the foundation for us.
If that is typical, if it typically takes four billion years from cell to civilization, then there may be very few planets in a typical galaxy stable enough for long enough for the process to finish. For all we know, we are on the fortunate end of evolutionary timescales. On a truly typical planet it might take three or four times as long, which would exceed the current age of the universe. Cox's guess: there are probably microbes all over the place. He would be delighted but not surprised to find microbes on Mars, Europa, Enceladus, even in subsurface oceans as far out as Pluto. But a galaxy full of complex, sentient life, things smart enough to build rockets and head for the stars, may average less than one civilization per galaxy. He recommends the famous book by Barrow and Tipler, The Anthropic Cosmological Principle, in which the authors suggest there might be one civilization in the entire observable universe, which would be us. He will not necessarily go that far, but he thinks civilizations are very rare. And then, with a grin, he adds that if he walked outside over Westminster and saw a spaceship hovering, he would not be in the least bit surprised. The paradox cuts both ways.
He closes the segment on the moral weight of it. There is a profound puzzle in why there seem to be so few civilizations around, and it shadows our own situation, because we have effectively handed everyone the means to destroy themselves. If we cannot find a way to stop arguing and make a complicated world of many countries, cultures, and political histories work together, the same intelligence that built the tools could end us.
Figure 2. Why Cox's bet is on a filter in the past. For three of the roughly four billion years that life has existed on Earth, nothing was more complex than a single cell. The jump to complex life waited on the eukaryotic cell, which seems to have evolved exactly once. If that bottleneck is typical, most galaxies may never produce a single civilization.
Why space is flat, and what that says about the size of everything
The conversation turns to cosmology. The universe is expanding, and we always see the same glow of the Big Bang coming from every direction. But there are deeper reasons to think it is far larger than the part we can see, and one of them comes from the theory of inflation. The best way to explain the properties we observe is that the universe is very much bigger than the piece visible to us.
The key observation is that space measures flat. Cox almost refuses to hedge: it is flat. Picture slicing the universe at a given moment into a giant sheet, like a tabletop, then another sheet, then another. Each sheet has a geometry. It can be flat like a tabletop, curved like the surface of a sphere, or curved the opposite way like a saddle or a bowl. We can measure which it is, and when we measure, it comes out absolutely flat. That is a very unusual thing for it to be. Einstein's theory of general relativity says the curvature of space is determined by the stuff inside it: put matter and energy into space and it curves, bends, warps, and stretches. What we find is that there is precisely the right amount of stuff in the universe to make it completely flat, the critical density.
The favored explanation is that the universe is way bigger than the piece we can see. It is like looking at a one mile square patch of the Earth. That patch looks flat. You have to look across distances comparable to the radius of the Earth to notice you are on a curved surface. We are just looking at a tiny piece of the cosmos, which is why it looks flat.
Cox gives a clean way to define flatness without three dimensions. On a flat tabletop, draw a triangle and the angles add up to 180 degrees. Draw a circle and the ratio of circumference to diameter is exactly pi. On the curved surface of the Earth, a big triangle's angles do not add up to 180 degrees, and that pi ratio changes. So flatness has a precise meaning, and it generalizes to three dimensions even though we cannot picture it directly. We actually do this measurement with light. The most distant light we can see is the cosmic microwave background radiation. Looking farther out means looking farther back in time, because light from a billion light years away took a billion years to arrive, so we see it as it was a billion years ago. We can see almost all the way back to 13.8 billion years ago, very close to the Big Bang, to light that began its journey before there were galaxies. That oldest light is, by the way, one of the answers to anyone who says they do not believe in the Big Bang: you can see it. We have pictures of it. There are ripples or structures in that light that act as a ruler on the sky, and because the light has traveled across the universe to reach us, we can see how that ruler has been distorted and infer whether space is flat, curved, or warped, purely from the measurement.
Figure 3. The three geometries space could have, distinguished by a single test: draw a triangle and add the angles. Measurement of the cosmic microwave background ruler comes back flat, the angles sum to 180 degrees, which is the strongest hint that we live inside a universe far larger than the patch we can see.
Quantum mechanics is the base layer, and it runs on probability
Cox is firm that quantum mechanics is the base theory of reality. People tend to think of it as something that applies to atoms and electrons and particles but not to us. That is not right. Quantum mechanics describes everything. The reason big things look classical is that when you apply quantum mechanics to large objects, it produces behavior that is more usefully, though not more accurately, described by tools like Newton's laws. You need quantum mechanics to understand how a transistor works, how a laser works, how a nuclear reactor works, or how the Sun works. It is not a theory of little things bouncing around. It is necessary to understand a great deal we take for granted.
And it is a strange theory because it has probabilities built in. It was Ernest Rutherford, who won the Nobel Prize at Manchester for discovering the atomic nucleus, who pointed out in the late 1800s and early 1900s that radioactive decay is a probabilistic process. Take a uranium nucleus, or even make a hundred identical uranium nuclei at the same instant, and you still cannot predict when any one of them will decay and emit radiation. It is fundamentally probabilistic. That idea was shocking, because it meant the world is not Newtonian clockwork. Cox's one line summary: quantum mechanics is a weird, probability based theory, and it is the way things actually are.
The true scale of the universe, told through Andromeda
Cox tries to give the scale in numbers and then admits defeat. The nearest large galaxy, Andromeda, is about 2 million light years away. Light travels at 300,000 kilometers per second, so from our point of view it takes 2 million years to travel from Andromeda to us. Go out away from city lights on a clear, moonless night with binoculars and you can see Andromeda. It is worth looking at, because the light entering your eye began its journey 2 million years ago, before we evolved on Earth. You can just about see it with the naked eye because it is big, a galaxy of about a trillion stars, and although very faint it is actually larger than a full Moon on the sky. He thinks that is the best handle on scale, because you can almost get your head around it.
But that is only the nearest galaxy. When you look at the beautiful images from the Hubble Space Telescope or the James Webb Space Telescope, you are seeing things 50 million light years away, 300 million light years away for something like Stephan's Quintet, and onward. Many of the galaxies in those photographs are so far away that their light began its journey before the Earth had even formed. That, Cox says, is incomprehensible. The honest answer is that he cannot explain or picture the true scale of the universe.
Mars: a rover taking samples right now
As they speak, there are several missions out there, and one is Perseverance, a rover currently on the surface of Mars. It is taking samples, including a lot from below the ground, and packaging them up for return to Earth in a program called Mars Sample Return. The reason is that life may have existed on Mars, or may still exist, because the conditions were right. It is almost certain that 4 billion years ago Mars was like the early Earth, with oceans and an atmosphere, around the same time life began here. So it is entirely possible. In fact, Cox says it would almost be surprising if life had not existed there, because that absence would tell us something important: that even given the right conditions, the step from geology to biology, whatever the origin of life actually is, is not high probability.
Quantum teleportation is real, and done with photons
Asked about teleportation, Cox says we genuinely do it in the lab, with photons, particles of light. The experiment relies on two photons that are quantum entangled together. The fastest way to describe entanglement is that the two behave as a single system, but you can separate them. You then arrange for a third photon to come in and interact with one of the entangled pair, so that the distant partner, which could be a light year away, ends up in the same quantum state as the incoming one. That is quantum teleportation. The incoming photon gets destroyed in the process, its state changed. So yes, this is real. The question then becomes how big an object you can teleport. In principle, Cox says, there is no reason you cannot teleport anything. But he stresses very much in principle.
Wormholes have been hiding in the math for a century
Wormholes first entered physics in a 1935 paper by Einstein and Nathan Rosen. They were studying the description of the distortion of spacetime around a star that Karl Schwarzschild had discovered in 1916. They found that if you take that description, let the star collapse into a black hole, and extend it infinitely into the past, imagining the black hole had always existed in an eternal universe, you get a wormhole. The description of it is literally inside Schwarzschild's mathematics: two universes linked by a wormhole, one side a black hole and the other a white hole. That is what an Einstein-Rosen bridge is, and it requires the black hole to have existed forever.
If the black hole is spinning, you get an infinite tower of wormholes. That is the Kerr black hole, discovered in the 1960s by Roy Kerr. The eternal solution for a spinning black hole has an infinite number of universes connected inside it by an infinite number of wormholes. Modern work on what happens to things falling into black holes suggests there may be a kind of wormhole connecting the interior to the exterior universe, so the singularity may look like a tangled web or network of wormholes. Cox hedges his language deliberately, because the language is not really known yet, but it is one of the possibilities the mathematics suggests for the interior of a black hole.
Finally there is something called ER equals EPR. ER is the Einstein-Rosen-Schwarzschild wormhole, and EPR is Einstein-Podolsky-Rosen, the famous question about quantum mechanics and what Einstein called spooky action at a distance, where entangled quantum particles seem to affect each other instantly, though not in a way that lets you transmit information. There is a picture in which you can describe those two entangled particles as being connected by wormholes, an idea suggested by Leonard Susskind and others a few years ago. So wormholes are very much on the agenda in trying to understand what space and time are, and they are not just science fiction.
Black holes: dark stars, escape velocity, and the long argument over whether nature would build them
The simplest description of a black hole is a region of space from which even light cannot escape. Predictions that such objects might exist go all the way back to the beginnings of relativity at the turn of the 20th century, but really, until the 1960s and perhaps even into the 1980s, many physicists felt that because of the intellectual challenges these objects posed, nature would refuse to create them. Cox notes that even the great Steven Weinberg once said he in some sense hoped they would not exist because they are so confusing. But we now know they do exist, so we have to face the challenges they pose.
The deeper reason black holes matter, going back to Stephen Hawking's work in the 1970s, is that they force us to think about quantum theory and general relativity at the same time. The quest to unify those two pillars into a quantum theory of gravity is a holy grail for theoretical physicists, but the problem has always been finding somewhere in nature where the two theories must be merged to explain what we see. Black holes are the unique place in nature where we can see a thing just sitting in the sky that demands both theories work together to reveal a deeper one.
The idea itself goes back to the 1780s and 1790s. Two natural philosophers had the same thought apparently independently: an English clergyman, John Michell, and the great French mathematician Pierre-Simon Laplace. Both were thinking in terms of escape velocity, the speed you must travel to completely escape the gravitational pull of a planet or star. For the Earth, escape velocity from the surface is about 8 miles a second, 11 kilometers a second. Go bigger and more massive, up to a star like the Sun, and the escape velocity rises with the gravitational pull at the surface, reaching about 400 miles a second for the Sun. Michell and Laplace imagined going bigger still: ever more massive giant stars, until the gravitational pull at the surface made the escape velocity exceed the speed of light. Then you would not be able to see them. There is a wonderful line in Laplace's paper that the largest objects in the universe may go unseen by reason of their magnitude. Cox calls these dark stars, the term Laplace used.
We now know such objects do not exist in the sense Michell and Laplace meant. But they missed something that sounds almost paradoxical: you can also raise the escape velocity at the surface by squashing the object. Take the Earth and squash it down and down until its radius is just less than a centimeter, and the gravitational pull at the surface becomes so great that light cannot escape. That is essentially the modern concept of a black hole. Michell and Laplace worked in Newtonian physics, pre-Einstein. In 1915 Einstein published general relativity, a better theory of gravity, and it turns out black holes exist there too. The idea that objects can be compressed until they trap light survives the upgrade from Newton to Einstein.
The first physicist to derive the mathematics describing such a thing, though he did not know they existed, was Karl Schwarzschild. He provided an exact solution to Einstein's equations describing the distortion of space and time around an idealized star, a perfectly spherical, non spinning ball of matter, way back in 1916, just after the theory was published. Schwarzschild's solution also describes what that space looks like if you completely remove the star but leave its imprint in the fabric of the universe behind, and that is essentially the model of a modern relativistic black hole. Yet people did not think in those terms for decades.
Figure 4. A black hole as Einstein and Hawking together describe it. The event horizon is where, from outside, time freezes, while an astronaut falling across it for a large enough hole notices nothing. At the center sits the singularity, which in pure general relativity is not a place but a moment, the end of time. And from just outside the horizon leaks Hawking radiation, the glow that means a black hole is not an eternal prison but something with a lifetime.
In the 1930s you see Einstein and his colleague Rosen exploring that spacetime and building models of what it might look like, but it is true to say that until the late 1930s, and arguably until the 1960s, most physicists thought such things would not exist in nature. They were theoretically interesting, perhaps not practically interesting. The reason is that you have to actually create such a thing. It is one matter to have a model of space and time describing this object, and another for nature to make it. Through the 1930s there were many papers. Robert Oppenheimer and his student Hartland Snyder wrote a very famous paper just before the war exploring whether a real star at the end of its life could collapse without limit to form this geometry. Under certain assumptions, they showed a star could behave exactly that way. But it was not really until the work of Roger Penrose, Stephen Hawking, and several others in the 1960s that it began to look as if nature would build these things. Cox recalls a line he treasures from Arthur Eddington, a colleague of Einstein, very English and very proper, who declared that nature will prevent such absurdities from existing. It turns out nature does not prevent it. We have now observed that stars do collapse to form black holes.
The event horizon, where time stops, and the singularity that is really a moment
To understand the conceptual problems black holes pose, Cox gives the pure general relativity picture. What do you see from outside? There is an event horizon surrounding the black hole, the boundary between the external universe and the interior. A bit hand wavy but reasonable: imagine a sphere in space, and if you cross the boundary into its interior, then even traveling at the speed of light you cannot escape. The horizon separates the inside from the external universe.
There is another description of the horizon that confused people throughout the history of black hole research, certainly into the 1930s and even post war: viewed from outside, the event horizon is a place where time stops. That is a direct prediction of relativity. If you watched an astronaut falling toward a black hole, from your external perspective you would see their time pass more and more slowly, until on the horizon you would see their time stop. That suggested to many people that a star could not collapse to form a black hole at all. If a star is collapsing, does it not freeze forever on the horizon? These early conceptual problems were eventually solved. The one sentence key to relativity is that time can stop from one perspective while passing at the normal rate from another. From the perspective of an astronaut falling into a sufficiently large black hole, like the ones at the centers of galaxies, they would notice nothing at all as they crossed the horizon. Time passes at one second per second on their watch, while from outside, time freezes on the horizon. These apparent conceptual challenges are not really challenges at all, just a central part of general relativity, and by the 1960s people understood what these things are.
There is one central problem still not solved: what lies at the center. Cox is careful with his language, because in pure general relativity it is not even right to talk about the center of a black hole. The thing we are picturing is the singularity. You might think of it as an infinitely dense point to which the massive star collapses, which is the natural way to imagine it. But even in pure general relativity, when you look at a proper map of a black hole, the so called Penrose diagram named after Roger Penrose, the singularity is not really a place in space at all. It is a moment in time, and in fact it is the end of time. One way to picture what happens when a star collapses is that space and time become so distorted that their roles swap. What we thought of as an infinitely dense point, a place in space at the center of the collapse, actually becomes a moment in time, the end of time, the singularity. But the nature of that thing was not, and still is not, understood. It is a great mystery, and it has long been accepted that we will need a quantum theory of gravity to explain it.
For many years people thought the singularity might stay a mystery for a century, because we have no access to it and no conceptual tools to explore it. The great revolution, beginning with Stephen Hawking's work in the mid 1970s, was to notice that the deep conceptual problems are not only at the singularity but at the event horizon itself. That was a real challenge, and an interesting one, because we strongly expect the laws of nature we currently understand, the ones we have full mathematical and conceptual control of, to apply at the horizon. That is why black holes became so interesting: the horizon is a place where we believe we fully control the physics, yet there is a fundamental clash of principle between our two basic theories of nature, general relativity and quantum mechanics. That clash is ultimately why the event horizon, and black holes themselves, have become so fascinating and so important.
Hawking radiation: black holes glow, shrink, and die
The modern revolution, still ongoing, begins with Hawking's mid 1970s work. In his own words, Hawking showed that black holes ain't so black. We had been picturing them as prisons, regions from which nothing can escape. What Hawking showed in a landmark pair of papers, a tour de force of calculation, is that if you apply quantum theory near the horizon of a black hole, they glow. They produce particles. They have a temperature. The thing we pictured in Einstein's theory as pure geometry, just distorted space and time, actually emits particles. It is called Hawking radiation.
Cox gives Hawking's own hand wavy picture from the 1974 paper, stressing it is not a full description; you need the mathematics for that. Zoom in on space right at the event horizon. If you zoom in on any piece of space, even the empty piece in front of your nose, and slow time down with a giant microscope, you can picture a constant churn of particles coming in and out of existence, so called entangled particles. This is a picture of what the vacuum of space looks like everywhere, even in the emptiest space imaginable. Now consider the case where one of a pair of these particles is just inside the horizon and its partner is just outside. The outside particle, instead of merging back with its partner, can escape into the universe, made real by the presence of the black hole. Its partner is interior to the black hole, and the escaping particle heads off into the universe carrying energy away from the black hole as it goes. This rain, this glow of particles, is Hawking radiation.
The upshot is profound. Picture this thing: a black hole, glowing, emitting particles, losing energy, therefore shrinking, which means that one day it will be gone. So black holes are not eternal prisons. They have a lifetime. One day, whatever is in there is returned to the universe.
The black hole information paradox
That immediately raised the central question. What happened to all the stuff that fell in? The way Einstein's theory describes it, that stuff goes to the singularity, the end of time, a region so convoluted we do not know how to describe it. But then one day the whole thing is gone, and all that is left in the far, far future is the Hawking radiation produced near the horizon. The question: is it possible, if you could collect all that radiation across the whole life of the black hole, that the information about everything that ever fell in is imprinted in that radiation? Is that true, or not?
Why does that question matter? Cox makes it concrete. Take anything in the room, a book, a table, the camera, and incinerate it. Throw it in a furnace, or into the heart of a nuclear bomb, completely destroy it. In basic fundamental physics, if you could collect every piece of the thing you detonated, every quantum of radiation, every photon, every particle, and you were clever enough, you could in principle reconstruct the thing you destroyed. Information is conserved in the universe as far as we know. Every law of nature we have says information is conserved.
The problem is that Hawking's initial calculation of how black holes evaporate said information is not conserved. It said black holes are erasers of information. Bluntly, once the black hole had gone, there would be absolutely no way, even in principle, to learn anything or reconstruct anything about what fell in, including the very star that collapsed to form it. That would make black holes the only information erasers known in nature. That was the initial picture, as Hawking understood it in the 1970s and 1980s, and it became known as the black hole information paradox: a direct contradiction between Hawking's own calculation and the bedrock principle that information cannot be destroyed. It is the unsolved problem the hour leaves you on, the clearest single example of why physics still needs to merge its two greatest theories.
Key takeaways
The Fermi paradox is not soft speculation. The galaxy holds about 400 billion suns and trillions of planets and has had over 10 billion years, yet we detect nothing. The numbers are measurements; only the explanation is open.
Cox catalogs the serious resolutions: rare Earth (really rare solar system), they are already here as undetectable machines, the galaxy is too big, the von Neumann probe argument that absence implies no one ever reached that stage, the dark forest, Frank Drake's rare orchid analogy, and the great filter lying ahead of us or behind us.
Cox's own guess: a filter in the past. Complex life waited on the eukaryotic cell, which evolved once, after three billion years of nothing but single cells. A typical galaxy may average less than one civilization, and Barrow and Tipler suggest there may be only one in the observable universe.
Space measures flat, the angles of a triangle summing to 180 degrees, which is the strongest evidence the universe is far bigger than the part we can see, confirmed using the cosmic microwave background as a ruler on the sky.
Quantum mechanics is the base layer of reality and is fundamentally probabilistic, as Rutherford saw in radioactive decay; you cannot predict when a single uranium nucleus will decay.
Andromeda's light is 2 million years old, predating us; the deepest JWST galaxies shine with light older than the Earth. Cox admits the true scale is beyond picturing.
Quantum teleportation is real and routine in the lab with entangled photons; the incoming state is destroyed as the distant one is recreated.
00:00 The Fermi paradox, set up: 400 billion suns, trillions of planets, total silence
01:40 Resolution one, nobody evolved: time, stability, and the rare Earth idea
07:30 Rare solar system, binary stars, and a property of the parent star
08:00 Resolution two, they are here: nano machines the size of an iPhone
09:20 Resolution three, the galaxy is too big and signals too diluted
10:00 The von Neumann self replicating machine argument against ET
12:00 Resolution four, the dark forest and quarantine hypothesis
14:30 Carl Sagan, wisdom, and the Star Trek prime directive
16:00 Resolution five, civilizations rise and fall: Frank Drake's rare orchid
18:00 The great filter in our future: nuclear weapons, climate, and AI
22:00 The great filter in our past: biology, the eukaryotic cell, Cox's own guess
26:00 Barrow and Tipler, one civilization per observable universe, the moral weight
30:00 Why space is flat and what flatness says about the size of the universe
35:00 The CMB as a ruler on the sky, looking back to 13.8 billion years
37:00 Quantum mechanics as the base layer, Rutherford, and radioactive decay
40:00 The true scale of the universe through Andromeda and the deep field
43:00 Mars right now: Perseverance, sampling, and Mars Sample Return
45:00 Quantum teleportation with entangled photons, real in the lab
47:00 Wormholes in the Schwarzschild and Einstein-Rosen math, Kerr, ER equals EPR
50:00 Black holes: Michell and Laplace, escape velocity, dark stars
53:00 Schwarzschild 1916, Oppenheimer and Snyder, Eddington's objection
55:00 The event horizon, where time stops, and the singularity as a moment in time
58:00 Hawking radiation: black holes glow, shrink, and die
1:02:00 The black hole information paradox
Notable quotes
"Notwithstanding the fact that there have been billions of years on billions of worlds for civilizations to arise, we see no evidence of any of them in the galaxy at all." Brian Cox, stating the Fermi paradox.
"On this planet an unbroken chain of life existed for almost 4 billion years, notwithstanding the fact that we live in a violent universe." Cox, on how rare the Earth's stability may be.
"Once a single successful replicator has been launched, then it is inevitable that over a few tens of millions of years the galaxy will be covered with replicators. And we don't see any evidence of them." Cox, on the von Neumann argument.
"Maybe civilizations are like rare orchids. And so they flower and die, and flower and die, and just because of the sheer timescales involved, none of them ever overlap." Cox, relaying Frank Drake's analogy.
"If I was to guess why we see no evidence of other civilizations out there, the so-called great silence, it is because there aren't any and there never have been. That's my guess." Cox, on the filter in our past.
"There might be one civilization in the observable universe. Which would be us." Cox, summarizing Barrow and Tipler.
"We measure space to be what's called flat. I don't even know if I'd say what's called flat, it is flat." Cox, on the geometry of the universe.
"Quantum mechanics is the theory that describes everything." Cox, refusing to confine it to the small.
"The light entering your eye began its journey 2 million years ago, which is before we evolved on Earth." Cox, on looking at Andromeda.
"Nature will prevent such absurdities from existing." Arthur Eddington on black holes, as quoted by Cox. "Well, it turns out nature doesn't prevent it."
"Black holes ain't so black." Stephen Hawking, quoted by Cox on Hawking radiation.
"Black holes are erasers of information. The only information erasers that we know of in nature." Cox, stating the information paradox.
Resources mentioned
Brian Cox, physicist and presenter, the speaker throughout
Most of this hour is settled physics, carefully labeled by Cox as such. Flat space, the cosmic microwave background, the probabilistic nature of quantum mechanics, the scale distances, lab quantum teleportation, and the existence of black holes with event horizons are all established and measured. The frontier items he flags himself: the Fermi paradox has no agreed answer and his "we are alone" view is explicitly a guess; wormholes and ER equals EPR are live theoretical ideas, not confirmed objects; the nature of the singularity is unknown and awaits a quantum theory of gravity; and the black hole information paradox, while widely believed to resolve in favor of information being preserved, remains the open problem the hour closes on. Cox is scrupulous about which is which, hedging his language exactly where the science is still being written.
Full transcript
The Fermi paradox is this. We know that we live in a big old galaxy in a big old universe. And let's for the purposes of this discussion confine ourselves to the Milky Way galaxy. The Milky Way galaxy we now know has something like 400 billion suns. And we now know that most of those suns have planetary systems around them.
So, trillions of planets. The galaxy's been around for pretty much the age of the universe, 10 billion years plus. And so, there's a lot of real estate, and there's been plenty of time for civilizations to develop in the galaxy. The Fermi paradox at its heart is the the statement that notwithstanding the fact that there have been billions of years on billions of worlds for civilizations to arise, we see no evidence of any of them in the galaxy at all. So, the paradox is why?
It's a paradox. And I think it's a very good question. It's an extremely good question. And there can be many answers. And and the great fun or the great I would say the intellectual value of the Fermi paradox is if we accept that we haven't seen any.
So, let's accept that. Let's accept that that you know there isn't a UFO sitting in some warehouse in Roswell or something like that. Let's accept that we haven't seen any. Let's accept Well, we have to accept the picture that I've just given you about the Milky Way galaxy and its age cuz that's a measurement, so we know that. The question is then, why?
How do we resolve these apparent contradictions and paradoxes? So, one answer to the Fermi paradox, this is the idea that we don't seem to see anyone, is that no one ever evolved. Right? So, so life didn't get complex. Why might we think that?
Well, what did we need to produce our civilization on this planet? Well, on this planet one thing we needed was time. We have good evidence that life was present on this planet 3.8 billion years ago, perhaps even earlier. The planet is 4 and 1/2 billion years old. So, we know as a matter of fact, as an observation on this planet, that life was present 3.8 billion years ago, but it took 3.8 billion years, give or take a few tens of thousands of years, to go from the origin of life to a civilization.
Let's say from cell to civilization, 4 billion years. Uh that's 1/3 of the age of the universe. So, a possible answer to the Fermi paradox, the question of why there are no civilizations, is because the Earth is pretty much unique in the Milky Way galaxy in that it was stable enough, the climate, the conditions on Earth were stable enough for long enough for life to go from cell to civilization. If you think about it, that's a big ask. What I'm saying is on this planet an unbroken chain of life existed for almost 4 billion years, notwithstanding the fact that we live in a violent universe.
The sun must have been stable enough for long enough. We know the output of the sun has changed over those billions of years, but it's not changed so radically that it managed to erase, destroy life on Earth. We know that we live in a violent universe. We know there are supernova explosions all over the place. It turns out that there have been no stars massive enough to explode as a supernova in a way that would damage life on Earth or erase life on Earth in this vicinity for 4 billion years.
We know that nothing has happened like a We know that there've been impacts on the Earth, right? We know that the famous impact that wiped out the the the large dinosaurs. There's been no impact big enough to destroy the unbroken chain or break the unbroken chain of life for 4 billion years. So, maybe maybe it's the case that whilst there are billions of planets which may have liquid water on the surface, may have oceans that can support life, it may be that none of those planets in the Milky Way galaxy have been stable enough for long enough to produce a civilization. So, that would be a property of the planet itself, the so-called rare Earth hypothesis.
Actually, I should say it's a property of a solar system. It's not really just a property of the planet. It's a property of the parent star. You could ask the question, well, if it was a binary system, for example, a binary star system. Many stars are binary star systems.
Is it possible to have a planet with a stable climate, stable enough orbit in a binary star system to support an unbroken chain of life for 4 billion years? Perhaps not. Well, when we talk about rare Earth, um I think I would like to talk about rare solar system. Another possibility with the Fermi paradox is that it's not a paradox, they are here. So, there are intelligent civilizations out there and they are present in the solar system.
It's possible. Let's think, for example, what such an intelligence might look like. Well, well, who knows? They could have sent nano machines to our solar system. There could be probes all over the place in the solar system, but if they're the size of an iPhone, then we'd have no way of detecting them.
So, it could be that technology of a sufficiently advanced alien species, a civilization, is so beyond anything we can comprehend or detect that we haven't seen it and we've been fooled into thinking that there are no advanced civilizations in the galaxy. And that's certainly entirely possible. Another possibility, another possible resolution to the Fermi paradox, is just that the galaxy is so big. The distances between stars are so great that if you imagine there's another civilization, let's say on the other side of our galaxy, even if they had the most powerful radio transmitters you could imagine, or even if they'd spread out to neighboring solar systems, then it may just be that the distances are so great that the signals are diluted that we can't detect them because they're too weak, or that it's just very, very, very difficult in an engineering sense to build interstellar spacecraft. And perhaps you can build a spacecraft that can hop a few light-years away to the nearby solar system, 4 light-years in our case to Alpha Centauri, but you can't build spacecraft that can traverse a galaxy.
That That's a possibility. One of the arguments against that for me is the argument, it's called the space travel argument against the existence of extraterrestrial life. And it's often framed in terms of self-replicating machines, so-called von Neumann machines. So, imagine it's possible for us, for a civilization, to build a machine, some kind of AI, that's sufficiently smart and capable that it can fly to a nearby solar system, reproduce itself, copy itself, and then send the copy out to the next solar system, and so on. So, you have, if you build a one successful replicator, you have an exponentiation of replicators.
You have one, and then two, and then four, and then eight, and so on, exponential growth. And you can show that even given our rocketry technology, you can cover a galaxy like the Milky Way in a reasonably short space of time. By reasonably short, I I I might even mean 100 million years. Right? That's reasonably short on galactic time scales.
But the key point is once a single successful replicator has been launched, then it is inevitable that over a few tens of millions of years, the galaxy will be covered with replicators. And we don't see any evidence of them. It's possible that we can infer that if we assume that we could detect them, then the absence of them may allow us to infer that no civilization has ever got to that point. And I think that's quite a persuasive argument. Now, it's possible that there are many civilizations out there, but that advanced civilizations choose to remain hidden.
So, that's called the dark forest hypothesis, the quarantine hypothesis. We've been asked to make moral judgments or judgments on how a civilization will behave, what they will choose to do, and that's of course impossible to judge. But um let's imagine civilizations, when they get technologically advanced, also get intellectually morally advanced. And let's say that they choose, perhaps for good reason, let's say they choose to remain hidden because they don't want to draw attention to themselves. Let's say it's It's inevitable that if you think about it carefully, and you think there are other advanced civilizations out there, then you choose to remain silent.
You hide yourself as best you can. It's possible that that's the way that a civilization would think. Maybe that's the logical thing to do. I find it difficult to believe, given human history, that that's the way that intelligent civilizations behave. We certainly haven't made any attempt to remain hidden so far.
We've broadcast radio signals out to the stars. The Arecibo message, for example, albeit weak ones. We've launched on our space probes like Voyager maps pulsar maps in that case, which shows the location of our solar system should any other civilization find it. So, at least at the moment we haven't come to the view, maybe a wise view, but we haven't got there, that we should remain silent. Quite the opposite.
We've tried at every opportunity to broadcast our existence. Maybe that's cuz Carl Sagan argued, I think, that a sufficiently advanced civilization, a civilization that can build interstellar spacecraft and communicate across interstellar distances, perhaps is is wise enough to have overcome those primitive instincts, the instinct to cause trouble, to fight wars, to to colonize, to walk over other civilizations. Perhaps it's inevitable that with technological advance ultimately comes wisdom. But it's hypothesis. Maybe it is.
Maybe it's just anybody sufficiently clever to build an interstellar spaceship will be also sufficiently clever to hide it and not draw attention to themselves. Maybe it's immoral. Maybe it's like Star Trek. Maybe it's the prime directive. Maybe it's it's morally certain that if you're sufficiently advanced, you decide to take the position that you will never introduce yourself or interfere with another civilization.
Maybe that becomes a kind of law of nature for sufficiently intelligent beings. Maybe that's conceivable as well. Another explanation for the Fermi paradox might be that civilizations live and die. They rise and then they fall. And because of the sheer timescales involved and the sheer size of the galaxy, no two civilizations ever overlap.
I once had the great pleasure of meeting Frank Drake the Drake equation, legend, in his house. And he also grows orchids. And I arrived at his house just coincidentally on the day that this rare orchid flowers. And it flowers for I think one or two days and then goes away again for the year and then flowers again the next year for one or two days. And he used it as an analogy.
He said, "Well, maybe civilizations are like that." So, maybe civilizations are like rare orchids. And so, they flower and die, and flower and die. And just because of the sheer timescales involved, none of them ever overlap. And so, there could be the wreckage the ashes, the fossils of civilizations out there. But of course, we would have no way of knowing until we explore the galaxy and maybe find the the ruins of these other civilizations.
Who knows? I mean, it's quite plausible if you think about it. Are we going to exist in 10,000 years' time? It's to a large extent in our hands. Maybe we're sufficiently stupid that we won't exist beyond the next century.
There's an idea in this field trying to explain the Fermi paradox called the great filter. Now, the great filter can lie in our future or our past. So, let's think about what it would mean for a great filter to lie in our future. That would mean that civilizations do arise in the Milky Way galaxy and get to somewhere like the position that we're at now. So, they develop rocketry, they develop nuclear power, nuclear weapons, for example.
They industrialize. But then, there's a filter in the future that stops them becoming true spacefaring civilizations. So, stops them becoming multi-planetary species and stops them ultimately traveling between solar systems to begin to colonize the galaxy. So, why might that be? Why might there be a filter waiting for us in our not-too-distant future that's going to stop us going on to Mars and stop us escaping our solar system?
What might stop us from becoming an interplanetary species and ultimately traveling out beyond our solar system? I don't think it's technology. As far as I can see, I don't see anything in the laws of nature in principle that would stop us from becoming an interstellar species. Might be a thousand years in the future, 10,000 years in the future, might be a hundred thousand years in the future. Even that, right?
A hundred thousand years is a blink of an eye in the lifetime of the universe, in the lifetime of a galaxy. So, I don't see any reason in principle why we couldn't become an interplanetary, interstellar species other than potentially our own stupidity. And I think that probably it could be one of the reasons why we don't see any other civilizations around. It could be that our knowledge, our scientific prowess exceeds our wisdom, exceeds our political skill. It could be that once a civilization develops the means to destroy itself in the form, for example, of nuclear weapons or biological weapons or maybe some kind of a lack of control of AI, who knows?
It may be that once a civilization acquires that technical know-how then it goes ahead and destroys itself essentially inexorably because it's just too difficult politically to run a civilization that has the power to destroy itself. If you look back through our recent history, there've been several occasions that we know about, that I know about and you know about, where we came very close to destroying ourselves or at least setting us back to the Stone Age, basically. The Cuban missile crisis, well-documented events in the 1980s, for example, where there could have been nuclear launches and weren't. I'm sure there are many others that we don't know about. There's the challenge of climate change.
We're completely incapable of coming together at the moment as a global civilization to address that challenge. That could set our civilization back. Biological weapons the threat of AI, we seem to be completely incapable of regulating those threats. So, it might just be almost a law of nature. Things like us, things that can build an industrial civilization, are just inherently too stupid to get out there to the stars.
And I wouldn't put that past us. My favorite's the other one. So, I'll do the other great filter. If I was to guess, and this is a guess, if I was to guess why we see no evidence of other civilizations out there, the the so-called great silence, is what astronomers call it, is because there aren't any and there never have been any. That's my guess.
The reason I guess that, and I emphasize it's a guess is biology. So, if you look at the history of life on Earth, then we see that life began 3.8 billion years ago, let's say. But then we see for the best part of 3 billion years on this planet that there's nothing more complex than a single cell. 3 billion years. It's only in the last billion years or so, perhaps a little bit less, that multicellular life has existed on this planet.
And there could be good biological reasons for that. One that springs to mind is the evolution of what's called the eukaryotic cell, which is the cell with the cell nucleus and all little organelles and chloroplasts in plants and all those things, which form all multicellular living things on the planet. Those cells, which seem to be prerequisite for complex, multicellular life evolved once on this planet as far as we can tell. Pretty widely accepted. It's called the fateful encounter hypothesis.
And so, it seems that there was a very unusual evolutionary event at some point, maybe a billion, billion and a half, even 2 billion years ago, that laid laid the foundations for us. If that's typical, if it typically is the case that it takes 4 billion years from cell to civilization then I think there may be very few planets in a typical galaxy which are stable enough for long enough for that process to to to proceed. And we could be, for all we know, on the fortunate end of evolutionary timescales. We don't know. Let's imagine that actually we were on the lucky side and really on a typical planet, if there is such a thing, then it takes three or four times as long.
That would exceed the current age of the universe. My guess is that whilst I think there might be microbes all over the place I wouldn't be surprised, I'd be delighted, but I wouldn't be surprised if we found evidence of microbes on Mars Europa Enceladus, even in the subsurface oceans of of places potentially even as far out as Pluto, right? In subsurface lakes, if they exist, liquid water below the surface, who knows? I wouldn't be surprised if we find microbes all over the place. But a galaxy full of complex living things, other planets with not only complex life, but sentient life things as smart as us, things smart enough to build rockets and head out to the stars, my guess is that a typical galaxy may have, less than, on average, less than one civilization per galaxy.
Let's put it that way. And actually, just to say, there's a very famous book I'd strongly recommend, Barrow and Tipler, called The Anthropic Cosmological Principle. It's a great book, one one the books I grew up with. And in that book, Barrow and Tipler say that in their view, there might be one civilization in the observable universe. Which would be us.
Right? So, I who knows whether we should go that far. But I think civilizations are very rare. Okay, now, we walk outside and over Westminster, there's a spaceship hovering. I wouldn't be in the least bit surprised.
I mean, there are deep questions also interesting cuz then we get a profound prompt puzzle about why why there don't seem to be many civilizations around. If we don't find a way of not compromising, but understanding that the world is very complicated. So, then it's not only a single country, it's all the different countries with different cultures and different political histories and different views. If we don't find a way of um sort of stopping arguing and trying to find the way to make that work, I've just given everyone the means to destroy themselves. Well, one point is that it's expanding and and we always see the same radiation out there.
So, the glow of the Big Bang. But there are some deeper reasons. Um the one uh from the theory of inflation, the the the what the best way to explain the universe, the properties that we see, is that it's very much bigger than the piece we can see. So, for example, we measure space to be what's called flat. I don't even know if I'd say what's called flat, it is flat.
So, if you imagine slices of space, let's imagine slices of them at different times. So, so you just slice the universe and and say there's a big sheet, like this table, like a table. There's a sheet of space and there's another sheet and another sheet. And it can it can have a geometry, right? It can be flat like a tabletop, or it could be curved like a sphere, or it could be curved in the opposite direction, sort of like a saddle or a bowl.
And we can measure that. And when we measure it, we see it's absolutely flat. And that's a very unusual thing for it to be like. Um it requires because what what Einstein's theory says is that the the shape of space, that the curvature of space is determined by the stuff that's in it. That's uh basically Einstein's theory of general relativity.
Put stuff in space and it curves it and bends it and warps it and stretches it and so on. And what we find is that we there's precisely the right amount of stuff in the universe to have a completely flat universe. And the the the explanation, the most favored explanation for that is the universe is way bigger than the piece we can see. And so, it's like looking at a piece of the Earth. Like you look at a little 1 mile square of the Earth, right?
Then it's it it's flat, right? You have to look at big distances, kind of a order of the radius of the Earth, or not, you know, bigger bigger than 1 km anyway or 1 mile to see that actually you're on a curved surface. And that's one of the ideas about the the universe and why it appears to be the way that it is, because it's way way bigger. So, we just we're just looking at a little piece and that's why it looks flat. And that's the best way to think about it is not to think of three dimensions of space cuz then we can't picture it.
Okay. But you can think of two, like this tabletop. And that's all right, we just forget the other one for now. And so, you know what flat is on this table. I mean, you could define it, so you could say for example that if I draw a triangle on the top of the table, then all the angles add up to 180°.
So, that actually defines flat. If you did that on the surface of the Earth with a big triangle, then the angles wouldn't add up to 180°. Um or you could draw a circle and say what's pi. So, pi is the ratio of the circumference of a circle to its diameter. That's only true on a flat surface.
It's different if the surface is curved. So, you can define flatness. Oh, yeah, there's a third dimension of space. Um but the it the the same applies. It's just a generalization of geometry.
Then so, you you can pic the the point is we can picture it in two dimensions. But you can you can draw you can quite literally you could imagine sending light beams out. And we do this measurement actually. We can look at the the the dist the most distant light we can see, which is something called the cosmic microwave background radiation, which is if you if you imagine looking out, if you look at the Andromeda galaxy, which we can see with the naked eye here in LA, you can see that. It's the most distant object you can see with the naked eye.
And it's about two 2 million light years away or so, which means the light took 2 million years to get to us. So, it's a long way away, but it's very big. So, if as you look further out into the universe, to more and more distant galaxies, you're looking further back in time. Cuz you look at something that's a billion light years away, then the light took a billion years to get to us. So, you see it as it was a billion years in the past.
And we can actually look so far out that we can see almost back to 13.8 billion years ago, which is very close to the Big Bang. So, we can look to light that began its journey before there were galaxies. And that's the the oldest light in the universe. Which is, by the way, one of the one of the pieces of evidence from people who say, "I don't believe in the Big Bang." The answer is, well, you can see it. So, you know, it's just there.
You can see it. We have pictures of it. Um that light, it turns out that there are sort of structures or ripples in that light, um which we can use as a ruler. So, quite literally as as a ruler on the sky. And then because that light's been traveling through the universe, we can see how that ruler's been distorted as as the light has traveled through space.
And so, we can infer whether space is flat or curved or how it's warped, if you like, just from that measurement. So, quantum mechanics is the base theory, we think. I think people tend to think of quantum mechanics as something that applies to atoms and electrons and particles, but not to us. And that's not right. So, quantum mechanics is the theory that describes everything.
Uh the the point is though, it produces, when you look at big things, it produces behavior which can be more accurately or more usefully, not more accurately, but more usefully described by things like Newton's laws. It is necessary to understand quantum mechanics to understand how a transistor works, for example, or how a laser works, or how of course a nuclear reactor works or the sun works. So, it's not just a theory that's a theory of little things bouncing around. It it's a theory that's necessary to understand a lot of things that we take for granted today. And it's an unusual theory.
It's it's strange because it's got probabilities in it. And that was actually it was Ernest Rutherford actually so famously got the Nobel Prize at Manchester for discovering the atomic nucleus, but he pointed out that in the late 1800s, early 1900s, when we start looking at radioactivity, you see that radioactive decay is a probabilistic process. So, it means that given given something like a uranium nucleus, you can't tell you can even if you made 100 uranium nuclei at the same time, you could just make them, then you can't predict when any one of them is going to decay away and emit some radiation. Basically, it's probabilistic. And so, that idea was shocking to people that it appears that the world is not like Newtonian clockwork.
So, I would say quantum mechanics the the my explanation is it's a weird theory, it's a kind of probability theory, and it's but it's the way that things actually are. So, you can give the numbers. So, so the nearest large galaxy, Andromeda, is about 2 million light years away, which means light traveling at 300,000 km per second from from our point of view takes 2 million years to travel from the Andromeda galaxy to us. So, if you go away from the city lights and and out into on a clear sky and when there's no moon particularly, then you'll be able to see Andromeda if you've got some binoculars. And it's worth looking at cuz the light entering your eye began its journey 2 million years ago, which is before we devolved on Earth.
So, that that gives you a sense and you can see that. You can just about see it with the naked eye cuz it's big. It's a big galaxy of about a trillion stars. And it's actually, although it's very faint, it's it's bigger than a full moon on the sky. So, it's a big thing.
I think that's the best explanation of the scale cuz you can just about, I think, get your head around that. But that's the nearest galaxy. So, when you start talking about galaxies that we see beautiful images with the Hubble Space Telescope or the JWST, for example, you're you're looking at things that are you know, 50 million light years away, 300 million light years away for a thing called Stephan's Quintet and onward. And those, you know, 300 million light years. And many of the galaxies we can see in photographs from with like the James Webb, for example, th- those those things are so far away that the light b- began its journey before the Earth had formed.
And I that's incomprehensible, I think. So, the answer is I can't explain the true scale of the universe or picture it. And as we speak, there's a we have several missions out there, but one of them is called Perseverance, which is a rover. And as we're speaking now, it's on the surface of Mars. It's taking samples.
It's It's taken a lot of samples below the ground and they're being packaged up for return to Earth in something that's called Mars Sample Return, because we think that it's possible that life existed on Mars or may still exist, because the conditions were right. It's certainly true that 4 billion years ago, almost certainly Mars was is like Earth. When when when life began on Earth, Mars was pretty much the same with oceans and an atmosphere and and so on. So, it's entirely possible. And it would almost be surprising if it hadn't existed, because then it would tell us that even given the right conditions, then maybe it's not there isn't a high probability that life begins or geology becomes bi biology or whatever the origin of life is.
So So it does in the sense that So it's been done with photons, for example, so particles of light. So So you can do the experiment where you essentially it's based on on on having two photons that are entangled, quantum entangled together. So they The best way to describe that in about a second is that they they they behave as a single system, basically, but you can separate them. And then you can arrange for another one to come in so it kind of interacts with this one such that it ends up this one over here, which could be a light-year over there, um ends up being the in this the same photon essentially as the one that came in or in the same quantum state. So So So it's teleportation.
And the incoming one gets destroyed in that process or its state is changed. So yes, we do quantum teleportation in the lab. So it's real. The question then becomes how big an object can you teleport? And um I think in principle there's no reason why you can't teleport anything, right?
In in but it it's very much in principle. So No, the first time they came into physics was in 1930. I think it's 1935 in a paper by Einstein and Rosen. So they noticed the the description of the distortion of space-time in the vicinity of a star, which was discovered by Karl Schwarzschild in 1916. They discovered that if you take that description and you you allow the star to collapse essentially into a black hole and then you extend it infinitely into the past.
So you remember you say, "Well, let's imagine this black hole had always existed." So this universe in which this thing exists is is an eternal universe. Then you get a wormhole. So a wormhole, the description of it is in the mathematics that Karl Schwarzschild discovered in 1916 for the distortion of space-time around a star. Literally, you have two universes that and they're linked by a wormhole. So one of them is a black hole and the other bit is a white hole.
So that's just formally in the mathematics. So So that's what a wormhole is in in Einstein's theory. That requires this black hole to have existed forever. So and by the way, if the black hole's spinning, then you kind of get an infinite tower of wormholes. So it the so-called Kerr black hole.
This was discovered in the 1960s by Roy Kerr for a spinning black hole. The solution to that, if it's this eternal solution, is that you have an infinite number of universes connected inside the black hole by an infinite number of wormholes. So the modern work on trying to understand what happens to things that fall into black holes might suggest that there's a kind of a wormhole that connects the interior of the black hole to the exterior universe. And so the singularity looks like this kind of network tangled web of wormholes of some description. It's I hedge my language cuz the language isn't really known yet, but it's one of the ideas, one of the possibilities suggested by the mathematics for the interior of a black hole.
And the other thing, the final thing to say is that you might have heard about um uh there's something called ER equals EPR. So that's Einstein-Rosen Schwarzschild wormholes equals EPR, which is Einstein-Podolsky-Rosen. And EPR was a question about quantum mechanics and what Einstein called spooky action at a distance where you can have entangled quantum particles and somehow it seems that if you do something to this one, then there's a in a in a in a very real sense, this one's affected by it, but not in a way that you can transmit information, but it's quantum entanglement. And there is a picture where you can picture that as the two particles being connected by wormholes. And that was suggested by Leonard Susskind and others a few years ago.
So that's called ER equals EPR. So So wormholes are on the agenda in trying to understand what space and time are and how they behave. Um so they're not just science fiction. These objects, which have been known I would say for 40 or 50 years, but theoretically for the best part of a century, have always been fascinating. They're odd things.
The simplest way to describe a black hole would be a region of space from which even light can't escape. Predictions that such objects existed go all the way back to the the beginnings of relativity back at the turn of the 20th century. But actually really, I would say into the 1960s, perhaps even into the 1980s, many physicists felt that because of the intellectual challenges that these predicted objects pose, many physicists felt that maybe nature would not create them. I even saw that the great physicist Steven Weinberg say that uh he in some sense hopes that these things would not exist because they're so confusing. But we now know that they do exist and so we have to face the challenges that they pose.
Black holes are interesting uh because going back to the work of Stephen Hawking in the 1970s, uh it turns out that they demand that we think about both quantum theory and general relativity together. And the quest to unify those two great pillars of 20th and 21st century physics into what's often referred to as a quantum theory of gravity is in some sense a holy grail for theoretical physicists. But the problem has always been, well, is there anywhere in nature that we can look to to observe something that requires us to merge those two theories together. And black holes really are the unique place, as far as we can tell, in nature where we can see a thing just sitting there in the sky that demands that we consider those two theories uh working together to hopefully reveal a deeper theory. The idea of black holes goes back a long way, actually, back into the 1780s and 1790s.
There were There were two physicists, mathematicians, natural philosophers, whatever you want to call them, working at the time that had the same idea apparently independently of each other. One was a clergyman, an English clergyman called Mitchell, and the other was the great French mathematician Laplace. And they were both thinking in terms of an idea called escape velocity. So the escape velocity is the speed you have to travel to completely escape the gravitational pull of something, a planet or a star. For the Earth, for example, the escape velocity from the surface of the Earth is around 8 mi a second, 11 km a second.
If you go bigger, you make a bigger, more massive thing. Let's go up to a star, for example, like the Sun, then the escape velocity increases because the gravitational pull at the surface increases. And actually for the Sun, it's somewhere in the region of 400 mi a second. Really fast. What Mitchell and Laplace thought, and I think it's a very beautiful idea, is they imagined in their mind's eye, "Well, can you go bigger?
Can you imagine more and more massive stars, giant stars, such that the gravitational pull is so large at the surface that the escape velocity exceeds the speed of light?" And then you wouldn't be able to see them. There's a wonderful quote actually in Laplace's paper where he says that the largest objects in the universe may go unseen by reason of their magnitude. And this is back in the 1780s or 1790s. So he's imagining stars where the gravitational pull is so vast that even light can't escape and you couldn't see it. Dark stars, I think he referred to them as.
But now we know that such objects do not exist in the universe in that sense, in the sense that Mitchell and Laplace meant. But actually they missed something, which is not surprising cuz it sounds almost paradoxical. But you can also increase the escape velocity at the surface of an object by squashing it. And it turns out that if you take the Earth and you squash it down and squash it down and squash it down until it's about that big, the radius just less than a centimeter, then the gravitational pull at the surface would be so great that light couldn't escape. And that is essentially the modern concept of a black hole.
Now, Mitchell and Laplace's calculations or imaginings were based on Newtonian physics, so pre-Einstein. We come forward to 1915, Einstein published his general theory of relativity, which is a different model, a better theory of gravity. And it turns out that black holes also exist in general relativity. Now, if we come forward to 1915, Newton's theory of gravity is replaced by a better model, a better theory, which is Einstein's general theory of relativity. But the idea that there can be objects that can be compressed such that they trap light is also present in Einstein's theory.
Uh the first physicist to predict such a thing, or at least derive the mathematics that describes such a thing as a black hole, although he didn't know that they existed, was a man called Karl Schwarzschild. What Schwarzschild did was provide a an exact solution to Einstein's equations that describes space and time, the distortion of space and time in the region of a star, or or at least an idealized star, which is a perfectly spherical non-spinning ball of matter. It's a model, a simple model of a star. That's what Schwarzschild described or discovered, the solutions to Einstein's equations that describes what happens to space-time outside such a thing. Uh way back, actually 1916, just after the theory was published.
What Schwarzschild's solution also describes though, although he didn't think in these terms at the time, was what that space looks like if you completely remove the star, but leave its imprint in the in the fabric of the universe behind. And that is essentially the the theoretical description or the model of a modern relativistic black hole. Now, while Schwarzschild's solution, and we're by solution I mean we're to picture a distortion in space and time, a distortion in the fabric of the universe. While Schwarzschild's solution does indeed describe the simplest possible black hole that we can model in the universe, people didn't really think in those terms at all until later. You see in the 1930s, Einstein and a colleague of his Rosen, for example, exploring that space-time and building models of what that space-time might look like.
But I think it's true to say that really, certainly until the late 1930s, and actually arguably post-war until the 1960s, most physicists thought that such things would not exist in nature. So they were theoretically interesting, perhaps not practically interesting. Um the reason is that you have to create such a thing. So it's one thing to have a model of space and time that describes this object called a black hole from which not even light can escape, but it's another thing for nature to actually make it. So if you go through the 1930s, there's a lot of papers.
Actually Robert Oppenheimer and a student of his Snyder had a very famous paper just before the war, which explored whether a real star in the universe at the end of its life could collapse, and collapse without limit to form this geometry, this thing that we call a black hole. Just before the war, Oppenheimer and Snyder showed that under certain assumptions a star could behave in such a way. Um but it wasn't really until the work of people like Roger Penrose and Stephen Hawking and several others in the 1960s that it really began to look as if nature would build these things. There's this great quote I remember with some fondness, actually Arthur Eddington, a colleague of Einstein's, who was very English, very proper physicist, and he said, "Nature will prevent such absurdities from existing." Just that's it. Nature will prevent it.
Well, it turns out nature doesn't prevent it. We now know, we've observed them, that stars do collapse to form black holes. And then theoretical physics moves on. So people accept that these things should exist, although it's true to say that we haven't actually imaged one until the 21st century. But in any case, people accepted these things should exist.
More and more evidence mounted that they do exist at the centers of galaxies and at the sites of collapsed stars. But then we have to face the consequences. What does it mean for our understanding of the universe if there are these objects where space and time behave in a very strange way, where where light is trapped, and where it would seem that anything that falls in is at the very least locked away from the universe forever? So to understand the conceptual problems that are faced if these things exist, then it might be worth just describing very briefly the Einsteinian description, the pure description in general relativity. A black hole, what do you see from the outside?
Well, there's an event horizon surrounding the black hole. In some sense it defines the boundary between the external universe and the interior of the black hole. The event horizon is um very simply, and a bit hand-wavy, but it's it's a reasonable description. It's just if you can imagine a sphere in space, and if you go across the boundary into the interior of this sphere, then even if you can travel as fast as the speed of light, you can't escape. So the event horizon separates the interior of the black hole from the external universe.
Uh we'll see a bit later why that's a bit of a hand-wavy description. But another description of the event horizon, which confused people all the way through the history of black hole research actually, certainly to the the early papers in the 1930s and perhaps even post-war, was the idea that the event horizon, when viewed from the outside, is a place in space where time stops. And that's a direct prediction of Einstein's theory of relativity. From the external perspective, if you watched, for example, an astronaut falling in towards a black hole, then from your external perspective, you'd see their time pass more slowly, slower and slower and slower as the astronaut approached the black hole until on the horizon you would see their time stop. That suggested to many people in the early days that a star couldn't collapse to form a black hole.
It confused them. If only if a star is collapsing, then does it not freeze forever in some sense on the horizon? So there were all sorts of sort of initial early conceptual problems, which ultimately were solved. The thing about relativity, it's the one sentence thing to understand, is that time can stop from one perspective, but time can pass at the usual rate from another perspective. And indeed, uh from the perspective of an astronaut falling into a black hole, for a sufficiently large black hole, like the ones that we find at the centers of galaxies, the astronaut would notice nothing at all as they fell across the horizon into the interior of the black hole.
So time passes at 1 second per second on the watch of an astronaut falling in, but from the external perspective, time freezes on the horizon. So black holes are full of these um apparent conceptual challenges, which are actually not conceptual challenges at all, they're just a central part of Einstein's general theory of relativity. So that confusion was eventually dealt with and solved and people understood, certainly by the 1960s, what these things are and how general relativity models them. There is a central problem though, which is still not solved, which is you you put it this way, what lies at the center, and I'll be careful with my language, what lies at the center of a black hole. Now, in pure, just in Einstein's general theory of relativity, actually, it's not right to talk about the center of a black hole really.
So what are we picturing? It's this thing called the singularity. You might think of it as an infinitely dense point to which this massive star collapses. It's kind of the natural way to think of it. But actually, just even in pure general relativity, when you look at a nice map of a black hole, the so-called Penrose diagram named after Roger Penrose, what you see is that the singularity is not really a place in space at all.
It's a moment in time. And actually, it's the end of time. So one way of picturing what's happened when a star collapses to form a black hole is that space and time is so distorted that in a sense their roles swap. And so what we thought of as an infinitely dense point, a place in space, at the center at the center of the collapse of the star, if you like, actually becomes a moment in time, in the end of time, the singularity. But the nature of that thing, uh was not and is still not understood.
So that's a great mystery. And it's been long accepted that we will need a so-called quantum theory of gravity, a deeper theory of gravity, in order to explain the singularity. And for many many years actually, until quite recently, then people thought, "Well, there we are. We have a problem with the singularity. We don't really have any access to it.
We don't have the conceptual tools to explore it, so it may remain a mystery for a century to come. This is not clear what to do." The great revolution in black hole research was to notice, and it began with Stephen Hawking's work in the in the mid-1970s, was to notice that actually there are conceptual problems at the event horizon of the black hole, not in this extreme place, not only in this extreme place at the singularity, whatever that might be, but at the horizon. Now, that was a real challenge, an extremely interesting, because we think, we strongly expect, the laws of nature that we understand now, laws of nature that we have full mathematical and conceptual control of, to apply at the event horizon. And so that's why why black holes have become so interesting, it's because we have a place where we think, we assume, and indeed we're right to assume it seems, that we have full control of the physics. We understand what's happening, but there is a fundamental clash of principle between our two basic theories of nature, general relativity and quantum mechanics, and that is ultimately why the event horizon of a black hole and black holes themselves have become so fascinating and so important.
The modern revolution in in our thought, which is still ongoing by the way, in understanding black holes and quantum gravity, really does begin with work that Stephen Hawking did in the mid-1970s. Stephen Hawking, in his own words, showed that black holes ain't so black. So, we've described them as prisons in a sense. We're picturing them as a region of space from which nothing can escape. What Stephen Hawking showed in in in a landmark couple of papers, a tour de force of calculation actually, is that if you consider quantum theory, quantum mechanics, in the vicinity of the horizon of a black hole, then you find that they glow.
They produce particles. They have a temperature. This thing that we've we we pictured in Einstein's theory as as pure geometry, as just just distorted space and time, actually emits particles. It's called Hawking radiation. And one way to picture that, and Stephen in his 1974 paper gives this.
He says it's a hand-wavy description. So, it's not a full description. You need the mathematics for that, but he gives this hand-wavy description, which is kind of a nice way to picture what's happening. The idea is to imagine, to zoom in on space in the vicinity of the event horizon of a black hole. If you zoom in on any piece of space, the piece of space in front of your nose right now, if you could zoom in and slow time down with a big microscope, you can picture what's happening as as a series of particles coming in and out of existence all the time, so-called entangled particles.
So, it's a picture of what the vacuum of space looks like. As just to emphasize again, it's not a complete picture, it's not supposed to be precise, but it's a reasonable model of what's happening. So, particles in and out of existence all the time, and that's happening everywhere, in space, everywhere that you look, in the most empty piece of space you could imagine. That's what's happening. In the you can have the situation where one of those particles, those pairs of particles, is on the inside of the event horizon and one is on the outside.
And then it can happen that the one on the outside, instead of merging back with its partner again, can escape into the universe. It's essentially made real by the presence of the black hole. So, the other partner is interior to the black hole, and this particle heads off into the universe, removing energy from the black hole as it goes. This rain of particles, this glow of particles, is called Hawking radiation. That has profound implications.
So, it's kind of a simple picture of what's happening. But the upshot is, imagine this thing. It's a black hole. It's glowing. It's just space-time geometry, but it's emitting particles, losing energy, therefore it's shrinking, which means that one day it will be gone.
So, black holes are not eternal prisons, they have a lifetime. One day, whatever's in there is returned to the universe. The question was, the central question that was immediately raised by those calculations, is this. What happened to all the stuff that fell in? The way I've described it, the way Einstein's theory describes it, is somehow that stuff goes to the singularity, whatever that thing is, the end of time, a region of space-time that's so convoluted and distorted that we don't understand how to describe it at all.
But then, one day, the whole thing is gone. All that's left in the far, far future is Hawking radiation, those particles that were produced in the vicinity of the event horizon. The question is, is it possible, if you could collect all that radiation, all the Hawking radiation through the whole life of the black hole, is it somehow possible in principle, that the information about everything that fell into the black hole throughout its history is imprinted in that radiation in the far future. Is that true? Or is it not true?
You might say, why did I ask that question? Seems like a bit of a random question. It's a very important question. Let's say that I take anything in in here, in this room, a book, a table, the camera, right, anything at all, and I set fire to it. I incinerate it.
I destroy it any way that I can. I could throw it into a furnace. I could I could put it in the heart of a nuclear bomb and explode it, or whatever, just completely incinerate it. In physics, in basic fundamental physics, then it turns out that if you could collect every piece of that thing that I detonated or incinerated, every quanta of radiation, every photon, every particle, everything, in principle, if I could just collect it all, and I was clever enough, then I could reconstruct the thing that I had destroyed. Information is conserved in the universe as far as we know.
So, every law of nature that we have says that information is conserved. The problem was that Stephen Hawking's initial calculation of the way those black holes evaporate away said that information is not conserved. Said that black holes are erasers of information. To put it very bluntly, the the calculation said that once that black hole had gone, then even in principle, there is absolutely no way you could learn anything, reconstruct anything about the things that fell in, including the star that collapsed to to to form it. So, information erasers, the only information erasers that we know of in nature.
That was the initial picture of black holes as Stephen Hawking understood them in back in the 1970s and 1980s. This became known as the black hole information paradox.
