Transcription
In your opinion, are we living in a simulation? It's a [music] question that might seem absurd, straight out of a science fiction movie. And yet, it's a question that very serious people ask themselves, philosophers [music] like Nick Bostrom at Oxford, who developed a statistical argument suggesting it's almost certain that we are simulated beings. But there are also scientists, physicists, who, by pushing the laws of the universe to their limits, find themselves facing this same dizzying question. Why? Well, because science for over a century has forced us to accept a fundamental idea. The world is extremely different from what our intuition suggests. Our everyday experience of life is a bit like a computer. When you're on a computer, when you navigate on a desktop, on the internet, it seems very simple. And this simplicity actually hides all the complexity of the programs, the printed circuits, the processors that work without us even realizing it. Well, reality might be a bit like that. That is to say, in fact, we have a simple experience of the world around us, but perhaps the world is infinitely more complex than our daily intuition lets it appear. Think about it, the time we perceive as an immutable and universal river turns out to be elastic. Einstein's relativity showed us that it can stretch or contract depending on our speed or the gravitational field we are in. Two twins can therefore age at different rates. Time is not absolute, it is relative, and quantum mechanics goes even further in its strangeness. Particles can seem to remain mysteriously connected, entangled, even if they are separated by billions of light-years. It's as if distance didn't exist for them. And this is not barstool philosophy. It is the most accurate, most tested, most validated description we have of the real world. And if our intuition is already so wrong about time and about the connection between particles, then it can also be wrong about something much more fundamental, space itself. And the question we will ask ourselves today is, how different can the world be from our intuitions? How far can strangeness go? If space, time, and matter are not what they seem, what is real? And one of the wildest, most profound theories that explores this question is the holographic principle. The idea that our universe with its three dimensions of space and its dimension of time would actually be just a projection, a hologram. In other words, perhaps everything we experience, this impression of solid reality, of moving in a 3D world, is just an illusion. Perhaps the true nature of the world is encoded on a distant two-dimensional surface. It would be a bit like an image on a cinema screen that gives us the illusion of depth. But be careful, holographic doesn't mean false or simulated. It means that the same reality can be described in two different ways. And we will understand this nuance together. But just before continuing this video, I would like to quickly talk to you about today's sponsor. If artificial intelligence intrigues you, then this should interest you. We are living in a period where AI is evolving at a crazy speed. Every week, new models, new technologies, new tools are released. And today, the real challenge is no longer just to be interested in it, but to keep up with the pace and know which tool to use for which task. Because in reality, each has its own strengths, its limits, its specificities, and we shouldn't settle for just one for everything. Well, that's precisely what Mamoutai offers. It's a French platform that brings together thousands of current models in a single interface, those from OpenAI, Google, Antropic, Mistral, but also Perplexity for web search or image AIs. You can switch from one to another with a single click, compare their answers on the same prompt, and choose the one that best fits what you were looking for. And all this starting from €10 per month. It's much more affordable than having to subscribe to each tool separately, which would cost you over €200 in total. So not only is it more efficient, but it's also much more economical. In addition, Mamutai complies with GDPR and does not use your data to train their models. So you can use it with complete peace of mind. In short, if you really want to take advantage of what AI can offer without scattering yourself and without breaking the bank, I recommend you take a look. The link is in the description. Now, let's get back to the video. To understand this idea that may seem absurd, we must first set the context. We must take a journey to the heart of one of the greatest mysteries of modern physics. A journey that begins with the most extreme, most primordial objects in the entire universe, black holes. It all starts with a monster, a cosmic monster so dense that nothing, not even light, can escape it. A black hole. For a long time, black holes were just a mathematical curiosity. [music] A somewhat bizarre solution to Einstein's equations discovered in 1915 by Karl Schwarzschild from the Russian front in the midst of World War I. No one thought they could actually exist. It was a mathematical artifact, nothing more. And yet today, we know there are billions of them in our galaxy. We even photographed one in 2019. A black hole. In fact, it's a region of spacetime where gravity is so intense that nothing can escape. Around it, there is an invisible boundary, a point of no return called the event horizon. Before this boundary, you can still turn back. After it, it's over. Whatever you do, even with the most powerful rocket imaginable, you can never go back. In fact, it's a one-way trip to the unknown, a journey with no possible return. And in the classical view of physics, a black hole is an object of disconcerting simplicity. In fact, it has only three characteristics. Its mass, its electric charge, and its rotation speed. That's all. No matter what you throw into it, a star, a book, a computer, an entire civilization, the black hole keeps no trace of what it has swallowed. It's a bit like a safe that would destroy everything you put inside. Physicists summarize this with a famous formula. A black hole has no hair, it is smooth. In fact, it means it has no memory. And that's where the problem begins, a huge problem because this idea that an object destroys information, well, [music] it conflicts with one of the most fundamental laws of all physics. It's a bit like discovering that a building stands up when its foundations have just been removed. Something doesn't add up. To understand this conflict, we must first talk about a concept that may seem abstract but is actually very simple. Information. In physics, information is everything that describes the state of an object, the position of a particle, its speed, its mass, its temperature. It's the complete recipe that allows an object to be described in the smallest detail. And one of the most solid laws of physics tells us that information can never be destroyed. Never. It can be transformed, scrambled, made unrecognizable, but in theory, it never disappears. Take a book, for example, if you burn it, the book is destroyed, the pages are reduced to ashes, the words have disappeared. But the information it contained has not disappeared. It has just been transformed. It is now encoded in the smoke, in the ash, in the heat, in the light emitted by its flames. But if you had instruments precise enough to measure every particle of smoke and every photon of light, you could, in principle, reconstruct the book. Not in practice, of course, but in principle, yes, because all the pieces of the puzzle are still there, [music] scattered throughout the universe. Nothing has been destroyed. The information has just changed form. It has become unrecognizable, but it still exists. It is one of the most fundamental laws of physics. Information never disappears. Yet, as we said just before, a black hole seems to violate this law. If you throw a book into a black hole, the information seems to disappear purely and simply. And for a physicist, that's a catastrophe. It's a bit like being told that 2 + 2 = 5. It can't be true. If it's true, then all of physics collapses. There must be something we're not understanding. And it's a young 25-year-old student, Jacob Beckenstein, who will provide the first piece of the puzzle. It's 1972. Beckenstein notices something curious. Every time an object falls into a black hole, the event horizon, that invisible boundary, grows a little. The black hole gets bigger, and Beckenstein thinks, "What if this is the trace of the information? What if, instead of disappearing, the information is somehow stored on the surface of the horizon?" A strange idea, because normally when we store things, we put them in a volume. The larger the volume, the more we can put in it. The bigger the hard drive, the more data it can hold. That's common sense. But Beckenstein suggests that for a black hole, it's different, fundamentally different. The information would not be stored in the volume of the black hole, but on its surface. It's as if all of a computer's memory were not stored in the machine but on its outer casing, as if the entire content of a library were encoded on the walls of the building. At first, no one takes him seriously, especially Stephen Hawking, the most famous physicist of the time. Hawking was convinced that Beckenstein was wrong, completely wrong. He then embarks on complex calculations to prove it. He wants to demonstrate mathematically that this idea of entropy on the surface is absurd. And then, a twist of fate. Not only does Hawking not prove Beckenstein wrong, but he discovers something completely unexpected. Black holes are not completely black. Against all odds, they emit radiation, a sort of extremely faint glow produced by quantum fluctuations at the edge of the horizon. This is what is now known as Hawking radiation. It is one of the most important discoveries of 20th-century physics and is engraved on Hawking's tomb at Westminster Abbey in London. This discovery confirms Beckenstein's intuition. Yes, the information of a black hole is indeed linked to its surface, but it creates a new, even more serious problem, because this radiation implies that black holes evaporate very slowly, but they do evaporate. A black hole the mass of the sun would take an incredibly long time to evaporate, much longer than the current age of the universe. But in the end, it does disappear. But when it disappears, what happens to all the information it contained? This is the black hole information paradox. One of the greatest mysteries of 20th-century physics. A mystery that has haunted the greatest minds for decades. And what's fascinating is that this paradox, born from a very technical problem about black holes, will lead decades later to an idea that challenges our understanding of reality itself. Because the solution to this paradox is precisely the idea that our universe could be a hologram. This paradox will trigger one of the greatest debates in the history of physics. Not a debate of a few months, but a debate of 30 years. On one side, Stephen Hawking, the most famous living physicist of the time. For him, the conclusion is simple: information is destroyed. Period. If it challenges the laws of quantum mechanics, then so be it, nature has spoken. On the other side, physicists like Leonard Susskind, an American from Stanford, and Gerard 't Hooft, a Dutchman who would later receive the Nobel Prize. For them, it's impossible. Absolutely impossible. Susskind would later recount in a book he wrote about this debate that the first time he heard Hawking's argument, he felt a deep unease, not an intellectual disagreement, but a visceral unease. Because if information can be destroyed, then we can no longer do science. Science is based on the idea that the past determines the future, that the laws of physics are coherent, that we can trace back the chain of events. If information disappears, then all of that collapses. It's like removing the foundations of a building. And it is Susskind and 't Hooft who will propose a completely crazy idea to resolve this paradox. The idea is so crazy that even they have trouble believing it at first. But it's an idea that will end up changing everything. The reasoning actually starts from Beckenstein's idea. We remember, Beckenstein had shown that the information of a black hole is stored on its surface, not in its volume. That's already a strange idea. But Susskind and 't Hooft go much further. They ask a simple question. Is this a peculiarity of black holes, or is it a fundamental property of nature? In other words, is it true only for black holes, or is it true for the entire universe? Their answer is that it should be true for everything. To understand this idea, imagine a room, your living room for example. In this room, there are furniture, objects, air, light. All of this exists in three dimensions. You can move forward, backward, left, right, up, down. In fact, it's a bit like your daily experience of the world. Now, imagine we film this room with a camera and project the image onto a flat screen. The screen has only two dimensions. It's a flat surface. And yet, by looking at the screen, you can see everything that is happening in the room. You see the furniture, the objects, the people moving. All the information of the 3D room is contained in a 2D image. Now, of course, it's only an imperfect analogy. A TV screen doesn't really capture all the information of the room. But the holographic principle says something much stronger. It says that all the information contained in a volume of space, absolutely all of it, can be described by what happens on a surface that surrounds this volume. Not an approximation, not a summary, but all the information without losing anything. It's as if you could describe everything that happens in your living room, every atom, every particle, every movement, solely by writing things on the walls. And that this description on the walls is complete, perfect, and contains absolutely everything. It may seem absurd. How can information on a 2D surface contain everything that exists in a 3D space? It's like saying that an architect's plan contains as much information as the building itself. And yet, that's exactly what the holographic principle suggests. The word holographic comes from holograms. These 3D images that we can see on some bank cards or in some museums. A hologram is an image that appears to have volume, depth, even though it is stored on a flat surface. All the 3D information is encoded in 2D. The holographic principle is this idea, but applied to the entire universe. And be careful, here we are not talking about a metaphor, we are not talking about a poetic way of looking at things. We are talking about an extremely serious physical hypothesis studied by hundreds of physicists in the best universities in the world. The idea that our 3D universe with everything it contains, you, me, the stars, the galaxies, all of this would be just a projection, a sort of three-dimensional image generated by the information that lives on a distant two-dimensional surface. This means that depth, this impression of being able to move forward in space, to move away from or get closer to certain things, all of that would be an illusion. [music] Not an illusion in the sense that it's not real, but an illusion in the sense that the fundamental reality would be organized in a completely different way from what we perceive. To understand, think about a video game. When you play a 3D game, you have the impression of moving in a world with depth, mountains, buildings, landscapes. But in reality, all this information is stored on a hard drive in digital data. The 3D world you see on the screen doesn't really exist in 3D. It's a reconstruction, a projection generated from data stored in a completely different way. The holographic principle suggests that our universe might work a bit like that, not because it's programmed by someone, but because its deep reality is organized in a radically different way from what our senses show us. This debate between Hawking and Susskind lasted for decades. Entire conferences were dedicated to it, hundreds of articles were published, and the stakes were dizzying. Because if Hawking is right, it means that quantum mechanics is wrong. The most accurate theory ever built by humanity, the one that makes your phones, your computers, your lasers work, would be fundamentally wrong. If Susskind is right, it's our idea of space that collapses. Space as we know it would not be fundamental. In fact, in either case, it radically changes how we perceive the world. For years, no one budged. Hawking remained inflexible. Susskind too. The debate was endless. Conferences followed one another, articles piled up. And then in 2004, a bombshell. At a conference in Dublin, Stephen Hawking took the stage and publicly admitted he was wrong. It was a historic moment. The holographic principle was taken seriously by the scientific community. But a question remained open, a huge question, because if the information is on the surface, how does it work? How can a surface with one less dimension contain all the richness of a three-dimensional universe? A mathematical model was needed, proof, not just a seductive idea, but equations. And it was a young Argentine physicist who would provide it. In 1997, Juan Maldacena, at 29 years old, was a physicist at Harvard, brilliant but still relatively unknown, and he made a discovery that would shake the world of physics. A discovery so important that his article is today one of the most cited in the entire history of science, with over 20,000 citations. During a conference where he presented his results for the first time, the physicists present in the room stood up and began to sing "Maldacena, Maldacena" to the tune of the Macarena. Theoretical physicists in suits singing and dancing in an amphitheater. It was unprecedented, absolutely unprecedented, in the history of theoretical physics. So, [music] what did he discover? Well, he found a kind of dictionary. A dictionary that allows translation between two completely different worlds. To understand, imagine two languages, French and Chinese, for example. These two languages have nothing in common. The alphabet is different, the grammar is different, the logic is different. And yet, with a good dictionary, you can translate any sentence from French to Chinese and vice versa. The meaning is preserved. The information is the same. Only the form changes. Well, Maldacena discovered something similar, but in physics. He shows that a theoretical universe with gravity, a universe where objects attract each other or space curves, can be entirely translated into a simpler theory, without gravity, that lives on the boundary of this universe with one less dimension, and vice versa. Everything that happens in one can be described in the other. It's the same reality described in two different languages. Let's take a concrete example to understand. Imagine you are in a closed room, and on the walls of this room, someone is writing equations. These equations describe everything that happens inside this room. The position of each object, the movement of each air particle, the temperature of each point, absolutely everything. And what Maldacena showed is that these equations on the walls, which live on the boundary with one less dimension, contain exactly the same information as the room itself. Not an approximation, not a summary, exactly the same thing. It's the holographic principle, but this time it's no longer a vague idea, it's a mathematical result with equations, predictions, things that can be tested. And this dictionary has an extraordinary property. Because what is complicated in one language becomes simple in another. It's like when you sometimes can't express something in your language, but by translating it into another language, you find the words more easily to express it. Well, that's exactly what Maldacena's dictionary allows. The most spectacular example concerns black holes. A black hole in a world with gravity is an incredibly complex object. But when translated into the world of the boundary without gravity, it corresponds to something much simpler, a very hot and very dense state of matter. And this hot matter, we know how to produce it in the laboratory. We can create it in particle accelerators like the LHC at CERN in Switzerland. By colliding atomic nuclei at speeds close to the speed of light, we obtain an extreme state of matter called a quark-gluon plasma. And here's what's incredible: by using Maldacena's dictionary to calculate the properties of a black hole, which is relatively simple, physicists have been able to predict the properties of this plasma, and these predictions have been confirmed by experiments at CERN. It works, the dictionary works, and that's why Maldacena's discovery had such an impact. Before him, the holographic principle was a seductive but vague idea, an intuition without a solid foundation. After him, it was a precise mathematical result with equations, verifiable predictions, a tool that physicists could use concretely. Every time a physicist uses this dictionary to solve a problem, and the solution corresponds to what is observed, it's another confirmation that the holographic principle could be true, that our universe could indeed be described by a theory living on its boundary with one less dimension. So, does this mean that our universe is a hologram? That everything we experience is just a projection of a reality that would live on a boundary with one less dimension? Well, not so fast. We must be honest. Maldacena's dictionary works with a theoretical universe, a universe called "AdS" that has very particular properties. This universe has a special shape and, above all, a well-defined boundary, an edge on which the theory with one less dimension can live. It would be a bit like a snowball. There's the inside of the ball with a snowy landscape, and there's the glass surface surrounding it. The dictionary translates between what happens inside and what is written on its surface. Our universe is not like that. Absolutely not. It is expanding. It is growing, and above all, it has no clear edge or boundary. There is no glass surface around our universe. So where would the hologram be? On what surface would the information of our universe be encoded? Well, that's a question that no one has answered yet. Hundreds of physicists are working on it in the best universities in the world, but for now, we haven't found the right dictionary for our world. We know that the principle works in an AdS universe, but we don't yet know if it works for ours. And even if we found this dictionary, what would it mean concretely? Would it mean that we live in a simulation, a bit like in The Matrix? Would it mean that someone programmed us? Well, no, absolutely not. And it's crucial to understand this well. A holographic world is not a world simulated by a giant computer. It's not a false or unreal world. It's a world that can be described in two different ways, and both are equally true. Think of a piece of music. You can listen to it, feel it as vibrations in the air, but you can also look at it as a musical score, notes written on paper, or as a digital file. A sequence of 0s and 1s on a hard drive. These are three completely different transcriptions of the same thing. None is truer than the others. They are simply different languages to describe the same reality. The holographic principle is a bit like that. It doesn't say that our 3D world is an illusion and that true reality is in 2D. It says that these two descriptions are equivalent, that the same reality can be told in two different languages, that both are equally valid. But then, if these two descriptions are equivalent, why is it so important? Why have hundreds of physicists spent their lives working on it? Because if it's true, it changes everything we thought we knew about the nature of space. It would mean that space, this thing in which we seem to move, this stage on which our lives play out, is not fundamental, that space would be an illusion, a construction that emerges from something deeper. To understand, let's take the example of temperature. When you touch a hot object, you feel heat. Temperature seems to be a fundamental property of the object. But in reality, temperature doesn't exist at the fundamental level. What exists are atoms that are moving. The faster they move, the hotter the object seems to you. Temperature is just a practical way to describe the collective movement of billions of atoms. This is called an emergent property, something that appears at a large scale but doesn't really exist at the most fundamental level. Well, the holographic principle simply suggests that space could be the same thing, an emergent property, something that appears at our scale but doesn't exist at the fundamental scale. Just as temperature emerges from the movement of atoms, space would emerge from something deeper, something that lives on a boundary with one less dimension. And this connects to the question we asked at the beginning of the video. Are we living in a simulation? Well, perhaps that question is poorly posed. Perhaps the real question is, what is reality at its core? And modern physics answers us with a disarming sincerity, something much stranger and much more subtle than anything we had imagined. Thank you to Mamou for making this video possible. All links are in the description.