Why the Universe Exists at All Leonard Susskind Explains

Added: 2026-05-07

[00:00:00]There was a night in 1973. I was in my office at Stanford, papers everywhere, three cold cups of coffee, and I wrote down an equation that I thought described nothing. Turned out it described everything. That is not a metaphor. That is what happened. And it changed how I think about the the question I am going to ask you tonight.

[00:00:21]The equation was not elegant. It was not one of those beautiful things that gets carved into stone and put on the wall of a physics building. It was a mess. It was a partition function for a quantum system that I was fairly sure was unphysical, a toy model, something I was using to understand a corner of string theory that nobody else cared about at the time. And what it told me, when I finally sat down and really looked at it, was that the vacuum, the nothing, the zero, the baseline state of a universe containing absolutely no matter, no radiation, no structure of any kind, was not actually a state of zero energy. It was fluctuating. It was alive. It was doing something.

[00:01:09]And I sat there with my cold coffee, and I thought, if nothing is not nothing, then maybe the question why is there something rather than nothing is not actually a question about how something got created. Maybe it is a question about what nothing actually is. And maybe the answer is that nothing, in any precise physical sense, does not exist, has never existed, cannot exist. That idea took me about 30 years to develop into something I was willing to defend in public.

[00:01:43]Tonight, I am going to try to walk you through it in 90 minutes.

[00:01:49]Fair warning, it is not going to be comfortable. The comfortable version of this talk ends with someone saying, "Well, God created the universe." Or it just happened. Or these questions are beyond science. I am not giving you the comfortable version. I have never given anyone the comfortable version, and I am not starting tonight. So, why does the universe exist? First, let us be precise about what we are actually asking, because the question, as most people carry it around in their heads, is already contaminated. When a person asks, "Why does the universe exist?"

[00:02:28]They usually mean one of three things.

[00:02:30]They mean, "What caused it?" Meaning what event or agent produced it? Or they mean, "What is its purpose?" Meaning what is it for? Or they mean, "Why is there anything at all rather than a pure empty void?" Those are three completely different questions, and conflating them is how you end up with bad answers. I want the third one, the hardest one, the one that does not dissolve into theology or teleology if you push on it hard enough. Why is there anything at all?

[00:03:00]The first thing to do with this question, the first thing any honest physicist should do, is check whether it is even well-formed, because a lot of questions that sound profound turn out, under examination, to be confused. What happened before the Big Bang sounds like a deep question until you realize that time itself may have begun at the Big Bang, at which point the question dissolves the same way what is north of the North Pole dissolves. The words are grammatically fine, but they are not pointing at anything real. Is why is there something rather than nothing that kind of question? Maybe, but I do not think so.

[00:03:40]And here is why.

[00:03:42]The question has genuine content precisely because we can define, at least formally, what nothing would mean.

[00:03:50]In physics, nothing would mean no space-time, no quantum fields, no energy, no information, no dimensions, no laws, absolute ontological zero. And the question is, why do we not have that instead of what we have? That is a real question. It is uncomfortable, but it is real. Now, the standard move at this point, the move that every philosophy undergraduate makes, and that many physicists make when they are not being careful, is to say that science can tell you how things happen, but not why they happen. Science, on this view, starts from a given universe with given laws and given initial conditions, and then it tracks the evolution of that universe forward. But it cannot reach behind the curtain and tell you why the curtain exists, why there are laws at all, why there is a stage for anything to happen on. That is wrong. Or more precisely, it was wrong, and the last 50 years of physics have made it increasingly, demonstrably wrong. The reason it feels right is that it matches the way physics was done for most of its history. You are handed a system, you write down a Hamiltonian, you solve the equations.

[00:05:02]The laws are externally given. God, or nature, or whatever you want to call it, writes them on a tablet and hands them to you. Your job is not to question the tablet. Your job is to do the math. But something started shifting in the 1970s.

[00:05:19]It shifted because of black holes, which is where everything interesting in modern physics happens. And I want to tell you exactly how it shifted, because the history matters here. It is not just decoration. The history is the argument.

[00:05:35]In 1974, Stephen Hawking proved something that, when I first heard it, made me feel physically ill. He proved, and this is real mathematics, not hand-waving, that black holes radiate. A black hole sitting in empty space will slowly emit particles, lose mass, and eventually evaporate.

[00:05:57]What made me feel ill was not the result itself, it was the implication. Because if a black hole radiates thermally, if it radiates like a hot body with a specific temperature, then it has entropy. And if it has entropy, then it contains information. And if it contains information, then what happens to that information when the black hole evaporates? Hawking said it was destroyed, gone. The information falls in, the black hole evaporates, and the information does not come out. It is simply erased from the universe. And for about 20 years, most of the physics community accepted this. Hawking was Hawking. You did not argue with Hawking.

[00:06:39]I argued with Hawking, not because I was brave, and not because I had a proof. I argued with him because his conclusion violated something I believed so deeply that accepting its violation felt like accepting that arithmetic was broken. He was saying that quantum mechanics, the most precisely tested theory in the history of science, verified to 10 decimal places, the foundation of everything from transistors to chemistry, quantum mechanics, is wrong, because quantum mechanics does not allow information to be destroyed. That is not an interpretation. It is not a philosophical preference. It is a mathematical theorem, unitarity. The information content of a quantum system is conserved, always. It is one of the load-bearing walls of the entire structure. Hawking was saying, "Pull out this wall, the building can stand without it." I said, "No, the building cannot stand without it, and one of us had to be wrong." It took 30 years to settle. In 2004, Hawking conceded, in public, at a conference in Dublin, he was wrong.

[00:07:49]The information comes out. It has to come out. We do not yet fully understand the mechanism. That is an open problem, and the people working on it are some of the smartest people alive. But the conclusion is established. Quantum mechanics wins. Information is conserved. Now, why am I telling you this story in a talk about why the universe exists? Because the resolution of the black hole information paradox did not just save quantum mechanics. It revealed something about the structure of physical reality that directly bears on our question. It revealed that information is not just a thing that happens inside the universe. It is, in some deep sense, what the universe is made of. This is not poetry. Let me make it precise. The holographic principle, which emerged from the information paradox, from the work of Gerard 't Hooft, from my own work, and later from the extraordinary work of Juan Maldacena, says the following. The maximum amount of information that can be contained in a region of space is not proportional to the volume of that region. It is proportional to the surface area, the area, not the volume.

[00:08:58]This is counterintuitive in the extreme.

[00:09:01]Your intuition, and your intuition is a reasonable Newtonian intuition that works fine for everyday life, says that a bigger box holds more stuff. Double the volume, double the possible content.

[00:09:13]That is just common sense. Common sense is wrong. The universe, at a fundamental level, is not a three-dimensional object with information filling its volume. It is, in some precise technical sense, a two-dimensional object. The three-dimensional world you experience, the room you are sitting in, the depth of the space around you, the fact that you can move forward and backward and sideways and up and down, is encoded on a lower-dimensional boundary, the way a hologram encodes a three-dimensional image on a two-dimensional film.

[00:09:50]Maldacena made this mathematically precise in 1997 with what we now call the ads CFT correspondence and it is the most important theoretical result in physics in the last 30 years, maybe longer. And here is where it gets strange. Here is where I want you to sit up in your chair. If the universe is holographic, if the three-dimensional structure of space is not fundamental, but is somehow encoded in or emergent from a lower dimensional description, then what does that say about the question of why anything exists? It says this, the question may be confused in a very specific and correctable way. We have been asking, why is there space, time, matter, energy and laws, treating these as the fundamental ingredients, but the holographic principle suggests that the space in particular is not a fundamental ingredient. It is a derived quantity. It is something that emerges from the underlying information structure, the way temperature emerges from a collective behavior of billions of molecules. No single molecule is hot.

[00:11:04]Temperature is a property of the aggregate. Similarly, no single quantum of information is spatial. Space is a property of the aggregate information structure. If space is not fundamental, if it is emergent, then the question why is there space changes character. It becomes what are the information theoretic conditions under which a structure like space can spontaneously appear? And that is a question physics can potentially answer.

[00:11:34]That is not a question that requires invoking creation ex nihilo or pre-existing divine will. It is a question about the self-organization of information, but I am getting ahead of myself.

[00:11:48]Let me back up and talk about another piece of this, one that is older and in some ways even more disturbing. I want to talk about the quantum vacuum. When I was a graduate student, and this was in the 1960s, which I realize makes me sound like a geological formation, but stay with me. The vacuum was considered simple. The vacuum was the state of minimum energy. You empty a box of everything you can empty it of, you cool it to absolute zero, you have the vacuum, nothing in it, end of story.

[00:12:17]Then quantum mechanics happened to the vacuum. More precisely, quantum field theory happened to it, and what quantum field theory says about the vacuum is one of the most startling things in all of science. The vacuum is not empty. It is seething. It is a roiling, fluctuating sea of quantum fields. Every field that exists in nature is present in the vacuum in a superposition of states, constantly creating and annihilating pairs of virtual particles, borrowing energy from the uncertainty principle and paying it back before anyone can catch it. The Heisenberg uncertainty principle, in its energy-time form, says you cannot simultaneously know the energy of a system and the time at which you measure it with perfect precision, which means a vacuum can spontaneously have energy, borrow it, use it, return it, as long as the loan is repaid quickly enough. And it does this constantly, everywhere.

[00:13:15]This is not theoretical, this is measured. The Casimir effect, two uncharged metal plates placed very close together, experience an attractive force that has no classical explanation, is a direct measurement of vacuum fluctuations. The plates alter the spectrum of fluctuations that can exist in the space between them, and the difference in pressure on the inside versus the outside produces a force. We have measured it, it is there. The vacuum is not nothing. Now, this creates an immediate problem for the question of origins. If the vacuum is not nothing, if even the lowest energy state of the universe's quantum fields contains this seething structure, then we cannot solve the problem of why is there something rather than nothing by getting the universe to spontaneously appear from a quantum vacuum.

[00:14:10]Because the quantum vacuum is already something. It already contains structure, energy, information, laws. It is not the baseline of absolute nothingness.

[00:14:22]It is already partway up the ladder of existence. This is where I took a wrong turn once. In the early 1980s, and I was not alone in this, Jim Hartle and Stephen Hawking were doing similar things, as was Alex Vilenkin. There was a great deal of excitement about quantum cosmology. The idea was, write down a wave function for the universe, not a wave function for a particle or a field, but a wave function for the entire universe, including its geometry, its matter content, its topology. The Wheeler-DeWitt equation is the equation of motion for this cosmic wave function, and the proposal was the universe tunneled into existence from nothing, quantum tunneling, uh like a particle tunneling through a barrier it classically should not be able to cross. The universe tunneled from a state of no space-time into a state of having space-time.

[00:15:17]Beautiful idea, wrong answer. Not wrong in the math, the math is fine, but wrong in the interpretation, because the nothing in this scenario is not nothing.

[00:15:27]The tunneling happens within the context of quantum mechanics. The wave function already exists. The laws already exist.

[00:15:36]The framework for tunneling, the probability amplitudes, the Hilbert space, the operators, all of it already exists. You have not explained why there is something. You have assumed a rich mathematical structure and then explained how a particular kind of space-time emerged within it. That is interesting physics. It is not an answer to the deep question. I recognized this about 10 years after I had been enthusiastic about the approach. That was uncomfortable. You spend a decade thinking you are working on the answer, and then you realize you have been working on a restatement of the question. The deeper question, the one that quantum cosmology does not touch, is why are there laws of physics at all?

[00:16:21]Why is there a mathematical structure for the universe to obey? Why is quantum mechanics the framework rather than something else or nothing? David Deutsch has argued powerfully that this is where the question ultimately lives, not why did something appear from nothing, but why does the mathematical structure of physics have the specific character that it has, a character that permits complex, stable, information-rich structures to exist. Max Tegmark pushes this further with what he calls the mathematical universe hypothesis, the claim that mathematical existence and physical existence are the same thing.

[00:16:59]Every consistent mathematical structure exists, not just as an abstract object, but as a physical reality.

[00:17:07]Our universe is not described by mathematics, our universe is mathematics.

[00:17:12]And in that case, the question why is there something rather than nothing has a clean answer, because nothing in the sense of no mathematical structure whatsoever is itself a mathematical structure, and a particularly degenerate one, and there is no reason to privilege it over the others. Everything that can exist does exist. I want to be honest about where I stand on Tegmark's proposal. I find it genuinely interesting. I do not know if it is right. It has a certain grandiosity that I am instinctively suspicious of. When an idea solves everything, I reach for my wallet, but I cannot dismiss it. The mathematical structures of physics are not arbitrary.

[00:17:54]The specific symmetry groups, the specific dimensionality of space-time, the specific form of the quantum mechanical framework, these things have a coherence, a unity, a depth that strongly suggests they are not an arbitrary choice out of some infinite menu of possibilities. They feel like they are the way they are for reasons.

[00:18:16]What those reasons are, I do not know.

[00:18:19]What I do know, what I am willing to defend on the basis of 50 years of looking at this is the landscape, and I realized that when I say the landscape, some of you think I am talking about real estate. I am not. The string theory landscape is one of the most controversial and one of the most important ideas in modern physics, and it bears directly on why the universe has the properties it has rather than other properties. Here is the situation.

[00:18:45]String theory, and I have been working on string theory since before most of the people in this room were born, so I am allowed to say this, string theory does not uniquely predict our universe.

[00:18:57]This was considered by many people for a long time to be a fatal flaw.

[00:19:03]A theory of everything that does not tell you what everything is, what good is it? I have had this argument.

[00:19:10]I have had it with colleagues who are much smarter than I am, and I will have it again tonight if anyone wants to. The answer to what good is it is this.

[00:19:21]It may be revealing something true and deeply important about the structure of physical reality. String theory does not uniquely predict our universe because there is not a unique universe. There is a landscape of possible universes, an astronomically, mind-bendingly, almost incomprehensibly large space of possible configurations of the vacuum, each with different values of the physical constants, different numbers of large space-time dimensions, different particle spectra, different laws of low-energy physics. The estimate for the number of vacua in this landscape is around 10 to the 500. That number has no name. It is not just large. It is large in a way that breaks your ability to reason by analogy with anything you have ever encountered. And the claim, which I believe, and which I want to be precise about, because people misrepresent it constantly, the claim is not that the all of these vacua exist simultaneously in some parallel universe that you can visit on weekends. The claim is that eternal inflation, the cosmological process by which the universe continuously generates new inflating regions, each of which can settle into a different vacuum state, implies that the universe is vastly larger than what we can observe, and different regions of it realize different points in this landscape. We live in the particular patch, the particular vacuum, where the constants have values that permit stars, planets, chemistry, biology, and physicists to exist and ask questions. Other patches exist in the same sense that other stars exist, but we cannot visit them, and they do not have observers asking why the cosmological constant has the value it has, because in those patches the cosmological constant has a different value, and there are no observers.

[00:21:13]This is the anthropic reasoning as applied to physics, and I know how it sounds. It sounds like giving up. It sounds like saying, "The constants are what they are because if they were different, we would not be here to notice." And that is the end of the explanation. I understand why that is unsatisfying. It is unsatisfying to me, too.

[00:21:33]But I want to distinguish between two versions of the anthropic argument. One version is lazy.

[00:21:40]Things are the way they are because they have to be this way for us to exist.

[00:21:45]Full stop. No further questions. That version is not science. It is a thought-stopper. I do not endorse it.

[00:21:53]The other version, the version I do endorse, is a selection effect argument.

[00:21:59]It is the same kind of argument that explains why the Earth is at the particular distance from the Sun that it is, not because some law of physics uniquely specifies that distance, but because in a solar system with many possible orbital radii, life arose on the ones where liquid water can exist, and liquid water requires a specific temperature range, and that temperature range requires a specific distance from the star. No mystery. No metaphysics. A selection effect. We are at this distance because we could only have arisen at a distance compatible with our existence. If the landscape is real, and I believe it is real, though I hold this belief with the appropriate uncertainty, then the same logic applies to the constants of physics. We observe the constants we observe because these are the values compatible with observers existing to observe them. The vast majority of the landscape has no observers. We are in the tiny corner that does. Does this answer "Why does the universe exist?"

[00:23:04]Partially. It answers "Why does our universe have the specific properties it has, rather than the slightly different properties it could have had?" But it does not answer the deeper question. It does not tell you "Why there is a landscape at all? Why there is a quantum theory of gravity at all? Why there is an eternal inflation at all?" You push the question back, but you do not dissolve it. And here is where I have to stop and be honest with you about what I do not know, what nobody knows. We do not know why there is a mathematical structure to physical reality. We do not know why quantum mechanics is the framework, why the world is described by complex Hilbert spaces and unitary evolution and Born rule probabilities, rather than something else entirely. We do not know whether something else entirely is even a coherent notion. We do not know whether the question why these laws, rather than other laws, has an answer, or whether the laws are, in some technical sense, necessary, not contingently true, like the boiling point of water, but necessarily true, the way 2 + 2 = 4 is necessarily true.

[00:24:17]Erik Verlinde has proposed that gravity, not just space-time, but the gravitational force itself, is an emergent phenomenon, a thermodynamic force that arises from the statistical behavior of information on holographic screens. I find this idea deeply interesting. I think Verlinde is onto something real. I also think the proposal is incomplete in ways that matter. The specific mechanism by which information on a screen becomes the force that holds galaxies together is not yet worked out. Saying gravity is entropic is a slogan. Making it a theorem requires showing that you can derive the Einstein field equations, not approximately, but exactly, in the appropriate limit from an information theoretic description. That has not been done. But the direction is right. The direction toward understanding space-time, gravity, and the laws of physics as emergent from something more fundamental, something information theoretic, something that lives at the boundary, rather than in the bulk, I believe that direction is correct. And if it is correct, then the question why is there something rather than nothing becomes, ultimately, a question about whether the information theoretic structure can exist in the absence of a physical instantiation, which brings me back to 1973 and that cold coffee and that equation.

[00:25:47]What I realized that night, though it took decades to formulate properly, is that the mathematical structure of physics does not seem to require a physical substrate in order to be consistent. The equations do not know they are describing a physical universe.

[00:26:02]They are just equations. They are true or false, consistent or inconsistent, by purely mathematical criteria. The question of whether they describe something real, whether there is a physical universe out there that the equations are about, is a question the equations cannot answer from inside themselves. And this is the strange loop at the heart of the question of existence. You cannot bootstrap physical existence from purely mathematical existence without some additional principle, some statement that says mathematical structures of this particular type, or above this particular level of complexity, or with this particular kind of self-referential coherence, are not just abstractly true, but physically instantiated. Tegmark calls this principle mathematical realism. Others have called it the principle of plenitude. Leibniz, and yes, I am going to cite Leibniz, because he was right about more than people give him credit for. Leibniz asked why God chose this world, rather than other possible worlds, and his answer was that God chose the best of all possible worlds. We do not have to invoke God, but the structure of the question is the same. Why this particular mathematical structure, rather than a different one, or none? I do not have an answer. I want to be completely clear about that. I have a direction. I have a set of tools.

[00:27:32]I have a research program that I believe is pointed at the right target. But the answer, a real answer, a satisfying answer, the kind of answer that makes you feel the problem is solved, and not just repackaged, that answer does not exist yet. It may not exist in my lifetime. It may require a conceptual revolution at least as large as the one that produced the quantum mechanics, and that revolution came from directions nobody anticipated, and it took decades and the combined effort of some of the greatest minds of the 20th century, and even now, 100 years later, we are still arguing about what it means. So, I want to leave you with this. Not a conclusion, a provocation.

[00:28:18]The fact that you exist, that you are sitting in this room, that your neurons are firing in patterns that constitute thought, that you have the experience of wondering why you exist, this fact is not separate from the question we have been discussing. It is the question. You are not an observer standing outside the universe looking in through a window and asking why the universe is there. You are the universe asking why it is there.

[00:28:44]You are a pattern in the quantum fields, a very complicated, very temporary, very improbable pattern that has achieved sufficient complexity to turn back on itself and ask about its own origin. And here is the thing that keeps me up at night. Here is the thing I have never found a satisfying answer to, after 50 years of trying. The fact that the universe is comprehensible, that the patterns in the quantum fields that constitute human minds are capable of understanding the mathematics that governs those same quantum fields, this is not obviously necessary. It is not a logical consequence of anything we know.

[00:29:27]There is no theorem that says a universe complex enough to produce life must produce life capable of understanding physics. Most complex universes, presumably, do not. The organisms that arise in them may be exquisitely adapted to their environments and still completely incapable of grasping quantum mechanics or general relativity or information we can grasp these things, we have grasped them imperfectly, incompletely, with enormous effort over the centuries, but we have grasped them. The universe is, to a degree that should astonish you, legible. The laws are not hidden behind a veil of irreducible complexity.

[00:30:05]They are hard to find, but once found, they are expressible in mathematics that human beings can learn and use and extend. Why? I do not know. I have thought about it for 50 years and I do not know. It might be a selection effect. Minds capable of physics are better at surviving and reproducing in complex environments. So, of course, the minds that are here asking the question are physics-capable minds. That argument has some force, but it also feels insufficient. The gap between good at navigating the environment and capable of deriving the holographic principle is vast. Evolutionary pressure can explain the first, I am not sure it explains the second.

[00:30:47]Wigner called it the unreasonable effectiveness of mathematics. He was being deliberately understated. It is not just unreasonable, it is bizarre. It is the kind of thing that, if you sit with it long enough, starts to feel less like a fortunate accident and more like a clue. A clue to something we do not yet have the framework to understand.

[00:31:11]That equation I wrote in 1973, the one I thought described nothing, what it was actually describing was the boundary theory of a particular kind of space-time. The nothing was a holographic screen and the everything was the bulk space-time that the screen encodes. I did not know that then. The mathematical structure that would reveal that the ads/cft correspondence did not exist for another 24 years. I was holding a piece of a puzzle without knowing the puzzle existed. Maybe that is where we are with the question of existence itself. Maybe the pieces are already in our hands. Maybe the equations are already written, scattered across the papers and chalkboards of a hundred physicists who do not yet know they are working on the same puzzle.

[00:31:57]Maybe the revolution that dissolves the question why is there something rather than nothing is already underway and the people driving it think they are working on quantum gravity or information theory or the emergence of space-time from entanglement and they are right. They are working on those things and those things turn out to be the answer. Or maybe the question is harder than that.

[00:32:20]Maybe it requires something we do not have yet, not just new mathematics, but a new way of thinking about what existence means, what it means for a physical universe to be real rather than merely mathematically consistent, what the relationship is between the mathematical and the physical that would make the second possible without the first. Maybe we are like Maxwell before Faraday's field concept existed trying to describe electromagnetism with particles and forces and getting it almost right, but not quite right because the concept we need has not been invented yet. I am 70-something years old. I am not going to be the one who solves it. That is fine. Science is not about the individual, it is about the argument and the argument goes on. The argument was going on before I was born and it will be going on after I am gone.

[00:33:13]And the fact that it will be going on, the fact that there will be minds in a universe that did not have to exist asking why it exists, I find that, on balance, more interesting than any answer could be. You came here tonight because something in you needs to ask this question.

[00:33:31]The discomfort you feel when you really sit with it, not the not the polite academic version, but the real version, the 3:00 in the morning version where you lie awake and think, "Why is there anything? Why is there something rather than nothing? Why does the nothing not just stay nothing?" That discomfort is not a symptom of confusion. It is a symptom of contact. You are touching the real question. Do not let anyone hand you a comfortable answer that makes the discomfort go away. The discomfort is the point. The discomfort is what doing physics feels like when you are doing it honestly. Go home and sit with it. It will not leave you. It has not left me.

[00:34:14]That's the whole point.