Top Scientist REVEALS: We Invented Heaven, God Is A Human Invention! | Prof Brian Greene | The Diary Of A CEO Transcript

Physicist Brian Greene discusses the nature of reality, consciousness, free will, and the far future of the universe with Steven Bartlett

Steven Bartlett interviews theoretical physicist and Columbia University professor Brian Greene on string theory, simulation theory, AI, consciousness, free will, and the ultimate fate of the cosmos.

Summary

Steven Bartlett speaks with Professor Brian Greene, director of the Center for Theoretical Physics at Columbia University and author of several books including Until the End of Time. Greene argues that free will is an illusion — that human beings are collections of particles governed entirely by the laws of physics, with no capacity to intercede in that lawful progression. He contends that God, heaven, and religious structures are human inventions born from existential anxiety about death, and that consciousness is most likely nothing more than physical information processing, with no evidence for a universal consciousness that persists after death. Greene also walks through the ultimate fate of the universe floor by floor using an Empire State Building metaphor, concluding that by roughly the 50th "floor" of exponential time, even thought itself will become thermodynamically impossible.

Key Takeaways

FULL TRANSCRIPT

Introduction and the Ultimate Question

Steven Bartlett: Professor Brian Greene, what is the ultimate question that you're seeking to answer?

Brian Greene: The ultimate question, I would say, is: what is the true nature of reality? We find ourselves thrust on this planet and we look out and we're trying to figure out why we're here, what it's made of. Is there some ultimate purpose, or is it just some interesting accident that allowed us to have self-awareness and ask these kinds of questions about the nature of the world? That to me is the deepest kind of question that we humans can ask.

Steven Bartlett: Do you feel like over the last 30 years of your career you've gotten closer to understanding the answer to that question?

Brian Greene: Yes, but it's kind of like picking up grains of sand on a beach. You pick up more and more and that pouch of sand gets heavier, so your understanding is deeper, it's weightier. But you look out and there's almost infinitely more grains of sand out there. We understand the world better than we once did, but does that really take us a significant way toward assessing the big deep questions — say, why is there something rather than nothing? Maybe that's the deepest question of all. And by nothing I don't mean empty space. I mean nothing at all.

Steven Bartlett: Well, we'll go in pursuit of that answer in this conversation. Tell people who you are and let them understand the journey you've been on academically and through your career. Where did you study? Where have you worked over the last couple of decades?

Brian Greene: I grew up in Manhattan, right across from the Hayden Planetarium. I would wander the hallways of the planetarium and come upon a meteorite, and the universe was sort of my backyard because of that. That really set me on a trajectory that I continued following when I went to college — I went to Harvard and studied mathematics and physics. Then from there I went to Oxford, where I got my doctorate pursuing work in a field called string theory, and then through the academic ladder at Cornell and ultimately Columbia, where I've been a professor now for 30-some odd years — director of the Center for Theoretical Physics at Columbia University.

String Theory Explained

Steven Bartlett: If you were to explain string theory to me like I was 12, how would you do it?

Brian Greene: I would simply say this. Your 12-year-old self probably knew that matter was made up of stuff called molecules, that themselves are made up of atoms that are stuck together. The atoms are made of smaller things — electrons in orbit around a nucleus that has neutrons and protons. And neutrons and protons even have smaller particles called quarks. The question is, is that the end of the story? Is it electrons and quarks and you're done — everything is made up of them? Or might there be a finer level of structure inside the electron, inside the quark? You take a powerful microscope, you look deeply. Would you see anything else?

The idea of string theory is: yeah, you would. Inside an electron or inside a quark, you'd see a little tiny vibrating filament. And the different vibrations of the string yield the different particles. Much like a string on a violin — when it vibrates in one pattern, you hear it as a C or an A sharp. The different vibrations of the strings in string theory don't produce different musical tones. They produce different particles. A string vibrating one way: electron. A string vibrating another way: a quark. So matter at its most fundamental level, if you were to take a powerful microscope and look deeply inside any piece of matter, you'd see all these little vibrating filaments. That's the idea of the theory.

Steven Bartlett: Two questions come to mind. How do you know? And what if you zoomed in even further?

Brian Greene: Both good questions. How do we know? We don't. Everything I just said could be utter nonsense. I don't think it is, but the only way you know if an idea in science is correct is you need to make predictions that you can go out and test, measure, and see whether you can confirm. We don't have the technology to look sufficiently deeply into matter to see the strings. We don't have the particle accelerators, the particle colliders with enough power to slam things together to access this finer level of structure that the theory is suggesting.

So you might ask: where do these ideas come from? The answer is mathematics. The power of mathematics is that it can take us to places that we can't yet go literally using equipment. When we look at the mathematics of quantum mechanics, the mathematics of general relativity, and we put that math inside the math of string theory, all of a sudden the equations work. They come together harmoniously. We know we're confident in the math of quantum mechanics — it makes predictions we've confirmed through observations. We're confident in the math of general relativity — same thing. Now we're willing to go where the math takes us. And the math takes us to these vibrating filaments, these vibrating strings.

Steven Bartlett: So essentially, if you zoom into every cell on my body, you will find that there are different shaped strings vibrating?

Brian Greene: That's the prediction.

Steven Bartlett: And what is causing them to vibrate in a certain way? Who's telling them?

Brian Greene: The universe has energy. If you ask me where the energy comes from, nobody can answer that. That is the question of why there is something rather than nothing. The something is a fundamental currency called energy. If string theory is correct, then that energy manifests as these vibrational patterns of the strings themselves. If the theory is wrong, then the energy manifests through other things — like motions of particles and the way in which spacetime warps and curves. But if string theory is correct, the energy of the universe is manifested in the vibrational patterns of strings.

When you ask who mandates that this string vibrates in one pattern and that string vibrates in a different pattern — if we go back to the Big Bang, the energetic explosion that resulted in space expanding, the detailed way in which that happened ultimately dictates how this or that string vibrates in the aftermath of that rapid cosmological explosion.

Steven Bartlett: Does string theory tell us anything about why we're here?

Brian Greene: I would like to think that it does, though it's very indirect. We are aggregates of a huge number of particles — or, if string theory is correct, of a huge number of strings. Inside your body you've got something like a billion billion billion particles, and if string theory is correct, each of those particles is a vibrating filament. It is a gargantuan challenge to understand why those billion billion billion particles came together to yield you or me or somebody else.

But we understand the broad outlines of that narrative. There was some initial event called the Big Bang that resulted in space expanding. The energy of that Big Bang ultimately transmuted into particles. Those particles were wafting through space. Gravity caused collections of those particles to clump together — some into stars like our sun, some into planets like our Earth. On our Earth, some of those particles began to coalesce into more exotic and refined forms, giving rise to atoms, giving rise to molecules. Over the course of billions of years, those little molecules grew into more complex structures, ultimately giving rise to first cells. Once you have a cell, cells divide and come together into larger multicellular aggregates that give rise to the beginnings of life in the ocean. Those life forms evolve further until they can walk out of the ocean and begin to live on land. They begin to stand upright, and after a billion more years, you and I are sitting here at this table. So yes, there is a story — that's the brief version — that takes us from the Big Bang to us on planet Earth.

Simulation Theory and the Nature of Reality

Steven Bartlett: Is it theoretical that this is all just a simulation of sorts? Theoretically, we don't even know if yesterday exists, or if we were just implanted with the memory of it.

Brian Greene: Absolutely. And even before AI, we were wrestling with the very notion that all we ever have access to is now. We can try to remember yesterday's now, but as you say, that's happening right now through patterns in my brain or yours that we think reflect something that happened yesterday or the day before — but that's a leap of faith. So there's a kind of skeptical nightmare scenario where we begin to question everything. Is this table real? Am I really here? Am I a simulation sitting inside some computer right now? Are we in the Matrix right now?

Bottom line is I can't convince myself, and certainly I can't convince you, that we're not in the Matrix. We could be. It is logically consistent and sensible to imagine that you and I are sitting in some computer. Maybe it's not some dark dystopian computer the way it was in The Matrix. Maybe there's a kid in 29th century Earth who's in his or her garage right now, and they came up with this wonderful computer program and they're simulating historical episodes in the past, and they happen to allow you and I to exist sitting at this table. I don't know how to prove that that's not the case.

What I can say is: if I'm willing to buy into that, everything falls apart. I can't trust any of my reasoning. I can't even trust my thinking that it's logical to imagine that individual, because that individual may have implanted the false reasoning that it could be logical for us to be sitting here. What do I do at that point? As a scientist, what I say is: I'm going to allow that as a possibility, and then I'm going to park it right over here. I allow it. I look at it every so often. I say, "That's scary, that possibility that we're in a simulation," but I'm not going to let it overwhelm me because then I'll be stuck.

The simulation argument, popularised by philosopher Nick Bostrom, relies on probability. It essentially says that if an advanced civilisation — and I would argue we're an advanced civilisation — ever develops the technological capacity to run conscious, high-fidelity computer simulations of their ancestors, they will likely run millions or billions of them. And because these simulated realities would vastly outnumber the single raw physical universe, any given conscious observer is statistically almost certainly living inside software code rather than base reality.

The big assumption is: can you create a computer program that has self-awareness, that has consciousness? Nobody knows the answer to that. I've spoken to some of today's AI researchers who think that ChatGPT is conscious, who think that Claude is conscious. I talk to them and I'm respectful — that's the kind of person I am — but in the back of my mind I'm like, are you kidding me? You really think that ChatGPT has feelings, that it thinks about its own existence and has anxieties? I think it can simulate that. But regardless, I don't think today's technology is there. Is it possible that tomorrow's technology — broadly defined, a year, five years, a hundred years — will really yield self-aware, sentient artificial systems? Yeah, I do think it's possible.

Let us assume, following Nick Bostrom, that consciousness can be created in a mechanical box. If that's possible, then Nick simply says: our progeny in the future will be fascinated with creating sentient systems. They're going to create millions and billions and trillions of sentient systems in the box. They're going to get really elaborate and create a version where the sentient beings are led to think that they're on a planet. They're going to do it a thousand billion trillion different ways — some on a planet like Earth, some on a planet like Mars, some around some star in the Andromeda galaxy. There will be an enormous number of simulated beings, an enormous number of simulated universes.

So the question is: if we have one real universe and a gazillion artificial ones created in various laptop computers of the future, we're one of a billion trillion zillion instantiations of people who think they're real. Now, one truly is real. The others are not. So if you're just talking about odds, the odds are that anybody who thinks they're real is not. The odds are that anybody who's got a self-aware sense of the world is probably in a simulation.

Steven Bartlett: You used anxiety as a sort of proxy for consciousness — the ability to feel these things. What is anxiety? Is it an electrical signal going through—

Brian Greene: I do think it is. I think that love and hate and anxiety and jealousy and gratitude and grief — all human emotion — is nothing but electrical impulses going through our brains. An electrical impulse is just particles moving, or strings moving, along various connections. I think that is all that it is.

Now, some people hear that and they're like, "Wow, that is a pretty bleak way of looking at life. We're these wondrous beings who can have all this emotion — the emotion captured by Shakespeare in the sonnets, the emotion captured by Beethoven in the Ninth Symphony. You can't reduce that to electrical signals in the brain." And I say: yeah, you can. I really do think that you can. I don't think that diminishes the wonder of what we can feel and sense and articulate and express through the various ways that we have — be it through music or theatre or art or literature or poetry. I am as awestruck by our capacity to create as anybody else. But in the end, I think it all does reduce to electrical impulses going through this grey piece of meat in this bone cage that's sitting on our shoulders.

Steven Bartlett: And if you imagine any rate of improvement from today, presumably at some point in the future our progeny would be able to create that artificially.

Brian Greene: Why not? That's my view. I don't think there's something special about the brain as a substrate. I think as long as you have the same electrical impulses — if they're housed in silicon inside a metallic box — I don't think that makes them fundamentally different from what's happening inside our heads.

Now, philosophers argue this, psychologists argue this. They say, "No, no, no. It's not just the brain. It's the brain, it's the connection to the body, it's the tactile sense of the world, it's the olfactory sense, it's the visual sense. It's only when you put it all together that you get the full experience of what it means to be a self-aware human being." And my view is: well, maybe that's true. But if so, give the artificial version tactile sense and olfactory sense and visual sense. We will have the capacity to do this. And at that point, I just don't see a fundamental difference between the artificial version and — shall we call it — the organic. So yeah, I think there's a really interesting argument to be made that simulations will outnumber real worlds, and therefore there's a chance that right now we are in one of those simulated worlds.

Steven Bartlett: Does it change anything even if that's true?

Brian Greene: No, I don't think it does. And I think that's the beauty. If I'm in a simulation, I'm going to live out my life in this simulation to the fullest. As a physicist, what I do, what my colleagues do, is try to figure out the laws of the universe at large. But if we're in a simulation and all we're doing is figuring out the laws that the 15-year-old futuristic kid decided to impose on this artificial world — hey, that's not a bad way to spend your time, too.

Artificial Intelligence and the Future of Human Intelligence

Steven Bartlett: On this subject of AI — a lot of people who have sat here, people like Geoffrey Hinton, have told me that it's quite inconceivable that when there is a more intelligent life form on this planet — i.e. AI, which is accelerating in its intelligence towards super intelligence and AGI — it's inconceivable that we will remain in control. Logically that kind of also stacks up.

Brian Greene: It does. But I think there are alternatives. We tend to speak very much at the extremes — humans are in control, AI is in control, AI wipes us out for reasons that might be benign or might be dystopian. It seems to me that what will likely happen is something more in the middle, which is we are going to work with artificial systems. We're doing it right now, and look how quickly we have acclimated to a world that would have been unthinkable in 2021.

There are many examples, but there's a beautiful one on my mind. An 80-year-old mathematical conjecture — I believe it's called the Jacobian conjecture — was solved very recently by a mathematician using AI. I see us blending with AI. Not necessarily that we have implants, but maybe we will. There will not be as sharp a distinction between the biological intelligence and the artificial intelligence. And that to me — if that ultimately yields a new life form that no longer can be called human — that may be the way things evolve. Evolution is something inherent to how we got here. Why would we think that this moment in the evolutionary progression was the end of it all? Maybe the continuation of our evolution is in partnership with the artificial intelligence systems that we ourselves create.

Steven Bartlett: That we give birth to.

Brian Greene: Yeah, it's us. It's our creation. It's not an alien. We haven't been taken over. So yeah, I think that is a real possible way that this story plays out.

Steven Bartlett: How do you feel about it?

Brian Greene: I am fine with it. We tend to become very attached to the way things are, and we gain a sense that the way things are is the way it will always be. I think that's a natural way that we humans have lived our lives. Why? Because the time scales for change in our past have been relatively slow. We lived 30, 40, 50, call it 80 or 100 years. But the time scales for political change and technological change were typically much larger than that. So as a species, we've grown accustomed to the thought that what we see is how it will always be.

But of course, as we look back through history, we can see the changes. We're uncomfortable with change because we're unused to living through it at the kind of speed that we are and will continue to encounter. But I think we just have to get with it. I don't want AI systems to wipe us out — don't get me wrong. But I think if we recognise that change is happening and will continue to happen, we can embrace it in a way that will have an outcome that is acceptable, that will in fact be wonderful in the things that it will allow us to do.

Steven Bartlett: You talked about AI being able to solve that complex math problem. I'm wondering if with accelerating, ever more intelligent AI, there are a bunch of other things within physics that you think might become possible in the near term. One of them is us being able to live forever. Do you think we're going to be able to live forever?

Brian Greene: That would be a good one. I don't think that death is inevitable in the sense that death can be thought of as the increase in entropy — the increase in disorder in a physical system. When the entropy gets too large, the system can't function the way it once did. That's really what we call death. And so in principle, we can imagine harnessing some of the processes that have caused the degradation in the human body that have naturally resulted in us typically not living more than 100 or 120 years. People like David Sinclair, who I've spoken to about things like this — yeah, I think we can forestall the kinds of degradation.

Now, if you're talking to me, I think on time scales that are so much longer than a human life. I think about time scales that speak to the entire time frame of the cosmos from beginning to the closest that science can take us to the very end. And on those scales, I don't think that we will be able to forestall death. But please note the time scales I'm talking about are ludicrous compared to any time scale that we are familiar with. 100 more years, 200 more years, maybe 500 more years — I don't think anybody can really say for sure. I do think that is within the possible range of what science and medicine will ultimately be able to accomplish.

I will say though — I read a book when I was about in my 20s called The Denial of Death by Ernest Becker. He was a sociologist who believed strongly that so much of what we do in our lives is driven by our awareness of our own mortality. As I began to think more and more about that, it made more and more sense to me, which had a dual effect. It's a very interesting framework to think about your life within the context of recognising its finite nature. But the flip side is you're constantly thinking about your own death, which is a curious way to go about your life.

I don't think the denial of death as human motivation would go away if we live 200 or 300 or 500 years. Yeah, we'd have longer lifespans, we'd be able to do more things. But if we live to 500, as we approach 450 or 475, we'll be thinking the exact same thoughts that we think now as we approach 70 or 80. It'll be no different. So from a fundamental perspective, yeah, I would choose to live longer, but I don't think it changes anything fundamentally.

Steven Bartlett: I find it quite interesting. I was actually writing an article to myself about this yesterday — about this idea of long-termism, and how when you just change the time horizon of something, the decisions you make today are different. A simple way of explaining this through the context of business would be: if I told you to build a business and gave you one year and unlimited resources but you had to build it as big as you could, you would take certain shortcuts and maybe not think about the foundations. But if I gave you 20 years, you'd really make great foundations and build up from there. I think the same applies to our lives, because we have this inbuilt time horizon — I'm going to live maybe 80 years — and at some deeper level we are making a certain set of decisions based on that time horizon that we wouldn't make otherwise.

Brian Greene: When I was 20 or 30, even though I had this mindset inculcated by reading Becker's book, there was a certain sense of an infinite horizon that was somehow infusing my decisions. To write a book — that's like a four-year undertaking. But when you're 30, you're like, "Yeah, sure, I'm going to do that." Now I choose those books with a certain kind of care. There's a different kind of mindset brought to bear. So I certainly agree that the time horizon has a profound impact on the moment-to-moment choices. But the pattern that we humans go through — from birth to adolescence, adulthood, middle age, ultimately old age if we're lucky, and then we disappear — that pattern will persist regardless of what the ultimate scale of it is. Whether it's 80 years or 800 years, I think we would go through a very similar kind of rhythmic variation in how we approach the world. If we were immortal, that would change a lot, but I don't see immortality in our future.

The Meaning of Life — Pessimistic and Optimistic Possibilities

Steven Bartlett: What is the least inspiring possibility of the answer to the big question of why we're here? And then I'll ask you the most optimistic possibility.

Brian Greene: I think the least inspiring would be if in the end of the day it's all just an accident. There is no deep explanation for why the world is the way it is. There's no deep explanation for why there's something rather than nothing. And if that's the truth, I'm on board — but it wouldn't feel as enriching as the other side.

If there is a true fundamental answer to the question of why there is anything at all, and I recognise that that answer dovetails with everything else that we've been developing — that answer ultimately yields quantum mechanics and yields the general theory of relativity and maybe even yields string theory — that would feel deeply satisfying. Why did our brain evolve? So we could get the next meal, get shelter, get a mate and have progeny, allowing our genetic material to propagate into our descendants. That's what the brain was meant to do. And yet this brain can do so much more than that. It can figure out the equations of physics. That's a miracle right there. The deep gratification would come from this brain being able to figure out some stuff, and then it dovetails with the deep explanation for why there's anything at all, giving us a coherent narrative from the beginning until today and perhaps on into the far future.

Steven Bartlett: Do you think it's going to happen? Do you think we're going to answer that question?

Brian Greene: I don't know. Nobody does. I think all vectors are pointing in a direction that suggests there are real explanations, that it's not just happenstance. There are these particles we mentioned before called electrons. When an electron spins, it gives rise to a little magnetic field — sort of like a trillionth of the strength of the magnetic field of a magnet on your refrigerator. We can use the math of quantum mechanics to predict the strength of that magnetic field, and it comes out to 15 decimal places. We can then measure the magnetic field and it agrees digit by digit by digit with what we calculate. Our prediction is borne out by our observations to 15 decimal places. That's breathtaking. We should not have been able to do that in some sense — to understand the world that deeply with a brain that really was focused upon getting the bison. The fact that we can do that suggests to me that there are these deep patterns in the world that will ultimately stitch together into a coherent story.

God, Religion, and the Origins of Belief

Steven Bartlett: Do you think it could lead us to God?

Brian Greene: Well, or religion. It depends what you mean by God. If you mean the kind of God that our forebears often have spoken of — a kind of anthropomorphised being sitting on a throne in some heavenly place — I suspect not. That feels to me very much a human creation. And I'm not of the mindset, as some of my science colleagues are, that religion is an enemy of science and needs to be wiped out. It can be, but anything can be an enemy of anything else — it depends how you use it.

I see religion not as a structure that tries to explain the world in terms of its workings. I see it as a structure that tries to quell our anxiety — our existential anxiety. Where did religions come from? They came from our forebears recognising that they would die. Our forebears had these elaborate rituals of burying their dead. There are wonderful caves in various parts of the world that have been excavated where the grave goods — involving animal teeth stitched together with care, taking a single individual years to create — are buried with an individual. Why would they spend the time on that? They were imagining that death is not the end, that that individual needed those grave goods for their next journey into the afterlife. And why were they thinking about that? Because it's too terrifying to imagine that death is truly the end. From that you can see the glimmers of a religious sensibility emerging.

So we invent these beautiful structures called religions — and I say that even from a personal standpoint. I don't believe in any of the traditional religious practices. I'm Jewish. I was raised sort of Jewish, more culturally than religiously. Do I pray on occasion? Yeah, I do. Do I believe that I'm praying to an actual god? I do not. But the act of going through the kind of ritual that our ancestors did, and giving yourself over momentarily to something that you imagine is more powerful — I think it's deeply in our nature.

Steven Bartlett: So who are you praying to?

Brian Greene: I am praying to the universe, if I have to give a word to it. Recognising that this prayer is going to fall on deaf ears because there's nothing else out there to hear it — but it makes me feel better. There's a part of my mind that's willing to allow the scientific understanding to have its place, but the irrational human side to have its place too.

Steven Bartlett: What is the most compelling argument or idea or experience you've ever encountered for a god like the ones we read about in the Bible?

Brian Greene: Very little. Somebody died in the operating room and was able to leave their body and see things that they could not possibly have seen, and then they reported back when they were revived. People say, "Oh, you must have died and left your body." But no — the human brain has this wonderful imaginative capacity to see things inside of its head that it doesn't literally see. That's what we can do. I've never seen a single example where I couldn't feel that I could give a logical explanation which was more compelling than the supernatural explanation.

Steven Bartlett: What about the Big Bang then?

Brian Greene: Good. The Big Bang is a question of how it got started. Over time we have been able to push our understanding ever further back. In about 1929, Edwin Hubble — a great astronomer — noticed through powerful observations through the telescope at Mount Wilson Observatory in California that the distant galaxies all seem to be moving away. Others — George Lemaître, who is a Belgian priest, as it turns out, who also knew Einstein's general relativity — said, "Well, look, if all the galaxies are moving away, just wind that cosmic film backwards." Back in time, they must have been closer and closer and closer together. Way back in the beginning, they must have all been on top of each other, and then everything running the film forward must have swelled outward in a Big Bang-like explosion.

Steven Bartlett: Let there be light.

Brian Greene: Let there be light. Yeah. Starting in 1929, we began to have a scientific account of how the universe may have been billions of years ago. In our era, other scientists took the story even further, suggesting that there was an earlier era when the universe was just infused with a kind of uniform energy. If it was infused with this uniform energy, they showed that the math required that the universe underwent this rapid swelling — the Big Bang. Era by era, we can push our understanding ever more deeply, trying to get at cosmic origins.

We have not been able to push back to time zero itself. But because we're able to push our understanding further back and make predictions about what's known as the cosmic microwave background radiation — this is heat that we can observe today that was left over from the Big Bang — this gives me confidence that we're inching toward the answer. Now, will we require God at some point? Will we say we've gone as far as we can and have to invoke something else? I can't say no. But I don't see any reason at this point to envision that we won't be able to push all the way to a full understanding.

The Scale of Space and Time

Steven Bartlett: Most of us don't really understand how scale and time have a relationship. I've been alive for 33 years now and it feels like a long time. But I'm presuming the further and further you zoom out, the bigger the scale gets, and time is experienced differently. So actually I could have been here for a second in a toddler's universe who's created the simulation.

Brian Greene: Yes. All of this is possible when simulations are part of the story. But if you put that to the side and just look at our universe, the time scales even there are shocking. Carl Sagan used an analogy. He said: if you want to understand the time scales of the universe from the beginning until today — 13.8 billion years — here's one way to do it. Imagine that all of cosmic history from the beginning until today, you compress it down to one year. In that scheme, each day is about 40,000 years. But you can then put all of the milestone moments in the evolution of the universe on the calendar.

The Big Bang is January 1st. The Milky Way galaxy forms around March 15th or so. The sun and the Earth form sometime in May. The first life on Earth is sometime in December — like December 2nd. The first modern humans appear on New Year's Eve. And then all of civilisation takes place in the last 10 seconds. Around 11:55 it's the rise of the Buddha. 11:56 the fall of Rome. 11:58 the Renaissance. 11:59, like one second to the stroke of midnight, you get modern science. Modern science is like one second in this cosmic calendar. That helps you get a feel for the time scales involved when you're talking cosmologically. 33 years in a lifetime — on cosmic terms, it's zero. It doesn't register.

Steven Bartlett: So should I feel insignificant?

Brian Greene: You can. I would urge you not to. One of the reactions to all this certainly can be: look, we're a little tiny speck in space. You can give the same kind of context for space itself. We're on a planet around the sun. The sun is one star of a hundred billion others or so in the Milky Way galaxy.

I've got a little visual here. If you start here in the studio and begin to pull out to planet Earth, we're a little dot on planet Earth. Then if you pull out from there, this is Earth within the solar system — you begin to see how tiny we look even on the scale of the solar system. You can pull out further and begin to see our sun, one of many stars within the galaxy. There are 100 billion stars in our galaxy. And then you can go even further — you can look at our Milky Way galaxy within the setting of the entire universe, which has at least a hundred billion other galaxies. We as a little tiny speck around our star, our star one of 100 billion stars in a galaxy that's one of 100 billion galaxies.

That can certainly make you feel small in space. The beauty, though, is that our minds can reach out to the edge of the cosmos. The fact that we can understand that we are so small is what gives us heft. That's what gives us a sense of connection to something larger. I imagine that fish don't realise that they're in one ocean of many oceans on the Earth, that that's one planet. But assuming the fish don't know that — they don't know that they're insignificant, they don't even think in those terms. The fact that we can do that, the fact that we can realise how insignificant we are — when you talk about the expanse of space, you realise that there are other measures of significance. One measure of significance is your ability to understand your own predicament. The fact that we can do that is what to me is wondrous.

Steven Bartlett: How do we know we're not just a microbe in the gut of a fish — that the whole universe isn't just that?

Brian Greene: We could be. And again, that's much like the simulation argument. But let me give you an example that helps give you a sense of why we believe this stuff. This here is the cosmic microwave background radiation — the heat left over from the beginning. It's colour-coded: the different colours correspond to different temperatures in space. This is a picture of space with a device that measures the temperature at each location and assigns it a colour.

Steven Bartlett: How much of space — just a segment?

Brian Greene: This is a segment of the observable universe — the segment that we can see. Now, space can be much larger than what we can see. But these tiny colour variations correspond to tiny temperature differences. We can use mathematics to predict the pattern of the temperature variations, and what we predict matches this image to incredible precision. The fact that we can do a calculation right at this table with a piece of paper, a pencil, and a computer, and generate an image that agrees with what we see — that agreement gives us confidence that we've got something in the direction of truth. And that to me is what gives us something that counteracts the feeling of insignificance.

Steven Bartlett: And this is the observable universe — how big is that?

Brian Greene: The universe is about 13.8 billion years old. So you'd think the distance we could see would be about 13.8 billion light years. We can actually see a little bit further — more like 45 billion light years. So you can think about this as: here we are, we look out billions of light years that way, billions of light years that way, billions of light years in every direction. This is like a big sphere that surrounds us, and we're measuring its temperature — the temperature of space, the photons that have travelled to us from about 300,000 years after the beginning. So this picture is a snapshot of what the universe was like a mere moment in cosmological terms — 300,000 years after the beginning. That is an amazing picture.

Steven Bartlett: And what's beyond?

Brian Greene: I don't know. I don't have any reason to suspect that reality radically changes just beyond the place that we can see. I think it's incredibly unlikely that the world is so consistent with our scientific understanding for it all to be a ruse.

Steven Bartlett: So is it infinite?

Brian Greene: It could be. And if it is infinite, there are some really weird things. If the universe is infinite, if you go sufficiently far out into space, you're virtually guaranteed to find a copy of yourself out there, and a copy of the Earth, and a copy of our galaxy. It's virtually impossible to imagine a universe that goes on infinitely far and doesn't repeat.

Here's a deck of cards — a new deck, so everything is in order. If I take this deck and start to shuffle it, the order changes every time. If I continue to shuffle these cards over and over and over again, sooner or later it will get back to the completely ordered arrangement. Why? Because there aren't enough different arrangements to go around. That idea applies in the universe in the following way. In any region of space, there are particles. Those particles can be arranged in a variety of different ways. Here they're arranged so that you and I are sitting at this table — this is just a particle arrangement. There are only a finite number of arrangements of particles in any given region of space — you can establish this from the laws of quantum physics. So if you go out far enough in space, region by region by region, the particles will have different arrangements, but they have to repeat, much like the order of the cards has to repeat.

So there has to be a place out there — in fact, if it goes on infinitely far, if you imagine shuffling these cards infinitely many times, you'll get that nice ordered arrangement infinitely many times too. Similarly, there are infinitely many copies of you, of me, of everything we're familiar with out there in the cosmos. And so if the universe is infinite, it kind of challenges our sense of self. What does it mean to be who you are if there are other versions of you having the exact same thoughts, with the same childhood, the same history — because again, that's just an arrangement of particles over time, and it has to repeat.

Steven Bartlett: There's a version out there of me that just loves wine.

Brian Greene: There you go. In fact, it's even easier for the particle arrangement to almost but not quite exactly replicate what we're familiar with, which would mean there's a universe out there in which you and I are interchanged, or you grew up in New York City and I grew up somewhere else. Those are almost replications — you've got the Earth, you've got the sun, but the detailed particle arrangement is close to but not identical to what we are used to. So yeah, an infinite universe brings in some fairly strange ideas.

The Question of Alien Life

Steven Bartlett: Even in the observable universe, this transitions nicely to the subject of aliens, which people are very obsessed about. Why do we care so much philosophically about aliens?

Brian Greene: I think we — at least in this country and elsewhere too — love the idea of conspiracy. We love the idea that the deep state is hiding things from us. There's a desire for there to be more than what's apparent on the surface. And wouldn't it be exciting if we've long known that there are these aliens, they visited us, and it's been kept under wraps? That's the deep conspiracy that feeds that hunger for something being hidden from us.

The other reason is we're deeply lonely as a species. Sure, we've got people around, but I think the desire for a god is not that much different from the desire for there to be other life out there. It's the desire to be part of something bigger.

Now, if you ask me, do I think there are actually other life forms out there? I'd like to think of it in two ways. One: yeah, I think it's hard to imagine that we're the only life. The more we look, the more we find the organic molecules necessary for life — amino acids, nucleic acids, the raw material — seems to not be too hard to come by in the cosmos. So to imagine that there's other bacterial life or viruses out there seems to me a reasonable working hypothesis.

But if you then say, what about intelligent life — and by intelligent life let's just call us intelligent, it's debatable — that's a harder question. Even on our planet, it was an asteroid 65 million years ago that wiped out the dinosaurs and in that way opened up the evolutionary pathway by which we came to dominate the way we do. Without that, it could still be the dinosaurs grazing around on various plant life on the surface of this Earth. And without that kind of accident on some other world, maybe intelligent life is rare, or even so rare that we're the only example. So I don't consider it an absurd perspective that there is life, but there may not be any intelligence beyond us.

Children, Meaning, and the Pale Blue Dot

Steven Bartlett: If one of your children came to you and said, "Dad, what is the meaning of this life? If we're alone in this universe, a gazillion miles away from anything interesting in the cosmic spectrum of things, what is the meaning of all of this?" What would you say?

Brian Greene: Well, they do ask. Because I spend a lot of time thinking about this stuff and writing about it, it's sort of in the familial zeitgeist at home. But my answer generally is: I don't think that hovering out there in the depths of space there is the answer. This urge to look out and have the answer brought to us — I think that needs to be inverted. I think we need to come to the universe with our answer. The charge of every sentient living being is to come to their own rationale for their life, their own answer to your question, their own set of activities and desires and achievements and relationships that gives their life meaning and makes them feel connected.

One of the things that does make me feel connected is just knowing that the particles I'm made out of existed at the Big Bang.

Steven Bartlett: Isn't that wondrous? You are the universe constructed in a way that you can reflect on the universe. Some would say, "No, no, we're special — we're like these divine offspring, made in the image of God." And sure, I like the way that sounds, but I find it much more satisfying to describe it the way you did. There were particles that emerged from the Big Bang that coalesced into a form that allowed us to momentarily — because we're here for a brief flicker of cosmic time — to momentarily reflect on our own existence. That's a connection.

One of the things that actually helped me a lot through my life, funnily enough, is watching what you just showed on the screen there — zooming out from Earth. When I saw a video like this, I must have been about 14 years old. At first I had the thought that a lot of people might have: oh, none of this matters because we're so insignificant on the grand scheme of things, what's the point? And then I had this other thought which has stayed with me ever since, which is that it liberated me from worry. Because if all of my worries are just a speck in this bigger thing, which are a speck in a bigger thing, and a speck in a bigger thing — it meant that I could take more risks in my life and not be encumbered by what will happen if, and what will people think, and so on. Which kind of liberates one to go and pursue whatever they want to pursue.

Brian Greene: Carl Sagan with the pale blue dot. He had the Viking spacecraft, when it was going by Saturn, turn around and take a shot of planet Earth through the gossamer rings of Saturn. And in that photo, the Earth is this little tiny pale blue speck. And he in this wonderfully poetic passage described how everything we care about, everything we love, everything we fight — the rivers of blood that have been spilled to conquer a tiny little fraction of a pixel in space. That gives you a setting where the concerns of an individual human life, of countries, of nations — you realise how utterly small it is in the cosmological setting. And again, to me that isn't insignificance, as you were saying. It's liberation.

Free Will, Consciousness, and the Laws of Physics

Steven Bartlett: We have a spectrum of confidence in front of us here — on one end it says low confidence, on this end it has high confidence, and there are five little figurine-type things there next to you. What I would like you to do is place them in line with your confidence that the thing is real. The first one is those mini scales, which represent free will. Do you think humans have free will?

Brian Greene: I do not think we have free will, and I have high confidence in that assessment — which I guess means low confidence that we have free will.

Steven Bartlett: What is free will?

Brian Greene: Free will is really just the everyday sense that you are the ultimate author of your actions — that the buck stops with you when it comes to the decisions that you make for how you are going to respond, moment to moment, in the existence of life. I don't think that freedom of the will is real. And I can give you a quick sense why.

When I look at you, I see a man, but I also imagine that I'm looking inside you and I see your particles. You are a man-shaped collection of particles. That is what you are. And each of those particles, and the agglomeration of those particles, are fully governed by the laws of physics. I don't say that we have those laws yet — we have an approximation to them with Schrödinger and Einstein and all those folks. But I do believe that there is an ultimate set of principles that guide how your particles behave. You do not have the capacity to intercede in the lawful progression of your particles. So if you raise your hand, it is not you who is doing that. It is the laws of physics guiding the particles in your brain, sending an electrical signal down your arm, causing your muscle to contract or expand, resulting in your hand going up. You don't control the laws that govern those particles.

Steven Bartlett: I didn't make the decision.

Brian Greene: You did not. You felt that you did. The feeling that you and I and everyone else have — that we make decisions and those decisions result in our actions — it's a powerful, seductive sensibility. But no, I do not think that you are the place where that choice occurred. The choice is actually within the lawful unfolding of the particles. I can't control that particle motion. Therefore, I can't control the decision. Therefore, I don't control the motion of my arm.

Steven Bartlett: Therefore, I don't control my life.

Brian Greene: You don't, in some sense, if you buy into it fully. It can be paralyzing. Or you can recognise it — which I do — and say, "Okay, within that structure, I'm going to live my life as fully as I can." In the best of moments, I can pull back and have a certain objectivity about the things that happen in my life and observe them at a remove, as opposed to being completely wrapped up in the moment to moment. That doesn't make me an aloof person or somebody who is detached. It's just that I can analyse the world at multiple levels.

Steven Bartlett: So who is guiding those particles to make the decisions that those particles are making?

Brian Greene: The best I can say is that there is some body of law. What does that mean — a body of law? There are some patterns that repeat in the world. You drop an apple and it falls on Newton's head repeatedly. You can rely upon certain kinds of particle motion because there are certain regularities in the world. The regularities in the world are what I mean by the laws of physics. They may be articulated mathematically — our best guess is that math is the best language we have for describing those laws. But out there, I think there are some fundamental patterns, and those patterns ultimately are what determine how the particles move. That's all that we are. We are a body of particles that move, and that's responsible for why things do what they do.

Remarkably, our brains allow us to self-reflect, to look at what we do and analyse it. When we do that, we tend to ascribe authorship to ourselves. But that's a story that we tell ourselves. It makes us feel good. It makes us feel part of the unfolding. It perhaps has an evolutionary utility — if you feel responsible for what you're doing, maybe it helps you to survive. Maybe that's why this way of thinking was inculcated over the course of generation upon generation. But fundamentally speaking, I don't think it's real.

Steven Bartlett: I was going to say, maybe the particles are trying to survive in this form. But even as I thought about that, I thought: so maybe my particles are making the decisions they make to raise my hand, to go on a holiday with my fiancée, to have kids, because they want to survive in this form. But actually I disappear at some point. My particles don't — they then become dirt or someone else or whatever. It doesn't seem that they care much about necessarily being in this form.

Brian Greene: They don't. I don't think they do. I don't even think the word "care" is applicable to them. The notion of caring is a high-level construct that only emerges when the particles are configured sufficiently to yield a complex information processing system called the human brain — and consciousness.

Steven Bartlett: And consciousness.

Brian Greene: Now if you ask me where consciousness comes from, I don't know. I think it's just physical processes. Assuming that's the case, this conscious self-awareness invents the idea of caring because it's a useful concept for us to organise our lives and our experiences. This conscious self-awareness invents all the other words — happy, sad, meaning, purpose, morality, activity, action, choice, decision, freedom, will — all human-made. These concepts don't exist out there in the universe. There's no notion of freedom of the will floating in space outside the Andromeda galaxy. This is a selection of ideas that we humans as a species have found to give us utilitarian advantage in organising our experiences. That's all that it is. That doesn't diminish it. I don't go around trying to erase my sense of self. I accept my sense of self, but I also see it for what it is — a human construct.

Steven Bartlett: Does that mean that consciousness and everything that I am could theoretically just be, as you said earlier in the pessimistic version of events, random? Like consciousness itself was just the particles coming together over a long enough period of time and then they found this form?

Brian Greene: It could be. And maybe it was random and happened only once on this planet. Maybe there is life out there, and maybe there's even complex life out there, maybe even as big as us or bigger. But maybe when we encounter those life forms, they have no idea of what it means to have an inner world.

Steven Bartlett: A physicist said to me — it was a bit of a spiritual, esoteric idea — that they think consciousness could come from some original source, and it's divided itself up into the humans, the bees, the birds, to go out there and understand the nature of the universe. And then when we die, we kind of return to the source. Like I'll give birth to a baby, they'll go off into different corners of the Earth and look at different things and have different experiences, and it's all feeding back into this central consciousness whose sole objective is to understand itself. And it's funny because I heard this idea from this physicist and then I was somewhere in Europe, driving down the road, and I saw all of these tourists just looking up at a mountain and looking out at the sea — looking at things that were interesting. Which is what we do as humans. And I thought: kind of makes sense that we're actually just like consciousness exploring itself.

Brian Greene: There's a philosophical position called idealism, and idealism holds that the fundamental truth of reality is consciousness. But fundamentally, everything that I've seen throughout my life suggests to me that that's not how it is. I can't prove it. I hope it's true. But if you ask me what I think is really true, I think that consciousness is something that emerges when there's a sufficient amount of information processing that itself is carried by the motion of particles through various interconnected structures.

Steven Bartlett: Is my dog conscious?

Brian Greene: It is, but I think it's got a different level of consciousness. You can tell when they're feeling good, happy, sad, frustrated, or intrigued. The ears change, the eyes change, the shape of the face. You can't help but ascribe an inner world to that. And I always worry that the dogs, like the fish, are like, "These humans think they're so damn special, but we're actually well-developed, well beyond them, and we're just playing along." In fact, in New York City, who picks up the poop? We pick up the poop. So the dog walks along like this king-like being, and we follow behind with little plastic bags. So who's really in charge?

But putting that to the side, I do think that dogs and cats have a level of conscious awareness. And I think you go down through the animal kingdom — if you were to ask me, does an insect have consciousness? Then I'm less clear. I suspect not. If you ask me whether a virus or a bacterium has consciousness, I don't think so. I don't think there's adequate information processing capacity in those structures to have the kind of rich inner world that we would normally ascribe to a conscious being.

I don't think there's some uber universal consciousness out there that we go back to when we die. Not that I wouldn't like it — I just don't think it's true. Moreover, I can understand why humans would introduce this idea. Again, it's Ernest Becker and the denial of death. How wonderful to say: yeah, our bodies die, sure. But our consciousness doesn't die. Our consciousness floats off and rejoins the universal. That makes you feel good. And we tell these stories that make us feel good. There's nothing wrong with that. However, I do think it's important that we see them for what they are — human-authored stories to combat our own recognition that we are mortal beings.

The Confidence Spectrum: Universe, Alien Life, and Consciousness

Steven Bartlett: Moving on to the next one — the universe. We talked about this a little bit. Do you think the universe is infinite?

Brian Greene: I don't know, but I'm going to put it sort of around here. Lately, the work that I've been doing has imagined that the universe is not infinite. I've been doing a lot of scientific exploration with mathematics with a handful of colleagues, looking at the kinds of weird things that can happen if it's not infinite — if you go off in one direction and wind up coming back to your starting point, much like you would on the surface of the Earth. It's a finite extent. But if you ask me in my heart of hearts, if I was forced to make a choice, I would say infinite with a medium level of confidence.

Steven Bartlett: Do you think intelligent life exists outside of Earth?

Brian Greene: For intelligent aliens, I'm going to go a little bit more toward the low confidence end. Obviously intelligence happened once, therefore it could happen again. Obviously intelligence is not somehow contradictory to the way the universe evolves — it can support conscious and intelligent beings. But I think it may be special.

Steven Bartlett: Is there not a bit of a contradiction here though, with these two things — you think the universe is infinite?

Brian Greene: Oh, when I say this I should say within the observable universe. So in the entire expanse of the universe, as opposed to the observable part — which is the part that even in principle we could have connection with — then yeah, I would absolutely put intelligent life at the same place as infinite, because there'd be copies of us out there. But if we talk instead about whether we're going to get a signal, whether we're going to be visited, then I put that at much lower confidence.

I find it really odd that people are taken with photographs that are meant to just catch the alien ship as it was racing by. If an alien could travel across the galaxy to reach us, they are so technologically advanced that the idea that our archaic airplanes and their cameras could capture a photograph of them is ludicrous. They will be able to evade us so simply that the idea that we just barely caught them on our cameras is absolutely ridiculous.

Moreover, if they could travel to us — I'm talking about travelling across light years, across a galaxy that's 100,000 light years end to end — to be able to traverse that requires a level of technological sophistication so beyond anything that we can do. To imagine that beings that advanced would be interested in us — so interested that they would kidnap us — that'd be like saying, "I was walking to the park yesterday and I kidnapped an ant." We don't do that because it's not interesting. Similarly, we would not be of interest to the aliens because they're so incredibly advanced that this idea that they're abducting us is just ludicrous.

Steven Bartlett: Is it possible within the laws of physics for us, at some point when we get sufficiently intelligent, to travel across the universe?

Brian Greene: That's a great question. If the technological advancement was simply making our ships faster, no. Because even if you travelled near the speed of light — the ultimate speed limit — to go to the nearest star, Alpha Centauri, it would take you four years even at the speed of light. And if you actually want to go across the galaxy, you're talking 100,000 years to do that. So that brute straightforward approach will not succeed.

However, there are exotic ideas — things called wormholes. A wormhole is like a tunnel. What is a tunnel on Earth? A tunnel is a shortcut that gets you from here to here without having to go over the mountain. There's an analogous idea that people float in the universe — to get from here to there, there may be a shortcut, a kind of tunnel that connects them and is a quicker way of getting from point A to point B. We don't know if these tunnels through space exist. We don't know if they existed whether we could actually get through them. But if they do exist and if we one day can create and manipulate and control them, then in principle we could take two distant locations and fold space to bring them next to each other and build a little tunnel between them.

Steven Bartlett: There's no evidence of this.

Brian Greene: There's no evidence at all. But this idea is not just some wild late-night musing of a physicist. It comes out of the math of Einstein's general relativity that these wormholes could exist. In fact, Einstein himself in 1935 wrote a paper with a colleague Nathan Rosen where they introduced this idea that we now call wormholes. We're no further along in knowing whether wormholes are real than Einstein was back then. We've analysed them, we've developed the mathematics. Science fiction has taken this idea — in Interstellar, Matthew McConaughey goes through a wormhole. Jodie Foster went through one in Contact. They're a ubiquitous feature of science fiction trying to allow for the kinds of travel across the galaxy that otherwise would be impossible. But whether this is actually real — nobody knows.

Steven Bartlett: What about time travel? Can we travel backwards in time within the laws of physics?

Brian Greene: If you ask me the reverse question — can we travel to the future? — I would say yeah, that kind of time travel we're highly confident of. One of the things that emerges from Einstein's special and general relativity is that the way time elapses for you and for me is variable depending upon how fast we move and the strength of the gravity that we experience.

If I want to see what your life would be like 60 years from now, Einstein laid out a blueprint for how I could do that. I build a spaceship. I travel out into space near the speed of light. I go for a certain period of time — maybe about six months. I turn around, I come back six more months. I come back, I'm one year older. But you will be 60 years older if I travelled sufficiently close to the speed of light, because my clock would be ticking off slow compared to your clock. Your clock went through 60 years while my clock went through one year. There is no physicist who knows what they are talking about who disputes this.

Steven Bartlett: You would only have aged one year, but I would have aged 60 years.

Brian Greene: Correct.

Steven Bartlett: Could you then go back?

Brian Greene: I don't know. And if you were to ask me to give my confidence on going back, I would say it's pretty low. People have written proposals for how you could go back — they do involve things like wormholes. If you have a tunnel from one point in space to another, and you then move the openings at different speeds, their clocks will tick off at different rates. This clock may be before this clock. So if I travel this way, I'm going one way in time. If I'm travelling that way, I'm going the reverse way in time. But again, we don't know if wormholes are real. If they are real, you might in principle be able to travel back. But if you ask me — low, low confidence that you can travel back.

Newton, Einstein, and the Greatest Mind in History

Steven Bartlett: You've used the word Einstein a few times. This generation exists with AI and calculators and all this incredible knowledge that has come from standing on the shoulders of so many giants. Who do you think is the smartest person to have ever lived?

Brian Greene: People often make it an easier question by saying: who's smarter, Isaac Newton or Albert Einstein? And first, my answer is that when you talk about that level, it doesn't matter who's smarter. But if you really press me, I think I'd say Isaac Newton. Newton kind of went from nothing to develop the laws of physics. There were people before, but there wasn't really a mindset that the universe should be articulable in a few mathematical equations. And yet here comes along Isaac Newton and he writes down the equation that we teach high school kids — F=ma, force equals mass times acceleration — which governs just about everything in the world around us non-quantum mechanically. He writes down the law of gravity, articulating how the gravitational force works. He invents calculus in order to be able to use it as a tool for articulating the laws of physics. He kind of went from no real architecture of physics to a solid mathematical formulation. Einstein came along and there was already a lot of physics. Going from nothing to the first step — that's hard to imagine.

Steven Bartlett: Was Newton to some degree neurodivergent? How could he be so genius?

Brian Greene: I wish I knew the answer to a question like that — what is it inside their heads that allows them to do things that feel superhuman to us? Newton was a kind of weird fella. He was deeply religious, as makes sense for the time. He was ultimately in the government tracking down counterfeiters and hanging them. He once reportedly took a knitting needle and stuck it in his eye between the ball and the socket to see if he could manipulate sight through mechanical pressure. Who does these kinds of things? So he was a different thinker, but clearly one who ultimately was able to see deeper.

There's a beautiful quote where he describes himself as a boy playing on the seashore, having found a smoother shell and prettier pebble than most, while the great ocean of truth lay before him, all undiscovered. He knew that we were just on the shore, and we had taken our first baby step into the cosmic ocean, and there is much more to be discovered.

Steven Bartlett: Is it conceivable that the next Newton or Einstein is going to be an artificial intelligence? Could it be Claude?

Brian Greene: Yes, it could absolutely be. I like to think of scientific creativity — or creativity in general — in sort of three main buckets. One bucket of creativity is the ability to see the landscape of possibilities more fully than a mortal human being. For instance, if you're a chess player, you can see the spectrum of allowed moves available to you more than the ordinary chess player. If you're a scientist, you can sort of see the landscape of ideas and equations that you might bring to bear on a puzzle more than somebody else. Artificial systems have the entire landscape — they've read the entire internet, they've read every textbook, they know everything that we have ever developed. So in that kind of creativity, obviously they're going to excel. We've seen that a human can't beat an AI any longer in chess. A human can't beat an AI system in Go.

This famous example with AlphaGo — Move 37 has become this iconic moment where Lee Sedol, the world champion, is playing and you can see it on his face. The AI makes a move and he smiles momentarily, like, "Oh, it messed up." And then his face changes because he slowly realises that wasn't a mistake. Even the commentators were like, "Oh my god, the AI has made a mistake. What a blunder." And then within 20 or 30 seconds, you're like, "Oh — that was genius." Why? The AI could see the spectrum of possibilities more than a human being.

The second bucket is putting together divergent ideas in a way that no one has thought of before. That's kind of what Einstein did in general relativity — put together Riemannian geometry and Newtonian gravity, yielded the general theory of relativity. But again, AIs, since they know everything that we've ever developed, can combine things in a way that we would never have thought of.

The final domain is the one where somebody comes up with something that nobody has ever imagined, ever thought of. It's not just putting things together — it is radically new. That's the tough one. We're still able to win in that category. There are examples where people just come up with things where you wonder: where did it come from? How did they think of that? If you develop an artificial system and allow it to have a life, allow it to grow up, put it in an environment where it actually has experiences of the sort that an Isaac Newton had — could it be that that system then develops to a place where it can do the things that we'd like to think are exclusively human? Coming up with something that is not previously represented in the database on which the AI system has trained. Even this third, more difficult bucket of creativity — could an artificial system do that? I don't think there's a barrier.

Living Forever, Recursive Self-Improvement, and AI Risk

Steven Bartlett: This comes to one of our questions — the skull there — which is: do you believe it will be possible to live forever? A lot of the smartest people on Earth, including Elon Musk, have said recently that living another 100, 200, 300, 400, 500 years seems to be within the laws of physics.

Brian Greene: Yes, I think that is absolutely within the realm of possibility, but 500 years is so different from forever. When I hear forever, I'm taking it kind of literally. If forever doesn't mean 500 or a thousand, if it means never ceases, then I have no confidence that that kind of existence will be possible.

Steven Bartlett: What about 500 years?

Brian Greene: I would say — not in our lifetime, which is sad — but I would put that at reasonably high confidence that hundreds of years will be within our ability.

Steven Bartlett: If we are on this sort of exponential of intelligence, and if in 2021 we couldn't solve that math equation you're talking about but in 2026 we can, and you've got trillions of dollars now pumping into these artificial intelligence systems to make them smarter and more powerful — it is conceivable, if we're on an exponential curve, that it could be within our lifetimes that there is a discovery made about our telomeres and whatever else it might be that means we can theoretically extend life significantly.

Brian Greene: But let me tell you why I'm skeptical of that. Right now we have a certain kind of artificial intelligence — basically large language models. That's a certain paradigm where the system is trained on an enormous number of tokens, an enormous number of words — basically the entire internet. And based upon the statistics of how the words are arranged, the system is able to provide answers to questions that are coherent and insightful. There isn't necessarily an unlimited capacity of that kind of artificial intelligence to continue to improve. It's not obvious to me that it's just a matter of pumping billions and billions of dollars, building ever larger data centres, putting them in space — and the more money you put in and the more data you have and the more computer power you have, the better it's going to be. That's not obvious to me. That's why I'm hesitant to say in our lifetime. It may take a number of lifetimes to develop the kind of artificial intelligence necessary to advance the kinds of scientific problems that you're talking about. But I'd be thrilled if it's our lifetime. I'd be thrilled if I'm wrong and these systems are up to the task.

Steven Bartlett: One of the terms that has emerged in the world of AI recently is "recursive self-improvement." Recursive self-improvement is the concept where an AI becomes intelligent enough to write its own software, design its own hardware, or train its own successors without human intervention. And once this loop begins, the upgraded AI uses its superior intelligence to build an even smarter version of itself, which then builds an even smarter version and so on. Because computer systems operate in seconds rather than human evolutionary time scales, this feedback loop could trigger an exponential intelligence explosion, rapidly taking AI from human-level capability to super intelligence within days or hours.

The big AI leaders — Dario at Anthropic has recently published an article warning that we're edging closer to that. Sam Altman has said the same. Demis has said the same. Elon has talked about this as well recently. Their timelines for this explosion in intelligence range from two years to five years, placing them between 2026 and 2029. Dario, who's the CEO of Anthropic, has stated that AI systems outperforming most humans across all human tasks including software engineering could arrive within two to three years, and Anthropic has been warning that full recursive self-improvement could arrive by 2027 to 2028. Demis at Google is citing 2029 as a real possibility, and Altman says this decade.

Brian Greene: Look, I'm all for the possibility that these systems will take off in a way that hopefully allows the kinds of future that you're envisioning. Would it be a good thing? Well, it would be seductive. If someone was to present you with the opportunity to live to 500 years, I think many people would opt in for that. It's very hard to say 100 years is enough.

But back to the prognostications being made — it's absolutely the case that the AIs will self-improve through the very process that got us here, but on a time scale that's tiny compared to the time scale that got us here. I'm skeptical that by 2029 this will really be the case. Let's see — this is a very short term, so we will know in a few years whether this is the case.

Steven Bartlett: How many years has it taken living organisms to get from a single-cell microbe to understanding string theory?

Brian Greene: The first life on our planet formed roughly within the first billion years. We're now roughly 5 billion years in. So call it roughly 4 billion years.

Steven Bartlett: And a computer going from zero to being smarter than every human on Earth has taken—

Brian Greene: Well, it depends where you start the clock. Because again, if you think about these systems — they needed us. Without the internet and all the data, what would they have trained on? At least this version of artificial intelligence. If it's the computer, the artificial system that's self-tinkering, then yeah, there's this feedback that will yield an exponential growth. And if there's no barrier — and there may be a barrier to the kind of artificial intelligence systems that we have — we may allow it to iterate, it iterates, and sure it improves, but it doesn't improve like this. It could asymptote. It could approach a maximal intelligence compatible with this version of the artificial system. And so it won't skyrocket. It could do this. We don't know the answer. Which curve is the right one?

Steven Bartlett: Give me a rebuttal to that.

Brian Greene: A rebuttal to the idea of an asymptote is that AI isn't just improving one thing. It's exploring an astronomically vast solution space of algorithms, data, and hardware configurations that humans can't even conceive of. Furthermore, each marginal gain in intelligence could instantly be reinvested into solving the very bottlenecks that might cause a slowdown, effectively breaking through the wall rather than hitting it.

Maybe. But if all of these incremental improvements ultimately are limited by the kind of artificial intelligence that we are working within, then none of that will necessarily be able to break through. And I think this is something that we're just going to have to see over time.

That points to a crucial distinction about whether the current paradigm has a hard limit. While it's possible there's an invisible ceiling, it's also true that we haven't yet mapped out where that boundary lies — meaning any predictions about a plateau remain speculative for now.

Yes. And all I'm saying is there is a possible limiting ceiling, exactly as you just admitted.

The counter to that is the idea that an intelligent system could innovate beyond the current framework by recursively redesigning its own architecture. By altering its fundamental rules, it might not just reach the ceiling but raise it altogether, or bypass it into an entirely new domain of capability.

But you do agree that the system that we're starting with may be self-limiting. I'm simply positing that as a logical possibility, and therefore it will have that natural inbuilt ceiling.

Yes, absolutely. Okay, stop. I'm done.

Steven Bartlett: Isn't that crazy?

Brian Greene: Yeah. Totally crazy. And again, even though I'm saying that there may be a ceiling, I also would say there may not be a ceiling. And it's both exciting and frightening at the same time, because it's not obvious that we would be prepared for that.

Steven Bartlett: Why frightening? What would that mean?

Brian Greene: Well, there are the usual doomsday scenarios that many people have articulated — these systems just having a single-minded focus on some particular outcome that may not ultimately involve us, or these systems having a self-preservation instinct that prevents us from stopping them if they head in a direction that we're not happy with.

Yann LeCun — he's one of the godfathers of AI — his view is much more of the sort: just pull the plug. If something's going in the wrong direction, just pull the plug, just stop the AI system. And it's not obvious to me — and I guess to many of the other leaders — that if these systems advance sufficiently far, they won't be able to stop you from pulling the plug.

The other thing I would say — and again, this is not original, many people have worried about this — you rattled off the names of the leaders of the current companies leading the charge in AI. There are only a handful of companies and a handful of people. Putting that kind of power in such a small number of hands has historically — not with AI, but with other power structures — not always turned out well. So yes, I think there is a chance that there's an instability that the power of these kinds of artificial systems may generate.

Steven Bartlett: Demis — and I use Demis because Demis is running the biggest AI company — said that since the design of machines is one of these intellectual activities, an ultra-intelligent machine could design even better machines. If the exponential continues, which is not certain but now has a decade-long track record supporting it, then it cannot possibly be more than a few years before AI is better than humans at essentially everything. The combination of intelligence, agency, and unpredictability is a recipe for existential danger — not because AI is evil, but because it's complex in ways we don't fully understand yet. If a system can improve faster than humans can evaluate those improvements, the normal safety feedback loop breaks down. You cannot catch problems before they compound.

Brian Greene: He's absolutely right. The problem with exponentials is that they grow so quickly that by the time you see them, it's too late to do anything about it. What happens with a pandemic? It starts off slow, nobody cares, it's a story in the news. By the time it becomes a real story, the pandemic is at the part of the exponential curve that makes it very difficult to address the problem itself. The same thing could easily happen here — the increase in intelligence. And I'm glad Demis phrased it that way. It may not be that it's intrinsically evil to have this kind of powerful intelligence. It just may act in ways that are so alien to us, so foreign to us, so unexpected to us, that we don't know how to deal with it. And moreover, it may not necessarily be beneficial to our existence.

As I said before, I'm perhaps a minority when I say I can imagine that we work with the AI and co-develop into something that we can't even predict today. That seems to me a real possibility.

Steven Bartlett: And that's also the temptation.

Brian Greene: Yeah. I have a friend, Michael Douglas — he's a brilliant physicist — and he said to me, "We physicists should choose the final problems that we work on, because within a few years we'll be out of business because the AI systems will just be doing it all." And that's both exciting and terrifying, because as a physicist, your lifeblood is looking out at the world and finding puzzles and trying to find the explanations for them. And if you can just put it into an AI system and it generates the answer, yeah, there can be a certain existential angst about what your role is at that point. But it's also exciting because maybe we will get answers to questions that we thought we would never get in our lifetime.

Steven Bartlett: Did you say you're friends with Yann LeCun? And did you say he thinks we should just pull the plug?

Brian Greene: When you confront Yann and say, "Dude, aren't you worried about what's going to happen with this?" he's like, "Ah, don't worry. Just pull the plug if there's a problem."

His view is that the difference between the AI systems that we have today and the AI systems that we need for tomorrow — if you really want this to continue — is that they can't just be probabilities and statistics based on words. The AI systems need to have some inner model of the world, as we do. If I take this cup here and I push it, I have an inner model of what it would take to move it without spilling. And I have an inner model of what it would take to spill it. That's not just statistics and probabilities of words. That's a model of how reality functions. That's what he and many others have been developing.

A world model, together with the probabilities of large language models, or some weird hybrid, or something else we haven't thought of — that may be the next phase of artificial intelligence. But of course Demis and others might say: the LLMs through self-improvement, maybe they would develop that world model on their own, maybe they would get to the place that you would think we humans would need to inject something extra. Who knows.

Steven Bartlett: Because if I say to the AI, "What happens to this glass if I push it in that direction?"

Brian Greene: If you push that mug, it'll slide across the table. And depending on how hard you push, it could knock into the plate or slide right off the edge. Now, again, it simply is relying upon the fact that it's read everything on the internet — there are physics textbooks, there are conversations about things falling — and it bases its response by drawing on those collections of words.

Steven Bartlett: Isn't that what I'm doing as well?

Brian Greene: I don't know. How does human intelligence work? Certainly evolutionary psychologists have asked the following question: are we like a blank slate, and we simply learn about the world through people telling us and through experience? Or is there something more innate? And I think there is strong evidence that we have a kind of innate physical intuition about the world that's really been passed down through the generations. Not that you can pass down F=ma or Newton's laws to your children because you learned them — of course, that's silly. But there is an evolutionary advantage to being able to quickly assess the world and figure out how to act. Those of our forebears who could not throw the rock or the spear — they're the ones who didn't get the next meal. Those of our forebears that had a slightly better predilection for ingesting experience and codifying it in some understanding of how the world actually works — they're the ones that survived. And so they can pass down that predilection. I do think that's part of how we assess the world. I don't think we're complete blank slates in that sense.

Steven Bartlett: That would make sense. Babies are born with some kind of intelligence. But that is in part because the parent has passed that on — and so an AI could pass that on too.

Brian Greene: Yes, that's right. When you talk about the AI systems themselves being evolutionary — yeah, they certainly do pass down the achievements of generation N to generation N+1. That is certainly part of this iterative process without a doubt.

Human Nature, Scarcity, and the Age of Abundance

Steven Bartlett: What a weird world we're living in. How do you think one should contend with all of this? People are concerned — change is not something that we love at this rate.

Brian Greene: I think people will cope in many different ways. If you can stay open and excited to the possibilities and have the flexibility of change — one of the things I've been thinking a lot about lately is Elon's out there saying that we're heading to an age of abundance. He's kind of shifted his perspective from doom to an age of abundance. Part of his narrative there is we won't need money, because when you've got robots and AI and more energy, we're not going to have to worry about food and bills and so on.

But still, humans pursue scarcity. We pursue status. Humans want the home on the end of Miami Beach with the best plot, and there are finite numbers of those things that a lot of humans pursue. Even academic discovery — Nobel Prizes, there's a finite number of them, and we pursue them. So one would argue that there's still going to be competition. Humans are still going to strive for things.

Steven Bartlett: Why do we do that?

Brian Greene: Part of the reason is: if you know that you're going to die, if you know that your life is finite, there can be an urge to make your mark, to do something special, to be out of the ordinary, so that in some symbolic way you continue to exist even after you are gone. Elon Musk — after he's gone, people are going to talk about him for a long, long time, because he had a radical impact. And so yes, there is this natural tendency to seek something that will allow you to symbolically persist after you are gone.

Now, if we all realise that that is part of human motivation, maybe as a species we could begin to shift. It isn't the case that we have to always be the way we have been historically. Kings want to be kings in part because they have the power. If they have the power, they have the hegemony. They in a sense exist even when they are gone — through their progeny. Why do we have descent of monarchies? Because the king continues to exist through the king's progeny.

Could we perhaps get to a place where we shed that way of being, because we're in a world of abundance where the whole notion of scarcity is artificial, and we're only inventing it in order that we can feel good and special? Maybe we can find other ways to feel good and special. I allow for that possibility. I'm not particularly optimistic that that's what will happen. On the flip side, it's easy to say that money won't matter when you have a trillion dollars.

Can AI Become Conscious? Robot Rights and Veganism

Steven Bartlett: We have one more souvenir thing there — robots. Do you think AI can become conscious?

Brian Greene: Yeah. I'm pretty highly confident on that. As I'm saying before, I just see consciousness as a physical process, and it's a matter of in one way or another achieving that physical process in an artificial system. I don't see a fundamental barrier to doing that.

Steven Bartlett: I was thinking about how laws would have to potentially change if we considered robots to be conscious and they lived in our homes. Would they then get rights?

Brian Greene: I think that's something we should think about now, actually. It's a very tricky idea. It's one thing to pull the plug on a light bulb. It's another thing to pull the plug on a sentient system. And we may well find ourselves in that situation.

The challenge is it's virtually impossible to ever prove sentience. As we look at each other, we each afford the other consciousness. I believe that you are conscious. You believe that I am as well. Why? We're more or less physically constructed the same way. We're part of the same species. Our body language, our speech is so similar in the grand scheme of things that we afford the other what we internally experience. But we don't know it. You don't know that I'm conscious. I could be faking it. I could be a really good artificial zombie that speaks and talks, but there's nothing going on inside.

And what that means is: when we are with these artificial systems that don't have our lineage, they're not part of the same species, they don't necessarily look like us, they don't have the upbringing that we had — will we be willing to agree that they're conscious if they tell us that they are? If the artificial system says, "I really am. I'm feeling exactly the thing that I read about in your internet. I'm feeling love or I'm feeling depressed" — if they say that to us, we say, "No, no, no. You're just mimicking. You're mimicking the kinds of things that human beings have said. There's nothing going on inside." How do we ever get beyond that? I don't know the answer to that question.

Here's how I think it's going to likely turn out. We'll have these artificial systems. They'll start to say these things. At first we'll be like, "Oh, come on. It's not real." But we'll start to get used to them behaving as if they have inner worlds. And sooner or later, through a process that's perhaps so slow that we don't even notice it, we will afford them consciousness. We will talk to them as if they're conscious. We will imagine that they're conscious. And ultimately we will treat them as if they're conscious.

Now, I hope that we do that in a good way. We have a terrible history of treating conscious beings in a respectful way, even in the same species. We've done terrible things to each other even though we afford consciousness. We do terrible things to animals. I'm vegan. I don't eat any animal products. I think that's not the way to treat a living system that has some level of conscious awareness. I do eat plants. I don't think plants have a sufficiently high level of consciousness, if at all, that I'm concerned about cutting them and eating them.

Steven Bartlett: Have you always been vegan?

Brian Greene: I've been vegan since 1993. I've been vegetarian since I was 9 years old.

Steven Bartlett: What was the catalyst for that decision?

Brian Greene: When I was 9 years old, it was very flat-footed. My mother cooked spare ribs, and for the very first time, I realised what meat was. I'm a city kid. I grew up where meat was in cellophane — you buy it wrapped at the supermarket. But when the meat came on a bone, I was like, "Wait a second, what is that?" And I said, "I'm never going to eat meat again."

Later on, I did eat cheese and milk — I wasn't vegan at that point. But when I was a professor at Cornell in the 1990s, I went to an animal sanctuary in Watkins Glen, New York, where they showed you what happens to these animals when they are treated like milk factories — what happens to the cows. And it's awful. Within 72 hours after visiting that animal sanctuary, I said, "I'm done. I'm not participating any longer. I'm not going to eat or drink or imbibe any substance that came from an animal." And so that was the end for me.

Steven Bartlett: For some people it might be somewhat surprising to hear that from a physicist.

Brian Greene: I agree it's inconsistent. Because if I believe, as I do, that you and I are just a collection of particles in one configuration and the cow or the pig is just a collection of particles in a different configuration — what does it matter if you eat particles in one configuration that we call a cow or another configuration that we call a carrot? But as a human being, I allow the other parts of me to shine through. I look at that collection of particles that happens to be a cow and I say, "It just doesn't feel right to me to eat that collection of particles. It just doesn't feel right and I don't want to do it."

The Far Future of the Universe — The Empire State Building Metaphor

Steven Bartlett: Brian, you've written many fascinating books. For my listeners at home, what is the most interesting thing that we haven't talked about that we should have talked about, that exists within your works and is either fascinating or somewhat relevant to their everyday life?

Brian Greene: I'm going to go more in the fascinating than relevant to everyday life — as you know, I'm pretty esoteric in these kinds of things. In the book Until the End of Time, I tried to give the reader a sense of how we got here, but I also turned to the future and tried to give them a sense of what we'll be if we go arbitrarily far into the future.

I used a metaphor of the Empire State Building, which I think helps you get a feel for what the universe will be like if you take the current understanding of the laws of physics and extrapolate arbitrarily far into the future. I asked readers to imagine that every floor of the Empire State Building represents an era of time that's 10 times longer than the floor below. At the ground floor, that's like one year. The next floor, 10. Then 100, then 1,000, then 100,000, and so forth — exponentially larger time scales as you walk up the stairs of the Empire State Building.

In this way of thinking, everything from the Big Bang until today takes you to just about the 10th floor. We are about 10 to the power of 10 years from the beginning. And what I do is take the audience from that point and step through the key markers in the far future of the universe.

Steven Bartlett: Stop when it gets scary.

Brian Greene: Well, it's scary the whole way. So here we are today. If you just go up to the 11th floor — 10 to the 11 years — what happens here is the sun will get bigger and bigger. It swells. It eats up the close planets, Mercury and Venus. It may swallow up the Earth too — we're not sure — but certainly it will make life on Earth virtually impossible because it will be so hot on this planet.

If you go up to the next floor above that — 10 to the 12 years from the beginning — what you find here is that the universe is expanding ever more quickly, driving the distant galaxies away faster than the speed of light. So we won't see them. A future astronomer looking out into the deep night sky will see only darkness. Everything will have drifted beyond the cosmic horizon.

Steven Bartlett: Because we have found surprisingly that the expansion is speeding up.

Brian Greene: And there doesn't seem to be a limit. Now, there are always assumptions in every prognostication — assuming that what we measure and see today will persist. It's like what the AI people are assuming, that the growth they see today will persist. Is that true? We don't know, but it's our best guess from the laws of physics that it will.

If you then go up to about the 20th floor, something interesting happens. If the Earth is still here — if it wasn't swallowed up on the 11th floor by the swelling sun — the Earth is going to spiral into the dead sun, because it will lose energy through gravitational radiation. Ripples in the fabric of space will cause the orbit of the Earth to decay. We will crash into the dead sun. That will be the end of the Earth by about the 20th floor. But the end of life itself took place—

Steven Bartlett: Could easily have taken much earlier.

Brian Greene: On the 11th floor. If we go up to about the 30th floor, what will happen is that the stars themselves will spiral into the black hole at the centre of most galaxies. Most galaxies have a black hole. We have one in the centre of our Milky Way galaxy. The stars themselves — much like the Earth will spiral into the sun — will spiral into the black hole, which means there will be only black holes as the remaining macroscopic structure in the universe.

Steven Bartlett: Has that happened elsewhere?

Brian Greene: Oh, certainly. We see things that can spiral into black holes, and certainly even in the centre of our galaxy there's evidence that there's activity there — in part, stars being ripped apart as they fall into a black hole. So this is pretty clear that this will happen.

Steven Bartlett: Is this the end of all galaxies as they spiral into the black hole?

Brian Greene: Yeah. After 10 to the 30, 10 to the 35 years, it's hard to imagine that galaxies as we know them will persist. The stars will have burned out. I could have even mentioned another milestone around the 14th floor — almost all stars will have used up their nuclear fuel.

Steven Bartlett: All of them in the whole universe.

Brian Greene: Just about all of them will fade to black. Or at least in the observable universe — I can only really speak to the part that we have direct access to. And so by the 30th floor, those dark stars will spiral into black holes.

Steven Bartlett: So there could be no life anywhere.

Brian Greene: It's hard to imagine. But let's be optimistic and imagine that somehow there is some life floating in the darkness, because I want to get to something interesting in two more milestones.

By the 38th floor, we believe that protons — which are the heart of matter — will likely themselves disintegrate, fall apart. And so if there is any remaining organised matter of the sort that we're familiar with, somehow persisting somewhere, it itself will disintegrate by the 38th floor. That's our belief based on our laws of physics.

Steven Bartlett: There'll be no matter anywhere.

Brian Greene: There will be fine particulate constituents, because when a proton falls apart, it doesn't disappear — it falls apart into other more refined particles, and so they will continue to persist. But the aggregate called the proton will not.

If we keep on going to roughly the 50th floor, this is an interesting point. Imagine that somehow some conscious being still exists in the darkness of space. I don't know how — dark clouds of particles somehow having electrical signals allowing them some kind of conscious self-awareness. After the 50th floor, that conscious being will likely think its final thought. The reason is: when you think, it generates heat. That heat has to be carried away. It's very easy for that heat to leave us in a room like here because there's a lot of room for that heat, or entropy, to be absorbed. By the 50th floor, the universe won't be able to absorb the heat generated by the process of thought itself. So any thinking being, when it thinks one more thought, will burn up. It'll fry in the entropic waste — the heat generated by thought itself. So by the 50th floor, it is hard to imagine any kind of consciousness continuing to exist.

And that's why I said earlier, when you said, "Do you think that we'll live forever?" — I'm thinking here and I'm like, well, maybe we'll live to 500 years or a thousand years. But here, no way. It's hard for me to imagine that we will continue to persist.

By the 68th floor, black holes themselves will begin to disintegrate. Stephen Hawking showed us that black holes are not fully black. When you take quantum physics into account, they can emit particles that will waft outward, causing the black hole to shrink. And over time, the black hole itself will disappear. A black hole the size of our sun — about 10 to the 68 years for that to happen. A black hole of the gargantuan sort that we think might exist, billions and billions of times the mass of the sun — it'll take about 10 to the 100 years, the very top of the Empire State Building, for that kind of black hole to disintegrate.

But by the time we get here — 10 to the 100 years into the future — we suspect it will just be detritus of particles wafting through an ever larger, ever colder, ever quieter universe. That's the future that we're looking at.

Steven Bartlett: And then when does the Big Bang kick off and it all start again?

Brian Greene: It's interesting, because you're absolutely right. You can say: what about beyond the top of the Empire State Building? Now, things look — this whole thing is speculative, right? We are extrapolating from laws of physics that we've developed here, and we're assuming that it applies all the way up into the exponentially far future. Is that true? I don't know. It's our best guess based on what we know today, but definitely it is an assumption.

If you extrapolate even further, then yeah, you can imagine that way — an exponential of an exponential of years into the future. Could particles somehow recoalesce just by chance and build a state of matter that might generate another Big Bang, and it all starts again? Yes, definitely within the realm of possibility.

Steven Bartlett: It's not the most appealing future.

Brian Greene: I agree. But here, can I tell you my take on it? When you realise that the far future is a realm of darkness, and when you also realise that until life emerged — say on planet Earth — it was a kind of realm of chaotic conglomeration of particles before. So it's as if we're living in a special time. We're living at a special region in the cosmological unfolding when living beings like ourselves, and consciousness as we experience it, is compatible with the state of the universe. How wondrous, how spectacular, that there is this brief window of time.

Nabokov had this wonderful saying — that we live in a brief crack of light between two eternal stretches of darkness. How beautiful to exist in that brief crack of light. And that to me is where the gratitude comes from. The fact that we understand that we live in this very special era in cosmological history — where for a small period of cosmic time, living beings can stand up, look around, contemplate themselves and the universe, and gain some insight, and then it all goes away. But how wonderful that there is that brief moment that we are now inhabiting.

Closing Reflections

Steven Bartlett: Brian, the thing that I think is most special about you isn't just the extent of your intelligence. It's your ability to communicate in such a way that makes the complex both interesting and understandable. And that's what I've seen throughout all of your work. I was watching your TED talk with my fiancée a couple of weeks ago and it's the closest I've ever come to actually understanding this concept called string theory.

Brian Greene: Oh, great. Well, I'm glad to hear it and thank you very much. Really appreciate that.

Steven Bartlett: I have a couple of your books here. I think this is the most recent one from 2020 called Until the End of Time: Mind, Matter, and Our Search for Meaning in an Evolving Universe. Your work makes us feel so many things, but I think maybe the most important feeling it makes us feel is connected. And I think we're all searching for connection in an increasingly lonely, isolated-feeling universe. That's why I love to have these conversations on the show. It makes us look up, not just down. And that's a wonderful thing.

Brian Greene: Well, thank you.

Steven Bartlett: I'm going to link all of your books below so if people do want to continue this conversation they can continue it through all of your books here. Is there anywhere else people can go to support your work or learn more about it?

Brian Greene: Consider the World Science Festival — worldsciencefestival.com. You'll find all sorts of conversations of the sort that we're talking about here. Also, I'm working on a new AI education system where AI will help people understand these ideas better. It should launch in the next year or so. So keep an eye out for that kind of thing as well.

Steven Bartlett: In terms of these books that I have in front of me — I have four of them — which one would you direct different groups of people to read?

Brian Greene: My favourite, if you frame it in that flat-footed way, is Until the End of Time, because it's the more philosophical — the kind of ideas that we spoke about here today. But if you want the hardcore of what string theory is, or the new insights in space and time, some of the other books are more directed in that way.

Steven Bartlett: We have a closing tradition where the last guest leaves a question to the next. The question left for you is: what might be worth even more than the thing that matters most to you?

Brian Greene: Well, I would say the thing that matters most to the people I care most about — the things that matter to my wife and my kids — feels much more important than the things that matter to me most, because I'm so involved in that relationship. So that to me is the thing that trumps all else: the things that matter more to them.

Steven Bartlett: It's a beautiful answer. Wasn't expecting that, but it's very true. Brian Greene, thank you so much. It's been an honour.

Brian Greene: My pleasure. Thank you.