The following is the full transcript of renowned theoretical physicist Professor Brian Greene’s interview on The Diary Of A CEO podcast with host Steven Bartlett, August 17, 2026.

EDITOR’S NOTE: In this episode of The Diary Of A CEO, host Steven Bartlett sits down with renowned theoretical physicist and string theory expert Professor Brian Greene to tackle some of the biggest unanswered questions in science and philosophy. From whether God and heaven are human inventions and the reality of free will, to the simulation hypothesis, AI consciousness, and the ultimate fate of the universe, Greene offers profound insights drawn from his work at Columbia University and books like Until the End of Time. It’s a wide-ranging, mind-expanding conversation that reframes our place in the cosmos.

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? I think 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, you know, 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 just almost infinitely more grains of sand that are out there.

We understand the world better than we once did, but does that really take us some significant way toward assessing the big, deep questions of, say, why is there something rather than nothing? Maybe that’s the deepest question of all. By which I don’t mean empty space. I mean nothing. Nothing at all.

Steven Bartlett: Well, we’ll, in this conversation, go in the pursuit of that answer. To tell people who you are and to 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: Yeah, sure. I grew up in Manhattan, right across from the Hayden Planetarium. I would wander the halls 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 it was to Oxford, where I got my doctorate pursuing work in a field called string theory. And then through the academic ladder, 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.

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, they even have smaller particles, 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 and you look deeply — would you see anything else? And the idea of string theory is, yeah, you would. Inside an electron or inside a quark, you’d see a little tiny vibrating filament. The different vibrations of the string yield the different particles.

So 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. So a string vibrating one way, electron. A string vibrating another way, a quark. Matter at its most fundamental level, if you were to take a powerful microscope and look deeply inside of any piece of matter, you’d see all these little vibrating filaments, these vibrating strings. That’s the idea of the theory.

Steven Bartlett: So I have two questions come to mind when you say that: how do you know? And the second one is, what if you zoomed in even further?

Brian Greene: 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 that 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 the ideas.

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 say, where do these ideas come from? Answer: 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.