- We have to talk a bit about the magical, the mysterious string theory, super string theory. - Sure. - There's still maybe this aspect of it, which is there's still, for me, from an outsider's perspective, this fascinating heated debate on the status of string theory. Can you clarify this debate, perhaps articulating the various views, and say where you land on it?

- So first of all, I doubt that I will be able to say anything to clarify the debate around string theory for a general audience. Part of the reason is because string theory has done something I've never seen theoretical physics do. It has broken out into consciousness of the general public before we're finished.

You see, string theory doesn't actually exist, because when we use the word theory, we mean a particular set of attributes. In particular, it means that you have an overarching paradigm that explains what it is that you're doing. No such overarching paradigm exists for string theory. What string theory is currently is an enormously large, mutually reinforcing collection of mathematical facts, in which we can find no contradictions.

We don't know why it's there, but we can certainly say that without challenge. Now, just because you find a piece of mathematics doesn't mean that it applies to nature. And in fact, there has been a very heated debate about whether string theory is some sort of hysteria among the community of theoretical physicists, or whether it has something fundamental to say about our universe.

We don't yet know the answer to that question. What those of us who study string theory will tell you are things like, string theory has been extraordinarily productive in getting us to think more deeply, even about mathematics that's not string theory, but the kind of mathematics that we've used to describe elementary particles.

There have been spin-offs from string theory, and this has been going on now for two decades almost, that have allowed us, for example, to more accurately calculate the force between electrons with the presence of quantum mechanics. This is not something you hear about in the public. There are other similar things, that kind of property I just told you about is what's called weak-strong duality, and it comes directly from string theory.

There are other things such as a property called holography, which allows one to take equations and look at them on the boundary of a space, and then to know information about inside the space without actually doing calculations there. This has come directly from string theory. So there are a number of direct mathematical effects that we learn in string theory, but we take these ideas and look at math that we already know, and we find suddenly we're more powerful.

This is a pretty good indication there's something interesting going on with string theory itself. - So it's the early days of a powerful mathematical framework. - That's what we have right now. - What are the big, first of all, for most people, probably, which as you said, most general public would know actually what string theory is, which is at the highest level, which is a fascinating fact.

- Well, string theory is what they do on the Big Bang Theory, right? (laughing) One, can you maybe describe what is string theory, and two, what are the open challenges? - So what is string theory? Well, the simplest explanation I can provide is to go back and ask what are particles, which is the question you first asked me.

- What's the smallest thing? - Yeah, what's the smallest thing? So, particles, one way I try to describe particles to people to start, I want you to imagine a little ball. And I want you to let the size of that ball shrink until it has no extent whatsoever. But it still has the mass of the ball.

That's actually what Newton was working with when he first invented physics. He's the real inventor of the massive particle, which is this idea that underlies all of physics. So that's where we start. It's a mathematical construct that you get by taking a limit of things that you know. So what's a string?

Well, in the same analogy, I would say, now I want you to start with a piece of spaghetti. So we all know what that looks like. And now I want you to let the thickness of the spaghetti shrink until it has no thickness. Mathematically, I mean, in words this makes no sense, but mathematically this actually works.

And you get this mathematical object out. It has properties that are like spaghetti. It can wiggle and jiggle, but it can also move collectively like a piece of spaghetti. It's the mathematics of those sorts of objects that constitute string theory. - And does the multidimensional, 11-dimensional, however many dimensional, more than four dimension, is that a crazy idea to you?

Is that the stranger aspect of string theory to you? - Not really. And also partly because of my own research. So earlier we talked about these strange symbols that we've discovered inside the equations. It turns out that to a very large extent, a dinkers don't really care about the number of dimensions.

They kind of have an internal mathematical consistency that allows them to be manifest in many different dimensions. Since supersymmetry is a part of string theory, then this same property you would expect to be inherited by string theory. However, another little known fact, which is not in the public debate, is that there are actually strings that are only four dimensional.

This is something that was discovered at the end of the 80s by three different groups of physicists working independently. I and my friend Warren Siegel, who were at the University of Maryland at the time, were able to prove that there's mathematics that looks totally four dimensional, and yet it's a string.

There was a group in Germany that used slightly different mathematics, but they found the same result. And then there was a group at Cornell who using yet a third piece of mathematics found the same result. So the fact that extra dimensions is so widely talked about in the public is partly a function of how the public has come to understand string theory and how the story has been told to them.

But there are alternatives you don't know about. (silence) (silence) (silence) (silence) (silence) (silence) (silence)