Thomas Lin is the Editor In Chief of Quanta Magazine and the Editor of two math and science books, “The Prime Number Conspiracy” and “Alice and Bob meet the wall of fire”. He joins Hari Sreenivasan to discuss how holographic principles are unlocking the secrets of our universe.

# Holographic Principles

## TRANSCRIPT

Thomas Lin is the editor in chief of Quanta Magazine and the editor of two math and science books, 'The Prime Number Conspiracy' and 'Alice and Bob Meet the Wall of Fire.' He joins us now to discuss how holographic principles are unlocking the secret of our Universe.

First, let's talk about what the holographic principles are and then kind of what the secrets are and how they're intermingled.

So how do we use holograms here?

Sure, and just to set it up, the problem that this is being applied to is one of the most fundamental questions, one of the big open questions in physics, which is the question of quantum gravity, which is how does gravity work at the smallest scales, at the particle level?

And so, somehow, physicists have come up with a holographic principle that can give some insights into, possibly, how quantum gravity works.

And basically the hologram idea is kind of like, imagine you're watching a 3-D movie, and so you have the glasses on and you see things that seem to be jumping out at you and things are happening and it seems like it's real life.

Mm-hmm.

And it's, like, a hologram, but you take your glasses off and it's a flat movie screen, and so all the information that you're seeing in 3-D with the glasses on is encoded on a 2-D surface.

And so that's the basic idea behind a physics correspondence called the AdS/CFT correspondence where, essentially, it's like a toy universe, where the inside of this universe has particles and black holes and gravity and things like that, but the surface, the 2-D surface, so to speak, doesn't, and the 2-D surface is directly mathematically mapped to what's happening on the inside, just like the 2-D screen in the movie is creating this 3-D hologram.

And so, you know, to sort of step back for a second, the problem is that physicists know how gravity works on a large scale.

Einstein came up with his theory of general relativity about a century ago and it describes perfectly the way planets move, the way all the things that we experience, in terms of gravity, work, but unfortunately, that description doesn't work at the particle level.

And so, at that level, we have a different theory, which is a bedrock of physics, called quantum mechanics, which describes particle interactions, but those two theories don't play well together, and we don't know how gravity works at that level.

So scientists have been looking for something to pull those two ideas together, saying, 'Well, gravity has got to... We have to have a rule that works in both cases, the big and the small.'

Absolutely.

Right.

So it's both because they want to know how all the final forces work together in all scales.

It's also because without a theory of quantum gravity, we don't know what's happening inside black holes, and we don't know exactly how the Big Bang happened because these are moments where there's such intense gravity happening at such small scales that we have to understand how that's working with quantum gravity in order to understand what actually happened.

So AdS/CFT, is that just a framework for how to map these 2-D, 3-D ideas?

Exactly, exactly because AdS stands for Anti-de Sitter space, which is essentially the higher-dimensional universe with stuff in it, and the CFT part of that correspondence is a conformal field theory, which essentially is a quantum mechanics field theory, which describes particles but does not have gravity.

And so being able to map this 2-D boundary to a three-dimensional or a higher-dimensional universe that has gravity inside shows, in theory, how you could get gravity from the interactions of a field theory like quantum mechanics.

It's almost like we're creating a laboratory, just theoretically...

Yes.

...to be able to even study how these things would interact because we technically don't have a world that would function like that anyway.

We're in the opposite of that.

Yes.

You could say that.

So how close are we to figuring this out?

I mean, I'm assuming there are researchers and scientists all over the world playing around in this AdS/CFT world and trying to figure out how gravity is going to work on this tiny, tiny scale as it works in the bigger world.

In terms of actually connecting it to the real world, though, unfortunately, physicists have sort of hit a wall in a way because there's no real way to probe at that level.

There hasn't been any way to, for example, find empirical evidence for what a gravity particle might be, which would be called a graviton, but nobody has actually detected that yet, and there's no sense that we'll be able to detect that any time soon.

And so, right now, physics is in this interesting state where there's this huge particle collider out in Switzerland called the Large Hadron Collider, and they've confirmed some of the particles of the standard model, which people were sort of expecting, but they haven't found the other particles that are needed to make sense of things like dark matter and quantum gravity and all these other things that are still missing in terms of our understanding of the Universe.

And there's still, again, many, many missing pieces and, unfortunately, we can't yet find those pieces, and so physicists have to work in this theoretical realm with math and with tools like the AdS/CFT correspondence.

So best-case scenario, let's say we figure out quantum gravity, what do we do with that information?

Okay.

So, again, this is one of the things that...at Quanta Magazine we cover these fundamental areas of research for which there's no necessarily a direct application right away, right?

It's just about gaining knowledge about the Universe, which ultimately, though, leads to all the technology that we use today, all of the medical advances and treatments that allow us to live longer and healthier lives.

That all comes from basic science from decades and decades ago.

Sure.

And so, we don't know yet.

I mean, we honestly do not know what the applications for something like that might be necessarily, but what we would know is, again, we would understand much more about how the Universe works, how the basic laws of physics work, what's happening inside black holes, which we have no idea right now about, how the Universe got started, and ultimately, knowing how quantum gravity works could potentially lead to other ways to take advantage of things at that tiny sort of particle level.

We're talking about changing or altering what we think of as space and time, altering what Einstein had at sort of his disposal?

That's a great question because essentially that's where physics is right now, right?

We have these two bedrock theories.

We've got Einstein's theory of general relativity, which works perfectly well at large scales.

Mm-hmm.

You have quantum mechanics, which works perfectly well at tiny particle scales, but they don't work well together, and so physicists have to think, 'Well, maybe there's a more fundamental theory than either of these.

Maybe one is right and really works in both scales.

Maybe neither is the more fundamental theory, and maybe there are things like, maybe space-time is emergent, right?

Maybe it's not even fundamental, too.' I mean, these are all different ideas that physicists have right now.

They have to work with them in order to try to get past this road block right now from where they are because, you know, it's been decades since the last major physics revolution, and there's a sense now that we need a new revolution to understand the underlying nature of reality.

The underlying nature of reality.

Nothing short of that.

Thomas Lin of Quanta Magazine.

Thanks so much.

Thank you so much.