- The speaker denies being a non-traditional or radical thinker in physics, stating that his ideas have become part of the mainstream.
- He mentions that his reputation may come from his willingness to consider alternative solutions when all other possibilities have been exhausted.
- The speaker cites Freeman Dyson and Gerard de Tufte as more outlandish and contrarian thinkers than himself.
- He acknowledges that mainstream thinking is often necessary in physics, but suggests that more free thinking could be beneficial.
- The speaker discusses how his ideas often stem from conflicts of principle or paradoxes in physics.
- He expresses doubt about the possibility of experimentally finding individual gravitons, but suggests it could be a technological problem.
- The speaker clarifies that he is credited as a creator of string theory, which has become mainstream, but was not when he started it.

- The particles known as protons and neutrons were found to be composites and capable of being spun and excited.
- The discovery of a mathematical formula led to the realization that these particles were string-like in nature.
- The speaker believes in string theory as a valuable laboratory for exploring ideas, but not as the theory of the real world.
- The goal is to develop a grand unified theory that combines gravity and quantum mechanics in a consistent manner.
- The speaker is focused on the puzzle of reconciling quantum mechanics and gravity, which is a major challenge in theoretical physics.
- There are also conflicts in understanding elementary particles, such as the gauge hierarchy problem.
- Experimental data from the Large Hadron Collider (LHC) at CERN has not provided new information, leading to a temporary halt in progress in particle physics.
- Other unanswered questions being pursued include problems in cosmology related to quantum mechanics and gravity.

- Cosmology is about quantum mechanics and gravity, but we don't understand how they fit together in the cosmological context.
- Dark energy is a puzzle because it is much smaller than expected.
- The universe being extremely big and varied may explain the small amount of dark energy.
- The anthropic principle suggests that we are where we can be in the universe.
- Almost all the matter and information in the universe is in the form of black holes.
- The holographic principle, which relates to black holes and information loss, is now mainstream in physics.

- The information that falls into a black hole is thought to be both stuck on its horizons and falling into the black hole.
- The idea is that the stuff on the horizon is a hologram of the stuff that falls in.
- When a black hole evaporates, the hologram evaporates with it and carries off the information.
- The entire universe could be in a black hole, and we could be inside it.
- The holographic principle suggests that the universe can be represented as information stored on the surface rather than in three dimensions.
- The holographic principle has become a tool in physics and is widely accepted.
- A hologram is different from a 2D image or a photograph, as it contains the same information but in a mathematically discontinuous way.
- The reconstruction of the information from a hologram would require advanced technology and knowledge of the precise rules of black hole evolution.
- The idea that we live in a simulation doesn't add anything substantial and doesn't provide satisfying answers.

- The speaker discusses Richard Feynman's dislike of philosophy and how he was actually a philosophical person in his own way.
- The speaker also mentions Feynman's moral conflict with participating in the creation of nuclear weapons.
- The speaker talks about the responsibility of scientists in discovering and understanding the world, but not necessarily in deciding how their discoveries are used.
- The speaker shares their own curiosity and motivation in continuing their career in physics.
- The speaker expresses their current interest in understanding the relationship between gravity and quantum mechanics.

- In 1935, Einstein wrote two papers: one on wormholes and one on entanglement.
- These two concepts were not believed to be connected until recent years.
- It was discovered that entanglement and wormholes (Einstein-Rosen bridges) are the same thing.
- The growth of these wormholes is related to complexity theory.
- Quantum teleportation cannot be used for intergalactic communication without sending classical information.
- Quantum theory has already shaped technology and will continue to do so, particularly in the development of quantum computers.
- Quantum computers can simulate quantum systems in a way that classical computers cannot.

- Quantum computers are more efficient for studying quantum mechanical systems than classical computers.
- Quantum computers can be used to understand the behavior of large chemical molecules and explore new materials.
- It is uncertain if quantum computers will be able to solve traditional computer problems.
- The speaker enjoys teaching and finds it helpful for formulating new ideas in physics.
- Teaching the public, particularly older students with some technical background, is especially gratifying for the speaker.
- The speaker is motivated by having fun and enjoying teaching, rather than conveying specific ideas to the public.
- The speaker's experience with his father's plumber friends influenced his desire to teach and differentiate real science from fake science.

- The speaker denies being a non-traditional or radical thinker in physics, stating that his ideas have become mainstream.
- He acknowledges the importance of mainstream thinking in physics but suggests that more free thinking could be beneficial.
- The speaker discusses his ideas on reconciling quantum mechanics and gravity, as well as other unresolved issues in physics.
- The holographic principle, which relates to black holes and information loss, is now widely accepted in physics.
- The speaker expresses their interest in understanding the relationship between gravity and quantum mechanics.
- Quantum theory has already shaped technology and will continue to do so, particularly in the development of quantum computers.
- The speaker enjoys teaching and finds it helpful for formulating new ideas in physics.

Leonard Susskind is a professor of theoretical physics at Stanford University and he’s regarded as one of the fathers of string theory. https://en.wikipedia.org/wiki/Leonard_Susskind

He’s written several books including: 
- The Black Hole War https://www.amazon.com/gp/product/0316016411
- The Cosmic Landscape https://www.amazon.com/Cosmic-Landscape-String-Illusion-Intelligent/dp/0316013331
- The Theoretical Minimum series https://www.amazon.com/Theoretical-Minimum-Start-Doing-Physics/dp/0465075681/

He also has over 100 lectures on YouTube https://www.youtube.com/playlist?list=PLQrxduI9Pds1fm91Dmn8x1lo-O_kpZGk8

The YC podcast is hosted by Craig Cannon https://twitter.com/craigcannon

***

Topics

0:00 - Being perceived as an outsider physicist  

4:00 - The perils of becoming too mainstream

5:45 - Where his ideas come from

7:00 - Claudio asks - Do you think the graviton can be experimentally found?

9:45 - The origins of String Theory

15:15 - Why should there be a grand unified theory?

16:30 - Quantum mechanics and gravity 

19:50 - Large unanswered questions in physics

27:30 - Holographic principle

38:00 - Simulation hypothesis

40:15 - Richard Feynman on philosophy

42:00 - Feynman and the bomb

46:00 - Improving the world by discovering what the world is

49:00 - ER and EPR - Black holes and entanglement

56:00 - Noah Hammer asks - Could quantum teleportation be used in the future as a means of intergalactic communication?

58:00 - rokkodigi asks - How do you think quantum theory will shape technology in the future?

1:01:30 - Why teach physics for the public?

What I wanted to start with is you've often been characterized as someone with like non-traditional You know kind of out there ideas some of which have become, you know part of the physics canon some of which who knows what happened

Who they all became part of the physics canon?

Of course, all right, well thanks for coming on the podcast.

You're the first person who's never made a mistake.

I was curious, who is your, who do you think is your most outlandish friend?

Wait, come back to your previous question for a moment.

This is some misconception, which I don't know how it happened, but my physics has been extremely mainstream.

It may have been that at the beginning of some of the ideas, people were not quite ready, but they very quickly caught on.

It's just not true that I was some kind of alternative, what should we call it?

Now I spent a lot of time thinking about, what should we call them, conflicts of principle.

Situations where things were not fitting together properly, and thought a lot about them, and eventually came to the conclusion that you had to change things, or that you had to break the mold a little bit.

And I think that's probably where this reputation came from.

But these were things that really there were no alternatives to.

When you've tried all possibilities, and I forget the exact, but roughly speaking, when you've tried everything and it doesn't work, whatever remains must be the truth, no matter how outlandish or something like that.

like Occam's razor, but in particular, when you've tried everything and it doesn't work, there may still be something left that you haven't tried yet because you thought it was too outlandish, well, you gotta try

And I think that probably is the source of some of this mythology about me as a radical.

am the most conservative physicist imaginable.

Okay, so what's an example then of a friend who is more outlandish, more radical than you?

I can't exactly call him a friend, but I know him a little bit.

He enjoys running against the grain, and he sometimes says some brilliant and smart things.

Like all contrarians, He's got a very large probability of being wrong.

My friend, Gerard de Tufte, I don't know if you know his name.

He's far more out there than I've ever been.

He was about as mainstream as you can be.

But also, he had his own very special scientific personality, and I suspect that's also true of me, that my way of thinking, my way of doing things is probably different than most people.

And so, Yeah, it did lead to this Contrary this view of me as a contrarian as a radical, but it was absolutely wrong.

Mm-hmm and do you see physics like kind of birthing more contrarians in the the modern paradigm where experiments are so expensive to you know, kind of execute at this point?

Or do they have to be kind of more mainstream to get things done?

Well, unfortunately I think you have to be really mainstream.

By mainstream now, I mean people are often trained within a framework which is fairly tight and rigid.

And I sometimes think maybe a little more free thinking out there might be useful.

Free thinking, but that doesn't mean being a contrarian.

A contrarian is somebody who is contrary just for the sake of being contrary.

Yeah, well because I mean, I read your Wikipedia page, I've listened to interviews with you and I heard about you being a plumber Working with your father.

Yeah But it's like and now here we are at Stanford and you're you know, kind of like of the industry, right?

Yeah, it took me a long time to feel part of the, to not feel an outsider.

Background was a little bit strange for, so it took me a long time to not feel like an outsider and then all of a sudden I found out I was the ultimate insider.

I spend most of my time thinking about it and Not agonizing about other things.

Then where do you go to think of new ideas?

No, I'm kind of curious where your ideas have come from over the course of your career.

They almost always came from some sense that things were not fitting together properly.

What I call a conflict of principle or a paradox.

1 of the early things that I worked on was what's called quark confinement.

Why don't quarks come out of particles and appear in the laboratory?

They seem to be part of the proton and neutron and so forth.

And they seem to be stuck inside and never come out.

And that appeared to be a paradox because from all that we knew about the subject as quantum field theory, the subject that governs particle physics, from all that we thought we knew, any kind of particle that exists should be possible to kick it out and observe it directly in the laboratory.

They seemed to exist, and yet they seem not to exist.

And that's the kind of thing that captures me and gets me going.

And I don't wanna let it go until I feel I understand it.

So someone from Twitter asked a question related to this.

Their name's Claudio, and they think, or they asked, do you think the graviton can be experimentally found?

Well, of course there's a sense in which it's already been, but this is, I think I know what they mean, but gravitons or photons, or some large class of particles, when they're in sufficient abundance, just behave like wave fields.

So the electromagnetic field is a collection of photons.

But that doesn't mean you can detect them as individual photons easily.

Radio waves, for example, would be very, very difficult to detect as individual photons.

All right, we've seen gravitational waves.

That means we've seen large numbers of gravitons.

I mean, I don't know how many, zillions and zillions and zillions of them.

I think what the person was asking about is the possibility of seeing them individually.

And in fact, I guess I don't see any route to it at all.

But ultimately, I think it's a technological problem.

If you could build an accelerator as big as the galaxy and so forth and so on, and harness 100,000 stars to take all of the energy that they produce and run the accelerator with it, you could make gravitons.

Okay, So it's a little bit like the space LIGO.

The space LIGO, wait, the space LIGO is a trivial technology, it'll be easy.

It's trivial in the sense that in principle, we can do it, we will do it.

And it probably doesn't involve any technological hurdles, which are insurmountable.

Building a machine that could produce gravitons at least for the next million years is gonna be insurmountable.

So you're credited as 1 of the creators of string theory.

Which is extremely mainstream, you'll notice.

So that's where the idea of me as a radical came from, but now it's mainstream.

Oh, well, The idea came from asking about the structure of particles which are known as hadrons.

These are protons, neutrons, mesons, they're common things that make up the nucleus.

And there was a lot of work, experimental as well as theoretical, which showed that these particles were not elementary particles, that they were composites of some sort.

The point is too small to, how do you, what does it mean to rotate a point?

Whatever protons and neutrons were, you could spin them up, you could increase their angular momentum.

They seem to be capable of being vibrated and excited in all sorts of ways.

There was some mathematical work, it was very mathematical and didn't have to do with strings, but which caught some of the properties of these hadrons.

And I got interested in it and just looked at it, looked at some of the formulas and said, oh, those formulas are interesting.

Look at it a little more and I said, oh, there's something vibrating.

There's some kind of concept of vibration going on.

And it was just a matter of thinking about it for a few weeks and saying, oh, the strings, they're elastic strings.

And with each of these, Like, were you deeply knowledgeable in the field before?

No, I was deeply knowledgeable about quantum mechanics.

At least, well, was I deeply knowledgeable?

Even that I think I was, but Yeah, I had a very, very good education, it was self-education, about quantum mechanics, about classical mechanics.

I did not have much of an education about particle physics.

Somebody showed me a formula, and it was a mathematical formula.

I knew what a proton was, I knew what a neutron was, I knew that if you collided, then stuff come out of them.

And I also knew that they had these properties of being capable of being excited and spun up and so forth.

I mean, I just told you and you now know it too.

And the mathematical formula had some pieces in it that I recognized, I'd seen it before.

I'd seen it in the context of basically elementary quantum mechanics.

And at first I thought, oh, this thing is just a pair of particles on the ends of a spring.

Meaning to say the mathematics of it was the mathematics of what's called a harmonic oscillator.

But I looked at it a little more, and a little more, and a little more, and eventually I realized that the formula was representing the interaction of particles which themselves were string-like.

String-like meaning elastic threads, let's call them.

And so I worked it out and published it And that was the story.

And then in your, the Cornell lectures from, is it 2014 or something like that?

You kind of like offhandedly said that, despite being 1 of the creators of the string theory, you weren't the biggest believer in the world right now.

I probably did say that, and what I had in mind was something like this.

I do believe in string theory in the following sense.

It's a mathematical theory, it's a consistent theory, and it contains both quantum mechanics and gravity.

That makes it a very, very valuable laboratory for trying out ideas.

It in itself doesn't mean it is the theory of the real world.

My guess is the theory of the real world may have things to do with string theory, but it's not string theory in its formal, rigorous, mathematical sense.

We know that the formal, by formal I mean mathematically rigorous structure that string theory became, it became a mathematical structure of great rigor and consistency, that it in itself, as it is, cannot describe the real world of particles.

It has to be put in a slightly bigger context.

So the exact thing which I call string theory, which is this mathematical structure, is not going to be able by itself to describe particles.

Will what does correctly describe particles be a small modification of it or a big modification?

But I do know the value of it as a laboratory for investigating quantum mechanics and gravity.

Because the question that I've been wondering, and it's sort of straightforward, but why does there have to be a grand unified theory?

I don't know what people think, I know what I think.

It's not tolerable to have inconsistencies in the theory of nature, where 1 piece of the theory says 1 thing, another piece of the theory says another thing, and they're saying inconsistent things.

At the present time, we're in the business of trying to put together a consistent framework for the combination of gravity and quantum mechanics.

Elementary particles, there are inconsistencies in what we know about elementary particles.

When we put them together and make a consistent story out of all of this, we'll call that a grand unified theory.

And it's inconsistent, I mean, sorry, it's intolerable not to have a consistent story.

You get different answers by doing different versions of it.

So when you look at physics as it stands right now, where do you see the cracks that you wanna be focused.

Is that like the most important thing you could possibly be working on right now?

At the moment, People like myself, John Preskill, Juan Maldacena, wonderful and great physicists, have gotten focused on the connection between quantum mechanics and gravity.

For many years, it was thought that quantum mechanics and gravity simply don't fit together, for a variety of reasons, including things that Stephen Hawking had said, which were brilliant.

I don't think correct, but brilliant anyway.

It really looked like there was an inconsistency between quantum mechanics and gravity.

Quantum mechanics governs all other parts of nature, but of course gravity also covers a large part of nature, and to have inconsistent theories is, as I said, intolerable.

So the puzzle of putting together quantum mechanics and gravity is the 1 which is front and center for me, and I think front and center for theoretical physics right now.

There are conflicts in our understanding of elementary particles.

We don't understand how they can behave certain ways that they do behave.

1 of the problems, it's just a name, but it's called the gauge hierarchy problem.

It's an apparent, almost inconsistency in the, let's call it the standard model of particle physics.

There are other questions about how it does fit together with gravity.

We made great progress in understanding elementary particles for a long time, and it was always progress that went hand in hand with experimental developments, big accelerators and so forth.

We seem to have run out of new experimental data even though There was a big experimental project The LHC at CERN what if that is A great big machine that produces particles and collides them.

And I would say, I don't wanna use the word disappointingly, Well, I will anyway.

Disappointingly, it simply didn't give any new information.

And so particle physics has run into what I suspect is a temporary brick wall.

It's been basically since the early 1980s that it hasn't changed.

And so I don't see at the present time for me much profit in pursuing it.

Gravity and quantum mechanics are what fascinate me.

DAVE Well, what are the other large unanswered questions that people are pursuing at this point?

Because clearly it's not just you working on this, right?

Well, in the context of there are huge problems in cosmology.

In all of this, cosmology is about quantum mechanics and gravity.

Early cosmology, so-called inflationary theory, is about how quantum fluctuations imprinted themselves on the universe and led to the things, galaxies, planets, and so forth.

So quantum mechanics and gravity are the foundations of cosmology, but we don't understand how they fit together at all.

Particularly in the cosmological context, we really just don't understand how they fit together.

The dark energy, the thing that's called dark energy, is a puzzle.

It's not the puzzle of why is there dark energy, it's the puzzle of why isn't there a lot more of it.

The dark energy is a tiny, tiny, miniscule fraction of what it could be.

10 to the minus 120 of what the natural expectation for it would be.

So for many years, people thought there was no dark energy.

We call it the cosmological constant, but it's the same thing as what people call dark energy.

So originally we thought it wasn't there at all.

It's also, yeah, Einstein invented the cosmological constant and then said it was his worst mistake because it doesn't seem to be there.

Well, it was there, but it was there at a level which was so minute that it took until the 1990s to discover any evidence for it.

It's measured astronomically and by modern observational cosmology, counting galaxy counts and all kinds of things, quasar counts, all sorts of stuff.

But the main point is, in the end, it turned out that it was there, this dark energy, but it was there at such a small, incredibly small value that it took all that time to get any evidence for, and we don't know why it isn't bigger, more of it.

Not why is it there, but why is it not there in larger abundance?

Well, the usual hypothesis is that, usual hypothesis, the only 1 that I think makes any sense, which is outlandish, There's no question it's outlandish.

No, it's not mine, but I think it's the only thing that does, at the moment, seem to make any sense.

This is to say the universe is extremely big, much bigger than we can see, and varied.

Varied means it has properties which are different from place to place.

It makes it, it does fit together with the equations and so forth, that the universe is vastly bigger than the part we can see, and that as you scan over the whole thing, you'll find places where the constants of nature are 1 thing, other places where the constants of nature are another thing.

Some places where this cosmological constant is more or less normal, which means much, much bigger than it is here in our neighborhood.

Some places where it might even be smaller.

But then the question becomes, in what kinds of environments can we exist and even ask the questions?

My friend Steve Weinberg, in around 1987, made an argument that if the cosmological constant were any bigger than a certain magnitude, that galaxies could not have formed.

And if galaxies couldn't form, stars can't form, planets can't form, we can't be here.

So he said, the answer is the universe is very big and varied, and we are where we can be.

That's called the anthropic principle and it's a widely hated idea among

Definitely among scientists, anything that just is.

Yeah, it's a widely hated idea, but it just might be right.

So I was listening to a radio interview with you and you said, similar to this, that there, there was a discovery that there are relatively few ways of organizing matter than we thought there would be.

See all Y Combinator transcripts on Youtube

Leonard Susskind on Richard Feynman, the Holographic Principle, and Unanswered Questions in Physics