Why string theory isn't real physics | Roger Penrose, Brian Greene, and Eric Weinstein

0:00String theory is the most potent and

0:02powerful approach that we have

0:04discovered as a species for blending

0:07general relativity and quantum

0:08mechanics. The theory to me is is just

0:12in the wrong direction and it's wasting

0:13a lot of time. I, like many string

0:16theorists, think that we live once. I'm

0:18not interested in spending my time on a

0:20theory that's wrong.

0:22>> The trouble is that they claim it's such

0:24a beautiful theory. As far as I can see,

0:26it's a great mess. But for two theories

0:29that were at loggerheads to naturally

0:32come together where one induces the

0:34other to emerge from the formulation,

0:38that's beautiful.

0:39[Music]

0:45We're first going to start off with

0:47Roger,

0:48who's going to tell us about his kind of

0:50three-minute pitch on whether it's time

0:53to move on from the dominance of string

0:55theory in our accounts of the universe.

0:57Well, I wasn't sure it was still

0:58dominant.

1:00But still, you know, when I first heard

1:02about it, in I guess the early 1980s,

1:05I found it a very exciting idea. It

1:07looks as though you could use these sort

1:09of well, these surfaces, Riemann

1:11surfaces,

1:12as a replacement for things like Feynman

1:15diagrams in talking about quantum field

1:17theory. And it looked very like an

1:19exciting idea to me. But then it rapidly

1:22became well, at least the story was, and

1:25I didn't understand it well enough to

1:27see why it had to be like this.

1:29This only works originally in 26

1:31dimensions. So, it meant that space had

1:3325 dimensions and time had one

1:35dimension. My reaction was, okay, well,

1:38that's the end of the end of it as far

1:39as I'm concerned, because space doesn't

1:42have that many dimensions. Of course, I

1:44understood that the idea is somehow that

1:46several of these dimensions are tied up

1:47into little knots and things like this.

1:50But it just never struck me that this

1:52worked, because

1:54well, the argument was somehow you could

1:56never excite the degrees of freedom

1:58which are involved in these tiny little

2:00extra dimensions. I just didn't seem to

2:03me that was correct, because even though

2:05it would take a lot of energy to excite

2:07these degrees of freedom, that amount of

2:09energy was, as far as the whole universe

2:11was concerned, I mean, it the

2:14you don't just do it locally. It would

2:16be affect the whole universe. And the

2:18amount of energy

2:20that here, over the whole universe,

2:21which amounts to, say,

2:24a Planck scale energy, which is the sort

2:26of thing they were talking about, is

2:27ridiculously small. So, I didn't see why

2:31this would

2:32why you wouldn't

2:34be

2:36notice that the number of space

2:38dimensions was wrong.

2:40I just never took it seriously for that

2:42reason. But then it became very much a

2:44an in thing.

2:46And so, I had to pay some attention to

2:48it and make critical remarks, which are

2:51probably too naive, I expect, in my

2:53various books. But I've never been a fan

2:56of the idea. As I said, initially, I

2:57thought it was a very beautiful idea.

2:59But since

3:00since it seemed to require all these

3:02unreasonable extra degrees of freedom,

3:05it's not just

3:06extra dimensions. There is There's all

3:08this stuff that you can't get hold of

3:10and doesn't affect the physics.

3:13So, it didn't make too much sense to me,

3:14I'm afraid. Okay, thank you. Um next, uh

3:18Brian Greene, uh is it time to move on

3:21from the dominance of string theory in

3:22our accounts of the universe?

3:25Uh happy to say a few words on it.

3:27Again, apologies that I couldn't join

3:29you in person. It looks like a a

3:31wonderful gathering.

3:32I would suggest at the outset that the

3:34framing of the question suggests a

3:38radically

3:40incomplete understanding of how science

3:43actually works. I mean, I'm happy to

3:46engage in these kinds of discussions. I

3:48think they can be illuminating for the

3:50general public and they're enjoyable to

3:53have. So, I've nothing against them.

3:56But the idea that we would sit here and

3:58pronounce how science should or should

4:01not move on from this or that idea

4:04misses the point of how science actually

4:07works.

4:08Science is an endeavor carried out by

4:11scientists who generally have an

4:14enormous amount of training, an enormous

4:17amount of experience, like Roger and and

4:21others in the discussion here.

4:23And the choice of what to work on by

4:27design is left to the individual. What

4:30they find interesting, what they find

4:32exciting. So, people will continue to

4:34work on any given subject so long as it

4:37continues to bear fruit and give

4:39interesting results that inspire yet

4:43other avenues of research. And that's

4:46what's happened in string theory and

4:47that's why people have continued to work

4:50on it. And when the theory and the ideas

4:53begin to dry up and they're no longer

4:56bearing fruit, then individuals will

4:59move on. And certainly, some string

5:02theorists of the past have moved on to

5:04other ideas because they lost interest

5:07in this or that avenue of research. And

5:10they do other things. And that I

5:12applaud. That's a wonderful way in which

5:14science can be rich and and

5:17self-propelling

5:19in order that the endeavor of trying to

5:21understand the universe will continue.

5:24When it comes to string theory itself,

5:28where have we gotten? I think that's a

5:30conversation that we will continue to

5:32pursue in our discussion here. But

5:34briefly put, in 30 seconds,

5:37string theory is the most potent and

5:39powerful approach that we have

5:41discovered as a species for blending

5:44general relativity and quantum

5:45mechanics. And putting these two pillars

5:48of scientific understanding together is

5:51vital to understand things like black

5:53holes and the Big Bang. And indeed,

5:55string theory has made its greatest mark

5:57in understanding black holes. So, is it

6:01something that is worthy of pursuit? I

6:04think the answer is yes. Obviously,

6:06thousands of others around the world

6:08think the answer is yes. But it's not

6:10for us to pronounce. It's for the

6:13marketplace of ideas to decide. And

6:16scientists will vote with the most

6:19precious resource they have, which is

6:21their time. And so, as long as

6:23scientists think that it's worthwhile to

6:25work on string theory, they will. And

6:27when they don't think it's worthwhile,

6:29they won't. That's about it.

6:32Thank you very much.

6:33Eric. So, rather depressingly, I very

6:36much agree with uh Brian, because it is

6:40a marketplace of ideas, at least in

6:41theory. And as long as there are

6:43equations and ideas and people who wish

6:45to explore them, the ideal of the

6:47scientific community is that we should

6:49do exactly that. And the problem is is

6:51that that is not exactly how science

6:53works. And in particular,

6:55uh this ignores the entire political

6:57economy of science, which has to do with

6:59jobs, claims, public relations, and

7:03effectively diverting the resources of

7:05the community, which are never uh

7:07particularly generous after the Cold War

7:10era, um towards string theory. And it's

7:13not just a question of, let's say,

7:14delivering better pizzas, but also

7:15slashing the tires of the opposing pizza

7:19parlor. And so, the real problem is is

7:21that we keep talking about what we're

7:22comfortable with, which is uh the nature

7:26of the ideas. Um what I'd like to do is

7:28I'd like to lead use the most of the

7:30three minutes for debate. So, I'm just

7:33going to say uh one thing about this.

7:35String theorists often say that's not

7:37how science works. And I'm The concern

7:39about string theory is that string

7:40theory works like no other science. And

7:43so, the biggest complaint that many of

7:46us have is is that string theory uh sets

7:49itself up as the judges and referees

7:51who's winning, who's losing, what's the

7:53best leading theory, and finding

7:55themselves virtuous in almost all uh

7:57contests. So, it's like playing a team

8:00versus of of referees uh who may not

8:02necessarily be better footballers, but

8:04tend to find that they always win the

8:05game. So, let's save the time for debate

8:08and uh let's get into it.

8:10Thank you very much. So,

8:12we've

8:14It's often said that uh mathematics is

8:16the language of physics. Um but physics

8:18also answers to uh experiment. How do we

8:21balance these two different factors? Is

8:24it kind of the the way the mathematics

8:25leads us um and the way that we uh find

8:28these out through experiment? Roger.

8:31Well, I think the trouble with string

8:32theory is that there isn't any

8:34connection with observation as far as

8:36I'm probably

8:38taking too strong a view here. But um

8:40the mathematics I mean, it's it's

8:42largely driven by the mathematics as far

8:44as I know, which is in itself is not an

8:46objection as far as I'm concerned. A lot

8:48of what I do is driven by the

8:49mathematics. But the trouble with string

8:51theory is it's supposed to be a theory

8:53of the way the world operates. And if

8:55the number of dimensions of space is

8:57just wrong,

8:59I can't take it seriously. And I can I

9:02mean, there are arguments why all these

9:04extra dimensions are hidden, and I don't

9:05think those arguments are right, either.

9:07So, I'm just less in a position where I

9:11don't The problem is not so much, you

9:13know, how much mathematics or how much

9:14physics or what. We're not talking about

9:17things which can be experimentally

9:18tested, anyway, as far as I can see. If

9:21you could have a theory which had

9:23predictions and these predictions could

9:24be seen to be right or wrong, that's

9:27what I understand about physics. But

9:29string theory seems to me driven largely

9:31by certain ideas, which

9:34have importance in mathematics, but sort

9:35of irrelevant to the physics, really. As

9:37far as I know, there's places where

9:39string theory has had an impact on

9:42mathematics. Sure, it's has. But it's

9:45not physics, as far as I can see. So,

9:49I have The main trouble I have with with

9:51string theory is I don't think it's

9:52physics.

9:53So, anybody wants to defend the idea

9:55that string theory is, in fact physics

9:57not mathematics. Okay. So, um I

10:00appreciate your remarks Roger.

10:02Obviously, I have a a different

10:04perspective on it. One, when it comes to

10:08the fact that various formulations of

10:10string theory do require extra

10:12dimensions, that's of course a

10:14long-standing program that existed long

10:17before string theory. The whole

10:18Kaluza-Klein program dating back to the

10:22early decades of the 20th century is one

10:24that considered the possibility that our

10:25universe might actually have extra

10:27dimensions. The issue of exciting modes

10:32in the extra dimensions is one that I

10:34think we understand reasonably well that

10:36the energy goes like one over the

10:38putative size of the extra dimension.

10:40So, if their size is sufficiently small,

10:43the energy that needs to be focused in a

10:44sufficiently small region to excite

10:46those modes is larger than any energy

10:49that we have access to. But, if we did

10:51have that energy, we would be able to

10:53excite those modes. I have a feeling

10:55that we're not going to be able to

10:56settle that in in this discussion here,

10:58but I'm not concerned. And in fact,

11:01moreover, I've spent decades thinking

11:03about those extra dimensions. So, I

11:05think it's actually very fruitful part

11:07of the theory as opposed to something

11:09that is a a detriment or something that

11:12is an embarrassment. It's a wonderful

11:14quality of the theory. In terms of

11:16whether string theory is physics, I

11:18would put forward two points. Number

11:20one,

11:21the whole reason why we are interested

11:23in string theory is that we believe the

11:26world is governed at least in certain

11:28energy scales by the general theory of

11:30relativity giving us our theory of

11:32gravity. We believe that on sufficiently

11:34small scales quantum mechanics shows its

11:38colors. And we believe there are realms

11:41in which general relativity and quantum

11:42mechanics need to play together.

11:45And without a theory of quantum gravity,

11:48physics is at a loss for how to proceed.

11:52And so, the value of string theory in

11:54physics is that it gives us a quantum

11:57theory of gravity. That's why we're

11:59interested in it and that's why it's

12:01part of physics. I agree fully that we

12:04don't understand the theory well enough

12:06yet to draw direct predictions for

12:09things that we can go out and measure.

12:11That's why it is a theory, very much a

12:14theory in progress. And how long will it

12:17take to extract those kinds of

12:20predictions?

12:21I don't know. I wish we had those

12:23predictions, too, because if we could

12:25verify them, great. If we could prove

12:27the theory wrong, fantastic, too. I,

12:30like many string theorists, think that

12:32we live once.

12:34I'm not interested in spending my time

12:35on a theory that's wrong. And so, being

12:38able to make that experimental judgment

12:41is is would be wonderful. We can't do it

12:45yet. I agree with that, but that's not

12:47some fundamental failing of the theory.

12:50It's a failing of us theorists to

12:52understand the theory well enough to

12:54make those predictions.

12:56Final point I will make is in terms of

12:57connection to physics. And there are

12:59others that I could bring up, but let me

13:00just focus on one.

13:02We've known, Roger, of course, since the

13:05work of you, work of Stephen Hawking,

13:07about, for instance, black hole entropy.

13:11And string theory is still the only

13:14framework within which we can calculate

13:17from first principles for a certain

13:19category of black holes and reproduce

13:22the beautiful formula that Stephen

13:25Hawking wrote down in terms of the area

13:27of event horizon divided by four. And

13:29so, there's a wonderful connection

13:32between a quality of the world that I

13:33think many of us agree is correct, that

13:35black holes have entropy governed by the

13:38area of the event horizon, and string

13:40theory's calculation

13:42from first principles for certain

13:44categories of black holes of that

13:46number. So, that, again, among many

13:49other things that we could discuss, are

13:51links to physics. So, I'm a little

13:53surprised that you would say there's no

13:56connection to physics, but perhaps I'll

13:59leave it there and happy to hear your

14:00response.

14:02So, Roger, um we've discussed how the

14:05evidence might be slightly different

14:07from string theory or not yet there in

14:09terms of observational evidence. Um what

14:12do you make of the appeal that people

14:13have to other types of evidence such as

14:15uh principles that guide us such as

14:17simplicity or naturalness or kind of

14:20mathematical beauty? Do you think these

14:22are things that can be used as evidence

14:24or they kind of supporting a fantasy? I

14:26think they can be used, but I don't see

14:28it in string theory.

14:30I mean, the trouble is that they claim

14:32it's such a beautiful theory. As far as

14:34I can see, it's a great mess. I mean,

14:35you've got the wrong number of space

14:36dimensions. You've got an argument to

14:38try and argue that you

14:41you don't get enough energy to excite

14:42the extra dimension. I I mean, I think

14:44that argument is wrong, actually,

14:45because

14:47the amount of energy is

14:49for the entire universe. It's not just

14:51your building accelerator to see get up

14:53to a certain energy if it's beyond the

14:55level of that accelerator. That's not

14:57the point. You've got that much energy

14:59and all over the universe there's things

15:01with great deal of energy and there's no

15:03reason why these shouldn't excite these

15:05extra dimensions. I think there's just a

15:07misconception in that.

15:09Um I don't want to go into it further,

15:10but I it's a bit technical, but I think

15:12it's a misconception.

15:14I also think

15:15that string theory is not all that

15:17beautiful. It's not beautiful

15:19even as mathematics, which is part of

15:21the argument.

15:22Uh you need all these extra dimensions

15:24which you don't see. Well,

15:27it it does nice things in mathematics. I

15:29I certainly I could go argue that there

15:32have been arguments

15:35which simplify certain mathematical

15:37expressions and

15:38calculate them further than have been

15:40done before using string theory

15:42arguments. That says nothing about

15:43physics. The theory to me is is just in

15:46the wrong direction and it's wasting a

15:48lot of time. And even the claim

15:51the claim that you get the Hawking

15:52temperature or the Hawking Hawking

15:53entropy out of this, we knew already

15:55what it was. Why does string theory if

15:57you can do it again using string theory,

15:59it doesn't impress me very much. If you

16:01got a different answer and somehow you

16:03could have observational evidence that

16:06that different answer was correct, that

16:08would impress me. But, there's nothing

16:10like that in in what's been done.

16:11There's a lot of hype in the subject, I

16:13think, which I I really

16:15don't go along with. And in a certain

16:18sense, there are too many theoretical

16:20physics communities which have

16:22wasted their time on this area rather

16:25than doing what I regard as more

16:26interesting topics. Even the question of

16:29quantum gravity, which has been brought

16:30up here,

16:32bringing quantum mechanics and

16:33gravitational theory together, sure,

16:34that's that's a very important thing to

16:36do. But, quantizing gravity, in my view,

16:39okay, it's an interesting exercise, but

16:42it's not the important thing in bringing

16:44them together. The important thing

16:46together in bringing them together, in

16:47my view, is to explain why quantum

16:50theory

16:51works. See, quantum theory works for

16:54tiny little things, if you like. But,

16:56when you do what's called making a

16:57measurement or you think talk about big

16:59things, it doesn't work. Because there's

17:01something missing, something

17:02fundamentally missing from quantum

17:04mechanics. And what's finally

17:06fundamentally missing, I believe, one

17:08can give strong argument, is that

17:11features of general relativity have to

17:12be brought in. And those features

17:15indicate the level at which the collapse

17:18of the wave function comes in. And that

17:20quantum mechanics needs help, I'm

17:22saying.

17:23And it needs help because we see things

17:25which, you know, if I take this glass

17:27and try to put it in superposition of

17:29here and there, you don't do it. It

17:30doesn't stay. I mean, it goes to one or

17:32the other. Um you quantum mechanics

17:34doesn't say that.

17:36Quantum mechanics

17:37is

17:38not just incomplete, as the

17:41some Einsteinian people would say, it's

17:44not really quite correct. It's not

17:46correct because uh for large objects or

17:49for large displacements of objects,

17:51you don't get the answer we see in the

17:53universe. Now, to get that answer, my

17:56current belief, uh for quite a long time

17:58I believe, that you have to bring

18:01principles from general relativity. So,

18:04that the combining of those two

18:05theories, the focusing on trying to

18:07quantize gravity, I think is

18:09is a mistake.

18:10Okay, we would be nice to see a theory

18:13of gravity which at the appropriate

18:15levels

18:17agrees with quantum mechanics. But,

18:19that's not where we need help in quantum

18:21mechanics. Where we need help is in the

18:23collapse of the wave function, which is

18:25a huge thing missing from the theory.

18:28And there are arguments which suggest

18:30very strongly that you need to bring

18:33gravity in. Right.

18:34>> And that feature is not anywhere to be

18:37seen, at least not by me,

18:39in what's been done in string theory.

18:41It's just going off, to me, in the wrong

18:43direction.

18:44And it's lots of people in physics

18:46departments, theoretical physics, um

18:49they're

18:50they're guiding this direction because

18:52lots of other people work in it. So, the

18:54the problem though uh Roger mentioned in

18:56terms of the the uh water glass is also

18:59famously known in terms of Schrödinger's

19:00cat, whether it's dead or alive. This is

19:02the kind of issue at the heart of

19:04quantum mechanics. Um and so, I wonder

19:07if our other panelists have any thoughts

19:09of whether this issue in quantum

19:10mechanics, can string theory help us

19:12with this problem? Or is string theory

19:14silent on this or is kind of orthogonal,

19:17an extra issue, not something uh string

19:19theory has its own issues separate from

19:20this?

19:21So,

19:23string theory does not, at the moment,

19:26answer the question of the quantum

19:27measurement problem. And I agree with

19:29Roger that that's a a critical question

19:31that has faced quantum mechanics really

19:34since its inception. So, I agree that

19:37there are open questions that even

19:39string theory, as it's currently

19:41formulated, doesn't doesn't address. As

19:45Roger addressed it with his approach,

19:47that aspects of general relativity will

19:49be vital to the collapse of the wave

19:51function, It's a powerful and

19:52interesting idea. I've studied it. I'm

19:55not yet convinced of it, but it is among

19:58many ideas that people working in this

20:01arena have put forward. I'm glad that

20:02people are working on this key question

20:05of quantum mechanics. But in terms of

20:07the importance of quantizing gravity, I

20:09would take a somewhat different

20:11perspective than Roger's.

20:13Quantum mechanics is an established part

20:15of the way the world works. General

20:17relativity is an established part of the

20:19way the world works. If they can't play

20:22together, if when you combine them, you

20:24get nonsensical results, which was the

20:27state of play before string theory gave

20:29us a potential solution, then your

20:32physical description of the world is

20:33fundamentally inconsistent. And I don't

20:36think the universe is inconsistent, and

20:39therefore it is vital that we find a way

20:41to put the pillars of our previous

20:43understanding into harmony with one

20:46another. Eric mentioned perhaps there

20:48are other ways of doing it. Fantastic.

20:50I'm open to those ideas, but string

20:53theory at least is one approach that

20:55does that. And it's important, it's

20:57vital, and string theory not only

20:59achieves it, but to the question of

21:01beauty, let me just put forward the

21:03following. You do quantum mechanics of a

21:06string without any knowledge of general

21:09relativity, and just through the process

21:12of studying that quantum mechanical

21:14description of a string, general

21:16relativity emerges. It's forced upon

21:20you. It comes right out of the

21:22mathematics. If Einstein hadn't

21:24discovered general relativity, string

21:26theorists studying the quantum

21:28mechanical motion of string would have

21:29found general relativity within the

21:32formulation itself.

21:34That's beautiful. That's breathtaking.

21:37That is what pulled many of us into the

21:40field. And so, these issues, sure,

21:43beauty is somewhat in the eye of the

21:46beholder, but for two theories that were

21:49at loggerheads to naturally come

21:52together where one induces the other to

21:55emerge from the formulation, that's

21:58beautiful.

21:59This is not a fair way to go about

22:01things. We're now having the

22:02conversation about beauty and ugliness.

22:04A giraffe is beautiful, as is a

22:063-year-old child. And if I cut the head

22:08off a 3-year-old child and sew on the

22:10head of a giraffe, that is in some sense

22:12beautiful in two separate ways. If I

22:15take space-time and I put a Calabi-Yau

22:17threefold, a complex threefold at every

22:20point, that has the feeling of a baby

22:22with the head of a giraffe. There's a

22:24certain way in which it's beautiful,

22:25particularly at the level of the

22:27analytics. So, what I hear Roger talking

22:29about very often with twisters is a much

22:31more algebraic and geometric view. What

22:33I hear Brian talking about as beauty is

22:35the beauty of a renormalizable spin-two

22:37field that's massless,

22:39falling out of the quantization of a

22:40string. These are different worlds.

22:43They're sewn together in a very

22:44unconvincing and ugly way. Uh we I don't

22:47want to rob Brian of the claim that

22:48there's beauty, because there's tons of

22:50beauty in the theory. There's also tons

22:52of ugliness and artificiality in the

22:53theory, as there is in general

22:55relativity, where the equations and the

22:57Lagrangian are beautiful, but the energy

22:59constraints are ugly and have a

23:00different origin or meant to keep

23:01Einstein out of trouble. So, what we

23:03have is a situation in which I feel like

23:06the audience never actually hears a

23:08debate about the underlying nature, and

23:11we stay on the surface level, where

23:13we're talking about it's beautiful, it's

23:14ugly, it it post-dicts, it predicts

23:16this.

23:17The real nitty-gritty of this thing is

23:20is that Brian and Roger are both

23:22correct, and they're not coming to

23:25loggerheads in front of an audience for

23:27reasons that actually escape me.

23:30Would Would you like to come to

23:31loggerheads now, uh I didn't know we

23:33weren't. I thought we were in

23:34loggerheads.

23:36I mean

23:37I was trying to say that all these extra

23:39dimensions are wrong.

23:41And that's fundamental to the theory.

23:43Surely that's a loggerhead of a as a

23:45sort of one. Isn't that a loggerhead?

23:48It's a very polite loggerhead, maybe.

23:52No. I mean it's it's completely against

23:54the way the string theory has gone.

23:56I mean

23:58originally, when I heard the idea of

24:00string theory in the space-time had the

24:01right number of dimensions, three space

24:03and one time, I thought it was a really

24:05nice idea.

24:07And it seemed to indicate that things

24:09could be finite, which would otherwise

24:11give you divergences and so on.

24:14Brings in beautiful ideas of Riemann

24:15surfaces and mathematics, which I like.

24:18I liked the idea. I thought this is a

24:20great place to go.

24:22But then they decided, "Look, it doesn't

24:23work unless you have 26 dimensions."

24:26So, I said, "Okay, you can work on that.

24:29I'm working on real physics."

24:32I mean

24:33you the reason to that they present that

24:36these extra dimensions you have to to

24:37excite all these degrees of freedom. I

24:39think that argument is not correct.

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24:49thousands more. Thank you for being part

24:52of the conversation.