How do string theorists reply to Roger Penrose

[kodama]

Roger Penrose’s Fashion,[/PLAIN] Faith and Fantasy in the New Physics of the Universe


"Arguing that string theory has veered away from physical reality by positing six extra hidden dimensions, Penrose cautions that the fashionable nature of a theory can cloud our judgment of its plausibility"

is finally published as i write this. in his section on fashion, Penrose considers string theory as fashion and considers problems with extra dimensions

that extra dimensions in string theory are unstable, and increase functional degrees of freedom. this makes string theory unlikely to be correct on purely theoretical grounds.

specifically for Penrose does not think having 4 large dimensions and 6 compactified dimensions is stable. that somehow the 4 large dimensions can be decoupled from the compactified dimensions, and Penrose does not see any reasons for this.

are there any string theorists counter arguments to Penrose extra-dimensions increase functional degree of freedom as he outlines in this book?

 

[mitchell porter]

I haven't seen the substance of Penrose's argument yet, but I will note two things.

First, unexpected instabilities of higher dimensions are not a new idea; Witten discovered a famous one called the "bubble of nothing".

Second, Penrose is talking about fields on a manifold, but in string theory that's only an approximation. Calculations in string theory are actually done from the 1+1 dimensional perspective of the "string worldsheet".

From this dual perspective, the dimensions of macroscopic space-time are just scalar fields on the string. So Penrose's instability might not apply because string theory, is in a sense, fundamentally two-dimensional.

 

[Urs Schreiber]

The parameters of size and shape of the compactified dimensions in string theory, and in fact in any Kaluza-Klein compactification, are called "moduli". Since they are part of the higher dimensional metric, they are components of the higher dimensional field of gravity and hence are dynamical fields that evolve. The problem of their stability, hence the question whether there are dynamical mechanisms that make for instance the size of the compactified space remain stably at a given value, is famous as the problem of moduli stabilization in string theory.

This problem used to be open until around 2002. Then it was realized that vacuum expectation values (VEVs) of the higher form fields ("fluxes") present in string theory generically induce effective potentials for moduli that may stabilize them.

For type IIB string theory/F-theory this was argued in the influential article KKLT 03. An analogous moduli stabilization mechanism was also argued for M-theory on G2-manifolds by Acharya 02 . It is the counting of all the many possible ways of stabilizing moduli via fluxes in type IIB that led to the now infamous discussion of the landscape of type II string theory vacua. So there is no lack of possibilities of solving the stability problem.

(This reply is now also at String theory FAQ -- Do the extra dimensions lead to instability of 4d spacetime?)

 

[Demystifier]

Fashions in "string" theory change fast. First it was fashionable that "string" really means string. Then it was fashionable to use name "string theory" for something which was really theory of branes or M-theory. After that, "string theorists" started to pay more attention to AdS/CFT duality. The most recent fashion is the study of gravity/entanglement duality. In all these phases, the researches discovered many conjectures and "miracles", but not so many solid theorems.

 

[kodama]

here is link

The Future of Theoretical Physics and Cosmology: Celebrating Stephen ...
https://books.google.com/books?isbn=0521820812
G. W. Gibbons, ‎E. P. S. Shellard, ‎S. J. Rankin - 2003 - ‎Science
10 On the instability of extra space dimensions Roger Penrose Mathematical Institute, University of Oxford 10.1 The issue of functional freedom Physical theories ...
On the instability of extra space dimensions - INSPIRE-HEP
https://inspirehep.net/record/608935/
Jan 28, 2004 - On the instability of extra space dimensions. R. Penrose (Oxford U., Theor. Phys.) Jan 2002 - 17 pages. Prepared for Workshop on Conference

 

[Urs Schreiber]

Thanks for digging out the reference!. The key point is actually in section 10.3 where he argues that, by his singularity theorem, in field theoretic KK-compactifications the small compact extra dimensions will quickly develop a singularity.

So this was written and published in 2002/03, essentially simultaneously with the claim by Achary, KKLT and others that it is the combination of two stringy effects, (fluxes and non-perturbative contributions) that leads to stabilization of the compact dimensions. I think it was generally accepted before that without such extra stringy effects KK-compactifications are unstable. This is after all the reason why KK-theory was abandoned back in the early 20th century, and the reason why one speaks of the "moduli stabilization problem" in the first place.

Now I suppose the arguments in the string theory literature leave room for being made more solid. For instance the literature typically concentrates on fluctuations keeping the special holonomy intact ($SU(3)$ or $G_2$). But if the argument is to be scrutinized, then it ought to be the one that applies to the string theoretic context, not to the field theoretic one. Because I don't think anyone claimed that pure field theoretic KK-compactifications can be stable. Which however is what the argument in that section 10.3 is all about.

 

[Urs Schreiber]

I have now expanded the nLab page on moduli stabilization a little more. Of course there is still lots of room to expand it much further...

 

[Haelfix]

Moduli stabilization is a big topic, which has tens of different approaches/scenarios with qualitatively different predictions and motivating calculations. KKLT is a popular one but there are other approaches.

For instance the technique of Kahler uplifting is currently popular in the literature
https://arxiv.org/abs/hep-th/0611332

Likewise the large volume scenario is another post 2002 scheme that has seen quite a lot of work.
https://arxiv.org/abs/hep-th/0502058

 

[Urs Schreiber]

And there are more generic schemes, not restricted to N=1 susy compactifications. There is the old Branderberger-Vafa argument (here) that string/brane winding modes spontaneously compactify 6 dimensions and decompactify 3+1. And, for what it's worth, there is more recently the claim (here) that computer Monte-Carlo simulations of the IKKT matrix model, regarded as non-perturbative type II string theory, show the same effect. (Hard to check this claim without access to the computer code.)

These are arguments for generic spontaneous 9+1 -> 3+1 compactification. They don't share what might be an issue with the CY compactifications: that only fluctuations that preserve CY-ness are considered.

 

[kodama]

de Sitter Vacua in String Theory
Shamit Kachru, Renata Kallosh, Andrei Linde, Sandip P. Trivedi
(Submitted on 29 Jan 2003 (v1), last revised 10 Feb 2003 (this version, v2))
We outline the construction of metastable de Sitter vacua of type IIB string theory. Our starting point is highly warped IIB compactifications with nontrivial NS and RR three-form fluxes. By incorporating known corrections to the superpotential from Euclidean D-brane instantons or gaugino condensation, one can make models with all moduli fixed, yielding a supersymmetric AdS vacuum. Inclusion of a small number of anti-D3 branes in the resulting warped geometry allows one to uplift the AdS minimum and make it a metastable de Sitter ground state. The lifetime of our metastable de Sitter vacua is much greater than the cosmological timescale of 10^10 years. We also prove, under certain conditions, that the lifetime of dS space in string theory will always be shorter than the recurrence time.
Comments: 12 pages, 2 figs, added comments on the thin wall approximation to tunneling
Subjects: High Energy Physics - Theory (hep-th); Astrophysics (astro-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Phenomenology (hep-ph)
Journal reference: Phys.Rev.D68:046005,2003
DOI: 10.1103/PhysRevD.68.046005
Report number: SU-ITP-03/01
Cite as: arXiv:hep-th/0301240

the paper's starting point is ". Our starting point is highly warped IIB compactifications with nontrivial NS and RR three-form fluxes."

is there any experimental or observational evidence such a structure exists, or that if the universe starts with 10 flat spatial dimensions, it is possible for 6 to curl into " highly warped IIB compactifications with nontrivial NS and RR three-form fluxes"

why aren't the large 3 spatial dimensions also highly warped, compactified with nontrivial NS and RR three-form fluxes?

did this happen before the big bang, during the big bang, or after the big bang?

 

[Urs Schreiber]

why aren't the large 3 spatial dimensions also highly warped, compactified with nontrivial NS and RR three-form fluxes?

That KKLT article considers just the question whether a semi-realistic setup could be stable at all, not how it would be selected dynamically. In fact that very article is the one that led people to the conclusion that there is no unique dynamic selection, just chance.

I'll repeat that I think there is room for improvement on the state of the art of KK-stabilization in the string theory literature. I just think that the argument which started this thread, about instability of KK-compactifications in field theoretic gravity, goes besides the point in that there were no claims that field theoretic gravity gives stable KK-compactifications. The claims are that it's stringy effects which make this happen. The details could probably do with some more scrutinization, but it's the stringy effects that would need discussion.

That said, I already mentioned above two arguments for how a stable compactification to 3+1 dimensions could dynamically occur by stringy effects:

The first is the "Brandenberger-Vafa mechanism" (here) which takes into account string and brane winding around compact dimensions. The second is a claimed Monte-Carlo computer simulation of non-perturbative type IIB string theory in the guise of the IKKT matrix model (here).

I suppose both of these need to be taken with a grain of salt. But that's the kind of arguments that would need to be sorted out for stability of KK-compactification in string theory.

 

[kodama]

can you clarify the beliefs of string theorists regarding the extra dimensions? that at some time before the big bang, the universe started out with 10 flat spatial dimensions and 1 dimension of time, and at some time, anti-d branes interacted with "Euclidean D-brane instantons or gaugino condensation" resulting in all but 3 becoming compactified, and that these "Euclidean D-brane instantons or gaugino condensation" have no effect on the remaining 3 large spatial dimensions?

 

[Urs Schreiber]

...regarding the extra dimensions? that at some time before the big bang, the universe started out with 10 flat spatial dimensions and 1 dimension of time, and at some time, anti-d branes interacted with "Euclidean D-brane instantons or gaugino condensation" resulting in all but 3 becoming compactified, and that these "Euclidean D-brane instantons or gaugino condensation" have no effect on the remaining 3 large spatial dimensions?

The field that cares about these questions is called "string cosmology", possibly not a particularly well developed field. If you are genuinely interested in seeing the arguments, you'll have to dig around the literature a little. For the arguments pre 2002, check out for instance section 3 "Superstrings and spacetime dimensionality" in Easson 01. For the meme that became popular among a vocal group of researchers after the KKLT article, you can do worse then check out Schellekens 06. For computer simulation of something argued to be the real thing, check out Kim-Nishimura-Tsuchiya 12.

But the brief summary is: the situation remains inconclusive.

 

[rude man]

I recommend everyone read Lee Smolin's The Trouble with Physics.

He argues convincingly that physicists have hit a 'dead end' and are basically just trying to justify their paychecks.

OK, so I'm an engineer and applied physicist but this string theory has no real significance to me . Isn't it enough to adopt and settle for the Copenhagen Interpretation which has been 'use-tested' almost 100 years, plus all that wonderful classical physics?

And in case you think Dr. Smolin is unqualified to judge, read http://leesmolin.com/about-lee-smolin/biography

 

[MathematicalPhysicist]

I recommend everyone read Lee Smolin's The Trouble with Physics.

He argues convincingly that physicists have hit a 'dead end' and are basically just trying to justify their paychecks.

OK, so I'm an engineer and applied physicist but this string theory has no real significance to me . Isn't it enough to adopt and settle for the Copenhagen Interpretation which has been 'use-tested' almost 100 years, plus all that wonderful classical physics?

And in case you think Dr. Smolin is unqualified to judge, read http://leesmolin.com/about-lee-smolin/biography

I do agree that they are trying to justify their paycheck, but it doesn't mean we need to stop searching for another theory, perhaps a different theory than string and LQG. But you won't find such a theory from an academic with tenure in universities (they only research in what they get funded for as Smolin put it).

As Einstein put it:"Science is a terrific thing but not for making a living out of it".

 

[rude man]

I do agree that they are trying to justify their paycheck, but it doesn't mean we need to stop searching for another theory, perhaps a different theory than string and LQG. But you won't find such a theory from an academic with tenure in universities (they only research in what they get funded for as Smolin put it).

As Einstein put it:"Science is a terrific thing but not for making a living out of it".

 

[Urs Schreiber]

string theory has no real significance to me . Isn't it enough to adopt and settle for the Copenhagen Interpretation which has been 'use-tested' almost 100 years, plus all that wonderful classical physics?

Apples have no significance to me, isn't it enough to settle for oranges?

There are various interesting points to criticize in string theory. But, sadly, popular discussion of the kind you are reflecting here rarley gets close to the real issues, losing itself in incoherent opinions based on some lack of substantial understanding.

I gather that it may be too disappointing to accept as a reality, but it is a fact that the truth about cutting edge research topics in modern theoretical physics cannot be learned from popular books, newpaper articles and internet fist-fights.

On forums such as this one here, this would be the place to have sober discussion, backed by actual technical insight, ambitioned to learn, not satisfied with cosy prejudices learned from the newsfeed.

Yes, there may be trouble with string theory, but, no, not at the naive level exhibited above. The actual trouble happens at a more sophisticated level. And as with all good theories in the past, if or where they fail, then it is interesting to accurately understand why they fail. We may learn from it.

So instead of spending time on the popular prose of those authors you refer to, why not drop all that, pick up Polchinski's textbook, open it on page 1, and start reading.Take a pen and paper with it, and try to follow the arguments. Such as to finally know what we are actually talking about.

Then, when done with volume one, close it and ponder what it said. Then make a list of technical points that worry you. Then we talk again.

 

[MathematicalPhysicist]

Apples have no significance to me, isn't it enough to settle for oranges?

There are various interesting points to criticize in string theory. But, sadly, popular discussion of the kind you are reflecting here rarley gets close to the real issues, losing itself in incoherent opinions based on some lack of substantial understanding.

I gather that it may be too disappointing to accept as a reality, but it is a fact that the truth about cutting edge research topics in modern theoretical physics cannot be learned from popular books, newpaper articles and internet fist-fights.

On forums such as this one here, this would be the place to have sober discussion, backed by actual technical insight, ambitioned to learn, not satisfied with cosy prejudices learned from the newsfeed.

Yes, there may be trouble with string theory, but, no, not at the naive level exhibited above. The actual trouble happens at a more sophisticated level. And as with all good theories in the past, if or where they fail, then it is interesting to accurately understand why they fail. We may learn from it.

So instead of spending time on the popular prose of those authors you refer to, why not drop all that, pick up Polchinski's textbook, open it on page 1, and start reading.Take a pen and paper with it, and try to follow the arguments. Such as to finally know what we are actually talking about.

Then, when done with volume one, close it and ponder what it said. Then make a list of technical points that worry you. Then we talk again.

Hold your horses, before he turns to read Polchinski he needs to read a lot more than just Polchinski.

I put it here just for a good laugh (but it's serious):

http://abstrusegoose.com/272

It's a classic comic...

 

[rude man]

Apples have no significance to me, isn't it enough to settle for oranges?

There are various interesting points to criticize in string theory. But, sadly, popular discussion of the kind you are reflecting here rarley gets close to the real issues, losing itself in incoherent opinions based on some lack of substantial understanding.

I gather that it may be too disappointing to accept as a reality, but it is a fact that the truth about cutting edge research topics in modern theoretical physics cannot be learned from popular books, newpaper articles and internet fist-fights.

On forums such as this one here, this would be the place to have sober discussion, backed by actual technical insight, ambitioned to learn, not satisfied with cosy prejudices learned from the newsfeed.

Yes, there may be trouble with string theory, but, no, not at the naive level exhibited above. The actual trouble happens at a more sophisticated level. And as with all good theories in the past, if or where they fail, then it is interesting to accurately understand why they fail. We may learn from it.

So instead of spending time on the popular prose of those authors you refer to, why not drop all that, pick up Polchinski's textbook, open it on page 1, and start reading.Take a pen and paper with it, and try to follow the arguments. Such as to finally know what we are actually talking about.

Then, when done with volume one, close it and ponder what it said. Then make a list of technical points that worry you. Then we talk again.

I let Dr. Smolin do my thinking for me in this instance. He's a lot better at it than I am, your kind suggestion to the contrary notwithstanding.

Good luck in your career.

 

[Urs Schreiber]

I let Dr. Smolin do my thinking for me

Sure, but why bother a physics forum with second-hand thoughts? We all know what Smolin said. For every argument, there is another second-hand-thinker with a different opinion, and a third one with yet another opinion. It's a cacophony of second-hand thoughts recycled indefinitely. What's the point? It's worse than politics. But here on Physics Forums we could use the opportunity to have scientific discussion. Why waste it? Why drown every attempt of people to understand something in noise?

 

[rude man]

Calmate. "We all know what Smolin said." Well, I didn't before stumbling on the book in a college bookstore and just in case anyone else didn't I thought I'd share that viewpoint, which happens to be my own. So, my bad I guess.

And I don't think these forums are or should be reserved for those few of us who can understand this level of pure physics. Much as you'd like them to be, seemingly.

 

[Nullius]

Calmate. "We all know what Smolin said." Well, I didn't before stumbling on the book in a college bookstore and just in case anyone else didn't I thought I'd share that viewpoint, which happens to be my own. So, my bad I guess.

And I don't think these forums are or should be reserved for those few of us who can understand this level of pure physics. Much as you'd like them to be, seemingly.

What is the point to come to a thread so as to shift goal posts, though? Sure, you have your reservations over string theory as much as others do, but the OP is not about that. The filibustering can wait lest you are a politician, right?

 

[ShayanJ]

I let Dr. Smolin do my thinking for me in this instance. He's a lot better at it than I am, your kind suggestion to the contrary notwithstanding.

Edward Witten is also better than you in this. So why Smolin and not him? How do you decide which one to "pick"? You just pick the one who wrote a popular book that you happened to see and buy?

Also, its OK for people like you that don't understand string theory to participate in discussions about it, but only discussions with people like yourself. In a thread that a mathematician(mathematical physicist?) is citing technical papers and explaining technical details, how do you find yourself eligible to criticize?

And Copenhangen is one of the interpretations of QM, and not a theory of Quantum Gravity! And "all that wonderful classical physics" is OK, but incapable of answering the questions that string theory was developed to answer. In fact even General Relativity and Quantum Field Theory are incapable of doing that otherwise we wouldn't need a theory of Quantum Gravity.

 

[JorisL]

Also, its OK for people like you that don't understand string theory to participate in discussions about it, but only discussions with people like yourself. In a thread that a mathematician(mathematical physicist?) is citing technical papers and explaining technical details, how do you find yourself eligible to criticize?

This might come of harsh to rude man.
But I think it's prudent to change the statement that one can participate in a more technical discussion by asking questions about the maths.
For example how a particular statement in a popular treatment translates to a more exact approach.

E.g. why the landscape is a problem according to some.
Or why we think there is something called M-theory.

By acknowledging that you are looking for a less/non-technical explanation we can tailor our responses to help you follow the discussion.
That is something physicsforums is almost uniquely qualified for. Quora has such questions but attract a lot of different types of answers and sometimes lower quality which is hard to spot for a layman. Stackexchange has more technical questions and equally technical responses.

 

[Ben Niehoff]

Smolin's book is woefully out of date with how string theory is currently done and how string theorists engage with the field. He mostly has straw-man criticisms.

 

[ShayanJ]

This might come of harsh to rude man.
But I think it's prudent to change the statement that one can participate in a more technical discussion by asking questions about the maths.
For example how a particular statement in a popular treatment translates to a more exact approach.

E.g. why the landscape is a problem according to some.
Or why we think there is something called M-theory.

By acknowledging that you are looking for a less/non-technical explanation we can tailor our responses to help you follow the discussion.
That is something physicsforums is almost uniquely qualified for. Quora has such questions but attract a lot of different types of answers and sometimes lower quality which is hard to spot for a layman. Stackexchange has more technical questions and equally technical responses.

Its harsh to experts to criticize something you don't understand based on an unfair sampling of the experts' opinions. He should have expected some harsh reaction. Although I don't think what I said was really harsh.

Anyway, you're right to some extent but its important to understand to exactly what extent. A layman having an opinion on something still controversial among experts is definitely out of that extent. And I know, rude man is an engineer and certainly is knowledgeable in some areas, but when talking about the current state of the research in Quantum Gravity, he is definitely a layman. Laymen can ask for non-technical explanations but can't expect to actually understand anything from those explanations. For example although the OP in this thread understands that string theorists have some points against the criticisms mentioned(which seems useless because as I said, its not a OK to have opinions on things you don't understand, as I, a physics student, don't have an opinion on different ways to cure cancer!), but I don't think s\he actually understands those points that Urs Schreiber explained. So what's the point of this thread? I don't know! Maybe it can serve to other more knowledegable people.

Another thing I don't get is the amount of criticism towards string theory because it doesn't make predictions,etc. . But other alternatives are not doing better. So why should people have these doubts only about string theory? Why isn't anyone asking whether people working on LQG can respond to criticisms? Its just because laymen are trying to have uninformed opinions based on unfair samplings of experts' opinions, they just listen to whoever shouts louder and Edward Witten seems to be more of the quiet type!

I should say that I'm not defending string theory or anything, I'm only a physics master's student with limited understanding of string theory and the mathematics involved and the technical explanations in this thread are completely a foreign language to me. But I do think that I understand how physics develops and that understanding tells me that string theory is fine! Its just a theory under construction, like all other alternatives.

 

[JorisL]

Another thing I don't get is the amount of criticism towards string theory because it doesn't make predictions,etc. . But other alternatives are not doing better. So why should people have these doubts only about string theory? Why isn't anyone asking whether people working on LQG can respond to criticisms? Its just because laymen are trying to have uninformed opinions based on unfair samplings of experts' opinions, they just listen to whoever shouts louder and Edward Witten seems to be more of the quiet type!

(underline mine)

This is exactly the case, Rovelli is another researcher that shouts hard claiming LQG is better. All the while I get the feeling that he follows exactly the same "philosophy" as the one underlying string theory research.

To me the important takeaway of these discussions is (and will remain for quite some time) that we need more physicists that are well versed in several of these candidates. Especially when (or if) we find predictions within our reach that have to be checked in other candidates. There are so many assumptions at play in the work done that it would be hard to translate the premise without having some profound knowledge of each theory.

 

[Ben Niehoff]

Regarding Penrose's comments themselves, I know people who work with Penrose and who also do string theory, so I'm not sure how seriously to take such comments. I think Penrose is just expressing his personal tastes.

In any case, in the UK, string theory is often subsumed under the mathematics department, and my opinion is that it sits somewhere between pure mathematics and "physics proper". It is a fascinating field of research that tells us a lot about possible worlds (even if it is still quite an open question whether it describes this world), and is certainly as worthwhile of study as any branch of mathematics. Moreover, string theory research has lead to innovations both in pure mathematics and in physics that relates to the real world (as a toy model for understanding, more so than as a literal application of string theory).

 

[Demystifier]

As Einstein put it:"Science is a terrific thing but not for making a living out of it".

Did he say that before or after receiving the Nobel Prize?

 

[kodama]

Smolin's book is woefully out of date with how string theory is currently done and how string theorists engage with the field. He mostly has straw-man criticisms.

true, it should be updated with the latest results from LHC and LUX. SUSY and WIMPS are a no-show

 

[ohwilleke]

I would be curious to know the extent to which there are particular innovations of string theory, if any, that are "severable" from the overall structure of M-theory.

If there are, that might justify the discipline. If all the work is irrevocably tied to the whole, the really undermines its usefulness as an intellectual endeavor.

 

[Urs Schreiber]

I would be curious to know the extent to which there are particular innovations of string theory, if any, that are "severable" from the overall structure of M-theory.

All of them are. It goes the other way around: there is a system of phenomena in string theory as such, and the observation that as a whole they seem to point to an overall structure to which they might all be connected.

 

[MathematicalPhysicist]

Did he say that before or after receiving the Nobel Prize?

I don't think anyone would quote someone who isn't famous, and Einstein wasn't famous before he won the prize, was he?

 

[Demystifier]

I don't think anyone would quote someone who isn't famous, and Einstein wasn't famous before he won the prize, was he?

First, Einstein became famous before he won the prize. Second, once he became famous, others may wanted to quote his statements that he wrote even before that.

 

[MathematicalPhysicist]

First, Einstein became famous much before he won the prize. Second, once he became famous, others may wanted to quote his statements that he wrote even before that.

When did he start being famous to the wide audience?