# I How do string theorists reply to Roger Penrose

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1. Sep 14, 2016

### kodama

Roger Penrose’s Fashion,[/PLAIN] [Broken] 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?

Last edited by a moderator: May 8, 2017
2. Sep 14, 2016

### 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.

3. Sep 14, 2016

### 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?)

Last edited: Sep 14, 2016
4. Sep 15, 2016

### 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.

5. Sep 15, 2016

### kodama

The Future of Theoretical Physics and Cosmology: Celebrating Stephen ...
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

6. Sep 15, 2016

### 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.

7. Sep 15, 2016

### 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...

8. Sep 15, 2016

### 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

9. Sep 15, 2016

### 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.

10. Sep 17, 2016

### 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.
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?

11. Sep 17, 2016

### Urs Schreiber

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.

12. Sep 17, 2016

### 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?

13. Sep 18, 2016

### Urs Schreiber

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.

14. Sep 20, 2016

### 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

15. Sep 20, 2016

### MathematicalPhysicist

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".

16. Sep 20, 2016

### rude man

17. Sep 20, 2016

### Urs Schreiber

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.

18. Sep 20, 2016

### MathematicalPhysicist

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...

19. Sep 20, 2016

### rude man

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.