# Are superstrings the only option? What about GR approaches?

1. May 25, 2006

### Robert100

Are superstrings (and related M-theory) the only option for a theory of everything? Wikipedia states:

"The only mainstream candidate for a theory of everything at the moment is superstring theory / M-theory; current research on loop quantum gravity may eventually play a fundamental role in a TOE, but that is not its primary aim. These theories attempt to deal with the renormalization problem by setting up some lower bound on the length scales possible. Also, early 21st century theories of everything tend to suppose that the universe actually has more dimensions than the easily observed three of space and one of time. The motivation behind this approach began with the Kaluza-Klein theory in which it was noted that adding one dimension to general relativity would produce the electromagnetic Maxwell's equations. This has led to efforts to work with theories with large number of dimensions in the hopes that this would produce equations which are similar to known laws of physics. The notion of extra dimensions also helps to resolve the hierarchy problem which is the question of why gravity is so much weaker than any other force. The common answer involves gravity leaking into the extra dimensions in ways that the other forces do not."

As far as I have been able to determine, no other ToE's are being worked on. I know that Peter Woit is (in)famous for his blog "Not Even Wrong", but is Woit offering any options? Are Woit and his supporters offering other paths to a possible ToE that is anywhere near as far along as superstrings? Are any of these other paths actually being worked on by any practicing physicists? Are there any recent (last 5 years) peer-reviewed articles on such alternate paths?

The Wikipedia article hints at other attempts to work out a possible ToE. Are any of the following accepted as being a possible ToE by mainstream physicists?

"There have been several attempts to advance the general theory of relativity as a theory of everything. As mentioned above, Einstein was responsible for one of these: in collaboration with Rosen he attempted to model particles as tiny wormholes, hence the term Einstein-Rosen Bridge. Wormholes have also been proposed at various times (for instance, by Shimony and by Durand [1]) to explain Bell violations not as superluminal influences but influences that take a shortcut through a wormhole. Such theories face a number of hurdles: the creation of wormholes changes the topology of spacetime by creating a new "handle" which implies violations of causality (see Hadley [2]), and the general theory of relativity predicts its own breakdown at a Gravitational singularity by theorems of Stephen Hawking and Roger Penrose. A recent effort to surmount this hurdle notes that the equivalence principle can be applied along curves rather than at a single point (Iliev [3]), which would imply that time dilation of (1 − v2) − 1 / 2 is indistinguishable locally (along the curve) from a relative velocity v and the unbounded time dilation observed as an event horizon emerges at the center of a collapsing star implies that the center is in reality as well as appearance receding at a velocity approaching the speed of light, producing a bubble-like local inflation of the star's interior (Monroe [4]). This approach skirts the trapped surface assumption of the singularity theorems of Hawking and Penrose. It posits that quantum behavior is an emergent phenomenon in general relativity caused by a stochastic gravitational background radiation (Calogero [5]), in which colliding gravitational waves create virtual particle pairs in the form of wormholes (Griffiths [6]), and the interference pattern in the two-slit experiment is caused by interference between gravitational wavefronts."

I would be really surprised if superstrings was the only ToE being worked on, yet I also get the idea that Woit doesn't have any actual alternatives, at least not that are developed and studied by peer reviw.

In a posting to Not Even Wrong Woit remarks:

"The SU(2) gauge symmetry is supposed to be a purely internal symmetry, having nothing to do with space-time symmetries, but left and right-handed spinors are distinguished purely by their behavior under a space-time symmetry, Lorentz symmetry. So SU(2) gauge symmetry is not only spontaneously broken, but also somehow knows about the subtle spin geometry of space-time."

According to Wikipedia, Woit believes that a proper investigation of what can be done using the geometry of spinors in just four dimensions (along with many other possibly fruitful ideas) has been prevented by an obsession with extra-dimensional speculations.

This seems to imply that no professional physicists are actually pursuing this, and it is really just a hunch. If so, then Woit seems to attacking superstrings without offering a better alternative! What's going on here?

Robert

2. May 25, 2006

### josh1

What do you think is going on Robert? Just visit the sites of physics departments of the top universities to see what's going on. You'll find it's string theory and not other approaches. Unfortunately, there are alot of people here, especially marcus and selfadjoint, who basically mislead people who don't know enough physics to know the difference. The reason these guys, especially marcus, don't like string theory is that it's simply too hard for them to follow. The following post by marcus is typical of the tactics used. Don't trust anyone at this site to be honest about why strings actually dominate. In fact, don't even trust me. You need to go directly to the scientists that actually do this sort of research. Peter woit is not one of them, and marcus certainly isn't and has never posted a single thing indicating he actually understands anything about string theory on a technical level. Ask him some real physics questions and he'll make excuses rather than just explaining the physics. On the other hand, I'll answer any physics questions you have, on string and any other approach to quantum gravity. But again, don't trust me or anyone else here. Email some of the researchers out there about what they think of peter woit or this forum. I'm pretty sure the term crackpot will come up alot. I think it's worse than that. For example, Peter woit is a liar. He repeatedly pronounces that string theory is a failure, but he knows full well that we don't yet understand string theory. All other approaches have been understood for a decade and it is clear to all but a very few that these other approaches we're pretty much dead on arrival.

Last edited by a moderator: May 25, 2006
3. May 25, 2006

### marcus

Wikipedia is not such a good place to start. Wiki's articles relating to quantum gravity have parts which have been repeatedly tug-of-war edited and re-edited so as to in some cases down-play the interest and importance of non-string approaches. IMO Wikipedia is not as authoritative or reliably unbiased in that department as it is in many other areas---some passages may give quite a wrong impression. In the case of several Wikipedia articles, I've found it interesting to click on "history" which gives the history of the article, how it was written, who changed what, who then changed it back---and so on. A kind of "scrimmage".

Peter Woit has his own research but his blog does not cover a wide spectrum of research news in non-string QG to any great extent. The blog is an interesting and fun place but if you want a broad introduction to non-string QG research efforts you should begin somewhere else.

the last big conference in non-string QG was October 2005 and the talks are on-line, at least were the last time I looked. go to the website of the Loops '05 conference and check it out. If the site is up, check out "program" and click on individual talks

recently quite a bit of emphasis has been given to including matter and deriving matter from the quantum space(time) models

a book to look out for, scheduled for 2006 publication by Cambridge University Press, is a collection of essays describing not just one approach but several different lines of non-string QG research. It also contains chapters by string theorists. The book is edited by Daniele Oriti. Around 10-or-so of the chapters are available online at the preprint archive. the book title is something like
Approaches to Quantum Gravity, towards a new understanding of space time and matter
(see for example the preprint chapter mentioned here:
https://www.physicsforums.com/showpost.php?p=980353&postcount=481)

several of the approaches have matter fields and spacetime geometry arise from the same "stuff"---non-string QG approaches are certainly developing towards a UNIFIED picture of quantum spacetime geometry and matter interactions----a quantum spacetime-matter dynamics. but they certainly are not there yet. the number of people researching in non-string QG is increasing (still quite small compared with the string research establishment however!)

for an introductory overview of nonstring QG there is a series of 25 video lectures by Lee Smolin and two other people. it is free for download at perimeterinstitute.com

neither Wikipedia (as currently edited) nor Peter Woit are really in the business of giving a broad accurate picture of non-string QG research or of the most active alternatives to string theory. Both of them are excellent and I have the highest regard for them as web resources. But Peter Woit has his plate full just keeping track of all the furor surrounding the String Landscape and the slide towards the Anthropic Principle, plus all the other stuff he watches and discusses with commentors. He would have neither the time nor the space in his blog to report the QG scene. It is a great blog but it is not its job to tell you about spin-networks, GFT (group field theory), spinfoam models, doubly special relativity, loop quantum cosmology, black hole singularity removal, quantum bounce replacing big bang, prospects for testing non-string QG models by gammaray burst observations and a lot more. If you want to know about alternatives to string you need a whole other news service and there almost isnt one

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4. May 26, 2006

### marcus

I was too long-winded before. I'll try to answer in brief
No, interestingly enough. Non-string QG is getting into the unification business.

On the contrary, alternatives are being worked on. But it is not clear to me that ANYBODY'S theory, as it stands, is going to work out as an option. Everything in the field is going to take more work. AFAICS the key step is TESTING. As science teacher, you know that a theory of spacetime and matter has to be brought to the point that it makes unambiguous predictions that can be tested. To be science, it has to bet its life that a practical future experiment will turn out a certain way, and if it turns out different, it dies. If a theory is so mushy that it can happily accomodate any possible future outcome of any future experiment then it is not predictive in the sense of making hard predictions---not falsifiable. Since science theories cannot be ultimately verified, testability amounts to being falsifiable.

So right now theorists are scrambling to get falsifiability for their theories. A recent string paper about this was by Distler. A recent non-string paper was by Smolin. (if you want archive numbers, just say).

And of course the non-string approaches are being pushed to get UNIFICATION too. At this stage the aim seems to be to get MATTER to arise from their models of spacetime. Laurent Freidel has in certain cases gotten the Feynman diagrams usual in the quantum field theory of matter to emerge in a spinfoam spacetime model. John Baez has two recent papers of a similar nature. No Feynman diagrams there yet, but matter cropping up in 4D spacetime models---i.e. no extra dimensions. May be some crossover with string theory, but without assuming extra spatial dimensions. Can't tell if any of the new non-string spacetime-and-matter approaches will be able to make it to the finish line. Too early.

No way! The non-string QG guys are really scrambling now. It's a fun field to watch. Do you want URLs to recent work to get a sample of it?

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5. May 26, 2006

### Chronos

String is still popular, but fading. The landscape/anthropic mess has disillusioned many who prefer to avoid blurring the line between science and philosophy.
Is the concept of refraining from 'ad hominem attacks' too hard to follow?

Last edited: May 26, 2006
6. May 26, 2006

### f-h

First of all, LQG is first and foremost a theory of Quantum Gravity not of Everything.

Matter may or may not arise in natural ways, but that's not the primary aim, and while it's an active field of inquiry it's far from established.
The following sentence:
"These theories attempt to deal with the renormalization problem by setting up some lower bound on the length scales possible."
Is of course wrong, the minimal length scale is not set up, but a consequence of the theory, implying why perturbative attempts to quantize Gravity failed.

Josh1, it's not difficult to see that above and beyond whatever scientific merits it might or might not have, there is a significant sociological inertia to the idea. This is normal but it's the first time in science that such an idea has become so dominant on the theoretical side without experimental evidence. This warrants suspicion from the outside, suspicion that can not be waved away by claiming that the critics do not understand the technical details. Once you understand the technical details of String Theory, you are looking for a PostDoc... in String Theory.

In fact the extreme technical sophistication makes me warry about ST. Compare to LQG, you go through a technically highly sophisticated quantisation procedure, but at the end (or at the first intermediate stop) you arrive at a physical picture that is simple and straightforward. To me the fact that technical complexity of the theory is not just increasing in time but comes back down again, too is an indication that something right is going on.

7. May 26, 2006

### josh1

No, strings are not fading. This is just the kind of B.S. peddled here as fact.

Not at all. The problem here is that there are all kinds of ways to behave rather badly towards other members without setting off any alarms, and some members here - two in particular - are masters of this.

Last edited by a moderator: May 26, 2006
8. May 26, 2006

### Careful

** What do you think is going on Robert? Just visit the sites of physics departments of the top universities to see what's going on. **

How many people were working on :
(a) discrete processes to explain'' the photo electric effect when Einstein came up with this ?
(b) relational approaches to physics when GR was born in 1915 ?
(c) the renormalizability of Yang Mills when 't Hooft proved this in his doctoral thesis?
(d) mathematics as the basic language for natural philosophy before Newton wrote his principia ?
etc...
Having settled that, let me enjoy

**
On the other hand, I'll answer any physics questions you have, on string and any other approach to quantum gravity. **

since I have plenty of questions about string theory (here are a few of them):

(a) where/when is it shown that string field theory is renormalizable ?
(b) does string theory reproduce the standard model of particle physics and why (not)?
(c) how dependent is string theory upon supersymmetry ? (one of these social concepts which can survive 30 years without any experimental support whatsoever)
(d) who has *any* clue whatsoever how to do string field theory in the strong field regime *nonperturbatively*?

There are of course the more burlesque questions such as why should matter be the vibration of a string now ? It would be much better if on this forum less people would say how great/unique their favorite approach is and explain more in a calm/straightforward way what the ups/downs are.

Cheers,

Careful

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9. May 26, 2006

### Mike2

I think more fundamentally String/M-theory needs to answer how strings or branes exist in the first place. How can it be physical to have fields existing only on submanifolds since this implies a discontinuity in the field as one approaches the submanifold (branes) from a space where the field is zero to the brane where the field goes instantly to some value. Can such discontinuities be physical?

And where does the flat spacetime come from which acts as the background to calculate vibration modes, etc?

10. May 26, 2006

### Careful

**I think more fundamentally String/M-theory needs to answer how strings or branes exist in the first place. **

I don't see any problem in taking the most simple proposal for matter and study it's consequences. One needs to assume something in order to make progress.

**
How can it be physical to have fields existing only on submanifolds since this implies a discontinuity in the field as one approaches the submanifold (branes) from a space where the field is zero to the brane where the field goes instantly to some value. Can such discontinuities be physical? **

One can joke that the big invention of string theory is the repacement of a \delta^3 by a \delta^2 and one could imagine that at the most fundamental level, things aren't a three dimensional continuum (string theory has to get rid of embedding space of course - see Dijkgraaff et al.). Again, going over to three dimensional continuum models (such as fast spinning fluids and so on) is much more difficult and you are immediatly left with the burden to explain why matter appears in quantized form to us (not that ordinary QFT explains that, it takes it as an assumption).

But *in the first place*, any reasonable candidate theory has to explain why accepted wisdom is a good approximation to experimental outcome. IMO, it is fine - and even necessary - to overlook certain theoretical desirata as long as this does not conflict common sense.

Careful

11. May 26, 2006

### George Jones

Staff Emeritus
Maybe string theory is not fading now, but a slow fade is inevitable if sharp evidence for string theory is not found. The fade will be slow, because, as you say, string theorits are firmly entrenched at top universities, and thus are firmly entrenched on hiring committees at top universities.

As f-h says,

An interesting exchange on sci.physics.strings occurred about a year ago between "We Pretty" and Lubos Motl. We Pretty posted first. Here is http://groups.google.ca/group/sci.physics.strings/msg/5ee7aff5697efe2f?hl=en&":

Even if string theory is correct, without experiemntal evidence, it will begin a slow fade. The question is when. For example, suppose the first hard experimental evidence is seen 50 years from now. Physics is a sociological as well as scientific endeavour, and even in this hypothetical "string theory is true" scenario, string theory will fade.

Of course, the LHC could soon find hard and accepted evidence for supersymmetry, in which case string theory will not fade.

I am neither pro string nor anti string, and I had hopes to work my way thoroughly through Zwiebach and Polchinski, but it now appears that this is not going to happen my lifetime. I do not, however, feel that such a programme is a waste of time.

Without further evidence, my feelings a decade from now might be different, though.

Regards,
George

Last edited by a moderator: Apr 22, 2017
12. May 26, 2006

### Mike2

It sounds like the problem still remains with $$\delta^2$$

13. May 26, 2006

Staff Emeritus
All of this discussion, pro and con string theory is just the sort of "here is my belief and it's better than your belief" stuff we are used to seeing from newbys on the philosophy subforums.

Let researchers continue to research and let onlookers refrain from making large comments, until we see what LHC has to show us (there might even be some shocks yet from Fermilab!).

And we on PF should continue to be VERY interested in any theoretical demarches that are announced, without nailing our hearts to any of them.

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14. May 26, 2006

### Careful

What specific problem are you alluding to here? Could you also provide some explanations?

15. May 26, 2006

### marcus

not to forget possible shocks from astronomical observations (such as nearterm by the gammaray burst instrument GLAST). Not seeing a slight play in the speed of light at very high energies over astronomical distances would cause real trouble for non-string QG

general agreement with your views on large pronouncements (but urge tolerance for outbursts evoked from any and all sportsfans in the heat of the moment, it's exciting stuff)

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16. May 26, 2006

### Mike2

I take it you mean that $$\delta^3$$ refers to the infinity seen while approaching zero distance to point particles from anywhere in 3D. And $$\delta^2$$ refers to the instantaneous change in fields while approaching a surface (of a worldsheet) from anywhere in 3D. If the first poses a problem in physics, I don't see how the problem is cured by the second.

17. May 26, 2006

### Careful

Ask that to a string theorist (!), it is not so obvious at all (think about the matrix models where you have this nonexisting double scaling limit).

Cheers,

Careful

18. May 26, 2006

### f-h

Maybe, but the problem is precisely one infinity smaller then with $$\delta^3$$ ;)

19. May 26, 2006

### josh1

Uhm, although I'm unsure about their appropriateness or accuracy, I'm guessing that with these historical remarks, you're making the point that judging the validity of research on the basis of whether or not it's mainstream is not a good idea. Yes, that would be stupid of me if I we're judging this way, but really, I was just trying to warn Robert that some of the members here try to convince people that string theory is no longer mainstream and other approaches are somehow knipping at it's heels. This isn't the case. So if someone does want to know truly what is going on, go visit the sites of some of the major research centers. That's all I was saying. However, LQG is certainly not mainstream compared to strings, and in this case at least, the reason is that people know that LQG is almost certainly unphysical.

I guess we’ll see.

I think it's time for you to leave the 70's and 80's behind and embrace the brave new world of effective field theories where theories that we're viewed as renormalizable in the original sense are now correctly viewed as including only the lowest order terms of a lagrangian that will in general contain infinitely many higher order terms, these just being suppressed at higher energies. If for each term there is a counterterm, then effective field theories are as renormalizable as one's that are renormalizable in the original sense.

We can understand why LQG is wrong from the effective field theory point of view in that the einstein-hilbert action is just the lowest order terms in an effective field theory. Yet the assumption of LQG is that these are the only terms, and this is impossible to reconcile (at least in any natural way) with the other interactions. It just makes no sense to expect the einstein-hilbert action will remain a valid basis for quantization of gravity all the way up to Planck energies.

There is sometimes a bit of confusion about what the significance of the term “nonperturbative” is when used in the same sentence as the term “string field theory”. String theory is formulated as a first quantized theory, these being inherently perturbative in form. On the other hand, quantum field theories are second quantized and are framed in a nonperturbative way - in the form of a lagrangian of field operators. So with string field theory, we’re looking for an inherently nonperturbative form of string theory, but we’ve not yet succeeded.

As is well known, we've been unable to produce the standard model using string theory. But this is very far from knowing that it's impossible to do so. For example, there are various compactifications of the weakly coupled E8xE8 heterotic theory that produce a picture too much like the one obtained from the standard model to be dismissed as insignificant in a theory as vast and poorly understood as strings. However, generating any kind of predictions from this particular model requires we either discover how to stably break supersymmetry at weak coupling, or pursue the strongly coupled theory. But at present there seems to be so many possible ways of handling the first problem that none of them seem natural. The latter on the other hand currently seems intractable.

In fact - and this is commonly not appreciated - string theory and supersymmetry are completely independent ideas. It's just that the nonsupersymmetric part of moduli space is far less tractable than the supersymmetric one.

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20. May 27, 2006

### Careful

** I was just trying to warn Robert that some of the members here try to convince people that string theory is no longer mainstream and other approaches are somehow knipping at it's heels. This isn't the case. So if someone does want to know truly what is going on, go visit the sites of some of the major research centers. That's all I was saying. However, LQG is certainly not mainstream compared to strings, and in this case at least, the reason is that people know that LQG is almost certainly unphysical. **

Right, but if that is all what you want to say, why make such a fuzz ? Bottom line is that (wrong) public opinion is not going to change anything to how research is done. Personally, I don't see any value in a discussion over which dog got the most juicy bone, or whether some nasty bugs are slowly consuming the latter.

**I think it's time for you to leave the 70's and 80's behind and embrace the brave new world of effective field theories where theories that we're viewed as renormalizable in the original sense are now correctly viewed as including only the lowest order terms of a lagrangian that will in general contain infinitely many higher order terms, these just being suppressed at higher energies. **

Bad start , first don't tell me how modern I should be (we are both intelligent people and make our own conscious well motivated choices), second my definition of renormalizability is the modern one (which you can find on Baez' webpage).

**
If for each term there is a counterterm, then effective field theories are as renormalizable as one's that are renormalizable in the original sense. **

So, here you tried to avoid my question about renormalizability of string field theory.

**
We can understand why LQG is wrong from the effective field theory point of view in that the einstein-hilbert action is just the lowest order terms in an effective field theory. Yet the assumption of LQG is that these are the only terms, and this is impossible to reconcile (at least in any natural way) with the other interactions. It just makes no sense to expect the einstein-hilbert action will remain a valid basis for quantization of gravity all the way up to Planck energies. **

That is not known, it *might* be that the renormalization flow around a nontrivial background (say de Sitter), has a non gaussian UV fix point (I never checked myself the work of Reuter though). Another possibility is that quantum mechanics is wrong at these gigantic energies. EDIT : another possibility is to quantize using a different state (eg. the Ashtekhar Lewandowski state).

**
There is sometimes a bit of confusion about what the significance of the term “nonperturbative” is when used in the same sentence as the term “string field theory”. **

Nonperturbative simply means that if you take a (quantum) field´´ theory (a theory with an infinite number of particles), split the lagrangian in a free and interacting part and treat the interactions as perturbations in your expression of interest (a state sum in statistical physics, vacuum expectation values in QFT), then either (a) the perturbation series converges (b) the series is the formal expression of a well defined, known, (non analytic) function.

**
String theory is formulated as a first quantized theory, these being inherently perturbative in form. So with string field theory, we’re looking for an inherently nonperturbative form of string theory, but we’ve not yet succeeded. **

Well, I am not a string theorist, but people have definetly studied the subject vis a vis its correspondance to 1+1 dimensional (topological) quantum gravity with topology change (without succes).

**
In fact - and this is commonly not appreciated - string theory and supersymmetry are completely independent ideas. It's just that the nonsupersymmetric part of moduli space is far less tractable than the supersymmetric one. **

Right, but you have to admit that analytic calculations get pretty fast hopeless once you abandon supersymmetry.

**

**

hehe, did it become difficult to imagine otherwise ? :rofl:

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