I Why String Theory Is Still Not Even Wrong

[kodama]

Horgan: Do you still think string theory is “not even wrong”?

Woit: Yes. My book on the subject was written in 2003-4 and I think that its point of view about string theory has been vindicated by what has happened since then. Experimental results from the Large Hadron Collider show no evidence of the extra dimensions or supersymmetry that string theorists had argued for as "predictions" of string theory. The internal problems of the theory are even more serious after another decade of research. These include the complexity, ugliness and lack of explanatory power of models designed to connect string theory with known phenomena, as well as the continuing failure to come up with a consistent formulation of the theory.

Horgan: Why do you think Edward Witten told me in 2014 that string theory is “on the right track”?

Woit: I think the conjectural picture of how string theory would unite gravity and the standard model that Witten came up with in 1984-5 (in collaboration with others) had a huge influence on him, and he's reluctant to accept the idea that the models developed back then were a red herring. Like many prominent string theorists, for a long time now he no longer actively has worked on such models but, absent a convincing alternative, he is unlikely to give up on the hope that the vision of this period points the way forward, even as progress has stalled.

https://blogs.scientificamerican.com/cross-check/why-string-theory-is-still-not-even-wrong/

any string theorists would like to reply, esp to the claim

"The internal problems of the theory are even more serious after another decade of research. These include the complexity, ugliness and lack of explanatory power of models designed to connect string theory with known phenomena, as well as the continuing failure to come up with a consistent formulation of the theory."

with specific evidence?

I want to be exposed to opposing points of view.

 

[martinbn]

Complexity and ugliness! How can these, especially the second which is as subjective as it gets, be problems?

 

[kodama]

Complexity and ugliness! How can these, especially the second which is as subjective as it gets, be problems?

bc ugliness violates occam's razor

 

[Drakkith]

bc ugliness violates occam's razor

I don't think I agree, but perhaps Woit's use of the word "ugliness" here means something less subjective that does fit Occam's Razor.

 

[julian]

String theory advocates, to this day, are guilty of trying to claim the full complexity of the world follows from an extremely simple picture. If they know this isn't true then stop doing this.

 

[kodama]

Complexity and ugliness! How can these, especially the second which is as subjective as it gets, be problems?

I don't think I agree, but perhaps Woit's use of the word "ugliness" here means something less subjective that does fit Occam's Razor.

String theory advocates, to this day, are guilty of trying to claim the full complexity of the world follows from an extremely simple picture. If they know this isn't true then stop doing this.

Standard Model-Axion-Seesaw-Higgs Portal Inflation solves dark matter, neutrino oscillations, baryogenesis, inflation and the strong CP problem with just standard QFT and in 4D w/o SUSY.

i.e start with the SM in 4D QFT then add the fewest number of additional particles and fields to address the remaining mysteries dark matter, neutrino oscillations, baryogenesis, inflation and the strong CP problem.

vs string theory which requires MSSM or some variation + GUT of some kind + extra dimensions, then finding a way to avoid falsification from LHC latest results.

 

[velocity_boy]

Agreed. String theory is not even, well...a Theory. At least not in the scientific sense of the word. Rather, it is a hypothesis at best.

A theory needs to have put forth postulates that were later confirmed and proven.

So far as I know, ST had not done this. It's all speculative in nature.

It actually poses more questions than it answers!The fairly recent discovery of the Higgs Boson at CERN did the ST hypothesis no favors, either.

I consider St to be Pop Physics.

It's fifteen minutes should be up soon. I thought it pretty much was, after the Brian Green Elegant Universe thing went out of Vogue

 

[haushofer]

bc ugliness violates occam's razor

Ugliness depends on your mathematical framework. Try doing QED without gauge invariance, or GR without a clear understanding of differential geometry.

As my mother once told me: never judge a theory from the outside.

 

[haushofer]

String theory advocates, to this day, are guilty of trying to claim the full complexity of the world follows from an extremely simple picture. If they know this isn't true then stop doing this.

How should we ever know whether this is true or not? I regard it as a matter of induction; looking at history, unification often kicks in when we go to higher energy scales. And that's what scientists do: try whatever worked in the past.

Personally, I think string theory has some remarkable properties which still makes it worthwile investigating in. One of them being the fact that it is a quantum theory which requires gravity to be consistent. Or, as is sometimes stated: it postdicts gravity.

 

[kodama]

How should we ever know whether this is true or not? I regard it as a matter of induction; looking at history, unification often kicks in when we go to higher energy scales. And that's what scientists do: try whatever worked in the past.

Personally, I think string theory has some remarkable properties which still makes it worthwile investigating in. One of them being the fact that it is a quantum theory which requires gravity to be consistent. Or, as is sometimes stated: it postdicts gravity.

gravity in 11 dimensions

 

[haushofer]

Well, not exactly; anomalies also disappear in 3 dimensions :P

But those extra dimensions are also necessary to make contact to realistic models. If string theory would be only be consistent right away in 3+1dimensions, we would have no hope to extract the standard model from it.

 

[kodama]

Well, not exactly; anomalies also disappear in 3 dimensions :P

But those extra dimensions are also necessary to make contact to realistic models. If string theory would be only be consistent right away in 3+1dimensions, we would have no hope to extract the standard model from it.

perhaps that's why string theory is wrong. the correct theory of nature should uniquely give rise only to the standard model in 3+1 dimensions.

 

[atyy]

Woit is wrong. String theory could be wrong. Even if it is wrong, its place in future physics is secure, and at least analogous to the place of Nordstrom's theory of gravity in today's physics. The developments from the Ryu-Takayanagi formula from around 2006 show that progress in string theory continues.

 

[kodama]

Woit is wrong. String theory could be wrong. Even if it is wrong, its place in future physics is secure, and at least analogous to the place of Nordstrom's theory of gravity in today's physics. The developments from the Ryu-Takayanagi formula from around 2006 show that progress in string theory continues.

does string theory make any falsifiable predictions such that if an experiment or observation is done, the theory is falsified?

e.g if we build a collider up to Planck scale energies, and no SUSY shows up, does this falsify SUSY and string theory?

 

[atyy]

does string theory make any falsifiable predictions such that if an experiment or observation is done, the theory is falsified?

e.g if we build a collider up to Planck scale energies, and no SUSY shows up, does this falsify SUSY and string theory?

Some parts of string theory have already been falsified. Yet these false models remain much studied because they are our best candidates for a complete theory of quantum gravity in some universe.

 

[Denis]

Some parts of string theory have already been falsified. Yet these false models remain much studied because they are our best candidates for a complete theory of quantum gravity in some universe.

This claim that strings give a theory of quantum gravity was questioned by Woit or Smolin (don't remember) too, with the claim that 1.) a proof is given only for a few low order terms, not for all, and 2.) this is anyway irrelevant because it requires exact supersymmetry, which is not viable. IIRC.

Has anything changed in this direction?

If not, what would be the point of QG being something in favor of strings? Anyway it does not work. While in fact QG is not a big problem if one thinks about it as an effective field theory. Throw away the problematic diff invariance, say, by introducing harmonic coordinates, and you have a normal field theory on some fixed background, then use a lattice regularization and you have some theory of QG.

So, having some theory of QG is not really problematic (one may not like this proposal, but so what), but string theory does not even have a viable well-defined one.

 

[arivero]

Horgan: Is it possible that the whole push for unification of physics is misguided?

Woit: In principle it's of course possible that the sort of unification present in our best current theory is all there is. There are however no good arguments for why this should be, other than that it's proving hard to do better. The lesson of history is not to give up, that seemingly hard problems of this sort often find solutions. Looking in depth into the technical issues, I don't see anything inherently intractable, rather a set of puzzling problems with a lot of structure, where it looks like we're missing one or two good ideas about how things should fit together.

Well, let's hope that at some time they will care to look to the lot of structure happening at the electroweak scale, instead of hoping to get all the rabbits out of the GUT scale hat.

 

[kodama]

How should we ever know whether this is true or not? I regard it as a matter of induction; looking at history, unification often kicks in when we go to higher energy scales. And that's what scientists do: try whatever worked in the past.

Personally, I think string theory has some remarkable properties which still makes it worthwile investigating in. One of them being the fact that it is a quantum theory which requires gravity to be consistent. Or, as is sometimes stated: it postdicts gravity.

how does string theory deal with the problem of time, with the different conception of time in GR vs QM?

 

[Arman777]

I found a picture of space-time in higher dimension (by Max Tegmark)

I don't know the time dimension prediction of the string theory.But here says after 3 space dimension (In 1 time dimension case) the universe becomes unstable.What this suppose to mean ?

 

[haushofer]

perhaps that's why string theory is wrong. the correct theory of nature should uniquely give rise only to the standard model in 3+1 dimensions.

Why 'uniquely' and why could the 3+1 not just be an effective description?

 

[strangerep]

Some parts of string theory have already been falsified. Yet these false models remain much studied because they are our best candidates for a complete theory of quantum gravity in some universe.

 

[kodama]

Some parts of string theory have already been falsified. Yet these false models remain much studied because they are our best candidates for a complete theory of quantum gravity in some universe.

why not also study quantum gravity in 2+1 universe

 

[haushofer]

how does string theory deal with the problem of time, with the different conception of time in GR vs QM?

I'm not sure, and I don't think this is fully resolved as there are still even technical issues with describing time-dependent backgrounds in string theory. I guess this is closely tied to the question of background-dependence, but I would have to think about it more carefully.

I just remark that in string theory, one starts out with a 2D conformal field theory, in which gravity and general covariance emerges.

 

[kodama]

I'm not sure, and I don't think this is fully resolved as there are still even technical issues with describing time-dependent backgrounds in string theory. I guess this is closely tied to the question of background-dependence, but I would have to think about it more carefully.

I just remark that in string theory, one starts out with a 2D conformal field theory, in which gravity and general covariance emerges.

so string theory doesn't reproduce all aspects of gravity i.e time-dependent backgrounds, which GR predicts and has been confirmed

 

[atyy]

why not also study quantum gravity in 2+1 universe

1+1 too.

https://cgc.physics.miami.edu/Miami2016/Witten.pdf

"The basic idea is due to Gurau and Rivasseau (a paper I found helpful is V. Bonzom, R. Gurau, A. Riello, and V. Rivasseau, Critical Behavior Of Colored Tensor Models In The Large N Limit, Nucl. Phys. B853 (2011) 174-195 ...)"

 

[kodama]

1+1 too.

https://cgc.physics.miami.edu/Miami2016/Witten.pdf

"The basic idea is due to Gurau and Rivasseau (a paper I found helpful is V. Bonzom, R. Gurau, A. Riello, and V. Rivasseau, Critical Behavior Of Colored Tensor Models In The Large N Limit, Nucl. Phys. B853 (2011) 174-195 ...)"

why not create a toy model QG in 3+1 that while doesn't describe our universe, has many attractive properties and is a self-consistent QG.

 

[arivero]

2+1 and 1+1 QG I have never understood why. The space-like part is very trivial, and then of course it has a lot of nice properties, which are not going to work in 3+1. And if you want to study pure math structure, then what about 0+1, 1+0 and 0+0?

 

[atyy]

why not create a toy model QG in 3+1 that while doesn't describe our universe, has many attractive properties and is a self-consistent QG.

Yes, string theory has good candidates for that too.

 

[atyy]

2+1 and 1+1 QG I have never understood why. The space-like part is very trivial, and then of course it has a lot of nice properties, which are not going to work in 3+1. And if you want to study pure math structure, then what about 0+1, 1+0 and 0+0?

That paper is about 0+1 without gravity and the relationship to 1+1 via LQG-related research.

People who attack string theory are attacking LQG.

 

[kodama]

Yes, string theory has good candidates for that too.

is there such a version in 3+1 spacetime?
i understand that strings only work in 9+1 to cancel anomalies.

 

[atyy]

is there such a version in 3+1 spacetime?
i understand that strings only work in 9+1 to cancel anomalies.

Yes, but the other 6 dimensions may not be relevant - eg. the best example of AdS/CFT is usually described as 4D CFT and 4+1D gravity. Actually the gravity there is 10D strings at high energy.

Similarly, there are versions of AdS/CFT for 3D CFT and 3+1D gravity.

 

[julian]

This is something I think I'd heard of before said by Abhay Ashtekar, I found it again here:

https://thewire.in/14279/good-scien...ut-great-scientists-know-whats-worth-solving/

I'm referring to the part:

"The Kaluza-Klein idea is that there are higher dimensions but because they are all wrapped up and microscopic, say, at Planck scale, we don’t see them. That’s plausible. But here, in AdS/CFT duality, they need the radius of the internal dimensions to be the same as the cosmological radius. If so, if I try to look up I should see these ten dimensions; I don’t. So, it can’t have much to do with the real world that we actually live in. These are elephants in the room which are not being addressed. "

 

[atyy]

https://arxiv.org/abs/1705.01964
Discrete Gravity on Random Tensor Network and Holographic Rényi Entropy
Muxin Han, Shilin Huang

Muxin Han is working on AdS/CFT!

 

[julian]

https://arxiv.org/abs/1705.01964
Discrete Gravity on Random Tensor Network and Holographic Rényi Entropy
Muxin Han, Shilin HuangMuxin Han is working on AdS/CFT!

In string theory when you consider the space $AdS_5 \times S^5$, it is the radius of the spheres $S^5$ that have to be of cosmological radius.

In Han's paper he studies one of these other versions you alluded to, in particular in this paper he is considering $2d$ CFT and $3d$ bulk spacetime ($AdS_3/CFT_2$) in Euclidean signature. Here there are no spheres, such models do then seem more physically reasonable.

 

[kodama]

In string theory when you consider the space $AdS_5 \times S^5$, it is the radius of the spheres $S^5$ that have to be of cosmological radius.

In Han's paper he studies one of these other versions you alluded to, in particular in this paper he is considering $2d$ CFT and $3d$ bulk spacetime ($AdS_3/CFT_2$) in Euclidean signature. Here there are no spheres, such models do then seem more physically reasonable.

Han then links it to tensor networks and LQG.

 

[atyy]

Han then links it to tensor networks and LQG.

Looks like there was an earlier paper from Han: https://arxiv.org/abs/1610.02134. We did put it in marcus's bibliography, but I didn't remember it.

 

[craigi]

I want to be exposed to opposing points of view.

A good succinct answer to the complaints exemplified by Woit can be found from Motl at stackexchange.

https://physics.stackexchange.com/questions/15/what-experiment-would-disprove-string-theory

In my opinion it's important not to get bogged down by the philosophy of falsificationism etc. Regardless of the motivation behind them, the popularity of these arguments has the effect of discouraging us from learning about the subject matter ourselves and making our own judgements on its validity. That can never be a good thing.

 

[Fra]

Even though i do not have faith in the constructing principles of string theory, i agree that a lot of the "not falsifiable critique" is simple minded and not the main problem with string theory.

The main problem I see is the existence of the landscape and without matching prior measures or evolutionary mechanisms that would make it sensible.

Horgan: Why do you think Edward Witten told me in 2014 that string theory is “on the right track”?

Woit: I think the conjectural picture of how string theory would unite gravity and the standard model that Witten came up with in 1984-5 (in collaboration with others) had a huge influence on him, and he's reluctant to accept the idea that the models developed back then were a red herring. Like many prominent string theorists, for a long time now he no longer actively has worked on such models but, absent a convincing alternative, he is unlikely to give up on the hope that the vision of this period points the way forward, even as progress has stalled.

From the “on the right track” interview:

Witten: "I hope the landscape interpretation of the universe would turn out to be wrong, as I would like to be able to eventually calculate from first principles the ratio of the masses of the electron and muon (among other things). However, the universe wasn't made for our convenience."
-- https://blogs.scientificamerican.co...l-thinks-string-theory-is-on-the-right-track/

I think must say this is a quite justifyable position as its the only hope, the real question is: does the string program (or witten himsel) has any CLUES as to HOW we can interpret the landscape in the right way? The landscape is there, no one doubts that. The question is, does the "multiverse" interpretation of the landscape really make sense, or are we seeing the problem in the wrong way?

If there is no such CLUE from the constructing principles but rather just a groundless hope then that is the main one reason to think that string theory is note a very useful "hypothesis generator", because it produces too many hypothesis. This is the problem, not falsification. A research program is hardly a "theory" its rather a way of reasoning that generates hypothesis. And this part of the scientific method is largely trivialized by Popper.

/Fredrik

 

[kodama]

Is there any experimental that can be performed to directly observe the higher dimensions or rule them out?

KK gravitons are a no show at LHC

 

[Fra]

I think these possible extra dimensions are likely existing at energy and length scales that are probably not reasonable to probe "directly" (we don't observe much of anything "directly" anyway if you think about this twice but let's not go there for now) the corroboration or falsification of these "constructions" are entirely in its implications.

What we are dealing with here is an abduction of "best explanation". If a theory that makes use of actions based on dynamics in higher dimensional spaces are the best explanation then that is all i ask for! Anything objections beyond that borderlines old times realist objections that i see are not helpful.

One need to discuss what is "best explanation", but that will het the thread off topic, but i consider this question relevant and tightly related to understanding the landscape problem (in GENERAL that is, not just the "string landscape"). Also it can not be the fuzzy occams razor or "beauty" silly arguments, i think we actually need a technical and physics basis for SELECTION among "best explanation", on par with fitness of organisms in biology.

Now if string theorist want to claim large extra dimensions that should show up soon at more reasonable energies, that's an option in theory maybe. But as string theory stands today it seems that's a possibility only, so falsifying a possibility does not falsify the program. This is of course the problem.

So as I see it, the most realistic chance to support or disprove ST in the, is to solve the landscape problem, and try to make some explicit predictions. In fact even without new physics at LHC, I would personally be MORE THAN please with if string theory could "just" postdict the standard model, and thus reduce the number of free parameters ;-) Thats the job of the string theorists, not ours :) But it would not quote be enough to show that the standard model is a "possibility". But they haven't even identified this possibility.

I am not even sure if there is some proof withing string theory that the landscape MUST contain the standard model + GR? Ie. an existence proof, without necessarily finding it. Does anyone know?

So from my perspective and interesting in this field, while new physics at LHC would be exciting and fun, we have more than enough work to find a consistent coherent picture of current physics. One might argue the point in doing that, ie why would a coherent theory be "better" than a patchwork of theories? I see many reasons for that, some of extend way beyond physics and connects to AI and brain research. So there is no lack of potential spinoffs.

/Fredrik

 

[haushofer]

is there such a version in 3+1 spacetime?
i understand that strings only work in 9+1 to cancel anomalies.

No. It also works in 2+1 dimensions due to proporties of the Lorentz group in 2+1 dimensions. See e.g. Paul Townsend.

 

[craigi]

Is there any experimental that can be performed to directly observe the higher dimensions or rule them out?

KK gravitons are a no show at LHC

It's hard to know what an experiment could even be like to observe hidden dimensions directly. To answer that, first we need to address what we mean by direct measurement. When we examine at all the measuring apparatus we use for what we consider direct measurement, it all involves conveying information using 3 spatial dimensions and/or 1 time dimension.

Would we expect to be able to perform an experiment which projects spatial detection within hidden dimensions into 3+1 space? Is that what we would consider direct measurement? If so how would we probe in these dimensions and then project into 3+1 space?