# I Why String Theory Is Still Not Even Wrong

1. May 5, 2017

### kodama

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.

2. May 5, 2017

### 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?

3. May 5, 2017

### atyy

Yes, string theory has good candidates for that too.

4. May 5, 2017

### atyy

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.

5. May 5, 2017

### kodama

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

6. May 5, 2017

### atyy

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.

7. May 7, 2017

### 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 refering 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. "

8. May 8, 2017

9. May 8, 2017

### julian

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.

10. May 8, 2017

### kodama

Han then links it to tensor networks and LQG.

11. May 8, 2017

### atyy

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.

12. May 12, 2017

### craigi

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.

13. May 17, 2017

### Fra

Even though i do not have faith in the constructing principles of string theory, i agree that alot 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.
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, noone 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

14. May 17, 2017

### 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

15. May 18, 2017

### Fra

I think these possible extra dimensions are likely existing at energy and length scales that are probably not reasonable to probe "directly" (we dont observe much of anything "directly" anyway if you think about this twice but lets 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, thats an option in theory maybe. But as string theory stands today it seems thats 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 havent 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

16. May 18, 2017

### haushofer

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

17. May 21, 2017

### craigi

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?

Last edited: May 21, 2017