Why I am REALLY disappointed about string theory

[ensabah6]

.. simply not true, a lot of people work in this field.

And as I said otherplace, this program remains a smaller blip on the radar screen due to a lack of convincing progress for many years, conceptional foundation, and scope, so that's why it didn't convince the majority of researchers. Take my word, the moment a theory would look really promising, and this not for you but to people who understand things at a deeper level, many people would start working on it. That this didn't happen is not due to sociological reasons, as Smolin & Co try to fabricate, but due to scientific reasons.

Consider how much hype string theory has received from the likes of Kaku, Greene, Hawking, etc., and there's no current evidence for SUSY and higher dimensions. HEP has bet the farm on a highly speculative program. The best universities all have string research groups.

Let's say hypothetically speaking SUSY and extra dimensions and GUT's are unrealized in nature. Nature is 4D without SUSY or GUT. Would it make sense for physicists to continue to pour research effort into strings?

 

[yossell]

That this didn't happen is not due to sociological reasons, as Smolin & Co try to fabricate, but due to scientific reasons.

But these scientific reasons are not empirical reasons, right? I'm not taking sides, but it seems to represent some kind of shift in physics that, after a long period when `shut up and calculate' was the official philosophy, such ``philosophical'' virtues as elegance, simplicity and generality should suddenly count for so much.

 

[atyy]

But these scientific reasons are not empirical reasons, right? I'm not taking sides, but it seems to represent some kind of shift in physics that, after a long period when `shut up and calculate' was the official philosophy, such ``philosophical'' virtues as elegance, simplicity and generality should suddenly count for so much.

If you look at say Smolin's http://arxiv.org/abs/gr-qc/9508064 , where he argues that gravity is not a conventional field theory, this is exactly what AdS/CFT provides - it is a CFT in one dimension less!

Of course, this does not model our universe, but it is the closest anyone has come to quantum gravity.

Also, he says:

"It is then very impressive that there is one context in which this problem has been definitely solved, which is perturbative string theory."

"It is then very interesting that, as was shown by Klebanov and Susskind, continuum string theory can emerge from a lattice field theory in which there is a cutoff in the transverse directions by means of a limit in which the lengths of the strings diverge while the transverse cutoff remains fixed."

"it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit."

 

[negru]

Why is it so important for the features of string theory (susy, extra dim) to be realized independently in nature? Maybe they could just be internal machinery of the theory? Quantum mechanics uses Hilbert spaces. Have any of you guys ever seen a Hilbert space? Or maybe you could argue that string theory is wrong since it uses the identity 1+2+3+...=-/12, which we all know is false. Leave the possibly internal stuff out of our universe.

If string theory can compute things correctly, that's all we need. If it uses susy, apples, or sand, why does it matter if we don't see those things as we would naively expect? The only question which we need to ask is : can it be used for anything?. And the answer seems yes.And about funding. Who exactly should decide who or what project should get funding (government funding, that is)? This is pretty big problem and it's not limited to string theory/high energy/physics/research in general.

 

[MTd2]

That sum is not wrong:

http://en.wikipedia.org/wiki/Ramanujan_summation

It works because you assume an ordering of operators. Notice that it appears on dimensional regularization after you assume that operators are time ordered.

 

[marcus]

Why is it so important for the features of string theory (susy, extra dim) to be realized independently in nature? Maybe they could just be internal machinery of the theory? Quantum mechanics uses Hilbert spaces. Have any of you guys ever seen a Hilbert space? Or maybe you could argue that string theory is wrong since it uses the identity 1+2+3+...=-/12, which we all know is false. Leave the possibly internal stuff out of our universe.

If string theory can compute things correctly, that's all we need. If it uses susy, apples, or sand, why does it matter if we don't see those things as we would naively expect? The only question which we need to ask is : can it be used for anything?. And the answer seems yes.
...

can it be used for anything? is a reasonable question to ask about whatever line of mathematics. And sometimes it's desirable to push ahead even if there is no positive certainty.

 

[ccdantas]

Why is it so important for the features of string theory (susy, extra dim) to be realized independently in nature?

You mean then that those features are not necessarily physical, and hence their eventual non-observation has no impact whatsoever on corroborating or not the theory? Then what is your criteria for considering a feature physical within a theory, or in other words, what would make it "important" or not (to the point where one could use it to falsify the theory)? Or would it be acceptable that everything would just be internal maths that magically gives a correct observable output?

If string theory can compute things correctly, that's all we need.

So again, what exactly do you want to calculate? Correctly to what precision (in order that you find that a given theory is acceptable)? Suppose someone gives you a black box that you cannot see inside what it calculates, and you see the output of the box which matches an observable up to a given precision. Would you be happy with that and finish your business? Then suppose that you are allowed afterwards to open the box and see that the internal calculations use some concept that have been falsified, or that something ad hoc was put in by hand, but nevertheless gives good results at some level. Would you still be happy with that and finish your business?

 

[tom.stoer]

String theory is crowding out other promising research programs, i.e LQG, and may not be physically correct (i.e 4D, non-SUSY)

That COULD be right, but it's definitely NOT the discussion I wanted to start in this thread. Sorry.

 

[tom.stoer]

Quantum mechanics uses Hilbert spaces. Have any of you guys ever seen a Hilbert space? ...

If string theory can compute things correctly, that's all we need. If it uses susy, apples, or sand, why does it matter if we don't see those things as we would naively expect?

I used the same example with Hilbert spaces in quantum mechanics. The difference is that with qm certain things in nature became calculable the first time - this is not the case with strings. And as I said there is another difference: SUSY is directly visible in the physical spectrum - but we do not see SUSY to be realized in nature.

 

[tom.stoer]

@ccdantas: there are a couple of things which should be calculated from a fundamental theory; I listed some of them is post #1.

And again I repeat my questions from post #1

• What are the major achievements of string theory?
• Are there predictions subject to (accessable to) experimental verification / falsification both in principle and in practice? Are there physical phenoma which (once observed) would kill string theory?
• Are there predictions specific for the string theory context (nothing that may follow from SUSY as SUSY could be true even w/o string theory)
• What are the short-term / long-term research programs?
• What are the major obstacles inherent to string theory preventing the theory from delivering on its promises?
• What will be the final theory in terms of strings - a theory, or a framework to create theories?

Sorry for insisting on that. The idea I had in mind when starting this discussion was to let string theorists tell us more about their theory, their specific achievements and issues - instead of always explaining them what we (outsiders) think about it.

 

[ccdantas]

@ccdantas: there are a couple of things which should be calculated from a fundamental theory; I listed some of them is post #1.

Yes, thanks, I know. I was just questioning negru in order that he would clarify his rationale.

But since I am not a string theorist and do not wish to contribute diverging from your interesting thread, I will just be following with no further comments.

Thanks.

 

[ccdantas]

Just one thing: I wonder who are really professional string theorists known to contribute at PF??

 

[tom.stoer]

Yes, thanks, I know. I was just questioning negru in order that he would clarify his rationale.

But since I am not a string theorist and do not wish to contribute diverging from your interesting thread, I will just be following with no further comments.

Thanks.

@ccdantas! This was not to forbid you to speak - sorry for that!

 

[tom.stoer]

Just one thing: I wonder who are really professional string theorists known to contribute at PF??

Very good question. Everybody is welcome. I would also invite members who are in contact with profession string theorists to contribute.

 

[ensabah6]

Very good question. Everybody is welcome. I would also invite members who are in contact with profession string theorists to contribute.

There's Lubos Motl

 

[ccdantas]

@ccdantas! This was not to forbid you to speak - sorry for that!

There is no misunderstanding It's just that it's really more appropriate to read how professional string theorists will address your questions than I write/question anything general for the moment. My concern with negru's comments is somewhat outside this thread.

 

[tom.stoer]

There's Lubos Motl

 

[marcus]

I got interrupted while I was writing this, and had to be away. I want to continue from here, and try to make some points related to what Negru said:

can it be used for anything? is a reasonable question to ask about whatever line of mathematics. And sometimes it's desirable to push ahead even if there is no positive certainty.

But there seems to be a lot of free-floating defensiveness. I'd like to understand that better. Who is supposed to be the enemy?

One has to distinguish between criticisms of the mathematics itself, and criticisms of the program (direction, emphasis...)

The most trenchant criticisms I can remember from recent times were from Nima Arkani-Hamed (November 2008) and from Murray Gell-Mann (I will try to find the links).

Gell-Mann was talking about the direction of the program (avoiding hard fundamental questions of principle in favor of increasing elaboration) and Nima was talking about what he suspects are mathematical limitations (not to expect it to say anything new about high energy physics, but maybe about gravity). I was surprised, a bit shocked, by both statements.

But we are told repeatedly about imagined bogeymen. "Armchair experts" who apparently were calling for a complete halt to string research 10 to 15 years ago!

I do see changes going on within the string research community (shifts in the makeup of new publications, the annual conference etc., the actual research focus of those traditionally considered top people, citation patterns...)
Surely transparency is a good thing and these trends should be reported.
I'm not sure that these changes should be considered problems or troubles. No matter what happens there will still be thousands and thousands of string theorists, many unable or disinclined to do any other kind of research.

If string is having trouble, it is surely not due to popular books by "Smolin and Co."
One should try to be serious. It is silly for real and interesting shifts going on in research to be blamed on "Smolin and Co." And it only deflects people's attention. A kind of noise--like banging on pots and pans. Maybe it allays some people's anxiety to focus their attention on an exaggerated image, but it doesn't make the real situation go away.

Personally I wouldn't label the changes going on in the stringy world as "trouble". I don't consider them a problem, just very interesting---something to observe and try to understand.

And it is way way overly dramatic to talk about "death" or "kill". Those words have been used in this thread. The "armchair experts" imagined as relentless, uncomprehending enemies, are supposed to desire the death or rejoice in the killing of string.
At most all we are talking about sociologically is a small percentage adjustment in the departmental pecking-order. A tiny adjustment in the prestige and self-importance index of a few academics. With thousands of theorists still on board and working world-wide.

 

[tom.stoer]

OK, sorry for using the word "kill". Let me explain: you can "kill" a theory in many different ways: cutting budgets, malicious gossip, ..., falsification. I would like to stress that I was always talking about falsification, nothing else. Sorry again.

 

[marcus]

Tom I understood the word when you used it! You simply meant empirical falsification, ruling out.
It was other people I thought were responding hysterically---with even a bit of paranoia.

My take on the empirical test issue is that we are looking at a large and versatile body of mathematics.
Freeman Dyson had a wonderful perspective piece on string where he talks about birds and frogs.
Anyway a great, admirable, complex, manyshaped, myriad-minded body of mathematics that has grown
(suddenly but in a sense naturally) out of the differential geometry and algebraic topology I learned about in grad school
back in the day.

You do not falsify that kind of thing. It is a self-supporting form of human creativity. Possibly even an addiction.

Maybe I'm wrong or my attitude is somehow inappropriate---if so I'm more than happy to retract what I just said.
And you may be right in asking for a unified coherent testable theory at some specified scale of interaction.

But what I am suggesting (at least right at this moment) is that we aren't dealing with a physics theory---something based on enunciated general principles---with a central main equation or two---that predicts new phenomena and you can compare with various critical future experiments.

What we are confronting is, instead, a vast mathematical grab-bag, which seems rich and applicable in several quite different areas of physics.
It somehow doesn't seem fair to ask it to be falsifiable.

And then there is the separate issue of a possibly dangerous fairytale: the anthropic multiverse. Essentially Susskind's 2003 reaction to the January 2003 KKLT paper.
Since that was excluded from Strings 2008 and played only a small role in subsequent Strings conferences, I am hopeful that Landscapism is now mainly for public consumption and that the community itself has avoided that route. String mathematics does not need Anthropics, it can flourish quite well without that IMHO. Anyone is of course welcome to correct me if I am wrong about that.

 

[MTd2]

I blame it on Marcus & Co.

 

[tom.stoer]

That's not the point: I had something in mind when I started this thread. I knew that it would become difficult. Therefore using "killing" was counter-productive, unrewarded and wrong.

 

[marcus]

I blame it on Marcus & Co.

Marcus & Co. loves and admires string. We have never criticized string mathematics. Why should we?
Personally I just report the news

But MTd2, let's not talk about each other! Tom has started a great thread. I am really interested in what people like Negru have to say that are actually embroiled in the string business. Maybe like Christine I will try to stay in the background and listen to others' opinions.

 

[marcus]

I had something in mind when I started this thread...

I will be sorry if my butting in somehow contributed to stalling the thread.
The initial series of exchanges between you and Negru was refreshing and constructive. Negru is a good spokesman: forthright honest un-defensive comparatively uncomplaining. I'm hoping to hear that conversation continue in some form, with the same or different parties, sometime soon.

There was also a mot juste metaphor about shampoo and justice,
a few words of which I copied and pinned to the wall of my computer space.

Of course I am prepared for answers regarding landscapes (Susskind) and mathematical universes (Tegmark). But frankly: I will never accept these arguments. This is regarding string theory, therefore I expect answers in the context of string theory (if my daughter has to go upstairs in order to shampoo I don't accept discussions regarding justice; that does not mean that I am unintersted in justice - I am - but not in the context of telling a six-year old girl to go upstairs in order to shampoo!)

 

[tom.stoer]

Hey marcus!

You like it? Good on you - and me :-)

Back to strings and to my intention. We had a couple of discussions here (and I was involved) where we discusses pros and cons of string theory. Unfortunately most threads ended with a variation of "string theory is unphysical" and "why can't you see that string theoriy is a great theory".

My impression was that were are (at least partially) unfair. We judged string theory from the perspective of completed or "working" theories although we should know that it isn't. (This become clear in this discussion a couple of times). So we should be fair and try to apply different standards. Nobody here knows how it felt 100 years ago when great physicists where looking for a solution of the quantum puzzle; maybe similar; one knew that there was something out there, but what exactly nobody could say.

So my conclsuion was that we (skeptics) should accept that string theory is work in progress and should listen to the argments of the string community carefully (we are no Woit's here :-).

But on the other hand the string community should accept some standards as well (at least for THIS discussion). Namely that one should not constantly change the rules of the game. That's why I was insisting on not talking about new standards in science, landscape and things like that. To be honest: it can very well be that string theory stays "unpredictive" in this sense; but (once proven) to overcome this situation is not a matter of string theory itself. It is really a matter of physics (as a whole). If string theory predicts that it cannot make predictions except for the prediction of the landscape, then we need a different and a larger context to deal with it. I do not say that I am not interested in this discussion, but not in the context of string theory - seen as an ordinary but still developping physical theory. Especially as I learn that string theory is work in progress and it may therefore very well be that there are clever ideas beyond the landscape to be discovered ... So I try to limit (focus) the context of this discussion - nothing else (and so far it worked, that's why perhaps it may have been better not to write this paragraph :-)

My conclusion was that it should be string theorists themsevles to assess string theory according to its value and its inherent problems. This is what I am still asking for.

 

[mitchell porter]

How I think about the string landscape:

First we need to find some vacua that do give us the particle-physics standard model and cosmological standard model (perhaps with some extra features). Remember that even that much has not been done! There are vacua which look like the MSSM, but no-one seems to have exhibited a construction providing the exact masses of the observed particles. It's all still work in progress.

Then, we need to embed this in a cosmological solution to string theory. I don't hold much hope for the idea of a unique, dynamically determined vacuum. The viewpoint of eternal inflation, with different regions of space exhibiting different particle spectra etc., seems like a natural cosmology for a theory with many different vacua. What's important is to show that this really is the natural cosmology of string theory.

In this regard, Susskind's original landscape paper (hep-th/0302219, pages 4-5) has a simple and vivid description of a string cosmos containing regions in different vacuum states. It's 11-dimensional M-theory, compactified on a 7-torus, leaving 3+1 large dimensions. The fundamental 5-brane of M-theory couples to a magnetic 7-form field. If you consider these branes and fields wrapped on 3-dimensional subspaces of the 7-torus, you are left with a 2-dimensional object coupling to a 4-form field in the 3+1 large dimensions. These 2-dimensional objects (partly compactified M5-branes) are then domain walls interpolating between regions of 3-dimensional space where the 4-forms take different values.

I presume the cosmological reality is more complicated than that, but it offers a starting point for thinking about the big picture.

 

[tom.stoer]

I agree that one has to come much closer to the xMSSM; but in parallel one has to find xMSSM at the LHC, otherwise the theoretical progress in string theory has no value.

 

[humanino]

I agree that one has to come much closer to the xMSSM; but in parallel one has to find xMSSM at the LHC, otherwise the theoretical progress in string theory has no value.

What happened to the misaligned superstring ?

 

[friend]

Isn't string theory about how strings and membranes vibrate on an pre-existing spacetime background which is just assumed? If so, then string theory is not background independent. Is this the reason for the landscape problem?

 

[tom.stoer]

Isn't string theory about how strings and membranes vibrate on an pre-existing spacetime background which is just assumed? If so, then string theory is not background independent. Is this the reason for the landscape problem?

I was thinking about that - and I hesitated to include the background-dependency into my list of questions and open issues. We had this discussion a cpouple of times and we came to the conclusion that string theory may be background independent - but not in the sense as excpected in ART.

I have a different opinion on the landscape: I guess that a completely different structure will emergy once one is able to perform full non-perturbative calculations. But this is implicitly assumed when a talk about a final set of definitions of the theory - which is currently missing.