String Theory State and History

[marcus]

... I'm glad that many professional high energy physicists on this forum see through his "Mr. nice guy" facade.

I try to report the facts and give pertinent links: real conferences, real schools and workshops, real collaborations between people, real papers. I try to get away from widespread preconception and engrained prejudice. So do quite a few other people here at the forum, who are also trying to see the situation as it is.

If anyone doesn't like the news (if it conflicts with their biases) and wants to call the news "spam" or "propaganda" that's fine with me. When you do that you show your own state of mind. In the long run, in a forum like this, we reveal what we are.

Smoit, this is really enough comment on personalities. The topic here is current scientific developments, not our personalities.

 

[Haelfix]

Well you see this is the sort of conclusion one might get from this forum but not necessarily anywhere else.

At least not anywhere in the US university system, apart from perhaps the cosmology center at Penn. Amongst the best physicists, the consensus has always been rather negative towards LQG and that's just the way it is. No amount of very cherry picked or naive citation count spam or links to skeptics taken out of context will change what's evident to anyone who works as a proffessional researcher in academia and who knows the situation first hand.

The situation with respect to String theory has changed a bit recently of course, most notably with respect to hiring and there are many reasons for that (almost none have anything to do with quantum cosmology), most notably the rise of interest in collider phenomenology and say potential direct applications like holographic superconductors or AdS/QCD is likely the most apparent reason, seconded by the sheer quantity of researchers already in string theory (the all the eggs in one basket problem).. Understand that the amount of quantum gravity positions offered at universities was always supposed to be thin, the reason String theory vastly surpassed that was b/c it has (or had) the potential to be important for accessible phenomenology. Since that has proven more challenging, department heads have obviously become a little bit concerned.

As for skeptics towards String theory.. Very little has changed in thirty years. A very small minority (several condensed matter physicists and a few idiosyncratic thinkers) have always been vocally against it. Most prominently Feynman, Veltman, Glashow, Laughlin and Penrose.

But that's pretty much it. Even long time disinterested observers like 'T Hooft, Coleman (when he was alive) and Weinberg have more or less stated that it probably has something to do with nature.

 

[atyy]

A very small minority (several condensed matter physicists and a few idiosyncratic thinkers) have always been vocally against it. Most prominently Feynman, Veltman, Glashow, Laughlin and Penrose.

I wonder what Penrose thinks about it now that twistors are linked to strings - if not technically, at least programmatically.

Some of his thoughts are found on these visually amazing transparencies from a few years ago, when things were just starting: http://people.maths.ox.ac.uk/lmason/Tws/Penrose2.pdf , but that's all I know of.

 

[martinbn]

This thread has derailed from the original topic and now is a bit of an argument, so I don't feel too guilty asking this. Is there a reasonable size non-popular overview of string theory? Preferably recent and addressed to mathematically mature readers.

 

[atyy]

This thread has derailed from the original topic and now is a bit of an argument, so I don't feel too guilty asking this. Is there a reasonable size non-popular overview of string theory? Preferably recent and addressed to mathematically mature readers.

I'm a biologist, so this is probably way too introductory for you, but just in case, I like

David Tong's http://www.damtp.cam.ac.uk/user/tong/string.html and John McGreevy's http://ocw.mit.edu/courses/physics/8-821-string-theory-fall-2008/lecture-notes/ .

Also, although not yet directly related to strings, but something I find very intriguing is the twistor business. http://pirsa.org/C10018

 

[petergreat]

The situation with respect to String theory has changed a bit recently of course, most notably with respect to hiring and there are many reasons for that (almost none have anything to do with quantum cosmology), most notably the rise of interest in collider phenomenology and say potential direct applications like holographic superconductors or AdS/QCD is likely the most apparent reason...

Will these trends will ever reverse? I can't imagine it happening any time soon, at least over the lifespan of the LHC.

 

[Haelfix]

Will these trends will ever reverse? I can't imagine it happening any time soon, at least over the lifespan of the LHC.

It very much depends what is discovered. If they find something completely unexpected, I suspect research will very much go back to string theory as people will try to work out the details and how string theory may or may not be applicable for shedding light on it or viceversa.

The more problematic scenario for string theorists is if they discover some vanilla variant of something we already know a little about (like the MSSM) but don't have enough details to really pin down the exact physics. So while it would obviously be a triumph for string theory's PR if SuSY is detected, actually making theoretical progress over and above the current situation might stay low paradoxically.

Really a theoretical breakthrough is most likely needed and these things are never entirely predictable.

 

[negru]

They will reverse if there's any progress on the vacuum selection problem. Which in my view is just a very trivial issue in the grand scheme of things.

 

[marcus]

At least not anywhere in the US university system, apart from perhaps the cosmology center at Penn...

The situation with respect to String theory has changed a bit recently of course, most notably with respect to hiring and there are many reasons for that (almost none have anything to do with quantum cosmology),
... the sheer quantity of researchers already in string theory (the all the eggs in one basket problem)..

... has (or had) the potential to be important for accessible phenomenology. Since that has proven more challenging, department heads have obviously become a little bit concerned.

As for skeptics towards String theory.. Very little has changed in thirty years.

... and Weinberg have more or less stated that it probably has something to do with nature.

Much that is correct in this general overview, I think. I'll comment on some minor details.
USA academia somewhat special, so you are right to restrict what you say to the USA.
Penn State is not the only place where you can do a PhD in nonstring QG. There have recently been a run of LQG faculty hires in places you might not think of including some in the USA ( U.Fla, LSU) Employment prospects have improved remarkably in the past 3 years, for Loop researchers.
But you are right that MOST of that is in other countries.

I don't think String "skeptics" are the issue. The issue is growing interest in Loop and allied QG. Why waste time criticizing String? What I'm reporting is growing activity in these other areas. Physics leadership in Europe is more positive and less static---UK, France, Germany, Netherlands and so on. Very different picture from most places in the US.

Contrary to what you say, very much has changed. We are not looking at the same scene as 30 years ago.

I heard Witten say, in 2006, that he "still hoped" String would turn out to have something to do with nature. But I don't recall Weinberg making that sort of statement recently. "Probably"? If you have a recent online source, say in the past 2 years, please give it to us!

You say that the decline in String employment prospects does not have to do with Quantum Cosmology. I agree---never said it did, of course. Don't get String decline confused with Loop gains. The QC developments are an important part of the increased visibility and success of LQG. More attention from phenomenologists, obserational cosmo people, is probably good for the field. I wouldn't be surprised if it has helped stimulate employment for LQG people. Ashtekar and Rovelli seem to have been steering a good many of the young reserarchers in that direction---or else they head that way spontaneously.

Also Loop numbers are small, at anyone time probably under 200 active researchers---most members of the community know each other on a face to face basis. Just because these people are finding more and better job openings (than they would have 5 or 10 years ago) does not harm String in any way. Or so I think anyway.

I think the reasons you give for the decline in String employment are reasonable and probably quite on target. I agree that Quantum Cosmology has very little to do with that
(but a great deal to do with how things look in the Loop community.)

 

[marcus]

...
Really a theoretical breakthrough is most likely needed and these things are never entirely predictable.

So many people have said this recently. String program is basically stalled (at least as far as fundamental or unification physics) and needs a breakthrough idea.

David Gross has opined (as far back as 2006, I think) that the completely new idea would be in the nature of space and time. In other words in the LQG department---although he did not say that. He suggested that "our" usual idea of space and time might not exist and "we" might have to get new ideas. I guess that means new mathematical representations of space and time.

Presumably something that is not a differential manifold. Maybe not any kind of manifold. Maybe not even a continuum.

To a LQG person that is not a very radical idea, or very surprising. But Gross was presenting it as radical and surprising. I respect him as a leader in the String program and as someone with vision.

What I wonder is, if they need a new concept of space and time to base String on, something not a manifold, why aren't they looking carefully at the new Loop formulation?
Loop has moved in the direction of GFT (fields defined on a group manifold) and quantum numbers as cell-complex labels. To me it looks like a rather elegant way of dispensing with the 1850 manifold concept of the continuum.

Maybe some String folks are already looking at LQG developments and seeing what they can adapt from that program, but I haven't seen any sign of it so far. Just David Gross crying out for a new concept of space and time, but no visible breakthrough idea taking shape.

 

[atyy]

I don't think LQG has moved toward GFT. Rovelli seems cool towards it, he's pushing TQFT.

Also, I think GFT has to learn from strings. The EPRL picture is too simple, if we think AdS/CFT is a hint. I wonder if Gurau is trying to see how close he can come to AdS/CFT with his 1/N expansion.

The emergence of space is far more subtle in strings with AdS/CFT and twistors.

Also, I'm not sure if Vidal, Singh, Swingle, Wen, Verstraete etc would consider themselves string (I think they are condensed matter guys!), but I'm sure they are keeping an eye on both loop and string. (I really should not be second guessing Physics Monkey who can give us the latest and greatest on this). Anyway, if non-QG (ie. condensed matter) folk are on the lookout, I would imagine the string pros are fully up to date with loop.

Incidentally, the Levin-Wen stuff is all about TQFT. I'm not sure Rovelli has that specifically in mind. I think his TQFT lineage goes back to Smolin (eg. http://arxiv.org/abs/gr-qc/9508064 yes, a cherry, but still sweet), who probably got it from Witten's pre-string work.

Now what's unclear to me is whether Rovelli's TQFT route and Freidel, Livine, Rivasseau's GFT route are divergent.

 

[marcus]

I don't think LQG has moved toward GFT. Rovelli seems cool towards it, he's pushing TQFT.
...

That's a nice sketch of where those various approaches are at present! I will explain what I meant about LQG moving in the direction of GFT.

The LQG Hilbertspace---the space of quantum states of geometry---is very basic to LQG. It used to be defined using a spacetime manifold.

Now Rovelli defines it essentially as a Lebesgue space of complex valued square integrable functions defined on a Cartesian product of SU(2).

So to me that looks like Group Field Theory, because the theory's basic Hilbertspace is functions defined on a group manifold. (You actually have to take quotient by an equivalence relation to get rid of some gauge redundancy but that's the idea.)

To me when I first saw it, it was a very refreshing and encouraging change. You eliminate the spacetime continuum from the picture and deal with pure information about geometric relationships embodied in your group manifold (simply the cartesian product of as many copies as you need of the underlying symmetry group).

And the Hilbertspace of squareintegrable functions, say on the unit interval, L₂[0,1] is one of the first nontrivial examples of a Hilbertspace you meet in undergraduate mathematics. It is a familiar and loved thing, like a teddybear is to a child.
So take a compact group manifold G, with Haar measure, and do the same thing.
L₂(G).

Haar measure on a compact group manifold is another familiar thing. It is the natural uniform probability measure that you get by using the group itself to spread out the measure evenly.

That is what the new LQG formulation's move toward GFT means to me. The group manifold with an old familiar measure and an old familiar Hilbertspace. The inner product is just the integral of the two functions multiplied together (one is complex conjugated). Probably what Hilbert himself had in mind when he axiomatized Hilberspaces.

It is not the FULL GFT, in your sense probably, but it puts a large part of the new LQG formulation conceptually within reach of an undergraduate math major. And probably a physics major as well. I like that, for sure.
The key to doing spacetime geometry that way, of course, is the graph. Each finite graph (with its links and nodes) has such a Hilbertspace associated with it.
Because each link of the graph can have an element of the Lie group SU(2) living on it! That's why you need the cartesian product---of as many copies of the group as there are links in the graph.

 

[Haelfix]

I heard Witten say, in 2006, that he "still hoped" String would turn out to have something to do with nature. But I don't recall Weinberg making that sort of statement recently. "Probably"? If you have a recent online source, say in the past 2 years, please give it to us!

Witten of course still works on String theory (last time you made a claim that he didn't, he promptly responded by writing a Stringy paper not two weeks later) and Weinberg is still very sympathetic to String theory.. From the CERN link at minute 58:

"If I had to bet I would bet that's not the case (referring to the existence of Asymptotic Safety for gravity), I would have to bet something on string theory... I am not against String theory by saying this ok!... but it (AS) is just a possibility to keep in mind"

Also Loop numbers are small, at anyone time probably under 200 active researchers. Just because these people are finding more and better job openings (than they would have 5 or 10 years ago) does not harm String in any way. Or so I think anyway."

I wouldn't necessarily call that small and I am aware that their teams have been growing. Good for them! I also agree they aren't threatening job offerings for String theorists.
But still ~200 papers a year is not in the same league with the Stringy research groups which have ten times that volume, with over two thousand papers written last year. Nevermind programs that are even larger than string theory that are 'Stringy inspired' like all the AdS/CFT offshoots, Randall-Sundrum phenomenology, deconstruction + Quiver gauge theory etc etc. Also, it is important to realize that activity level and citation count, aren't necessarily correlated with whether people believe a theory is correct or not. For instance, low energy nuclear physics is essentially a dead field activity wise (even though there is much theoretical work left to be done) but that doesn't mean that people don't believe in QCD or the assorted semi empirical models that are out there. Instead its just a statement of the fact that making progress in that field is really hard!

String program is basically stalled (at least as far as fundamental or unification physics) and needs a breakthrough idea.

It needs a breakthrough idea to rekindle the type of numbers it had in the 90s, but I wouldn't say 2000+ papers a year is a 'stalled' research program. The point is that on this board you make it sound like there is some sort of dead end in research, that all the top people are leaving and that research has dried up. It's simply not true, the amount of research still completely dominates the landscape of theoretical physics and I am sure the experts are quite satisfied with their productivity.

I would say that the nature of the productivity happens to be a little more esoteric and technical so that it is more difficult for nonexperts to follow, but then that is to be expected too. It's hard to make quantum leaps in mature research areas.

Presumably something that is not a differential manifold. Maybe not any kind of manifold. Maybe not even a continuum.To a LQG person that is not a very radical idea, or very surprising. But Gross was presenting it as radical and surprising. I respect him as a leader in the String program and as someone with vision. What I wonder is, if they need a new concept of space and time to base String on, something not a manifold, why aren't they looking carefully at the new Loop formulation?

B/c as usual the devil lies in the details, not in vague hopes. Look, everyone is sympathetic to wishing for a formalism that preserves manifest background indepandance (suitably defined), and it would be cool if there was something concrete with the quantum foam ideas. However those very ambigous and subtle concepts don't necessarily imply LQG. Indeed I would say most people have problems not with the foundational ideas of LQG, but rather the direct mathematics of the implementation, which was and still is rather fishy. Likewise in the strongly coupled sector of string theory (where you can get these non geometric, quantum foam like structures) it is entirely unclear what exactly is going on. What you aren't allowed to do, is simply posit a structure that suits your fancy. String theory is far too theoretically rigid (which is a good thing here) and won't allow you to write in something that it doesn't predict. Further, dualities and all the UV/IR correspondances murkies these waters tremendously anyway.

 

[Haelfix]

I wonder what Penrose thinks about it now that twistors are linked to strings - if not technically, at least programmatically.

I don't like speaking for him, but if I had to guess my impression is that Penrose is amongst the few remaining physicists who still hold onto the case that there is something wrong with the interpretation of quantum mechanics. That it must be replaced by something else eventually.

Since quantum gravity is the land where all fairytales have the possibility of being true, I think he believes (for whatever reason) that this physical regime will have something to say about his idiosyncratic ideas about the measurement problem and so forth.

Since string theory, and many other proposals more or less keep quantum mechanics intact, I would guess he would stay skeptical, regardless of whether or not several of his other pet ideas have been successfully implemented.

 

[Pythagorean]

Perhaps they were talking about string theory as applied to QCD

In it's origins, string theory apparently competed with QCD as an explanation for the strong force (well, according to this uncited paragraph anyway):

http://en.wikipedia.org/wiki/Gabriele_Veneziano#Research

But the physicists in the popsci show where saying more general stuff like "it doesn't predict anything. Is that theory? Or philosophy?"

 

[Pythagorean]

This thread has derailed from the original topic and now is a bit of an argument

I think the arguments and the behavior in this thread do help to answer the question about the current political state of string theory. I don't know if the sociology forum would have been able to produce such a transcendental experience; even if marcus' knowledge would have been there, there likely wouldn't have been the (revealing) conflict that's been produced here in the "beyond" subforum.

I'm tempted to peak at atyy's links, but I should probably get through my next poster session before I start burying myself in a new subject.

 

[unusualname]

This thread has derailed from the original topic and now is a bit of an argument, so I don't feel too guilty asking this. Is there a reasonable size non-popular overview of string theory? Preferably recent and addressed to mathematically mature readers.

There's an historical overview lecture from Ed Witten at the IOP site:

http://www.iop.org/resources/videos/lectures/page_44292.html

basically this is Witten's position:

"I guess it's possible that string theory could be wrong. But if it is in fact wrong, it's amazing that it's been so rich and has survived so many brushes with catastrophe and has linked up with the established physical theories in so many ways, providing so many new insights about them."

Witten is a very smart guy so we have to hope his intuition is as well-tuned, otherwise a whole lot of people might have wasted a whole lot of time.

(More quotes from Witten http://www.icelebz.com/quotes/edward_witten/)

 

[martinbn]

I'm a biologist, so this is probably way too introductory for you, but just in case, I like

David Tong's http://www.damtp.cam.ac.uk/user/tong/string.html and John McGreevy's http://ocw.mit.edu/courses/physics/8-821-string-theory-fall-2008/lecture-notes/ .

Biologist, that's interesting. The level is fine, but the volume is a bit more than what I am looking for. Also these are regular lectures, I would like to see overview articles. Thank for the links, if there is nothing else and my curiosity wins over my natural laziness I will try to read them.

I think the arguments and the behavior in this thread do help to answer the question about the current political state of string theory. I don't know if the sociology forum would have been able to produce such a transcendental experience; even if marcus' knowledge would have been there, there likely wouldn't have been the (revealing) conflict that's been produced here in the "beyond" subforum.

I'm tempted to peak at atyy's links, but I should probably get through my next poster session before I start burying myself in a new subject.

I was just justifying me posting a question which is not on topic.

There's an historical overview lecture from Ed Witten at the IOP site:

http://www.iop.org/resources/videos/lectures/page_44292.html

basically this is Witten's position:



Witten is a very smart guy so we have to hope his intuition is as well-tuned, otherwise a whole lot of people might have wasted a whole lot of time.

(More quotes from Witten http://www.icelebz.com/quotes/edward_witten/)

Thanks, I will take a look.

 

[atyy]

Also these are regular lectures, I would like to see overview articles.

Maybe http://www.blau.itp.unibe.ch/stqg.pdf

 

[martinbn]

Maybe http://www.blau.itp.unibe.ch/stqg.pdf

That's interesting, I'll read it. But this is too general, I know I am picky, sorry.

I am looking for something that is not more than 100 pages, less is better, non-popular in the sense that it doesn't try to avoid mathematics and the result is something imprecise. It should also give a general view of the theory. Something along the lines, what are the fundamental objects, what are the states of the system, how are they modeled mathematically. What is the time evolution of the states. What are the basic problems considered and some examples of such problems solved. If any of that makes sense, of course.

 

[Physics Monkey]

Maybe Joe's Little Book of String?

http://www.kitp.ucsb.edu/~joep/JLBS.pdf

 

[mehul ahir]

by knowing everything about universe even at the t=0 what was there every charatctristics up to now can we know what will hapen tomorrow?

 

[MathematicalPhysicist]

by knowing everything about universe even at the t=0 what was there every charatctristics up to now can we know what will hapen tomorrow?

If your'e god then yes, but we are mere mortals.