String Theory State and History

[Pythagorean]

First off, I've always been under the impression that string theory was crackpot (with absolutely zero reasoning on my part, just from authority) but the way I've seen it discussed here in the last years (and I've heard the LHC has some experiments pertaining to it) maybe my impression was wrong?

I'm curious what the current politi-scientific state of string theory is (is it falsifiable?)

but I'm also curious, in the way of history, why string theory is so appealing to people; how did the idea come up?

 

[MathematicalPhysicist]

Well there's the PBS programme with Michael Green which tells its history, check youtube.

It I remember correctly, it started with Venziano.
http://en.wikipedia.org/wiki/Gabriele_Veneziano

 

[suprised]

First off, I've always been under the impression that string theory was crackpot (with absolutely zero reasoning on my part, just from authority)

What kind of authority might that have been?

 

[revnaknuma]

string theaory is not physics but an ultra high level mathematics.
it is a mathematical success. no physics. it is false.

 

[marcus]

...I'm curious what the current politi-scientific state of string theory is (is it falsifiable?)
...

The best way I know to get a reading on String's current politi-scientific standing is to listen carefully to what Steven Weinberg says in this video:



Drag the time button to around minute 48. He will start commenting on the current state of the String program at around minute 50.

For background, as you may know, Weinberg is a world class physics elder statesman who worked on String back in 1980s but eventually got out of it and is now mainly in Cosmology.
At the beginning of the YouTube video the conference chairperson gives an introduction to him and his work. Central figure in the science Establishment.

It's an especially nice video for several reasons:
He is talking to science writers, a smart non-specialist audience at a level between beginner and expert.
He has a calm balanced view, not partisan, does not take sides in some petty squabble.
He chooses his words carefuly.
He is kind to theorists still working in the String program.
He expresses openness to the idea that the program could still succeed (in arriving at a single explicit testable theory) although he doesn't see how that could happen.
He describes the hopes for String that "some people" have, in a clear understandable way.

 

[marcus]

string theaory is not physics but an ultra high level mathematics...

Hi Revnaknuma and welcome! This is the first time I've seen a post by you. Your profile says you just joined the forum this year. I think you have an interesting point of view as a physics grad student who already has a career in ethnic music (and traditional musical instruments)---someone who is studying physics at graduate level because of pure interest in the subject.

I hope you watch the relevant part of that Steven Weinberg video. He uses much softer language than you---he does not say "crackpot" and "false"---but is perspective has something in common with yours.

The difference is in the words used. Your statement could be taken as an attack. Weinberg's mild carefully chosen words can be heard not as an attack but as measured and sympathetic understanding.

I'm not saying that it is wrong to attack! In some situations it can be the appropriate thing, especially if you want to stimulate a hot debate. But it's interesting to listen to the difference in tone of voice between you and Weinberg.

As an expert in traditional Central Asian musical instruments you undoubtably have a good ear for tone!

 

[marcus]

The best way I know to get a reading on String's current politi-scientific standing is to listen carefully to what Steven Weinberg says in this video:



Drag the time button to around minute 48. He will start commenting on the current state of the String program at around minute 50.

For background, as you may know, Weinberg is a world class physics elder statesman who worked on String back in 1980s but eventually got out of it and is now mainly in Cosmology...

Fzero just transcribed part of what Weinberg said after minute 50 on the video. It looks to me like a faithful accurate transcript and I really appreciate Fzero taking the trouble!

Here is the whole of what Fzero copied:
==quote Weinberg 2009 Science Writers Conference talk on LHC and the state of physics==
It's developed mathematically, but not to the point where there is anyone theory, or to the point where, if we had one theory, we would know how to do calculations to predict things like the mass of the electron or the masses of the quarks. So, I would say, although there has been theoretical progress it's been….I find it disappointing.

One of the hopes would be that the LHC would provide a clue to something we're missing in superstring theory and I think there supersymmetry is the most likely place to look. One of the troubles with superstring theory is that, although in a sense the theorists think there's only one theory, there are an infinite number of approximate solutions of it and we don't know which one corresponds to our world. But at least in a large variety of the solutions of superstring theory there is supersymmetry visible at low energies. And if we see supersymmetry at low energies, the superstring theorists may be able to derive from it some type of clue as to how to try to solve these theories.

But I haven't talked about it in this lecture because I don't see how that would work. But I couldn't say that that was likely with any sincerity and certainly the LHC and any other accelerator that we can imagine being built would not get up to energies which are high enough so that we can directly see the structures that are described by superstring theory: the strings or the D-branes or whatever it is. Those will not be accessible at the LHC, so any clue that we get will be very indirect.

I have myself, well I was working on superstring theory in the 80s and gave it up because I moved into cosmology, which, in the last couple of decades, had the excitement that elementary particle physics had in the 60s and 70s: A wonderful coming together of theory and observation. Cosmology now reminds me of the excitement that I felt when I was younger and doing particle physics.

It's a pity that superstring hasn't developed better. I still think it's the best hope we have. I don't know of anything else. My own work very recently has been trying to develop an alternative to superstring theory as a way of making sense of quantum gravity at high energies. But even though I'm working on this, I still find superstring theory more attractive, but not attractive enough.
==endquote==

If you have time to watch it, you might enjoy the whole 55 minute slide talk. Great sldes, clear non technical explanations of several physics frontiers (particle physics AND cosmology)

 

[negru]

Well, i'd hardly call Weinberg a "Central figure in the science Establishment", when discussing these newer developments. Not even younger people in the field are afraid to call him a crackpot these days. Not that it necessarily means anything, but he certainly hasn't been an authority on strings since the 80's or so.

 

[Pythagorean]

Thank you for your in-depth replies Marcus. I will watch the video later. Shallow response mode.

What kind of authority might that have been?

It was a NOVA show interviewing scientists. I think they were quantum chromodynamicists.

 

[MTd2]

Hi revnaknuma! Is this talented musician you?
http://www.youtube.com/user/revnaknuma#p/a

 

[petergreat]

Well, i'd hardly call Weinberg a "Central figure in the science Establishment", when discussing these newer developments. Not even younger people in the field are afraid to call him a crackpot these days. Not that it necessarily means anything, but he certainly hasn't been an authority on strings since the 80's or so.

Weinberg published two articles last year on pretty modern hep-th topics:
http://arxiv.org/abs/1009.1537"
http://arxiv.org/abs/1006.3480"

 

[marcus]

Another interesting Weinberg talk to transcribe an excerpt from would be his July 7 2009 colloqium talk at CERN.
http://cdsweb.cern.ch/record/1188567/
As I recall the last 10 or 12 minutes were especially enlightening, The first 55 minutes were largely devoted to summarizing the history of quantum field theory.

Weinberg was also one of the invited speakers featured at the annual Strings conference, Strings 2010. (Along with e.g. Strominger, Arkani-Hamed, Witten, Ooguri, Kachru, etc...)
He presented his (non-string) research in "Gravity at High Energy".

 

[negru]

Another interesting Weinberg talk to transcribe an excerpt from would be his July 7 2009 colloqium talk at CERN.
http://cdsweb.cern.ch/record/1188567/
As I recall the last 10 or 12 minutes were especially enlightening, The first 55 minutes were largely devoted to summarizing the history of quantum field theory.

Weinberg was also one of the invited speakers featured at the annual Strings conference, Strings 2010. (Along with e.g. Strominger, Arkani-Hamed, Witten, Ooguri, Kachru, etc...)
He presented his (non-string) research in "Gravity at High Energy".

yeah they also invite Horova to string conferences and everyone makes fun of him and attends his talks like it's the new circus in town.

 

[negru]

It's also difficult to believe he's so central while pushing his pet AS theory, which just completely disregards everything we've learned about gravity in the past decade or so. But I'm sure we've discussed that many times before here.. I don't remember what conclusion we reached however.

Anyway, Weinberg is just one guy, I don't understand why his view is so important, or relevant at all.

 

[marcus]

Well, i'd hardly call Weinberg a "Central figure in the science Establishment", when discussing these newer developments. Not even younger people in the field are afraid to call him a crackpot these days. Not that it necessarily means anything, but he certainly hasn't been an authority on strings since the 80's or so.

yeah they also invite Horova to string conferences and everyone makes fun of him and attends his talks like it's the new circus in town.

It's also difficult to believe he's so central while pushing his pet AS theory, which just completely disregards everything we've learned about gravity in the past decade or so. But I'm sure we've discussed that many times before here.. I don't remember what conclusion we reached however.

Anyway, Weinberg is just one guy, I don't understand why his view is so important, or relevant at all.

Not @ negru---@ others more prepared to understand: Watch the beginning of the 7 July 2009 talk at CERN where he is being introduced by Luis Alvarez-Gaumé, Director of the Theory Division at CERN.
http://cdsweb.cern.ch/record/1188567/
Listen. It's evident that Alvarez-Gaumé is not just "being nice".

 

[martinbn]

string theaory is not physics but an ultra high level mathematics.
it is a mathematical success. no physics. it is false.

I wouldn't say it is mathematics of any level.

 

[petergreat]

Not @ negru---@ others more prepared to understand: Watch the beginning of the 7 July 2009 talk at CERN where he is being introduced by Luis Alvarez-Gaumé, Director of the Theory Division at CERN.
http://cdsweb.cern.ch/record/1188567/
Listen. It's evident that Alvarez-Gaumé is not just "being nice".

He mispronounced "effective" as "composite". What's he thinking about? It sounds like a deliberate joke.

 

[marcus]

It's almost a reflex: If someone says "fundamental or..." what do you think?
theories are either fundamental or effective
but particles are either fundamental or composite.
It could have been a joke (letting the theorist know he was among experimentalists?)
and if so Weinberg took it in urbane good part and was not ruffled.

Luis A-G: "...he's probably one of the deepest physicists we have in the last hundred years..."

 

[Pythagorean]

Ok, so what I gather from the video and the replies here is, "it's complicated". The history of Venziano is interesting.

I don't see myself getting embedded in the formalism anytime soon, though. From where I'm standing, it appears to be a very complicated subject.

 

[atyy]

It was a NOVA show interviewing scientists. I think they were quantum chromodynamicists.

Perhaps they were talking about string theory as applied to QCD?

There are two "uses" of string theory. One is as a theory of quantum gravity, perhaps not of our universe, but of some universe, which will hopefully yield clues about the correct theory of quantum gravity for our universe. I think this part is pretty uncontroversial.

People have also tried to use strings to talk about QCD via the conjectured gauge/string duality. Although it's a conjecture, gauge/string duality is pretty well supported. The problem is that it's unclear if the sorts of gauge theories include QCD. I don't know what the status of AdS/QCD is at the moment.

 

[unusualname]

Who the hell is negru and why is he allowed to post his silly opinions on this forum?

String Theory is a mathematically mature subject that is struggling for a breakthrough idea to make it all work physically. A big mystery is what happened to Matrix String Theory which sounded so promising but controversially predicts a discrete nonlocal microscopic structure - perhaps that's why it's in hiding - until it's perfected.

Anyway, wait for supersymmetry confirmation or no confirmation over the next couple of years to really stir things up, everyone's a bit stuck in all the QG approaches at the moment.

 

[marcus]

I don't think everyone's stuck in all the QG approaches at the moment. There's been a speedup in progress since 2008.

But I like your concise snapshot of the current situation in String (with special mention of matrix theory). Mathematically mature subject in need of a breakthrough idea to make it physically relevant.

 

[negru]

Who the hell is negru and why is he allowed to post his silly opinions on this forum?

Just some guy who doesn't appreciate the fact that people (especially younger students) might get completely wrong impressions about what is going on in the HET field right now. Every time someone asks some stringy question Marcus will show up, post 20 links to LQG papers, as well as his favorite quote from the random string theory skeptic of the month.

You can't expect students who probably barely know any QFT or whatnot to formulate informed opinions on string theory or LQG, yet they will be immediately swamped with papers, quotes from nobel winers and the Pope himself.

I usually find it appropriate that when someone asks about the current state of a field, you also provide some opinions from the people actually still working in the field, preferably of those involved in developments more recent than 1980, since pretty much nothing is the same as it was back then with strings.

That said, I realize that there are probably fewer pro-string people on this forum who would start spamming papers and quotes left and right to balance Marcus, but my hope is that he will balance himself, at least when talking to non-experts.

I mean, I really wouldn't want someone to read his comments on LQG getting big at Berkeley, and then find himself ridiculed by the actual faculty there.(matrix theory isn't hiding, it just turned out not being good enough to answer what needs being answered)

 

[marcus]

... comments on LQG getting big at Berkeley, and then find himself ridiculed by the actual faculty there.
...

:yawn:
You misrepresent what was said. Nobody said "big at Berkeley." Berkeley is now on the LQG map because of Haggard's research with Littlejohn as physics faculty advisor. Evidently you can do a Phd thesis in LQG here now.

 

[suprised]

What negru says is very reasonable. I can judge it since I work more than 20 years in string theory; and never have seen such a silly biased anti-string propaganda anywhere on the planet! This here turned again into a completely unscientific discussion, a complete waste of time. Thank marcus & Co!

 

[marcus]

Here's the thread referred to:
https://www.physicsforums.com/showthread.php?t=477171

It's kind of interesting that Berkeley physics faculty Littlejohn visited at Rovelli's institute at Marseille in 2008 and 2009, and on his second visit he brought his student Haggard along. The thread notes a paper by Haggard and one of the Marseille group (Gene Bianchi).
In a world where PhD students sometimes have narrowly limited options for their thesis research topic it's nice to see a new option take root at Berkeley. I should think most of us would welcome this development.

It does no good to deny simple facts, as happened when I reported this. See thread post #2 and my reply #4.
https://www.physicsforums.com/showthread.php?p=3164318#post3164318
In my experience, Negru's posts tend to be misleading or unreliable (as here) so Unusualname's question makes sense.

Who the hell is negru and why is he allowed to post his silly opinions on this forum?...

From my point of view Negru is a welcome as comic foil but I don't always want to take the time to reply to him.

 

[unusualname]

@surprised &negru

marcus always seems reasonable to me, and posts many relevant links to refer back to professional work backing up what he says. That this can't be done on the pure String Theory side is telling imho, too abstract too difficult and too out of touch with any other mathematical or physical theory. Negru's comment about Matrix String Theory is an example - please post a link to a paper or discussion explaining where Matrix theory was not "good enough" [to give the answers] (hint: there is none, it just drifted into the overly complicated and never came back to an understandable applicable model)

 

[negru]

@surprised &negru

marcus always seems reasonable to me, and posts many relevant links to refer back to professional work backing up what he says. That this can't be done on the pure String Theory side is telling imho, too abstract too difficult and too out of touch with any other mathematical or physical theory. Negru's comment about Matrix String Theory is an example - please post a link to a paper or discussion explaining where Matrix theory was not "good enough" [to give the answers] (hint: there is none, it just drifted into the overly complicated and never came back to an understandable applicable model)

well I don't think there's any paper saying that, I just said why people sort of realized that although useful, it's just not good enough. what sort of paper do you expect? "here we attempt to use matrix theory but show that we fail miserably because we just can't do the calculations?"

 

[marcus]

... I can judge it since I work more than 20 years in string theory...

My goodness! You have quite a substantial career investment in the String approach, not so?

I try to be balanced and objective in reporting QG developments, which may be easier for me than it would be for you because I have no career investment in any of the different approaches.

 

[negru]

You might not have a career investment Marcus, but you quite clearly have an emotional one.

I was just pointing out that the general atmosphere on this forum is way way different from the one in the usual academic settings, and that as a result people will get a totally wrong impression about the current state and future of research in HEP. Which will (fortunately or unfortunately) remain pretty string oriented for quite some time. Saying or suggesting otherwise is simply not a "balanced" view.

 

[Kevin_Axion]

LQG is incomplete, String Theory is incomplete. Both are reasonable and are making progress, obviously more in LQG then String Theory in the last few years. I like String Theory and LQG but I think the researchers in String Theory need to take a step back and understand the pieces from a distant perspective, there's a lot of ideas being thrown around some of them are a little eccentric and possibly unphysical.

 

[negru]

Both are reasonable

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

 

[marcus]

LQG is incomplete, String Theory is incomplete. Both are reasonable and are making progress, obviously more in LQG then String Theory in the last few years. I like String Theory and LQG but I think the researchers in String Theory need to take a step back and understand the pieces from a distant perspective,..l.

I think that's a fair summary. Thanks Kevin! I want to remind people of the Quantum Theory and Gravitation all-approaches conference this summer at Einstein's alma mater (the ETH Zurich). It is about getting all QG people, non-string and string alike, talking to each other. To me it represents the change that's happening and it's also the most interesting conference planned for this year that I know of.
http://www.conferences.itp.phys.ethz.ch/doku.php?id=qg11:start
I'll list the topics:

Topics to be covered include:

• General quantum theory, relativistic quantum theory, emergence of space(-time)
• General quantum field theory, including deformations of QFTs
• QFT on curved and NC space-times
• Canonical quantum gravity and supergravity
• Regge calculus
• String theory and M-theory
• Loop gravity, spin foam
• Quantum cosmology

 

[smoit]

Just some guy who doesn't appreciate the fact that people (especially younger students) might get completely wrong impressions about what is going on in the HET field right now. Every time someone asks some stringy question Marcus will show up, post 20 links to LQG papers, as well as his favorite quote from the random string theory skeptic of the month.

That said, I realize that there are probably fewer pro-string people on this forum who would start spamming papers and quotes left and right to balance Marcus, but my hope is that he will balance himself, at least when talking to non-experts.

Marcus will never balance himself. His views are as balanced as those of Fox News. I've been watching the "watcher" for a while and it's pretty clear that he's got an agenda to discredit string-oriented research and promote LQG and related stuff. He's been spamming this forum with links to various LQG and related papers and has been hyping that area of research for years. I'm glad that many professional high energy physicists on this forum see through his "Mr. nice guy" facade.

 

[Kevin_Axion]

Marcus will never balance himself. His views are as balanced as those of Fox News. I've been watching the "watcher" for a while and it's pretty clear that he's got an agenda to discredit string-oriented research and promote LQG and related stuff. He's been spamming this forum with links to various LQG and related papers and has been hyping that area of research for years. I'm glad that many professional high energy physicists on this forum see through his "Mr. nice guy" facade.

It's science, let people believe what they want to believe. In fact, your perspective is itself fallacious for you feel to much attention is being brought to LQG on this forum and more to String Theory. Marcus feels the opposite and has made an effort, a damn good one. If you want the roles reversed do something about it.

 

[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?"