# Experimentalists 1, Theorists 0

At Fermilab the Tevatron is producing record amounts of luminosity, see here for a story about a celebration of this. Things also appear to be going well at the LHC, as the cooldown remains on schedule, and only a tolerable number (12) of PIMs needed to be replaced in the sector recently warmed back up. See here and here for some discussion of current planning for the next year. The machine should be cool and ready for beam commissioning in late June, and if all goes well, by September an initial physics run with 5 TeV beams at relatively low luminosity may begin. At these luminosities and energies, the stored energy in the LHC beam will be no greater than at the Tevatron (although the important number for physics, the per-particle collision energy, will be 5 times higher). The plan is to run until December, with a heavy-ion run at the end, then shutdown until April 2009. During the shutdown the magnets will be trained, allowing beams at the full energy of 7 TeV during the 2009 run.

Particle theory, especially string theory, is not doing as well. Data recently compiled about top-cited particle physics papers from 2007 shows only one [note added: should specify string theory here, at 27 and 31 are phenomenology papers from 2002 and 2000] theory paper from this century making the list of 51 most heavily cited papers, and that was the KKLT paper which is referenced by all “landscape” and “multiverse” studies. The sad state of string theory has even made it deep into the popular consciousness. Last week’s episode of “The Big Bang” featured a brilliant young prodigy explaining to the particle theorist character that his work on string theory was a “dead-end”, due to the landscape problem. Even economists are dissing the subject:

Modern financial theory as applied ranks with string theory in physics as one of the greatest intellectual frauds of our time. Whereas the vacuous pretensions of string theory have finally been exposed (we now know that the theory never generated a single falsifiable prediction), those of “financial engineering” are just beginning to be exposed both in the press and in lawsuits alike.

At Santa Barbara, Jennifer Ouellette reports on a workshop about “how to come off better during TV appearances”:

Joe Polchinski (inventor of D branes in string theory, and one of the few permanent members at KITP) also agreed to be mock-interviewed, revealing a sly sense of humor in the process. For instance, asked if there was any controversy about string theory, he deadpanned, “Oh no. Everybody agrees that string theory is correct.” It cracked up the room.

This workshop unfortunately didn’t seem to include the advice to just say no when asked by TV producers to participate in a short stupid comedy skit making fun of science and scientists. See here, here and here, for reports on Wednesday’s “Root of All Evil” show from Comedy Central, which featured a mercifully short segment making fun of scientists as incomprehensible geeks. Participating in things like this does about as much to help the image of science and scientists as appearing on a Spike TV segment about the use of physics to determine whether women can crush beer cans with their breasts.

Given that things are going very well with the LHC, and badly with string theory, string theorists are doing the logical thing: advertising their activities with graphics of strings superimposed on a picture of the LHC. See here and here.

Update: Minutes from the LHC Installation and Commissioning Committee April 11 meeting are here. They include the exchange:

L.Evans asked if the cryogenics teams are still on track for having the whole machine at operating temperature in mid-June. S.Claudet replied that taking the figure of 6 weeks from room temperature to 2K, and allowing 2 weeks of cryo tuning, sector 45 would be ready for hardware commissioning in the first half of July.

This indicates that beam commissioning is likely to begin in July, not June.

Also discussed was what to do about possible stray plastic parts in the beam tube:

Any pieces of plastic would be vaporised by the beam so we should not delay start-up to search for these.

Update: Commentary on this posting from Lubos here, including

I am amazed by the people who deliberately keep on opening the pile of manure called Not Even Wrong – it must be due to a really nasty deviation of theirs that dwarves pedophilia.

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### 140 Responses to Experimentalists 1, Theorists 0

1. Peter Woit says:

dan,

The problem with the idea that the LHC will provide experimental backing for string theory is that string theory doesn’t predict anything at all about what the LHC will see.

2. Coin says:

However, at this point I don’t want to say more for fear of being accused of self-promotion.

Erm, we don’t know who you are so we wouldn’t have known it was self-promotion unless you’d said that, now would we have?

nbutsomebody, you can find models purporting to be of the exact type you’re requesting by typing variations on “mssm string” into Google. Here’s one, title “The Exact MSSM Spectrum from String Theory”, abstract starts with “We show the existence of realistic vacua in string theory whose observable sector has exactly the matter content of the MSSM…”. I am not qualified to evaluate whether these models deliver on their claims.

3. dan says:

Hello PW,
I guess it’s an issue of semantics

you are obviously right that some string theorists from Michio Kaku and Lubos Motl are betting that string theory implies LHC-accessible SUSY partners to show up, with others like Eva Silverstein saying it won’t show up (but still working on string theory nonetheless)

so string theory as you say doesn’t make a prediction about whether SUSY shows up in the LHC,

but as far as prediction goes
1- all susy string theoris predict SUSY. it does not offer any details about the nature of SUSY that can be predicted a priori. So while string theory cannot predict whether SUSY will show up in LHC, or any of its values, if SUSY does show up at LHC, it would be evidence consistent with susy-string theory.

The semantic issue is this: string theorists say some models of string theory predicts susy at LHC energies, the LHC finds susy, a prediction is verified. If LHC does not find SUSY, some classes of string theory are falsified. So string theory does make predictions (or some classes do)

If LHC were to see SUSY, string theorists will continue its dominance of HEP and QG for decades on end. I do wonder what will happen if LHC does not find SUSY — Eva Silverstein clearly feels this is likely.

If Eva Silverstein is right and that string theory predicts SUSY will not be seen at LHC energies, but the higgs is, what then stabilizes it from quantum corrections, if it is not SUSY?

I take it from what you described that we’re looking at 2010 for quality large data sets to come out to show higgs and other exotics from LHC?

4. A.J. says:

If Eva Silverstein is right and that string theory predicts SUSY will not be seen at LHC energies, but the higgs is, what then stabilizes it from quantum corrections, if it is not SUSY?

Fine tuning?

5. nbutsomebody says:

monkey,
It does not matter who is the author. A objective truth is a scientific progress, and there is no self-promotion involved. I just like to learn. If you are an honest scientist, please give the arxiv number. Well, I am not an expert in compactification, but I would very much like to discuss the work with somebody who knows more.

6. chethan krishnan says:

> “…Besides, we are all clutching for strINGS here!…”
>
> You had a typo. I’ve fixed it for you.

Very cute. Now run along and bother mommy.

To Peter:
I wasn’t implying that YOU are a bitter person. I meant that I came here expecting disagreement with string theory, but what I also find is a lot of bitter hostility towards string theorists. The former of course I might debate with, but the latter seems a little …

In any event, about the scientific issue: I think it is fair to take a moderate stand about string theory, but I simply disagree with your implication that string theory is almost provably useless in our quest for quantum gravity.

7. nbutsomebody says:

Coin,

Many thanks. I am not also qualified enough to judge these models, but it seems they do not talk much about moduli stabilization and particle mass and cosmological constant. Getting the gauge group is not enough, one has to get the Higgs and/or sponteneous symmetry breaking. ( That is a tough job considering our inability calculate particle masses.)
Unfortunately just now I do not have access to guys who work on string compactification.
I really like to interview somebody like Kachru, Douglus etc., and put up a faq on what is known and not known about landscape and compactification.

8. nbutsomebody says:

chetan wrote,
“…but what I also find is a lot of bitter hostility towards string theorists.”
It is true that some people are to some extent hostile to string theorists. Even considering the fact that the string theorists have a rare trait of perceiving hostility when people are simply differing in their opinion, there are still some who really are hostile. The reason is simple. People are pissed off by the extreme stupidity and stubbornness of seemingly intelligent string advocates.

When a bunch of religious fundamentalists make bogus claims, people may forgive and overlook them as mere idiots. When a bunch of iv-league faculties does the same, people kick b*tt.

Peter, extremely sorry for being rude…..

9. Duncan MacGregor says:

So the LHC can vaporise plastic but (remembering an old news story about the LEP) how would it do against beer bottles?

10. Peter Woit says:

chethan,

My claims that string theory has failed has to do with the idea of using it to do particle physics. If you want to use it purely to do quantum gravity, the problem is that it’s unclear what “success” would actually look like. You’re not going to get anything you can test experimentally, and thus get stuck in the situation an earlier commenter referred to of near religious warfare between competing “quantum gravities”, with no good way of distinguishing which is “better” than the other.

As for the hostility towards string theory, some of it is, well, hostile people, or those who are hostile to things they don’t understand. But a great deal of it is coming from the behavior of string theorists. The amount of unprofessional behavior, conviction that anyone who disagrees with you is an idiot, unwillingness to admit a mistake or that things are going badly, etc. that has been on display over the last few years coming from the string theory community has been shocking.

11. PN says:

>Particle theory, especially string theory, is not doing as well. Data recently >compiled about top-cited particle physics papers from 2007 shows only one >theory paper from this century making the list of 51 most heavily cited papers, and >that was the KKLT paper which is referenced by all “landscape” and “multiverse” >studies.

Hmm… Particle phenomenology papers (#27 and 31) are not counted to be THEORY anymore?

12. nbutsomebody says:

Peter said,
“the problem is that it’s unclear what “ success” would actually look like. You’re not going to get anything you can test experimentally, and thus get stuck in the situation an earlier commenter referred to of near religious warfare between competing “quantum gravities”, with no good way of distinguishing which is “better” than the other.”

The comment is not entirely correct in my opinion. A quantum theory of gravity should have the following properties, which put strong restrictions of them. At the primary
level this conditions can be used to distinguishing which is “better” than the other.

1) As a quantum theory of GRAVITY, it should at least reproduce general relativity at low energy.

2) It should have a meaning full perturbative expansion.

3) It should reproduce the formula of black hole entropy calculated by Hawking in 1972.

4) It should say something about what happens near the initial singularities like big bang.

Once more than one theories at least satisfy first three of the four criterion, then only one has the luxary of a “religious warfare” between competing “quantum gravities”. As far as I know String theories are the only one to fullfill (partially) these criterion ,

1) What we get from a naive low energy limit of String theory is Dilaton gravity in 10D. It is not exactly GR in 4 dim but it is certainly GR+other fields in 10dim.
It is possible that there are compactifications exists for string theory which gives 4dim GR. That will bring us to the controversial arena of landscape.

2) It seems that a finite perturbation series exists, but yet to be demonstrated.

3) Using AdS/CFT black hole entropy has been successfully calculated in various cases. Even for some non-supersymmetric black holes.

4) There are some preliminary result.

At least it seems to me that string theory is partially successful as a MODEL theory of quantum gravity. There may be other theories which are equally consistent and it may be possible that in time experiment will prove some of them to be correct.

13. DB says:

The blurb for the upcoming Kalvi conference “Anticipating physics at the LHC ” is remarkably free of explicit string theory tie-in or the usual hype, considering the number of string theorists slated to appear:

http://www.kitp.ucsb.edu/activities/auto/?id=916

David Gross will open the conference.

Is this (along with Strings 2008 at CERN, and Witten spending next year on sabattical at CERN) part of a strategy to try to bring string theorists into closer contact with their experimental colleagues?

While it’s difficult to see how string theory can effectively interface with the LHC, bringing both communities together seems like a good idea.

14. Peter Woit says:

PN,

Oops, sorry I missed those two papers. Still, the latest of these is from more than 6 years ago…

DB,

I don’t actually see many string theorists listed as talking at that conference. While paying attention to what experimentalists are doing should be encouraged among all theorists, I don’t think that string theorists claiming to be “string phenomenologists” is such a great idea, although it may work for them as a career move. Witten is an interesting case. I believe he’s intensely interested in what is going on experimentally, and that’s one motivation to visit CERN, but his own research remains more mathematical, presumably since that’s where he sees a way to make progress absent new hints from experiment.

nbutsomebody,

Surely you’ve seen a similar list from Lee Smolin about the state of quantum gravity, in which LQG comes off better than string theory. I don’t want to get into that argument. I’ll just point out that this shows there is an obvious problem with such lists and the attached claims about how one’s favorite theory is doing better than someone else’s. One common problem with claims about string theory is that they neglect to mention that they’ve solved a problem in the wrong number of dimensions.

15. nbutsomebody says:

Peter,
Thanks.

“Surely you’ve seen a similar list from Lee Smolin about the state of quantum gravity, in which LQG comes off better than string theory.”

Sorry I do not, but will try to find the reference,,,

“One common problem with claims about string theory is that they neglect to mention that they’ve solved a problem in the wrong number of dimensions.”

Certainly , that is what I mentioned too.

16. Peter Woit says:

The Smolin article I was thinking of is:

http://arxiv.org/abs/hep-th/0303185

17. high school physics teacher says:

Might I offer into this discussion of Bohr’s contribution my own first hand account of Dirac’s position on the issue? In the 1982 conference at Loyola New Orleans, on the occasion of his 80th birthday, Dirac was asked from the audience if it was possible for him to name one name whose contribution to the development of QT was most important. Dirac’s answer, “Bohr.”

18. Chris Oakley says:

nbutsomebody,

The only property that a quantum theory of gravity must have is your #1, i.e. that it must reproduce GR in the classical limit. The rest … maybe … maybe not.

(That’s why I never got interested in the subject).

19. sanity says:

Quantum field theory as a whole makes no
phenomenological predictions,
until you specify the content and couplings of the theory.

String models are in an early stage, but have already
led to predictivity (in the same sense as QFT, namely
after specifying a controlled and stabilized corner of string
theory with specific field content) in inflation, a subject which
is sensitive to quantum gravity effects and requires
“top town” constructions. Upcoming data will
falsify a subset of the known mechanisms.

Although there is a large landscape, there are many criteria
that must be satisfied by a theoretically and observationally
viable model, and it is not yet known which of
these effects wins and whether string
theory can or cannot be as predictive as QFT, correlating
signatures with features of the internal dimensions. This will
simply take more time to determine, and there will be
important interplay between experiment (LHC and CMB) and
theory. The reason no one makes a prediction one way
or the other regarding low energy SUSY (the comments
of Silverstein and others that you quote
were essentially vacuous on this point, not
committing to a prediction pro or con) is that there is no
such sweeping prediction to make and they are trying to be
conservative.

20. Peter Woit says:

“sanity”,

I suppose I really should get around to writing that FAQ, including the answer I’ve given here many times to the claim that “string theory is just as predictive as QFT” (the short version is that what is predictive is gauge theory, it is predictive because the simplest gauge theories agree with a huge number of different observations, make huge numbers of testable and verified predictions. In string theory on the other hand, simple backgrounds disagree with experiment, so people end up choosing more and more complicated backgrounds until they can evade confrontation with experiment. Sure, you could make QFT unpredictive my constructing more and more complicated and ugly QFTs designed to not actually say anything about physics you can observe. But you don’t do this. You do do it in string theory).

One reason I haven’t gotten around to this is that I have a hard time believing that anyone is making this argument seriously. On the one hand you have the most successful scientific theory ever developed, one that makes an infinity of highly specific and testable predictions, thousands of which have been verified to high precision. On the other hand you have a theory that predicts absolutely nothing, is completely untestable and designed to remain so indefinitely. You want to argue these are “the same”. Why should I bother to take the time to answer such an argument? It’s obvious sophistry on its face.

And your claims that string theories make “predictions” about inflation are every bit as much nonsense as claims that they make “predictions” about particle physics at LHC energies.

21. Peter Woit says:

“sanity”

Oh, and doing this anonymously doesn’t help. It indicates that you have so little faith in your own arguments that you are not willing to put your name to them.

22. nbutsomebody says:

Chris Oakley,
I agree to you partially.

No. 2) of my point is what is meant by a quantum theory of gravity. However I do agree that even if there is no perturbative expansion the theory may be defined non-perturbatively. But one has to conclusively answer such question in his/her theory of quantum gravity.

No 3) Yeah, There is a possible that the black hole entropy calculated by Quantum theory of gravity may not match with Hawkings result. But one need such a calculation in ones theory.

23. nbutsomebody says:

Peter,

oh thanks!, I read the article when it came out. Anyway I do not agree with it completely. Primarily for the reason I do not think GR at low energy has not come out of LQG yet. Plese see http://arxiv.org/abs/hep-th/0501114.

Also the AdS/CFT is making steady progress and people are now even calculating entropy of Non-SUSY black holes. That does NOT prove the correctness of string theory in four space time dimension, but shows it to be a consistent quantization of some extension of GR .

24. anon. says:

John March-Russell’s answer in that Telegraph article — new particles with technological benefits for energy production — is the weirdest LHC prediction I’ve ever seen. (OK, maybe not weirder than Nielsen/Ninomiya’s….)

25. chris says:

>Oh, and doing this anonymously doesn’t help. It indicates that you have so >little faith in your own arguments that you are not willing to put your name to >them.

no, it probably just means he doesn’t have tenure. it’s a sad world.

26. Christine says:

Indeed, the Bohr model is in fact not correct. Rather, the quantum theory itself was pieced together bit by bit using these experimental data as guidance.

Just one more observation concerning Bohr, if I may.

“Two important heuristic principles have guided quantum physicists during the period 1913-1925, viz. Ehrenfest’s Adiabatic Hypothesis [term due to Einstein] and Bohr’s Principle of Correspondence”. (…) “The research work during the years 1919-1925 that finally led to Quantum Mechanics may be described by systematic guessing, guided by the Principle of Correspondence”. B. L. van der Waerden (Sources of Quantum Mechanics).

Bohr’s contribution was fundamental from that alone, in the case one has some reservations towards statements of the sort: “the synthesis of the Rutherford’s atom model with Planck’s quantum hypothesis was [his] great achievement” (ibid).

One can also consider Bohr’s [and Kramers’] ideas on the statistical conservation of energy and momentum and the statistical independence of the processes of emission and absorption in distant atoms (described in the paper by Bohr, Kramers and Slater), which turned out to be wrong (as already proved by experiments by Geiger and Bothe in 1924; Slater, the third author, was actually against those ideas and his contribution to that paper was more on the idea of ‘virtual radiation field’). Those concepts were sources of great debates, I mean, productive debates in physics, and were tightly connected with what one could test experimentally.

We are at different times.

Christine

27. Peter Shor says:

As far as I can tell, string theory as a model of quantum gravity has so far not produced a complete resolution to the black hole information paradox: that is, nobody has explained exactly how information gets out of a black hole convincingly enough to be widely believed. What is more disturbing, some string theorists say they consider this problem to be “solved.”

28. David B. says:

Dear Peter (Shor):

The most conservative claim in string theory is that there are examples, like in the AdS/CFT correspondence, where the complete non-perturbative definition of quantum gravity (realized as a string theory) on some spacetime is equivalent as a quantum system to a dual unitary quantum field theory without gravity on the asymptotic boundary of the spacetime. The claim is then that unitarity
is preserved in the full dual theory and therefore the information gets out of the black hole, somehow. In these examples the information presumably gets so scrambled by interactions that it becomes a complicated dynamical problem to see how much information one can extract about the initial state from a few obersvations after a small black hole has evaporated. This problem is not too different from burning a piece of paper with some writing on it, and reconstructing the writing from the smoke that one collects afterwards.

In the end, no one has a proof for the general case. But in essence the problem is solved in examples in the same sense that an abstract proof of existence of some mathematical object gives you very litttle information as to what the object actually can look like: it is a non-constructive proof that information gets out. I think everyone agrees that it would be a lot nicer if one could understand the same problem by solving the dynamics.

29. Coin says:

This indicates that beam commissioning is likely to begin in July, not June.

If this is true then when are we looking at for first collisions?

30. Peter Woit says:

Coin,

Their nominal beam commissioning plan refers to taking 30 days of work on the beam to get ready for physics, with likely down-time meaning it would take on average two calendar days for each day of work. So, this puts them with a physics run starting in September. Maybe it will go more quickly, maybe they’ll run into problems, we’ll see.

31. nbutsomebody says:

David B. and Peter Shor,
It is true that AdS/CFT provided a way to understand information paradox, but we certainly need to have a better understanding of the information paradox in the gravity itself. Like what is really happening in a AdS black hole?. Gauge theory should guide us in this manner. But the whole picture is not yet clear.

32. Moshe says:

Peter Shor, in addition to what David is saying, let me ask you a question, which I ask everyone skeptical of claims regarding the information paradox. Suppose we accept for the moment the picture of black hole evaporation suggested by AdS/CFT. In that picture the process is no more mysterious, but no less complicated, than any detailed dynamical process in strongly coupled many-body system. Under these circumstances, what kind of result or argument would you consider to be a satisfactory solution to the information paradox?

33. oohay says:

Moshe, Prof Shor’s observation has two parts. Your point addresses the first part very well indeed; but it only makes his second point all the stronger:

“What is more disturbing, some string theorists say they consider this problem to be “solved.””

The point is that the AdS/CFT resolution of the problem relies completely on the structure of AdS, which is an utterly unrealistic background. In fact, it would be quite reasonable to guess that, since the asymptotic structure of the *real* world is nothing like that of AdS, string theory probably *does not* solve the information problem in the real world! That is, the dependence of the string-theoretic explanation on the specific structure of AdS is both a plus and a minus. Now actually I myself don’t believe that this guess is right. A fair assessment is that there are promising signs but that we are still far, far, far away from solving the problem for black holes on realistic backgrounds. [Of course work has been done, but the problem with it is precisely that it does not rely on AdS/CFT !]

34. Aaron Bergman says:

It all depends on what question you are asking. If the question is, does black hole evaporation violate unitarity, string theory tells you that the answer is no. Ergo, no paradox. However, if you ask exactly where the semi-classical intuition breaks down and where the information is encoded, then string theory has not yet produced a definite answer. However, there do exist proposals (note 2 links).

35. Moshe says:

I was not making a point, I was asking a question. I think a person with experience dealing with quantum information may come up with an answer to my question I may not anticipate as a high energy theorist, so I thought I’d ask…

36. nbutsomebody says:

oohay,
Questions like black hole entropy and Hawking radiation is probably a local property of the black hole and does not in essence depend on the asymptotic geometry. I may be mistaken but I would suggest that you consult Hawking’s original papers and recent reviews.

Aaron Bergman,
Exactly!

37. Dan says:

PW,

Quick question — based on what you’ve read and your experience, when do you think LHC will be ready to start collecting quality data, which should shed light on EW breaking, SUSY, condensates etc.

thanks
Regards,
Dan

38. Peter Woit says:

Dan,

http://online.kitp.ucsb.edu/online/lhc08/huston/

My understanding is that the easiest thing to see is colored superpartners, since they are strongly interacting, and if these exist within the LHC reach, they should be found quickly, even in data from the hoped-for 2008 first physics run. As an uninformed guess, if there is significant data in 2008, sometime in 2009 the experiments would be ready to announce such a discovery.

I hear the Higgs is much harder, requiring a lot of luminosity and good understanding of backgrounds. If it’s there and the LHC works well in 2009, maybe there would be a discovery announcement in 2010.

If there is no Higgs, but something else is going on, unless there’s a dramatic signal, I’d also guess it wouldn’t be until at least 2010 before something was announced.

But I’m no expert on this, would prefer to hear from one…

39. piscator says:

I’m no expert either, but I have heard it said that the missing energy trigger, which is crucial in many of the standard susy searches (as to cut the SM background you trigger on the large missing E_T carried off by the stable LSPs), will take a long time (maybe a year) to commission. This is because at first lots of detector bugs, such as a malfunctioning calorimeter, will look like missing E_T.

I recall being in a talk where someone gave the timescale from when the Tevatron run II started to when they first published data using a missing E_T trigger. I forget the exact timescale but seem to recall it as being 18-24 months.

So even if susy is present, and even if it is light, and even if it is on the SPS point 1A slope, it doesn’t mean that it will be discovered through the first three months of data taking. And of course: LHC discovers new coloured stuff’ is not the same as LHC discovers susy’.

40. Peter Shor says:

Aaron – many thanks for the excellent clarification.

Moshe – if you take an ordinary dynamical process (burning an encyclopedia, say), there is no discrepancy depending on which reference frame you are in. If you consider the viewpoint somebody falling into a black hole, what they experience (nothing, when they pass the horizon) seems at odds with what is experienced by an observer outside the black hole (the information contained in the object falling in is somehow transferred to the Hawking radiation that is exiting). There are a number fo proposals about how to reconcile these viewpoints, but (a) they seem contradictory and (b) as far as I can tell, the community hasn’t really accepted any of them.

41. chethan krishnan says:

Dear Peter Shor,

I am not very sure how the community views this, but it seems pretty reasonable so I think it is not too controversial.

Chethan.

42. Moshe says:

Hi Peter, as Aaron says it depends what you call the information paradox, and I thought “breakdown of predictability in gravitational collapse” pretty much sums it up. In that case AdS/CFT suggests there is no such breakdown for an outside observer, though maybe it is interesting to make the case (that the information is fully contained in the Hawking radiation) more explicit.

The view of an in-falling observer is indeed much less clear, but I am wondering what is the precise problem implicit in “discrepancy” (beyond just seeking a convenient mental image, aka a classical limit). In any event, I am not sure any such issues are conventionally referred to as the information paradox. Maybe I’m wrong, it’s just semantics anyhow.

(As a side comment, Mathur’s proposals are of course fascinating, but in my mind have not reached the point of being uncontroversial).

43. Peter Shor says:

The question is: how does the information get transfered from the infalling object to the Hawking radiation when, in the semiclassical view, in the infalling objects’ reference frame, it never actually interacts with the outgoing Hawking radiation? Obviously, if string theory is a consistent theory of quantum gravity, something must be wrong with the semiclassical view (or else something much more mysterious is going on, which is what Leonard Susskind seems to believe). But exactly what?

44. Moshe says:

Thanks Peter. Yeah, that is a good question, wish I knew the answer…

One thing about observers and black holes which has bothered me for a long time is this. Hawking says that area and entropy are the same. However, area depends continuously on the observer since it undergoes length contraction, whereas entropy is the discrete number of state which presumably is the same from every observer’s point of view. So, in which reference frame is the black hole’s area equal to its entropy?

46. nbutsomebody says:

Good question. Continuity of area may really be due to semiclassical approximation and in a quantum theory of gravity area may be discrete. That is what one gets from LQG.

There are other issues also. e.g. entropy is a discrete quantity only when one using micro-canonical ensemble to calculate it, whereas an entropy calculated from a canonical ensemble is a continuous quantity. They agree for a large system but may differ for smaller ones. Hence it is a question of what ensemble one should use to get a matching with the area law. This is not really settled yet. However it does not matter for a large black hole anyway.

47. Moshe says:

Thomas: stationary observer far away from the black hole sees the entropy proportional to the black hole horizon’s area. Both quantities transform under the asymptotic Lorentz transformations.

The entropy is a logarithm of an integer, and the area is the leading approximation to that entropy for large black holes. Subleading terms in this asymptotic expansion (in black hole inverse mass) are known geometric quantities (due to work of Wald and collaborators). In string theory this expansion is calculated in various cases, where comparison to exact formulas gives precise agreement. The subject is pretty recent and goes under the name of precision counting.

48. Sidious Lord says:

Thomas and Moshe: I don’t think the area of the horizon transforms under asymptotic Lorentz transformations and under diffeomorphisms. The entropy does not transform either, so I don’t think there’s any issue of observer dependence here.

49. Haelfix says:

Assume that quantum gravity in reality is governed by a renormalizable Yang Mills theory. Lets say someone stumbled upon it by chance and showed a correct GR limit and so forth with only minor, unobservable corrections (so not quite Einstein-Hilbert but something close). This is at first glance, a perfectly plausible possibility (forget for a moment that such a thing probably does not exist).

The situation would be no different for predictivity, since you would still need to input some couplings into the theory, taken from experiment. So we would never know if it was or was not true, but just that it was minimal, nice and made good sense.

The only difference between that hypothetical and where ST is currently is that physicists already know the mathematics of YM is correct and that it has in the past described real natural processes.

But, if string theory can be independantly shown to be correct in different contexts (implying that the math is at least consistent), I fail to see the problem. Take AdS/QCD as a motivating example.

Point being, the QFT analogy is pretty good.

50. anon. says:

Haelfix, a Yang-Mills quantum gravity theory which ‘showed a correct GR limit and so forth with only minor, unobservable corrections’ might have similar lack of falsifiable predictions to string theory. I don’t see the point of the analogy.

The critics of ‘not even wrong’ ideas aren’t irrationally singling out string theory for criticism just because it is called string theory, but because it’s become a massive public relations exercise that eclipses other ideas which contain fewer unobserved extra dimensions.

For you to invent a failure of an imaginary Yang-Mills quantum gravity to make falsifiable predictions, and to imply that this makes criticisms of string theory bogus because the failure of string theory is no more serious than the failure of the imaginary QFT you wish to use for comparison, seems specious.

It just seems a lame excuse for string theory to say that certain other theories may be failures too when it comes to experimental verification. (Otherwise, the courts would need to excuse people of misdemeanours which have also been committed by other people, on the basis that only unique crimes deserve punishment!)