Questions About String Theory: Get Answers Here

[marcus]

To be perfectly fair, String theory developed a lot since Feynman's death. They are able to write down models that can predict masses and so forth.
The problem is they have way too many models, very few have clean profiles without exotics (and then there are usually residual problems), and they haven't been able to constrain phenomonology like it was once hoped for from first principles in the theory, there are just too many possible scenarios.
...
...
Now, its not just string theory that has this problem, field theory in general is a mess at the moment. There are literally thousands of models that nature could plausibly pick out, and its a hard task classifying the various ideas, much less distinguishing them experimentally or finding a mathematical flaw. Thats why they call it the LHC lottery game.
Personally I am getting rather pessimistic ...

Juan I often enjoy your posts a lot, but you quoted that one sentence from Haelfix out of context. Out of context it sounded like a defense, which you then answered in polemical style. But actually I don't think the polemic adds much to what he said. Haelfix was not writing in defense of string concepts, instead he was trying to give a balanced realistic broad perspective, from a physics graduate student on the ground. Anyway that's how I see it. Instead of arguing against what he says, maybe we can benefit more by listening carefully.

 

[marcus]

Hey, I wanted to ask you guys a few questions about string theory.
(0) Why study string theory?
-----
(1) Is string theory (still) an active area in physics? Have there been any major advances in the field since the "revolutions" of 1984 and 1995 (dualities)?
...
...
(4) How many string theorists are there in the U.S? World?
Thanks for any help!

just a reminder of the orig. post's questions
question (0) was recently addressed at the Strings '05 conference panel discussion on the future of the field. I gave a link to the video earlier and pointed to some relevant time segments. Apparently it is something string theorists wonder about and try to answer. Andy Strominger had some frank and courageous words on the subject.

About questions (1) and (4), you can get some idea by doing a simple search at the Harvard abstracts: adsabs.harvard.edu
I checked recently how many papers were published each year (October thru September) with the words "brane" or "M-theory" or "AdS/CFT" in the abstract.

2001   1067
2002    967
2003    868
2004    846

That is, according to the Harvard service search engine between October 1, 2004 and September 30, 2005 the number of papers published was 846. This is not the whole spectrum but it is a good representative chunk. The trouble is if you put in keyword "string" you get papers on protein sequencing, cryptography, ASCII character strings, braille readers, polypeptide chains, and so on.

It is very hard to say how many string thinkers there are who are really active---either in US or in world. I suppose roughly on the order of 1000. Could even be 2000, but that estimate is very rough and someone else may have a better one.

The Strings '05 conference in July in Toronto was a big important conference and it drew between 400 and 500 (if I remember right). You could check for yourself at the website if you are interested.

An active researcher typically authors or co-authors at least one (probably more often several) papers a year. So if you are seeing 1000 papers per year being published then it is not wildly unreasonable to estimate that there are roughly on the order of 1000 people working in the field.

For comparison sake, about 150 people participated in this weeks Loops '05 conference and an arxiv.org keyword search shows roughly 150-some papers per year in that field (maybe as many as 180 but keyword searches are not precise). This includes all nonperturbative background independent quantum gravity (essentially non-string QG) worldwide, to the extent that I can get a figure for it.

I will get some conference links so anyone who is interested can check the number of participants---they usually have a list. BTW you might like the graphics at the Strings '05 website!
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/
I see that Strings '06 will be in BEIJING! Beijing Normal has some people who recently got into Loop gravity and gave presentations at Loops '05. I wonder if the Chinese will be clever enough to include some LQG in their program (as Loops '05 included talks by string people Dijkgraaf, Bahns, Thiesen) good practice.

Here is Strings '05 program and list of participants
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/program.html
http://www.fields.utoronto.ca/programs/scientific/04-05/string-theory/strings2005/participants.html
I would estimate 450 participants, but could be wrong. Does anyone want to count?

 

[selfAdjoint]

Marcus did you try "superstring" on the Harvzrd search? Also "heterotic" would find some papers; not everybody is doing branes or Ads/CFT, almost every day on hep-th you see papers on the older areas of string theory.

 

[marcus]

I'll be glad to try. The main thing is just to have a fixed set of keywords you can apply year by year, to get the trend in papers with those keywords.
In accordance with your suggestion, I checked how many papers were published each year (October thru September) with the words "brane" or "M-theory" or "AdS/CFT" or "superstring" or "heterotic" in the abstract.

2001   1202
2002   1097
2003    970
2004    959

 

[marcus]

one has to be cautious about interpreting simple keyword searches like this:
they can miss stuff you might like it to get, and pick up bogus stuff (a paper in some other field that mentions a string term)

the main thing I notice here is not the absolute numbers but the rough size of the trend, regardless of what criteria or keywords are used. With either set of keywords, the 2004 output looks roughly like 80 percent of the 2001 output, for whatever reason.

one way it's 959/1202, which is about 80 percent
another way it's 846/1067, which is also about 80 percent.

depending on how you look at it, it's not a big decline----you might say it's holding approximately steady----but on the other hand it is not a marked increase either.

 

[selfAdjoint]

Given the publicity attached to the landscape problem, it will be interesting to see what the next year figures are. In a normal field you have a steady changing of the guard, where the numbers would be sensitive to new people coming in with a flurry of papers, while older contributers slow down. But physics does not seem to be like mathematics, "over at 35", older people like Wilczek still do things, and I well remember the late Hans Bethe, still involved with neutrino oscillations at 90.

 

[Juan R.]

Juan I often enjoy your posts a lot, but you quoted that one sentence from Haelfix out of context. Out of context it sounded like a defense, which you then answered in polemical style. But actually I don't think the polemic adds much to what he said. Haelfix was not writing in defense of string concepts, instead he was trying to give a balanced realistic broad perspective, from a physics graduate student on the ground. Anyway that's how I see it. Instead of arguing against what he says, maybe we can benefit more by listening carefully.

Hi Marcus, perhaps you are right -i read again i said and was puzzling- but let me explain better my point before obtaining conclusions.

I split Haelfix comeent into two parts

To be perfectly fair, String theory developed a lot since Feynman's death. They are able to write down models that can predict masses and so forth.

I think that Haelfix was attempting to claim that now string theory is predictive and therefore initial Feynman's claims would be not applicable. As already explained that is not true.

String theory predicts absolutely nothing. String theorists are very 'carefull' here and often they talk in public of string theory predicts BH entropy, string theory predicts the number of families, string theory predicts gravity, etc.

'Predict' in string theory literature means

know the result you may obtain first and then modifies ad hoc string theory or introduce by hand the needed adjusted parameter for that string theory was compatible with that result

Next Haelfix adds

The problem is they have way too many models, very few have clean profiles without exotics (and then there are usually residual problems), and they haven't been able to constrain phenomonology like it was once hoped for from first principles in the theory, there are just too many possible scenarios.
...
...
Now, its not just string theory that has this problem, field theory in general is a mess at the moment. There are literally thousands of models that nature could plausibly pick out, and its a hard task classifying the various ideas, much less distinguishing them experimentally or finding a mathematical flaw. Thats why they call it the LHC lottery game.
Personally I am getting rather pessimistic ...

This part is exclusively related to Landscape, which is not the only i discussed in above post. I also discussed Landscape but i was not only emphasizing that. The fact that string theory cannot predict is not only related to Landscape.

That is, one thing is that string theory cannot predict/explain things we know. Other thing is that there actually many models of string theory. Are very related but are not the same!

The problem of Landscape is basically, and in plain language, that we cannot select correct vacuum and, therefore, there are many different string theories like vacuum choosed. Initially it was thought that Landscape was finite and one could find the correct vacua characterizing our universe. After was thought that via statistical methods one could find some correct or quasi-correct vacua. Last works suggest that possibilities are infinite and therefore none statistical method will be help. Then some people (half the panel) suggest that string theory cannot select the correct vacua and therefore we cannot predict from first principles. In short, using language of quantum chemists:

string theory will be not ab initio, will be a semiempirical theory

Now the problem string theory cannot predict/explain things we know goes beyond Landscape trouble.

Imagine that Landscape is solved and someone find the correct vacua, fix parameters of string models and obtain a single string theory (forget now background problems and so). Would be string theory able to predict masses and particles of the standard models?

No!

Because ONLY exact supersymmetric masless states are permitted in the current theory. There is not mechanism for obtaining masses of real particles like electrons from ST (unless that one forces the model previously, but this is not related to Landscape and its 'many models'). There is not mechanism for breaking the exact supersimmetry required by the theory for avoding tachionic behavior in none of models. Particles we know have not superpartners and even if one finds superpartners in the HLC, still string theory would explain why do not arise at low energies doing the standard model non-supersimmetric.

I wait now to have expressed my view better i did. It was no my aim to present a distorted view of Haelfix's claims!

 

[Juan R.]

Perhaps this recent post in Woit blog can be useful

http://www.math.columbia.edu/~woit/wordpress/?p=219

I reproduce here Woit initial posting

Is String Theory About to Snap?

The August issue of Discover magazine is out, with a cover story entitled “Is String Theory About to Snap?”. The editors of the magazine describe how they recently became aware of the controversy over string theory when they organized a celebration of Einstein in Aspen last summer. They quote Lawrence Krauss as telling them “String theory may be in a worse position now regarding being testable than it has been at any time in the past 20 years.” To get a response to this, they asked Michio Kaku to write something for them. They refer to him as a “cofounder of string theory”, which I suspect some people might object to. Presumably they meant to repeat what is in their profile of him, which calls him a “cofounder of string field theory.”

Kaku’s article is entitled Testing String Theory, and is a thoroughly intellectually dishonest piece of writing, designed to mislead anyone without expertise in what is at issue here. He succeeded in misleading whoever wrote the blurb for the article which goes: “No experiment has ever allowed us to test whether any of the assumptions of string theory are true. That is about to change.” No it’s not. None of the experiments Kaku mentions will “allow us to test whether any of the assumptions of string theory are true”.

As I’ve explained in detail on other occasions, the simple fact of the matter is that string theory does not make any predictions, unless one adopts a definition of the word “prediction” different than that conventional among scientists. A scientific prediction is one that tells you specifically what the results of a given experiment will be. If the results of the experiment come out differently, the theory is wrong. String theory can’t do this, since it is not a well-defined theory, but rather a research program that some people hope will one day lead to a well-defined theory capable of making predictions.

At places in the article Kaku qualifies his claims of “predictions”, for instance saying near the beginning of the article that certain experiments “could provide significant evidence that would support string theory” (note all the qualifiers in this phrase: “could”, “significant evidence”, “support”) but that “the rub is that all the new evidence, no matter how compelling, will still provide only indirect proof.” He soon abandons his qualified language and starts talking about the following topics:

1. Gravitational waves: He says of gravitational waves created in the Big Bang: “String theory predicts the frequencies of such waves”, and that this prediction will be tested by LISA. I don’t know specifically what he has in mind here, but I know of no way to use string theory to make a specific prediction of the spectrum of gravitational waves that LISA will see. The only things he mentions are inflation and epkyrotic scenarios, the first of which has nothing to do with string theory, the second very little.

2. The LHC: Kaku discusses the possibility that superpartners exist, but does note that you don’t need string theory to have these. He also discusses possible Tev-scale particle physics effects of extra dimensions, without mentioning that string theory makes no predictions at all about what these extra dimensions are like, or even what their size is. There is absolutely no reason other than wishful thinking to expect extra dimensions in string theory of a size invisible until now, but visible at LHC energies.

3. Laboratory tests of the inverse-square law: Kaku claims: “according to string theory, at small scales like a millimeter, gravity might hop across higher dimensions and perhaps into other, parallel universes”. This is a load of nonsense. String theory predicts no such thing. It may be consistent with this, purely because it is consistent with anything. He does go on to say “Perhaps the additional dimensions would show up only on smaller scales — string theory is still somewhat vague about this prediction.” “Somewhat vague”? As far as I know string theory makes no prediction about this at all, except that most string theorists expect effects to show up below 10-33cm, not 10-1cm.

4. Dark matter searches: according to Kaku “Once particles of dark matter are identified in the laboratory, their properties can be analyzed and compared with the predictions of string theory.” Only problem is string theory makes no such predictions. He’s talking about neutralinos, but in string theory the neutralino mass could be absolutely anything. After discussing these string theory”predictions” about dark matter, he goes on to speculate that maybe there is no dark matter anyway, just “huge clumps of shadow matter in a parallel universe, causing our galaxies to form in mirror-image locations”, then admits that such an idea is incapable of ever being experimentally tested.

After going through all this, he saves the real kicker for the end: “Some theorists, myself among them, believe that the final verdict on string theory will not come from experiments at all”. So he doesn’t even believe in any of the nonsense he has been spouting. He admits that “The principal reason predictions of string theory are not well-defined is that the theory is not finished.” So the earlier talk of “predictions” is now no longer operative. He goes on to invoke the pipe dream that someday someone will come up with a finished version of string theory that will predict precisely the standard model, neglecting to mention that there’s not the slightest evidence that this is a realistic possibility. On the contrary, all the evidence now points to the conclusion that, if string theory makes sense at all, it has an infinity of different vacuum states, and is probably a radically non-predictive theory. Impressive that Kaku could write a whole article about the prospects of string theory, and somehow neglect to mention the huge and very relevant controversy surrounding the idea of the landscape. Do you think he hasn’t heard about it?

 

[ossito_the-diracian]

String Theory in wrong hands

I was reading this thread and found it interesting how many people replied to a supposedly dying field. Witten leaving ST would be unfortunate but not the end. IMO I believe that the field requires some physics students with a very high math background, especially in group theory. I graduated last May in physics and math and I believe that if I strengthen my math skills there will be no limit for me in theoretical physics. Witten as everyone surely knows is one heck of mathematician. Which to me explains how he was able to advance another dying field, twistor theory. Someone else will come along sooner or later and adapt/modify Wittens theories. ST may die only in name but grand unification will never die.

 

[ZapperZ]

3. Laboratory tests of the inverse-square law: Kaku claims: “according to string theory, at small scales like a millimeter, gravity might hop across higher dimensions and perhaps into other, parallel universes”. This is a load of nonsense. String theory predicts no such thing. It may be consistent with this, purely because it is consistent with anything. He does go on to say “Perhaps the additional dimensions would show up only on smaller scales — string theory is still somewhat vague about this prediction.” “Somewhat vague”? As far as I know string theory makes no prediction about this at all, except that most string theorists expect effects to show up below 10-33cm, not 10-1cm.

This isn't true. The Arkani-Hamed et al. scheme of compaction appearing at millimeter scale was published several years ago (and received a lot of attention.[1, 2] In such a scheme, a deviation of the standard gravitational law would start to become apparent at the millimeter scale. However, what was left out is the fact that several very precise measurements of the gravitational interaction (especially those coming out of U. of Washington[3]) have been measured all the way down to the micrometer scale.[4] In none of these has any deviation of the Newtonian gravity been detected.

So yes, a possible detection has been proposed, but it has failed miserably so far. If Michio Kaku did not mention these, then he is either thoroughly out of date with the experimental results, or simply in denial.

Zz.

[1] N. Arkani-Hamed et al., Phys. Lett. B v.429, p.263 (1998).
[2] I. Antoniadis et al., Phys. Lett. B 436, 57 (1998).
[3] C. D. Hoyle et al. PRL v.86 , p.1418 (2001).
[4] J. Chiaverini, et al., PRL v.90, p.151101 (2003).

 

[George Jones]

However, what was left out is the fact that several very precise measurements of the gravitational interaction (especially those coming out of U. of Washington[3]) have been measured all the way down to the micrometer scale.[4] In none of these has any deviation of the Newtonian gravity been detected.
[3] C. D. Hoyle et al. PRL v.86 , p.1418 (2001).

Noises now coming out of Washington indicate that they may have found something. See Runner up at
http://www.symmetrymagazine.org/cms/?pid=1000189"
Very tentative.
Regards,
George

 

[ZapperZ]

Noises now coming out of Washington indicate that they may have found something. See Runner up at
http://www.symmetrymagazine.org/cms/?pid=1000189"
Very tentative.
Regards,
George

I have heard about that, but considering the very lack of even vague details of it, I didn't think I want to spread the rumor. They would certainly be in competition/contradiction with the recent verification out of University of Mainz at the nanoscale regime from a neutron drop experiment, which was reported at this year APS April Meeting. I did not highlight this earlier because, again, I haven't seen any details on this yet.

Zz.

 

[George Jones]

If it's in Symmetry, the cat's already out of the bag.

It will interesting to see if these results go the way of the 17 keV neutrino.

Regards,
George

 

[ZapperZ]

If it's in Symmetry, the cat's already out of the bag.
It will interesting to see if these results go the way of the 17 keV neutrino.
Regards,
George

... or the pentaquarks!

:)

Zz.

 

[Juan R.]

This isn't true. The Arkani-Hamed et al. scheme of compaction appearing at millimeter scale was published several years ago (and received a lot of attention.[1, 2] In such a scheme, a deviation of the standard gravitational law would start to become apparent at the millimeter scale. However, what was left out is the fact that several very precise measurements of the gravitational interaction (especially those coming out of U. of Washington[3]) have been measured all the way down to the micrometer scale.[4] In none of these has any deviation of the Newtonian gravity been detected.
So yes, a possible detection has been proposed, but it has failed miserably so far. If Michio Kaku did not mention these, then he is either thoroughly out of date with the experimental results, or simply in denial.
Zz.
[1] N. Arkani-Hamed et al., Phys. Lett. B v.429, p.263 (1998).
[2] I. Antoniadis et al., Phys. Lett. B 436, 57 (1998).
[3] C. D. Hoyle et al. PRL v.86 , p.1418 (2001).
[4] J. Chiaverini, et al., PRL v.90, p.151101 (2003).

In my opinion Woit knows he is saying.

The relevant part is not if in string theory one can construct models or not. The relevant part is

String theory predicts no such thing. It may be consistent with this, purely because it is consistent with anything. He does go on to say “Perhaps the additional dimensions would show up only on smaller scales — string theory is still somewhat vague about this prediction.” “Somewhat vague”? As far as I know string theory makes no prediction about this at all, except that most string theorists expect effects to show up below 10-33cm, not 10-1cm.

Adjusting different parameters ad hoc one can construct models where deviations from Newtonian physics are observed at mm-range or beyond. That is not a prediction.

In fact, note that your "So yes, a possible detection has been proposed, but it has failed miserably so far." has not stopped string theory popularity. String theorists invoke a new change of scale, or a new scalar field, or some extra dimension, etc. each time that a 'prediction' from string theory is invalidated in experiment.

Some time ago, string theory WAS the only explaining of dark matter and energy, now after the revolutionary paper suggesting that GR does not need dark matter or dark energy, string theorists do not claim that were wrong in the recent past. I find this curious.

If Michio Kaku did not mention these, then he is either thoroughly out of date with the experimental results, or simply in denial.

I cannot wait 'honesty' from a man who claimed that evident absence of aliens in the universe was a proof of string theory was correct. The "argument" proposed was that aliens were very advanced and using string theory developed an engine for traveling via hidden dimension to a parallel universe.

 

[ZapperZ]

In my opinion Woit knows he is saying.
The relevant part is not if in string theory one can construct models or not. The relevant part is
Adjusting different parameters ad hoc one can construct models where deviations from Newtonian physics are observed at mm-range or beyond. That is not a prediction
In fact note that your "So yes, a possible detection has been proposed, but it has failed miserably so far." has not stopped string theory. string theorists invoke a new change of scale or a new scalr field or some extra dimension etc each time that a predictions from string theory is invalidated.
Some time ago, string theory WAS the explaining of dark matter and energy, now after the revolutionary paper suggesting that GR does not need dark matter or dark energy, string theorists do not claim that were wrong in the recent past.
I cannot wait honesty from a man who claimed that evident absence of aliens in the universe was a proof of string theory was correct. The "argument" was that aliens were very advanced and using string theory developed a engine for travel via hidden dimension to a parallel universe.

I do not want to get into this with you, because in my personal opinon, you have a rather unhealthy obsession with this thing. I am NOT a fan of String Theory, and I know many prominent physicists who aren't either. But they don't go out of their way and have the compulsion to intrude into string theory discussions.

My point was that there WERE attempts at trying to come with some measureable effects. It's only fair to point this out, even when it has failed. This is what experimentalists like me would want to test, and they have.

Done!

Zz.

 

[Juan R.]

Noises now coming out of Washington indicate that they may have found something. See Runner up at
http://www.symmetrymagazine.org/cms/?pid=1000189"
Very tentative.
Regards,
George

Do you refer to the Eöt-Wash group?

If yes, let me say that measured deviation (if it is correct) from Newtonian behavior goes AGAINST string theory.

Basically, string theory 'predicts' an increase of Newtonian force due to extradimensions

F = 1/r^(2+D)

with D the number of additional dimensions D= 6, 7 in string M-theory. However the experiment -that is being repeated- measured a decreasing of the force strengh.

Curiously a decreasing of Newtonian force could be compatible with other models of quantum gravity. For example, in triangulations, recent simulations suggest that at small scales dimensionality reduces from 4 to 2. See also Baez comments on this in sci.physics.research

Formally, we can take the number of extradimension to be -2 in the triangulation model discussed by Baez

then

F = 1/r^(2-2)

One finds both decreasing of force and elimination of divergences at r=0.

Of course, this is not rigorous, but it offers an idea of "not all is string theory". There are very revolutionary and interesting views outside of string theory dogma.

 

[selfAdjoint]

Curiously a decreasing of Newtonian force could be compatible with other models of quantum gravity. For example, in triangulations, recent simulations suggest that at small scales dimensionality reduces from 4 to 2. See also Baez comments on this in sci.physics.research

This thought had also occurred to me, but I lack the background to follow it up. Would a low dimension at small scales (=high energy) imply a weaker gravity? In the Randall-Sundrum model the weakness of gravity relative to the other three forces is explained by allowing it to propagate in more dimensions. Plus, in a two dimensional world, the force would only fall off as the inverse first power, so it would be stronger, not so?

 

[Juan R.]

I do not want to get into this with you, because in my personal opinon, you have a rather unhealthy obsession with this thing. I am NOT a fan of String Theory, and I know many prominent physicists who aren't either. But they don't go out of their way and have the compulsion to intrude into string theory discussions.
My point was that there WERE attempts at trying to come with some measureable effects. It's only fair to point this out, even when it has failed. This is what experimentalists like me would want to test, and they have.
Done!
Zz.

I did not say you was a FAN nor nothing of the rest.

i simply explained that Woit's comment was correct, because string theory predicts nothing. Only that!

I do not doubt that one can construct 'models' on string theory, but string theory predicts nothing since models are constructed ad hoc. What supersimetry is not detected where suggested by string theorists no problem! one change the scale until a new acellerator again proves that superimmetry was not wher was 'predicted'.

The same holds about the rest of 'predictions' of string theorists, including deviations from Newtonian law.

 

[maverick280857]

This question, which is perhaps trivial for the forum (and may not be germane to the present discussion, in which case please accept my apologies), is from an enthusiast's point of view who is interested in learning more about string theory: What are the prequisites for studying and understanding string theory and quantum field theory? At what stage does one become eligible (formally, or on paper) to delve into string theory?

 

[Juan R.]

This thought had also occurred to me, but I lack the background to follow it up. Would a low dimension at small scales (=high energy) imply a weaker gravity? In the Randall-Sundrum model the weakness of gravity relative to the other three forces is explained by allowing it to propagate in more dimensions. Plus, in a two dimensional world, the force would only fall off as the inverse first power, so it would be stronger, not so?

Basically, if gravity 'dilutes' between other dimensions doing that at macroscopic scales we can only detect the 3D-part then at small scales it may be stronger. This is said by string theory.

See http://www.aip.org/pt/vol-53/iss-9/p22.html

for the force, the dependence is 2+D instead of (D+1). D is the number of extradimensions. If you reduce from 4 to 2 the spacetime, then the number of extradimensions is, formally, D = -2 and therefore the force is more weak at small scales -whereas string theory 'predicts' the contrary-.

when r --> 0

1/r^2 >> 1/ r^1 >> 1/r^0

what 1/r^0 to small scales appears to eliminate 'divergences' of quantum gravity has been also pointed by J. Baez.