The editors at Seed magazine have started a new blog about science called sciencegate, which contains a wide variety of interesting material. One of the recent postings is called Strung Out on the Couch; it’s by Joshua Roebke and not exactly complimentary about string theory. Here’s his analogy for the current situation of string theory:

*Think of it this way, a precocious little genius, who everyone has been touting would do great things in the world, finally grows up. Now imagine he’s 30 years old, living at home having not accomplished much, and his mom keeps going on about how great he is and is still going to be. You’d probably just want to tell him to grow up and make something of all that potential instead of just talking about how he’s going to get off the couch.*

Before Lubos and others start the usual personal attack on any string theory critic as not knowing anything about the subject, it’s worth pointing out that Roebke spent several years as a graduate student working on string cosmology before leaving academia.

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what is the status of BH entropy in LQG (and BI-like CDT) gravity?There is some debate on this point. There has been a calculation done that obtains a number proportional to the area with an unfixed constant of proportionality. I don’t think there’s a real sense that this calculation was done in LQG; it’s done in a sort-of hybrid formalism.

From my reading of the calculation, I think that the proportionality to the area is something assumed by the particular formalism used. In other words, I don’t think it is true to say that black hole entropy has been derived to be proportional to the area in LQG. Others disagree.

I discussed this a bit with Lee Smolin here — scroll down a bit.

I discussed this a bit with John Baez here.

I know of no discussion of black hole entropy in CDT.

I do not understand why, when talking about String Theory, I always have to choose the optimistic version, whereas when talking about most other scientific theories, I am allowed to tell it like it is.

If we have to select, among the various candidate String theories, the ones that contain Einstein’s equations, then Einstein’s equations are not a “prediction” of String theory.

Please explain why it is more complicated than this.

Although I find it hard to believe that ST will ever be a useful tool in doing anything other than duping funding agencies, I do at least agree about group theory – indeed, if you can be bothered to look through the archives of this blog, you will find me advocating teaching the theory of continuous groups to physics undergraduates. Also, in my own attempts to understand QFT – e.g. here, I try to give group theory as central a role as I can.

Wolfgang,

Assume that the LHC (or ILC) will provide strong empirical evidence, e.g. for a MSSM or NMSSM model.What would be your reaction?

I think one should formulate the answer symmetrically.

Low-energy SUSY would not prove string theory, and absense of low-energy SUSY would not disprove it.

Low-energy SUSY would be a strong argument in favor of string theory, and absense of low-energy SUSY would be a strong argument against it.

Do you agree?

I have an analogous question about LQG, which somebody (Christine perhaps) may be able to answer. LQG apparently predicts (or may predict) Lorentz violation. If this is observed in Auger or GLAST (or whatever the relevant experiments are called), it would be a great triumph for LQG. But is it a real, falsifiable prediction, in the sense that no Lorentz violation at Auger or GLAST would disprove LQG?

Thomas Larsson Says:

I have an analogous question about LQG, which somebody (Christine perhaps) may be able to answer. LQG apparently predicts (or may predict) Lorentz violation. If this is observed in Auger or GLAST (or whatever the relevant experiments are called), it would be a great triumph for LQG. But is it a real, falsifiable prediction, in the sense that no Lorentz violation at Auger or GLAST would disprove LQG?According to Lee Smolin, there are two scenarios that specify how lorentz invariance is treated in LQG, see: “An invitation to Loop Quantum Gravity” (hep-th/0408048; section 5, “The near term experimental situation”). From a reading of this material, it is not really clear that LQG offers clean-cut, distinguishable low-energy physical predictions at the moment, exactly because of these 2 possible scenarios. But that is my understanding.

I think Smolin offers a somewhat optimistic view, which is not something particularly bad, I even share it. That is acceptable if you are not stuck by it and are able to contrast it with, e.g., Amelino-Camelia et al.’s (gr-qc/0501053) more direct words expressing that the main difficulty is:

“

the fact thatthe techniques for obtaining the classical limit of the theory have not yet been developed. Since our phenomenology will usually be structured as a search of corrections to the classical effects, this is a very serious issue. However, several authors [16, 17, 18, 19], guided by the intuition from working with some candidate quasiclassical states, made analyses that led to the expectation that Lorentz symmetry should be broken in Loop Quantum Gravity, and as a result the Maxwell and Dirac equations should include extra terms of higher derivatives. But clearly these violations from Lorentz symmetry still cannot be viewed as a “prediction” of Loop Quantum Gravity because of the heuristic nature of the underlying arguments, and indeed there are some authors (see, e.g. Ref. [20]) who have presented arguments in favour of exact Lorentz symmetry for Loop Quantum Gravity.”Two more papers I find relevant are:

* “Falsifiable predictions from semiclassical quantum gravity” (by Lee Smolin, hep-th/0501091).

* “On Loop Quantum Gravity Phenomenology and the Issue of Lorentz Invariance (by Martin Bojowald et al., gr-qc/0411101).

I have a somewhat biased view that LQG is — or will eventually be — a falsifiable approach/theory, and the current problems of LQG concerning the issue of predictability are of a very different nature as those already pointed out in the case of string theory. That is my biased view but I am attempting to broaden my understanding of this matter of course.

The overall conclusion I have at the moment is that

LQG phenomenology is a very, very incipient one, but a possible one. I have a much more pessimistic view concerning ST, since, as already stated,“string theory may predict many new low-energy effects, but it can also be easily tuned to avoid all of them”.I do not think that is the case for LQG, but I may be in error of course.Ed Witten often has said in open forums and media that string theory pre-dicts gravity.

Of course, this is a nonsense and then Witten openly admits that that really with that phrase is that he means a

post-dictionbecauseNaturally, an ‘uniportant’ detail for stringys!

Source: Interview with John Horgan on the in-famous book

The End of science.Note1: i obtained the Spanish version (pag 97), perhaps the english quote is not exactly equal.

Note2: perhaps i would remember readers as Jose that only ‘gravity’ contained in superstring theory is a perturbative spin-2 (masless) graviton over a flat classical spacetime. String states are masless and, therefore, even a basic description of e-e gravitational scattering is absent.

Of course, a spin-2 perturbation over a flat background is NOT general relativity.

How many times, will string theory be mistified until it was abandoned as a complete failure?

Juan R.

Center for CANONICAL |SCIENCE)

Christine Dantas, it appears that LQG already offers expressions for alpha coefficient and some bounds for beta, but I have not clear still is at what extension the proporcionality constant (L_p) is fixed on LQG.

Moreover, not forget that still nobody has proven that LQG was the correct classical limit. Why would we speculate about future AUGER, GLAST, etc. predictions being unable to explain current data (classical gravity)?

Even if tomorrow LQG offers corrects values for possible Lorentz violations, will continue to be an incorrect theory if anybody does not prove that when h–> 0, LQG –> GR or similar.

The same about ‘stringys’, very anxious waiting indications of supersimmetry at HLC without worry that string theory has not the correct low energy limit?

Is this science?

Juan R.

Center for CANONICAL |SCIENCE)

Juan R. Says:

I would like to know that too. I have a lot of information yet to process. Could someone answer that?

Undoubtedly yes. I have never thought otherwise.

According to Frank Wolfs’ text,

So the answer would be yes if we believe we are in the process of construction without forgetting the 3 magic words: reliable, consistent and non-arbitrary. However, of course this is not all. Let us review some concepts. He continues:

So it is my opinion that ST and LQG are both hypotheses for sure (at least in the sense highlighted above). I am optimistic that LQG can evolve towards item 3, because item 2 seems within its reach, if not already. I am not sure about string theory and other approaches, but one obvious thing that none of them can be seen as theories.

Christine

If we have to select, among the various candidate String theories, the ones that contain Einstein’s equations, then Einstein’s equations are not a “prediction” of String theory.We don’t have to do so. It’s very hard to write down a string theory that doesn’t contain gravity. Sometimes it’s even annoying. Much of the excitement about the twistor string, for example, was tempered when it turned out that it was computing amplitudes in conformal supergravity rather than ordinary super Yang-Mills.

Aaron Bergman Said;

Any inexpert reader would be convinced that string theory works except by a ‘small’ detail you forgot.

As explained above on reply to Jose, the “gravity” contained in string ‘theory’ is not the gravity contained on general relativity not Newtonian gravityJuan R.

Center for CANONICAL |SCIENCE)

Christine Dantas,

In science a hypotesis is a premise can be falsified via laboratory. The hypothesis, beyond any distance reachable by present or future telescopes stars are pink cubes is not scientific since is not testable. It would a metaphysical hypotesis.

Therefore string theory is not a scientific hypotesis since it is not (in rigor) testable; in fact, some leading string theorists openly call it a kind of church (i can obtain this cite).

A priori, i think that LQG may be a scientific hypotesis. However, my emphasis on the classical limit of LQG was not superfitial.

The failure for finding the limit is not due to thecnical difficulties. I mean that the classical limit obtained is not compatible with correct classical limit and this is the basis of the general criticism to LQG: it has not the correct classical limit.

Therefore, as one scientist, i consider ‘stupid’ the construction of detailed models and predictions for that “we will observe at HLC” when TODAY we cannot explain already know and explained by general relativity, for example Mercury perihelion or binary pulsar data.

That is, first to develop a candidate to quantum gravity with the correct classical limit and then

only thenpredict it will be observed at HLC. Do not follow this basic scientific methodology is, in my opinion, a waste of time.Juan R.

Center for CANONICAL |SCIENCE)

Juan R. Says:

Yes, I agree with you. I do not consider it (the classical limit of LQG) a minor problem, on the contrary. It is a very serious one of course.

Best wishes

Christine