Why does Stephen Hawking omit all mention Loop Gravity?

[ensabah6]

Is it a violation of lorentz symmetry if a version of LQG predicts that higher energy photons travel SLOWER than c (w/lower energy photons traveling at c)

Does inflationary cosmology, where the early universe expanded faster than c, violate QFT and lorentz symmetry?

Anyhow aren't there versions of string theory which allow photons to travel faster than c via extra dimensions?

 

[ensabah6]

Unless hoping you'll have him show up to answer, why do you care ?

Since he writes of string theory in his popular books, why not mention some alternatives?

 

[Civilized]

Is it a violation of lorentz symmetry if a version of LQG predicts that higher energy photons travel SLOWER than c (w/lower energy photons traveling at c)

Yes, this would be a violation of special relativity and QED, and it is a violation of lorentz symmetry.

Does inflationary cosmology, where the early universe expanded faster than c, violate QFT and lorentz symmetry?

The calculations of inflationary cosmology are entirely based on QFT in a curved spacetime i.e. QFT + einstein gravity. Here is a classic review paper: http://prola.aps.org/abstract/RMP/v57/i1/p1_1"

The expansion of the universe in general does not violate lorentz symmetry, because once we accept general relativity (which we all do) we are talking about local lorentz symmetry which means the symmetry holds in regions of spacetime which the curvature is locally flat.

Anyhow aren't there versions of string theory which allow photons to travel faster than c via extra dimensions?

No way, lorentz symmetry is one of the main constraints in string theory. This is a critical reason why string theory can be seen as building on the last 50 years of physics without starting anew. For technical reasons gravitons cannot be point particles in QFT, so the next simplest thing would be for them to 1-dimensional (strings) instead of 0-dimensional (point particles). In the modern view there are not different versions of string theory, only different solutions to a single string theory, and all of the solutions contain gravitons and respect lorentz symmetry.

 

[tom.stoer]

There is a subtlety: The modified dispersion relations from DSR violate Lorentz symmetry in an interesting way: Lorentz symmetry is not broken, but it is realized non-linearily, thast means the algebra SO(3,1) is deformed.

There is no (spontaneous symmetry) breaking in the sense of special points or directions.

Anyway: I am not so sure if LQG still insists on predicting these effects. As far as I remember there were no rigorous proofs. I haven't seen papers talking about deriving DSR from LQG for years. Does anybody know about the status?

 

[marcus]

There is a subtlety: The modified dispersion relations from DSR violate Lorentz symmetry in an interesting way: Lorentz symmetry is not broken, but it is realized non-linearily, thast means the algebra SO(3,1) is deformed.

There is no (spontaneous symmetry) breaking in the sense of special points or directions.

Anyway: I am not so sure if LQG still insists on predicting these effects. As far as I remember there were no rigorous proofs. I haven't seen papers talking about deriving DSR from LQG for years. Does anybody know about the status?

So far LQG does not predict Lorentz violation. There was hope back in 2004-2005 that one could prove LQG implies some form of DSR. Because of the subtlety you mentioned, this would not create problems for LQG and would produce a testable prediction about gammaray bursts that could be tested.

There was considerable optimism about this, because the desired result was shown in D=3 case by Laurent Freidel and Etera Livine. For some time people tried to extend to D=4, but as you suggest, they seem to have gotten discouraged.

There may be a renewed effort in this direction if Fermi spacecraft observes dispersion. This would serve as guidance to LQG research, but because there is no outstanding prediction would not constitute a test.

The status is that LQG has been proven compatible with Lorentz invariance (Rovelli 2003) and the rumor that its spacetime is discrete and therefore incompatible is a canard. The underlying spacetime is a continuum---operators corresponding to area and vol measurements have discrete spectrum.

Smolin-Amelino have a recent paper about testing DSR---not about LQG directly although Fermi LAT data will provide guidance.
Rovelli has a recent offering a conjecture as to what the physical basis of DSR might be, if the dispersion effects are observed. It is not a specifically LQG paper, but I think quite interesting. Does anyone want links to any of these? The Freidel-Livine 2005, for example?

 

[nrqed]

Why do all discussions here end up like this ? Why do people always side to this or that ? Is it because after studying one for a couple of days, you think you understood something but you are tired and do not want to study what other people have done ? Would you mind showing some respect towards the research work of people more competent than you ? Who cares that you prefer Weinberg to Rovelli ? Are you confusing horse race bets with science ?

I strongly agree with this point of view!
This is why it is tiresome to see so much cheerleading for LQG whihc has nothing to do whatsoever with the actual science, but more to do with how many papers have been published on this or that, who is working on this or that, who was the student of this or that reseracher, who has given a talk at this or that conference.

I would be delighted to see you express the same opinion with regards tp the LQG cheerleading.

Regards

 

[MTd2]

So far LQG does not predict Lorentz violation. There was hope back in 2004-2005 that one could prove LQG implies some form of DSR.

There is still hope, if there is some kind of drag from the quantum foam. Same for string theory.

 

[nrqed]

You presume a lot about me Humanino, and you are attacking me personally. You choose to use personal attacks instead of responding to my evidence that Smolin's LQG violates Lorentz symmetry. I am not an amateur physicist, I am a professional. I am not interested in personal attacks. I do not care about Rovelli or Weinberg as people, I look at their arguments.

I, for one, appreciate very much all your insights, Civilized! It's great to see discussiosn on physics instead of discussions on people which is unfortunately the main emphasis on this board (and completely unscientific). I guess it's because it is far easier to find out how many papers a person has published or who's graduate student he/she was than it is to try to understand the physics.

I think your attitude is the correct one. We shoudl not care about who wrote a paper or gave a talk, we shoudl just discuss the physics. II agree that it is hard to avoid complete neutrality, in the sense that I can't help putting more weight on a paper by Witten than on a paper from someone I have never heard of, but we should strive to put as little emphasis on the person as possible and as much emphasis as possible on the science. Unfortunately, this board does the exact opposite.

Than you for your very informative posts and interesting links!

Regards,

Patrick

 

[marcus]

...
I would be delighted to see you express the same opinion with regards tp the LQG cheerleading...

I'm not sure what you mean by cheerleading. It is important to get the facts out. We now have much more interest by string people in what LQG and other nonstring QG research is turning up. People are crossing over. People are attending the same conferences.
So something has to be done to counter the outdated stereotype of string folks sneering and dismissing nonstring QG.
The way to do this is with facts.
Like the Planck scale conference that just occurred. Like the Ellis-fest to be held in Cape Town in August. Like the new money the ESF is pumping into non-string QG via two networks headed by Barrett and by Loll.
The change is more evident in Europe, Canada, Australia than in the USA, where most departments are simply not equiped to handle grad students who want to pursue QG.

There still is a lot of minsinformation of the Motley sort but it has become less important I think. I don't know if it is as necessary to point out inaccuracies as it was in the past.

Anyway in case anyone wanted the links I mentioned in previous post:
Freidel-Livine
http://arxiv.org/abs/hep-th/0512113
http://arxiv.org/abs/hep-th/0502106
(They managed to get DSR in the 3D case.)

Rovelli 2003
http://arxiv.org/abs/gr-qc/0205108
The preprint is 2002 and the publication in Physical Review D was 2003.

 

[MTd2]

This is the apparent lorentz symmetry break, within superstrings:

http://www.ift.uni.wroc.pl/~planckscale/lectures/3-Wednesday/6-Mavromatos.pdf

 

[MTd2]

Like the Planck scale conference that just occurred. Like the Ellis-fest to be held in Cape Town in August. Like the new money the ESF is pumping into non-string QG via two networks headed by Barrett and by Loll

You forgot to mention the countless string meetings, including string 2009, where works on Horava Gravity are being presented.

 

[Civilized]

There is still hope, if there is some kind of drag from the quantum foam. Same for string theory.

No, there is no hope of lorentz violations in string theory, unless the theory turns out to be mathematically inconsistent (which seems unlikely after 20+ years of study but could conceivably happen), because lorentz symmetry is one of the primary axiomatic inputs.

This is the apparent lorentz symmetry break, within superstrings:

Thanks for providing a link to your claim. Here is a technical paper by the same author which elaborates on the approach:

http://arxiv.org/abs/gr-qc/9906029"

From page 2:

As commented earlier, one may expect
Lorentz invariance to be broken in a generic theory of
quantum gravity. In the context of string theory, this en-
tails the exploration of non-critical string backgrounds,
since Lorentz invariance is related to the conformal sym-
metry that is a property of critical strings.

Of course, a non-critical string theory is a string theory that is not in the critical dimension, e.g. strings in 3 + 1 d spacetime. These theories can only exist if we relax the constraint of lorentz symmetry.

It may be a matter of semantics Mt2d, and we can agree to disagree, but without any context I think "string theory", "superstrings", etc refers to the mainstream theory worked on by Susskind, Kaku, Greene, Randall, Vafa, Witten, etc which obeys lorentz symmetry without a doubt. It would be fair to say "there are non-critical string theories which violate lorentz symmetry" but since most people on this board won't know the jargon "non-critical" I still think this might be misleading.

Incidentally, some people might want to reflect on the fact that lorentz invariance is considered so important that the mainstream physicists have choosen to preserve this in string theory rather than choosing to preserve d = 4 spacetime.

You forgot to mention the countless string meetings, including string 2009, where works on Horava Gravity are being presented.

The idea of giving a serious new theory a fair chance is not so new, and I hope everyone who believes in conspiracies about string theorists supressing outside reasearch take this as evidence that they always give a new approach a chance until it is shown to be unviable. There have already been a few papers pointing out problems with Horava gravity, and if that trend continus unabated the mainstream will move on in a year or two.

There is still hope, if there is some kind of drag from the quantum foam. Same for string theory.

As an aside, why would anyone hope for lorentz violation? As I have said this is one of the deepest principles QFT is based on, and was essential for the development of the standard model, it would be disappointing if this symmetry were violated and it would reduce Einstein's theories and QFT to merely effective approxmations.

I am skeptical of the phrase "quantum foam" applied to string theory. I know Greene uses this phrase in his popularizations, but as far as I can tell that's just his way of communicating the nature of quantum mechanics / quantum field theory. I just want to clarify that spacetime is smooth in the mathematics of ST, even though I think Greene sometimes makes unclear comments about this for the sake of attracting a popular audience.

 

[MTd2]

I read Greene's book in Portuguese many years ago, so I am not aware about his terminology in English. I recall the use of "quatum foam" from Ashok Sen or Sergei Gukov from some colloquim slides from around 2005. I really never read books or articles about strings for the layman. I rather consult Polchinski's book or the older, Superstrings, for a deeper mathematical fundation.

I never talked about violation of lorentz symmetry, but an apparent one, if you just mind the possible effect of having a delay on the arrival time of a photon from a source. QFT and lorentz symmetry, this one even in the case of a "lorentz violation" are just fine from both of the sides of string theory and LQG, specially in the string side. QFT in string theory is effective in the low energy regime of the target space, but it is the natural foundation of both bosonic and susy 1+1 worldsheet...

As I said, in both cases, the delay is caused, in my view, by an increase of the vacuum peameabilty. Not because of fundamental reasons, but as in the same reason as light gets slower in a crystal, that is, it "bumps" more frequently with atoms, or in the other case, with "variations" of the background geometry.

 

[humanino]

This thread asked the question "why Hawking does not mention LQG". I and Haelfix have answered : he did not work on it. If someone claims "because Hawking thinks, like me, that LQG is wrong" please post a reference proving he thinks the same as you, otherwise please stop polluting this thread.

 

[ensabah6]

This thread asked the question "why Hawking does not mention LQG". I and Haelfix have answered : he did not work on it. If someone claims "because Hawking thinks, like me, that LQG is wrong" please post a reference proving he thinks the same as you, otherwise please stop polluting this thread.

Has Hawking worked on string theory since he devotes so much on it in his popular books (and does not appear to regard it as speculative in so much as fact)

 

[Civilized]

This thread asked the question "why Hawking does not mention LQG". I and Haelfix have answered : he did not work on it. If someone claims "because Hawking thinks, like me, that LQG is wrong" please post a reference proving he thinks the same as you, otherwise please stop polluting this thread.

Suppose someone created a thread, "why does Hawking omit all mention of the Planck scale green-goblin pastry theory?"

And the fans of goblin theory said "it's because he hasn't worked on it, and to say anything otherwise you need a reference."

I would ignore that fan because green-goblin pastry theory is so bogus, that although Hawking has never commented on it, it is still possible for educated physicists to suggest probable reasons why he hasn't worked on it. In particular, it's bogus.

You see, when you study physics you learn that "why?" questions like the title of this thread can be answered on multiple levels. The answer "because he has not worked on it" is not a very deep answer, because it immediately begs the question "why hasn't he worked on it?" Some of us work in theoretical physics, which involves using our minds to make logical inferences based on limited information. I would love to see quotes from Hawking about LQG, but if none are availible than the only way to answer the question on a deeper level than "because he didn't" is by make educated guesses into why he didn't.

Hawking's lack of comments about LQG cannot be a good thing for LQG, and this is a reason for LQG fans to end the thread. Afterall, by the trichotomy of "likes" the possibilities are:

(1) Hawking doesn't like LQG.

(2) Hawking doesn't care about LQG.

(3) Hawking likes LQG but never mentions it in his scientific or popular publications.

I think we can all see that (3) is unlikely, therefore LQG fans would like to stop us from discussing the reasons why (1) or (2) might be true. The fans might attack these comments as an invitation to "speculate", but all I am offering are informed inferences based on (a) my personal experiences talking to the professional American high energy physics community and (b) comments by leading mainstream physicists of similar stature as Hawking. I think it is important for us to discuss this because Hawking is a major part of the public image of physics, and the fact that he has ignored LQG my help some students to look at LQG more critically, that in particular it is a non-mainstream theory and it does not compete with string theory in the mainstream.

I would also call on Humanino to provide a reference to the statement in bold:

You mentioned that the conjecture is already used as a mathematical tool, and by the way no just for RHIC physics. However, in string theory it applies to SUSY QCD for instance, not QCD. And independently a good chunk of the models have 5 dimensions not 11.

 

[humanino]

And the fans

Your problem Civilized is that I do not care about LQG. My personal taste is obviously much more inclined towards string theory. My assumption is that my personal tastes are of no interest to anyone. Your assumption is that I am a LQG fan. It is just another wrong opinion of yours.

 

[Civilized]

Your problem Civilized is that I do not care about LQG. My personal taste is obviously much more inclined towards string theory.

Let's review some of your past comments:

You keep repeating that violation of Lorentz symmetry as a bad thing.

How can someone who is "obviously much more inclined towards string theory" not see that violation of lorentz symmetry is a bad thing?

However if LQG predicted Lorentz voilation unambiguously, it would be a falsifiable prediction making it instantly in better scientific shape than string theory.

Why would some who is "obviously much more inclined towards string theory" be unaware that string theory unambiguously predicts SUSY and extra dimensions or be unaware that these predictions of string theory are a least as definite and testable as the lorentz violations Smolin is suggesting? And even if you were unaware of these string predictions, it is still unlikely that someone who is "obviously much more inclined towards string theory" would make such a smug comment as "...would be a falsifiable prediction making it instantly in better scientific shape than string theory", because as I explained, the same faulty logic would make Planck scale green-goblin pastry theory "instantly in better scientific shape than string theory. "

If you are inclined towards strings, then you must not know the first things about them based on the two quotes of your above. Otherwise I would be happy to hear your explanation for those statements, and I am still waiting for you to retract or justfy your statement about (the supposed existence of) gauge/gravity dualities in 5 dimensions.

Lastly, I recognize an argument pattern. Often my opponents attempt to use the same tricks that I use on them, in this case the trick is of the general form "You assume a lot about me, ... , you're wrong." I don't claim to have invented the trick, but I used it on Humanino early in this thread and now Humanino is attempting to use it on me. The difference is that my "assumptions" were actually "inferences" based on evidence i.e. the quotes I have given above. Another common argument pattern is for my opponents responses to become increasingly glib and superficial, ignoring most of my points, not I'm not saying that that's happening here...

 

[suprised]

..There was considerable optimism about this, because the desired result was shown in D=3 case by Laurent Freidel and Etera Livine. For some time people tried to extend to D=4, but as you suggest, they seem to have gotten discouraged.

Right, and for good reason. Namely gravity is special in 3D, in fact in a sense topological and non-dynamical, and the tricks used in 3D just don't work in 4D. AFAIK there has been no proof so far that LQG would generate general relativity at large distances... after 20(?) years of research!

You forgot to mention the countless string meetings, including string 2009, where works on Horava Gravity are being presented.

By now it should be clear to anybody following the field that there are serious problems with that model. Various recent papers have shown that unphysical degrees of freedom do not decouple, as had been anticipated by many. Even Horava in his recent talk at strings stayed clear from this point as far as I know.

So ... this is _not_ an "alternative" theory, even if some ppl here continue to claim so. And we should be glad about that, namely if all those simple minded modifications/deformations would make sense, we'd have even a bigger mess to figure out which is the right theory. Same applies to Lisi's theory... it is manifestly wrong, conceptually (obvious) and technically (needs some work, see Distler), though it seems that some amateurs are still busy with trying to "assign" elementary particles to roots of E8... though somehow that thread died off over the last year. Just how come?

It seems that the main theme of all of this spin doctoring is "we don't care whether it makes sense, all what we want is that it is NOT string theory". Well, good luck. Let me conclude, again, with a remark Witten made in one of our discussions of Horava-Lifgarbagez (not verbatim but in the sprit): "it is extremely hard to find a modification of a sensible theory that makes sense, and there is a good reason why we should stick to the models we study".

 

[atyy]

As an aside, why would anyone hope for lorentz violation? As I have said this is one of the deepest principles QFT is based on, and was essential for the development of the standard model, it would be disappointing if this symmetry were violated and it would reduce Einstein's theories and QFT to merely effective approxmations.

But isn't that GR and the standard model are just effective theories a motivation of string theory, ie. perturbative calculations indicate that GR is not renormalizable and cannot be a theory with a continuum limit, suggesting the need for a theory in which GR is not fundamental, but which emerges at low energies?

So Asymptotic Safety explores the possibility that GR is non-perturbatively renormalizable (though even AS does not assume that the Einstein-Hilbert action is correct, just that the gravitational field has a geometric interpretation, and could include higher order curvature terms). Whereas string theory, Horava gravity, condensed matter approaches explore what one needs if GR is emergent - in which case they have to deal with the Weinberg-Witten theorem and change dimensions, Lorentz invariance or something.

 

[MTd2]

Various recent papers have shown that unphysical degrees of freedom do not decouple, as had been anticipated by many.

But it seems several of these articles also argue that these unphysical degrees are in fact dark matter.

I think it is ok to study non stringy theories, because when SUSY is ruled out, we will have more options.

 

[ensabah6]

But it seems several of these articles also argue that these unphysical degrees are in fact dark matter.

I think it is ok to study non stringy theories, because when SUSY is ruled out, we will have more options.

If SUSY is never detected, and is not detected at LHC nor any astrophysical observation, and is thus not an explanation for stabilization of the electroweak scale and the higgs field, (something else explains Higgs field or electro-weak breaking) is there any reason for popular writers like Hawking, Greene, Kaku, Weinberg etc. to continue to promote strings as the only game in town?

 

[atyy]

If SUSY is never detected, and is not detected at LHC nor any astrophysical observation, and is thus not an explanation for stabilization of the electroweak scale and the higgs field, (something else explains Higgs field or electro-weak breaking) is there any reason for popular writers like Hawking, Greene, Kaku, Weinberg etc. to continue to promote strings as the only game in town?

It's probably a caricature to say that Hawking, Greene etc "promote strings as the only game in town". However, string theory is the only known consistent quantum theory of gravity at the moment - asymptotic safety is undemonstrated, and condensed matter approaches including Horava gravity have difficulties such as giving the graviton a mass or the wrong dispersion relation. Furthermore, string theory has invigorated QFT because of AdS/CFT. Weinberg, 't Hooft, Wilczek, Hawking have all mentioned non-string approaches, and that string theory may not be ultimately correct, but they have all also acknowledged the importance of string theory.

 

[tom.stoer]

If you are really interested why Hawking does not mention LQG you should ask him :-)

 

[atyy]

But it seems several of these articles also argue that these unphysical degrees are in fact dark matter.

Could you post some links? I looked through Marcus's thread, but there are too many for me to find them.

 

[humanino]

[...]

Thanks for the good laugh. I hope you realize you are trying to convince me than you know myself better than I do... Also, I have enough ego not to consider you as an "opponent". I do appreciate your disagreements, at least they are honestly frank. I do not pay attention to your claims of my ignorance, but do pay attention to your scientific comments.

 

[humanino]

I am still waiting for you to retract or justfy your statement about (the supposed existence of) gauge/gravity dualities in 5 dimensions.

I mentioned the existence of MODELS inspired from Maldacena conjecture entirely restricted to 5 dimensions. It is very different. For instance
Light-Front Holography: A First Approximation to QCD
Phys. Rev. Lett. 102, 081601 (2009)
Guy F. de Téramond and Stanley J. Brodsky

 

[humanino]

string theory unambiguously predicts SUSY

How about misaligned SUSY ? I thought it provides an opportunity for SUSY on the worldsheet but not necessarily on the target space. I guess that does not count as "mainstream"...

 

[humanino]

No way, lorentz symmetry is one of the main constraints in string theory.

Another "non-mainstream" approach
Branes on Charged Dilatonic Backgrounds: Self-Tuning, Lorentz Violations and Cosmology
JHEP 0108 (2001) 005
DAMTP-2001-45, LBNL-48377
C. Grojean, F. Quevedo, G. Tasinato, I. Zavala

We construct an n+q+2 dimensional background that has dilatonic q-brane singularities and that is charged under an antisymmetric tensor field, the background spacetime being maximally symmetric in n-dimensions with constant curvature k=0,+1,-1. For k=1 the bulk solutions correspond to black q-branes. For k=0,-1 the geometry resembles the `white hole' region of the Reissner-N"ordstrom solution with a past Cauchy horizon. The metric between the (timelike) singularity and the horizon is static whereas beyond the horizon it is cosmological. In the particular case of q=0, we study the motion of a codimension one n-brane in these charged dilatonic backgrounds that interpolate between the original scalar self-tuning and the black hole geometry and provide a way to avoid the naked singularity problem and/or the need of having exotic matter on the brane. These backgrounds are asymmetrically warped and so break 4D Lorentz symmetry in a way that is safe for particle physics but may lead to faster than light propagation in the gravitational sector.

 

[MTd2]

Could you post some links? I looked through Marcus's thread, but there are too many for me to find them.

In fact, Marcus just post a tiny part of what is posted about Horava gravity. I guess this article is not there, so, here it goes:

http://arxiv.org/abs/0905.3563 and its citations.

This one is more recent, from the same author, and make a nice job of reinterpreting Horava Gravity bugs as nice features:

http://arxiv.org/abs/0906.5069

If you want some heavy criticism, this one is the best, uploaded last week:

http://arxiv.org/abs/0907.1636

It hits the theory at its most fundamental base, that is, about the existence of a fixed point. But these authors make a constructive criticism, since they point out possible solutions to solve them. I personaly think that the nature of said solutions will lead to a convergence to the assymtotic safety situation similar to that of einstein quantum gravity.