Lorentz violating severely restricted: Mqg/Mplank > 1200

turbo

It is interesting to see how such a small observational sample set can be touted as "proving" or "falsifying" anything about LQG, String, etc. Why not wait for more observational data and see what trends (if any) evidence themselves? The GLAST project was pushed back over and over - let's see what we get now that the probe is functional.

Fra

I find this type of judging and aggresive reasoning which apparent lack to humbleness to also be more alarming. It doesn't contribute to a good creative atmosphere.

It seems to me (my personal impression from your writings) an overall quality of your reasoning here and elsewhere to often make, from your point of view, very probable inferences, appear as bulletproof and unquestionable deductions, thereby clearing the fog that may exists by those who doesn't make the same inferences as you. It seems you often suggest that anyone that doesn't see it has inferior intelligence and should step aside.

I get the impression that you you think everyone who does see that string theory is the only reasonable way, must by conclusion, have inferior intelligence? :)

/Fredrik

lumidek

Dear Heinz, I can only subscribe to Finbar's answer.

If by general relativity, you mean a theory with a metric tensor and diffeomorphism-invariant action, i.e. one composed out of polynomials of the Riemann tensor (and perhaps its non-polynomial i.e. nonlocal i.e. nonperturbative extensions; and from the gauge-theoretical field strength and other matter), then yes, the (effective) action has to have this form.

However, it's not true that the Einstein-Hilbert action "R" is the whole story. The higher-derivative terms, such as R^n, are really the rule and are included (and have to be included) with appropriate coefficients whose magnitude may be guessed from dimensional analysis.

Cheers, LM

lumidek

Fra, in this particular case, the right conclusion about the intelligence of the writer can be obtained without any measurement of anyone's knowledge of string theory: grammar is enough. ;-)

lumidek

Dear Turbo, I think you are very confused. This whole thread is about the newest result of GLAST that was renamed to Fermi one year ago:

http://motls.blogspot.com/2008/08/gl...t-results.html

In some sense, this is the ultimate result of Fermi, the culmination of its ability to measure and decide things: the future measurements will be qualitatively less important because they will be essentially repeating what we can see in this paper.

Also, I want to emphasize that in science, one properly done observation is enough to falsify theories and whole frameworks, and we're just seeing a good example here.

Cheers, LM

lumidek

This comment of yours is ludicrous.

If a symmetry is violated even perturbatively, it is pretty much guaranteed that it is also violated nonperturbatively, unless there is a cancellation of perturbative and nonperturbative terms which would imply that the whole perturbative expansion is impossible - and in this case, it would also mean that it is impossible to define the theory from any classical starting point.

Perturbative expansions remain one of the main tools to gather the information about theories and your hostility towards this very method shows that you have no clue about physics. See also http://motls.blogspot.com/2009/08/wh...y-remains.html

Cheers, LM

lumidek

Dear Finbar, there are several other illuminating posts on Jacques' website, e.g.

http://golem.ph.utexas.edu/~distler/...es/000648.html
http://golem.ph.utexas.edu/~distler/...es/001585.html
http://golem.ph.utexas.edu/~distler/...es/001609.html

I think that these insights are shared by virtually all the sane people who have thought about this issue but it's not being published by anyone because it's considered a part of the general lore. See e.g. page 4 of Polchinski's book where he explains why this guess about the UV fixed point is not pursued there.

The assumption is, of course, that the terms that behave nicely only behave nicely because they're either removable by field redefinitions, renormalizable, or topological, and the true difficult contractions of powers of the Riemann tensor, i.e. those arising from higher-loop divergences, would falsify the safety - and add infinitely many new parameters in the UV.

These are technical reasons and there may exist a simple proof that this doesn't work. But I personally have very different primary reasons to be sure that gravity can't be described by asymptotically safe UV theory - namely black hole thermodynaimics, holography etc. Field theory just doesn't reproduce the right high-center-of-mass spectrum (which should be dominated by black hole microstates). Also, the black hole information loss paradox requires some nonlocality for the information to get out of the hole, so a field theory with an exactly definable metric tensor and the corresponding causal structure can't be right.

MTd2

I merely quoted and summarized part of the conclusion. So, you mean that paper is ludicrous. So, why did you even bother coming up with that paper?

turbo

I am not confused. I know that the probe/instrumentation was renamed in honor of Fermi. I also remember that Fotini Markopoulou suggested that the highest-energy gamma rays might be slowed (energy-dependent time dispersion) by interacting with the fine-scale structure of the vacuum, AND suggested that with a large enough spread in energies over very long distances, Glast might be able to detect such dispersion. Let's see what happens when more bursts are analyzed. Collecting a few photons at a time and analyzing their energies and arrival times is not a trivial exercise and much can rest on the way that the data are analyzed.

As to the bolded text: you are assuming that all future observations will be similar (in contrast to the possible dispersion found by the MAGIC consortium). That may or may not be true, and it is in bad taste (IMO) to trash the careers of others who are keeping an open mind about this subject. Rarely do we get the opportunity to test cosmological theories with direct observation. Fermi may allow us to do just that, and we should make many observations and look for trends in the data.

lumidek

Dear MTd2, indeed, the whole research of LQG has always been ludicrous, but there are different levels of its being ludicrous. Because you asked a question about LQG, I had to come up with a paper about LQG. Sensible papers don't talk about LQG, so I couldn't give you a quite sensible paper.

So I took a paper that was sensible relatively to the ludicrous question you were asking, and this paper also has different levels of quality of physics. It contains some actual calculation, and it contains verbal paragraphs filled with absurd wishful thinking that is justified by nothing whatsoever. The latter is clearly more ludicrous that the former, but it also happens to be much more attractive for you. It seems that you're choosing the worst garbage out of the worst paper that you may find in the worst corners of the dumping ground of physics.

humanino

Very well, let's read :
So it is not known, and there are reasonable points such as history and unification. This is not "certainty" or a mathematical theorem. Just reasonable points. The book was written in 1998, and it is not clear whether Polchinski considered reformulating this in later editions. What is clear, is that when you claim "Weinberg advertises AS because he came up with the idea", at the very least you did not read the paper or attend the talk, or did not understand them, because he explains simply that there are other very interesting reasons. At the very worse, you chose to present only the aspect supporting your position, which amounts to ... well, I would rather let you qualify what it would amount to, since you are so talented for names.

Also, please note that you ignored my answer where I notified you that you did not understand Bojowald's paper, or possibly consistently chose to present things in a biased manner (ooops, you did it again with Polchinski). Please note that I have no reason to be surprised, reading your blog suffices to realize quickly what one can expect beyond mathematical computation, from a human point of view.

ensabah6

Lubos, or any string theorist or anyone


doesn't string theory compatification presented as a 6-dimensional yau-calibi manifold in every point in 4D spacetime imply discrete spacetime? If spacetime in string theory is infinitely smooth and continuous and infinitely divisible (even below the Planck length) how then can you speak of a 6-dimensional yau-calibi manifold in each point in spacetime:?

Do you know for a fact that neither SUSY breaking mechanism nor moduli stabilization schemes like KKLT don't break lorentz invariance?

lumidek

Dear ensabah, nope, the existence of a 6-dimensional manifold at each point of the 3+1-dimensional space doesn't imply any discreteness.

In topological string theory, the sizes of the hidden manifold are quantized. In the full physical string theory, they can't be. Everything is continuous. With a B-field, one can get a noncommutativity on the hidden manifold which effectively makes the space of functions on the manifold finite-dimensional, as expected from N points. This is the closest point to a "discreteness" but you can never imagine that they're real "points" and the manifold is made out of edges, triangles, or simplices.

I don't understand why you think that there's a contradiction between the existence of a Calabi-Yau space and the continuity of space. There's no contradiction. The Calabi-Yau manifolds are perfectly smooth and dividable to arbitrarily small pieces, too.

Below the fundamental scale, the usual geometric intuition breaks down. But it is surely not replaced by an even more naive intuition, such as a space constructed of edges and triangles. The physics that replaces the usual long-distance physics is much more subtle and requires somewhat complicated mathematics that is not equivalent to any simple presentation for the laymen.

Neither SUSY breaking nor any moduli stabilization or any other process that is essential in the KKLT or other famous groups of stringy vacua breaks the Lorentz invariance at the fundamental scale. The Lorentz invariance at the fundamental scale is a universal principle valid according to string theory. All symmetry breaking mechanisms for similar symmetries are cases of spontaneous symmetry breaking in string theory: it means that the symmetry holds at high energies (short distances) and is being broken at low energies (long distances), below the symmetry-breaking scale.

Analogously, moduli are "massless at high energies", meaning that the masses are negligible relatively to these high scales, but they do acquire small potentials and masses that matter for long-distance physics. Also, supersymmetry breaking splits the supermultiplets, making the unknown superpartners heavier than their observed counterparts. But these mass differences are small relatively to the Planck scale which means that at short distances, when we care about big energies only, SUSY is restored. The same principle applies to electroweak, GUT, or any other similar symmetry breaking.

In the LQG and similar discussions of Lorentz symmetry, the opposite direction of the symmetry breaking is assumed: the symmetry shouldn't exist at high energies but it should be restored at low energies. This is infinitely unlikely because the short-distance physics is fundamental, and the long-distance physics is its consequence. You can say that long-distance physics may be calculated from - i.e. evolves from - short-distance physics. This evolution is analogous to the evolution in time, and restoration of symmetry is analogous to a low-entropy state. In thermodynamics, low-entropy states don't normally evolve from generic high-entropy states in the past. In the very same way, symmetric effective long-distance laws of physics usually don't evolve from asymmetric short-distance laws unless there is a reason to expect that the symmetric point is an attractor, which is not the case for Lorentz symmetry of realistic effective theories.

Finbar

I think that yes Distler has a point. But if you read the posts he does not give reasons why the fixed point doesn't exist. Instead he is concerned withe the reliability of the ERG. These concerns are valid but its the best tool we have to find evidence for a fixed point. Whats more if we were to find that by adding a term to the truncation destroyed the fixed point we would surely of found proof(?) that gravity is nonperturbativly nonrenormalizable which in turn would support string theory.


Actually I'm very interested in your comment about non-locality. Are you saying that string theory should allow information travel outside the light-cone? Or how do you see this non-locality? A "stretched horizon" of order the Planck length maybe?

heinz

Ok, thank you for the clarification - as usual straight and to the point. In fact, this yields two issues.

1. Woodard in http://arxiv.org/abs/0907.4238 says that general relativity cannot be
changed by adding higher derivatives. What is wrong in his reasoning?

2. Does the GRB measurement also provide limits for the magnitudes of these higher order terns in the Lagrangian?

heinz

Finbar

Can you elaborate on 1. or give the page number because clearly by adding extra terms in the action we change the theory. So what do you mean?

heinz

The argument is on page 31 and subsequents, called "the problem with higher derivatives". Woodard says that higher derivatives make the theory very unstable.

heinz