lumidek said:There is no asymptotically safe theory of gravity, because of technical RG reasons and because of wrong scaling for the entropy at high energies that should be dominated by black holes. And even if there were one, CDT couldn't be its approximation.
lumidek said:There is no asymptotically safe theory of gravity, because of technical RG reasons and because of wrong scaling for the entropy at high energies that should be dominated by black holes. And even if there were one, CDT couldn't be its approximation.
Well, even without this paper, it seems very improbable that the photonlumidek said:Every single model marketed as loop quantum gravity, spinfoam, causal dynamical triangulation, Horava-Lifgarbagez gravity, and dozens of other names violates the Lorentz symmetry by first-order terms, with a coefficient of order one, and is simply safely dead after this paper.
lumidek said:
Hans de Vries said:Now, while agreeing with you, how would you explain that your favorite theory
doesn't exhibit the same problem? Why doesn't the photon propagator become
"fuzzy" with all these complicated geomet
Regards, Hans
lumidek said:http://arxiv.org/abs/gr-qc/0411101[/QUOTE]where it is explained (or wished) that the breaking is an artifact.
Just like with Pauli bashing Yang because Pauli "knew" that nonabelian gauge theories were "sick", very little discussion is possible against no-go theorems until loopholes are found. And just like Yang, LQG people are not blind but quite aware of those difficulties. Yes, LQG has difficulties, and is much less attractive than string theory, especially considering how much the latter is developed. By itself it does not justify calling people names.
I failed to find a published reference to http://arxiv.org/abs/gr-qc/0411101 answering negatively to the (putative ?) hopes conveyed there. If Lubos has such an obvious answer, he would contribute positively to public money saving by publishing a Letter instead of making short statements on a blog.
For example, because the asymptotically safe (and other) field theories have a unique vacuum while the vacuum in (Minkowskian) triangulated or otherwise discretized models of gravity is highly non-unique, creating an entropy density that goes to infinity in the continuum limit.atyy said:I understand the plausibility of the first two statements - but why can't CDT be an approximation to an asymptotically safe gravity, if such a thing existed?
Could you please stop emitting this noise and lies? The paper, much like all others, calculates Lorentz violation in the dispersion relations. Openhumanino said:Throw a list of reference and drown the fish. It is amusing that Lubos would repeatedly quote where it is explained (or wished) that the breaking is an artifact.
Just like with Pauli bashing Yang because Pauli "knew" that nonabelian gauge theories were "sick", very little discussion is possible against no-go theorems until loopholes are found. And just like Yang, LQG people are not blind but quite aware of those difficulties. Yes, LQG has difficulties, and is much less attractive than string theory, especially considering how much the latter is developed. By itself it does not justify calling people names.
I failed to find a published reference to http://arxiv.org/abs/gr-qc/0411101 answering negatively to the (putative ?) hopes conveyed there. If Lubos has such an obvious answer, he would contribute positively to public money saving by publishing a Letter instead of making short statements on a blog.
Dear Finbar, see, for exampleFinbar said:Hi Lubos,
Can you give some references to back your claims about asymptotic safety in gravity? What are these technical RG reasons?
Thanks.
Dear Heinz, it is somewhat strange to call these propositions "reasoning" because they seem to be rather isolated propositions and don't follow from each other in any way (and are not proved in your comment). But all propositions you wrote are correct. ;-)heinz said:Lubos,
is the following reasoning correct?
1) There are no deviations from special relativity's Lorentz symmetry in nature.
1b) All proposals which state such deviations are wrong.
2) There probably are no deviations from general relativity's diffeomorphism invariance and other symmetries.
(For essentially the same reasons that 1) is correct: continuity of space-time holds.)
2b) All proposals that state such deviations are wrong.
Heinz
Actually, I printed it the other day. As written in the abstractlumidek said:Could you please stop emitting this noise and lies? The paper, much like all others, calculates Lorentz violation in the dispersion relations. Open
the paper questions whether it is possible that the violations are an artifact of the approximation scheme. It is also illustrated with kindergarden examples on the harmonic oscillator how a the discretization can yield such artifacts and miss nonperturbative terms in the corrections.Furthermore, by contrasting Hamiltonian and Lagrangian descriptions we show that possible Lorentz symmetry violations may be blurred as an artifact of the approximation scheme. Whether this is the case in a purely Hamiltonian analysis can be resolved by an improvement in the effective semiclassical analysis.
It seems like you do not even expect your readers to check your references. Again, please note that the reason I am asking is because of the possibility of you saving my time by convincing me (and everybody) that reading LQG literature is a loss of time. This is your own claim, so I hope you can back it up.These examples have important hints for the calculation of corrected dispersion relations and the issue of Lorentz covariance. Since only higher order corrections will be seen when a Hamiltonian is perturbed, Lorentz violations are bound to appear as a consequence of this way of doing the calculation. Space and time derivatives of the classical fields have to be related in the Lagrangian in a way dictated by the symmetry. If those terms are torn apart, because one computes the Lagrangian from a perturbed Hamiltonian which only sees higher space derivatives but not higher time derivatives in its corrections, Lorentz invariance will be violated. This kind of violation of Lorentz symmetry is not a consequence of the theory but of the way to perform perturbative calculations.
lumidek said:Dear Finbar, see, for example
http://golem.ph.utexas.edu/~distler/blog/archives/001585.html
Wrong. P. 12 "This kind of violation of Lorentz symmetry is not a consequence of the theorylumidek said:Look at pages 8-13 where the calculation is hidden. The conclusion is at the end of the section on page 13 and the conclusion is that the result "does break Lorentz invariance".
lumidek said:... But all propositions you wrote are correct...
heinz said:Lubos,
do I understand you correctly that if the symmetries of general relativity
are correct at all scales, as you stated, then also general relativity itself
is correct at all length and energy scales?
heinz
lumidek said:Give me a break with the arrogance. I am just alarmed that some people want to dilute this experimental result and its consequences on physics. But physics is all about direct and indirect comparisons of observations with theories. And this observation happens to be extremely clean and settles the question. It proves that people like me have always been right and people around loop quantum gravity have always been wrong, using their poor education, weak intelligence, and lacking intuition to study questions that go well beyond their abilities. The result proves that all sponsors and foundations who have funded theories building on the assumption that Lorentz symmetry will have to be broken have wasted the money, and as soon as they care about the empirical data, they should learn a lesson and fire all these people.
I will not allow anyone to create fog about this very clear situation.
Dear Heinz, I can only subscribe to Finbar's answer.heinz said:Lubos,
do I understand you correctly that if the symmetries of general relativity
are correct at all scales, as you stated, then also general relativity itself
is correct at all length and energy scales?
heinz
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. ;-)Fra said: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
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:turbo-1 said: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.
This comment of yours is ludicrous.MTd2 said:Wrong. P. 12 "This kind of violation of Lorentz symmetry is not a consequence of the theory
but of the way to perform perturbative calculations."p.13 :"It should
however be kept in mind that the calculations done up to now (including the model of
the previous section) can only yield preliminary results and that a definite answer to the
question of Lorentz violation by loop quantum gravity definitely has to await a more complete
treatment, possibly along the lines sketched above."
So, there is no certainty. And by the time the calculations were done, it was a problem with the hamiltonian perturbative expansion.
Dear Finbar, there are several other illuminating posts on Jacques' website, e.g.Finbar said:I've read that sometime ago. But in light of this paper
http://arxiv.org/abs/0902.4630
"Yes, that would also serve as a good test"
Distler would have to admit the AS program past this test i.e. finding a fixed point when non-perturbativly renormalizable terms are included in the truncation.
Do you have any other references? If there are good RG reasons why AS cannot work it would be interesting to see if these could be formulated into some "no-go" theorems.
lumidek said:This comment of yours is ludicrous.
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.lumidek said: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/glast-first-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
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.MTd2 said: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?
Very well, let's read :lumidek said:See e.g. page 4 of Polchinski's book where he explains why this guess about the UV fixed point is not pursued there.
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.There are two possible resolutions. The first is that the divergence is due to expanding in powers of the interaction and disappears when the theory is treated exactly. In the language of the renormalization group, this would be a nontrivial UV fixed point. The second is that the extrapolation of the theory to arbitrarily high energies is incorrect, and beyond some energy the theory is modified in a way that smears out the interaction in spacetime and softens the divergence. It is not known whether quantum gravity has a nontrivial UV fixed point, but there are a number of reasons for concentrating on the second possibility. One is history — the same kind of divergence problem in the Fermi theory of the weak interaction was a sign of new physics, the contact interaction between the fermions resolving at shorter distance into the exchange of a gauge boson. Another is that we need a more complete theory in any case to account for the patterns in the Standard Model, and it is reasonable to hope that the same new physics will solve the divergence problem of quantum gravity.
Dear ensabah, nope, the existence of a 6-dimensional manifold at each point of the 3+1-dimensional space doesn't imply any discreteness.ensabah6 said: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 said:Dear Finbar, there are several other illuminating posts on Jacques' website, e.g.
http://golem.ph.utexas.edu/~distler/blog/archives/000648.html
http://golem.ph.utexas.edu/~distler/blog/archives/001585.html
http://golem.ph.utexas.edu/~distler/blog/archives/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.
lumidek said: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.
heinz said: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 said: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?
If that's all you have to say about the emergence of time and the relations with quantum mechanics and entropy, I guess you have already dismissed in your blog Connes and Rovelli 94 paper. It's just an example illustrating that, once again, sweeping away the problems with general arguments and a flavor of superior contempt is not very constructive.lumidek said:This evolution is analogous to the evolution in time, and restoration of symmetry is analogous to a low-entropy state.
lumidek said:Dear ensabah, nope, the existence of a 6-dimensional manifold at each point of the 3+1-dimensional space doesn't imply any discreteness.
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.
s.