Lorentz violating severely restricted: Mqg/Mplank > 1200

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In summary, this conversation discusses the recent detection of high-energy emission from a short gamma-ray burst, GRB 090510, using the Fermi Gamma-ray Space Telescope. This emission shows a significant deviation from the Band function, suggesting two distinct spectral components and challenging the prevailing gamma-ray emission mechanism. The detection of a 31 GeV photon during the first second of the burst also sets the highest lower limit on a GRB outflow Lorentz factor, indicating that the outflows powering short GRBs are highly relativistic. This photon also sets limits on a possible linear energy dependence of photon propagation speed, requiring a quantum-gravity mass scale significantly above the Planck mass. However, this result does not disfavor loop quantum gravity or other
  • #36
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.

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?
 
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  • #37
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.

Hi Lubos,

Can you give some references to back your claims about asymptotic safety in gravity? What are these technical RG reasons?

Thanks.
 
  • #38
lumidek 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.
Well, even without this paper, it seems very improbable that the photon
propagator could emerge from any theory with path/geometry randomness
at the Planck scale.

Being on the light-cone with such an extreme precision , what mechanism
could cancel out all the contributions from the random geometry paths
which are not on the large-scale lightcone?

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 geometry paths at the Planck scale?

The (not so well known) "photon self-propagator" which has the photon
field itself as a source, rather than the current j, does a wonderful job in
canceling out the contributions on all paths other than the light-cone path
(see sect 1.19 of: http://physics-quest.org/Book_Chapter_EM_basic.pdf )
but it needs a flat geometry at Planck's scale.Regards, Hans
 
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  • #40
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

I was just going to ask Lubos this,

how do you know that the compatification of the additional 6 dimensions and the landscape and various mechanisms such as KKLT doesn't break lorentz invariance, or in some way affect the speed of light at the plank scale, or SUSY breaking mechanism?
 
  • #41
Throw a list of reference and drown the fish. It is amusing that Lubos would repeatedly quote
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.
 
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  • #42
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?
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.

This situation differs from normal lattices for QFTs because the shape of the lattices is fixed and the fluctuations of the degrees of freedom living on the lattice sites are universal in the UV. For triangulations, there's no real "UV", the metric is dynamical, and one always sums about all kinds of stuff.

Second, the "causal" in the causal triangulations refers to an artificial truncation of the configurations to a subset that satisfies a "causal" global condition on the geometry. Such a truncation can never generate a field theory because almost all (in the measure sense) individual configurations that are summed in the path integral of any quantum theory are acausal. For example, a point-like electron is moving along trajectories that are superluminal almost everywhere, and causality is only restored when all these paths are summed over.

Truncating paths that are superluminal anywhere (i.e. almost all of them) would completely damage the short-time behavior (the power laws etc.) and it would really break the uncertainty principle because in the path integral formalism, the uncertainty principle is only possible because almost all trajectories contributing to the path integral are non-differentiable.
 
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  • #43
humanino 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.
Could you please stop emitting this noise and lies? The paper, much like all others, calculates Lorentz violation in the dispersion relations. Open

http://arxiv.org/PS_cache/gr-qc/pdf/0411/0411101v1.pdf

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".

There have been many papers showing that LQG is worthless crap in detail and they have even become the most cited LQG papers of the year (like Nicolai et al. 2005). But no one really cares about it because for every correct result and sensible scientist, it has become politically correct to promote one (or two) wrong result and fund one (or two) crackpots.
 
  • #45
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
 
  • #46
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
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. ;-)

In the case of diffeomorphisms, it is even more obvious. Whenever the dynamical metric tensor is a part of the description, there must exist an exact diffeomorphism symmetry, otherwise the negative-normed (negative probabilities!) unphysical modes of the metric tensor wouldn't be decoupled, and probabilities could become negative.

With the diffeomorphism symmetry being a fact, the Lorentz symmetry is there, too (because it is a subgroup of diffeomorphisms, at least on the sectors with the right topology). The nontrivial part of the statement is that the flat vacuum is invariant under such a Lorentz group, or at least it is "locally" invariant so that the Lorentz group can only be at longer distance scales L by positive-energy structures/objects whose typical length is L.

These facts also imply that the spectrum of positions or lengths or areas, whenever it may be definable, must be continuous. In realistic theories of quantum gravity, however, the lengths or areas are good observables only in the long-distance approximating theories. The true calculable quantities are scattering amplitudes for particles with fixed momenta etc.
 
  • #47
lumidek said:
Could you please stop emitting this noise and lies? The paper, much like all others, calculates Lorentz violation in the dispersion relations. Open
Actually, I printed it the other day. As written in the abstract
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.
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.
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.
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.
 
  • #48
lumidek 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.
 
  • #49
lumidek 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".
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.
 
  • #50
lumidek said:
... But all propositions you wrote are correct...

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
 
  • #51
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

what do you mean by general relativity? general covariance is probably correct at all scales(in string theory?) but not the dynamics of gravity i.e the Einstein equations.

Lubos,

Would I be correct to say that in perturbative string theory one allows strings to "live" on a background spacetime such that the physics is generally covariant. Nonetheless exact non-perurbative string theory is fully background independent?
 
  • #52
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.
 
  • #53
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.

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
 
  • #54
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
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
 
  • #55
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
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. ;-)
 
  • #56
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.
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
 
  • #57
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.
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/why-perturbation-theory-remains.html

Cheers, LM
 
  • #58
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.
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.
 
  • #59
lumidek said:
This comment of yours is ludicrous.

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?
 
  • #60
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
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.
 
  • #61
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?
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.
 
  • #62
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.
Very well, let's read :
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.
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.
 
  • #63
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?
 
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  • #64
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?
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.
 
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  • #65
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.

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?
 
  • #66
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.

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
 
  • #67
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


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?
 
  • #68
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?

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
 
  • #69
lumidek said:
This evolution is analogous to the evolution in time, and restoration of symmetry is analogous to a low-entropy state.
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.
 
  • #70
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.

what is the fundamental scale in string theory? What is the distance between one calabi-yau space and the adjacent one, in flat 3+1 space? Does curvature bring them closer or farther apart?
 
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