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Lorentz violating severely restricted: Mqg/Mplank > 1200 
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#55
Aug1809, 12:51 PM

P: 92




#56
Aug1809, 12:54 PM

P: 92

http://motls.blogspot.com/2008/08/gl...tresults.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
Aug1809, 12:58 PM

P: 92

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...yremains.html Cheers, LM 


#58
Aug1809, 01:11 PM

P: 92

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 higherloop 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 highcenterofmass 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
Aug1809, 01:12 PM

PF Gold
P: 1,963




#60
Aug1809, 01:23 PM

PF Gold
P: 7,363

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
Aug1809, 01:24 PM

P: 92

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
Aug1809, 01:26 PM

P: 2,828

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
Aug1809, 01:27 PM

P: 716

Lubos, or any string theorist or anyone
doesn't string theory compatification presented as a 6dimensional yaucalibi 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 6dimensional yaucalibi 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? 


#64
Aug1809, 01:57 PM

P: 92

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 Bfield, one can get a noncommutativity on the hidden manifold which effectively makes the space of functions on the manifold finitedimensional, 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 CalabiYau space and the continuity of space. There's no contradiction. The CalabiYau 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 longdistance 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 symmetrybreaking 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 longdistance 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 shortdistance physics is fundamental, and the longdistance physics is its consequence. You can say that longdistance physics may be calculated from  i.e. evolves from  shortdistance physics. This evolution is analogous to the evolution in time, and restoration of symmetry is analogous to a lowentropy state. In thermodynamics, lowentropy states don't normally evolve from generic highentropy states in the past. In the very same way, symmetric effective longdistance laws of physics usually don't evolve from asymmetric shortdistance 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. 


#65
Aug1809, 02:22 PM

P: 343

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


#66
Aug1809, 02:26 PM

P: 63

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
Aug1809, 02:34 PM

P: 343

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
Aug1809, 02:41 PM

P: 63

heinz 


#69
Aug1809, 03:44 PM

P: 2,828




#70
Aug1809, 03:46 PM

P: 716




#71
Aug1809, 04:37 PM

Sci Advisor
PF Gold
P: 1,594

A better way to think of it is this: A cylinder is really just a line, with circles attached to it at every point. These circles are linked together in a continuous fashion. If the cylinder is all wobbly instead of straight, we can regard this as having the circles changes shape and size (in a continuous way) as we move from point to point along a line. The curvature form of the resulting surface encodes the information as to exactly how these circles are connected together; hence all the talk of "connections". In string theory, spacetime is similar, but replace our "line" with "4D spacetime", and instead of attaching "circles", use "CalabiYau manifolds". The CalabiYaus are glued together, in a continuous fashion, at "right angles" to both themselves and to the 4D spacetime. There is a 10dimensional curvature form that encodes information as to how everything is connected together. 


#72
Aug1809, 08:47 PM

Sci Advisor
P: 8,792

This paper has interesting comments on Lorentz invariance (or lack of) in spin foams and in the appendix, on condensed matter approaches to quantum gravity.
http://arxiv.org/abs/0901.4009 Quantum Histories and Quantum Gravity Joe Henson 


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