Lorentz violating severely restricted: Mqg/Mplank > 1200

atyy

I do - my favourite crackpot theories - Visser, Volovik, Wen - I still like them though

marcus

QG with a condensed matter perspective! I should have thought of that! I don't know much about VV&W's work but I wouldn't cry crackpot just yet.
BTW the analog models people seem to have put up a strong showing at Vancouver.

atyy

I mean "crackpot" as a high compliment BTW, although AdS/CFT was derived from a different viewpoint, I would actually count it as instantiating the "emergent philosophy" of condensed matter - I think they've also got a session at Vancouver.

Micha

Sorry, Marcus.

there is no particular sentence in your reply which I can point to and say this is wrong. But all in all I have the impression I am arguing with a rubber band, always stretching so much, it is forced to in the light of new results.

The problem I have is I read Smolin's book (the one whose sales figures you analyzed here for months) and while I do not remember every single detail, I clearly remember the general line of thought. On the one hand there was string theory not being able to make any predictions and sticking exactly to Lorentz symmetry. On the other hand there were all these new approaches like DSR and LQG, where experimental results were around the corner, an energy dependent speed of light being the most important one.

Yes, I remember clearly, that Smolin also said in his book, that in 3+1 LQG there were so far no clear predictions about an energy dependent speed of light, but he would like to have them, before experimental results were coming in.

You can always put some ifs and whens, but according to you, the new experimental result makes not the slightest difference on how you view the different quantum gravity approaches. Not the slightest disappointment. No reference to Smolin's book. Nothing. Now Smolin suddenly is a figure on the sideline of LQG (which he might even be today). This makes me wonder what result ever would make an impression on you.

marcus

Micha, I'm glad you remember so clearly from that popular 2006 book! I read some of it when it came out. It had an important function, but it was not something to learn all about LQG from! In this case what you remember is right. As we know, a number of people tried hard including top people like Freidel and Kowalski-Glikman. The result just doesn't seem to extend to 4D! Maybe sometime we will understand the underlying reason. Or even it could happen that some new approach to QG will actually imply some kind of Lorentz bending, and it will check out observationally! We are at the beginning stages of observational QG. Theories don't stand still either.

I have an idea to propose to you. Since you show signs of being a thoughtful reader, how about you try Rovelli's 2008 review of LQG?
This is a scholarly review article, not a popular-written book. The popular-written books can not and do not properly characterize an intellectual discussion, obviously.

BTW my introduction to QG was basically the online 2003 draft of Rovelli's book. That book is still worth looking at, for its depth. But I mention the review article because shorter and more up-to-date. It is true that I tend to see LQG "thru Rovelli glasses" so to speak. I also follow AsymSafe QG of Reuter/Percacci/Weinberg and the Triangulations QG of Loll's group. These are all exciting QG approaches in a phase of rapid growth and change.

It is useless to harp on the past. Like what looked hopeful in 2003 to someone writing an informal memoir for a poplar online magazine
LQG has gone thru enormous change even since 2006. A new spinfoam model is now the basis of a new dynamics. (Before there was no satisfactory dynamics, and no good grasp of the semiclassical limit.) The cosmo subfield LQC has also been completely revolutionized since 2006. Basic equations of LQC changed, and fresh results. It is really time now for a new Smolin-type book that will bring the general audience readership up to date.

I guess I should point out that my personality or your personality or what we think of each others thought process is not the issue. We are talking about the real research world out there.

Micha

I agree with that.

But it is also true that real research is paid by the general society. So the public deserves to get an accurate picture about the status of various research approaches. It is a moral requirement and public perception surely also has an effect on funding agencies and hiring at universities and so on, although you will never be able to exactly quantify that. And the physics forum is a small piece of this public perception. So therefore it is not unimportant, what is written here. And as you are a key contributor to this forum, I think, that you have a responsibility as well.

Best regards Michael.

PS: I haven't anything against you personally. To the contrary. I am just curious about scientific truth.

MTd2

Unfortunantely, this forum is not supported by any public agency.

Micha

I don't think, this invalidates any of my points.

MTd2

Of course it does not invalidate your points. But many of us do not have much time left because of our jobs, family, etc. And many of the recent developments in most of new areas did have a proper conference on the subject, so there is not much consolidation even among experts.

MTd2

What if lorentz violating effects were supressed like 1-sech(Mpl/Mqg^2)? I guess that wouldn't rule out DSR, right?

Edit: perhaps, a 1-erf^2

http://en.wikipedia.org/wiki/Error_function

Micha

Hi MTd2,
as any smooth function, the Lorentz violating effects, in this case the deviation of the speed of high energy photons from the known speed of light as a function of energy, can be developed into a Taylor expansion. So whatever the exact form of the function is, it is zero in the low energy limit (the speed of light of low energy photons is what it is) and then what you first would notice are linear effects. These have been excluded by the measured result now (if confirmed). Quadratic or higher order terms can't be excluded so far. Maybe quadratic terms can be measured or excluded by the experiment in the future, but not yet. So you can save a Lorentz violating theory by having it only predict quadratic effects at maximum. This however smells like a trick, if you don't have a good reason, why the linear term should be zero.

MTd2

Hi Micha, look at this:

http://backreaction.blogspot.com/200...grb090510.html

At 12:06 PM, August 25, 2009, Daniel de França MTd2 said...
You mean, the linear dispertion was not ruled out?

At 12:25 PM, August 25, 2009, Lee Smolin said...
Dear Daniel de França MTd2,

This is a key question which was covered in many places, for example my paper with Amelino-Camelia or my post above. See those for details.

The linear correction is parity odd in the case of Lorentz symmetry breaking and parity even in the case of DSR. The former is ruled out by several orders of magnitude because it leads to rotations of planes of polarization, see the reference by Gleiser and Kozemeh I mentioned above. The latter, DSR, parity even case is not ruled out, although further observations by Fermi may be able to do that.

The distinction between broken and deformed lorentz invariance is a key point in this whole discussion.

Thanks,

Lee

Thanks,

Lee

Micha

Yes I read it.

When I read your discussion of the error function, I just thought, that I remind people of the Taylor expansion. It is a trivial point of course.