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Percacci Calmet Hossenfelder find min length implicit in AsymSafe gravity

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marcus
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Aug19-10, 07:56 PM
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At first sight this seems like a beautiful paper. Or at least a refreshing one (like opening a window on a good day.) MTd2 spotted it for us.
http://arxiv.org/abs/1008.3345
Deformed Special Relativity from Asymptotically Safe Gravity
Xavier Calmet, Sabine Hossenfelder, Roberto Percacci
(Submitted on 19 Aug 2010)
"By studying the notion of a fundamentally minimal length scale in asymptotically safe gravity we find that a specific version of deformed special relativity (DSR) naturally arises in this approach. We then consider two thought experiments to examine the interpretation of the scenario and discuss similarities and differences to other approaches to DSR."

In AsymSafe gravity, the key constants run with scale. As the momentum k --> infinity, the Newton G goes to zero and the cosmological Lambda gets large. Because their dimensionless versions must converge to finite numbers.

So with the running of G and Lambda, unexpected things can happen. Percacci et al find that a minimum length emerges.
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atyy
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Aug19-10, 08:26 PM
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"We have investigated the implications of this by considering two thought experiments and concluded that there is no modification for free particles, but scattering processes in the superplanckian regime will be modified."
marcus
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Aug19-10, 09:09 PM
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Quote Quote by atyy View Post
"We have investigated the implications of this by considering two thought experiments and concluded that there is no modification for free particles, but scattering processes in the superplanckian regime will be modified."
That's a good point. They say that no one has so far been able to derive DSR from any (4D) version of QG, so their derivation from AsymSafe gravity (ASG) is a first.
Plus the form of DSR (Lorentz-compatible minimal length) that they do derive from ASG does not predict an energy-dependent speed of photons.

However, the form of DSR they derive from ASG is testable. At least in principle. It is testable by particle collision experiments---or by scattering experiments.

It sounds like a good thing for QG people to chew on, doesn't it?

atyy
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Aug19-10, 09:16 PM
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Percacci Calmet Hossenfelder find min length implicit in AsymSafe gravity

Well, string theory has long been testable by this criterion.

I think one of the initial hopes of AS was "Compared to the effective field theory framework the main advantage lies not primarily in the gained energy range in which reliable computations can be made, but rather that one has a chance to properly identify ‘large’ quantum gravity effects at low energies. Indeed the (presently known) low energy effects that arise in the effective field theory framework, although unambiguously defined, are suppressed by the powers of energy scale/Planck mass one would expect on dimensional grounds. Conversely, if there are detectable low energy imprints of quantum gravity they presumably arise from high energy (Planck scale) processes, in which case one has to computationally propagate their effect through many orders of magnitudes down to accessible energies. " http://relativity.livingreviews.org/...ticlesu20.html
marcus
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Aug20-10, 10:12 AM
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Quote Quote by atyy View Post
Well, string theory has long been testable by this criterion.
...

Well, I agree that the in-principle testability that Percacci et al offer in this paper does not seem very satisfactory. But two observations are in order:

They are not saying that this is the ONLY possible way to test the AsymSafe gravity (ASG) idea. And this is just a first paper.

As I understand it, heir main purpose was not to exhibit some practically testable ASG prediction. It was to take a careful look to see if anything like DSR or minimal length follows from ASG. I think this is preliminary work and doesn't by any stretch settle the question. But it suggests that maybe something DSR-like follows from asymptotic safe gravity. To me, it seems like an intriguing possibility--and this is the first I've heard of it.
MTd2
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Aug20-10, 10:50 AM
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Not really untestable. Given that what is being probed is the presence of a scale, not specific particles, looking for effects of DSR in high energy process is possible by looking at modification from standard theories of neutron star merges, GRB and the big bang itself.
MTd2
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Aug20-10, 10:59 AM
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Quote Quote by marcus View Post
But it suggests that maybe something DSR-like follows from asymptotic safe gravity. To me, it seems like an intriguing possibility--and this is the first I've heard of it.
Maybe then the DSR Smolin found on LQG is of this kind, but he wrongly interpreted as the old DSR.

Weinberg suspects that Dynamical Triangulations are related to AS. LQG can be parametrized by triangulations, although of different kind. Maybe what we have here is a full circle saying that non-stringy QG all have a coherent unit.
marcus
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Aug23-10, 07:19 PM
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More on this from Percacci, following up on his paper that appeared just a week earlier.
http://arxiv.org/abs/1008.3621
Asymptotic Safety, Emergence and Minimal Length
R. Percacci, G. P. Vacca
20 pages, 2 figures
(Submitted on 21 Aug 2010)
"There seems to be a common prejudice that asymptotic safety is either incompatible with, or at best unrelated to, the other topics in the title. This is not the case. In fact, we show that 1) the existence of a fixed point with suitable properties is a promising way of deriving emergent properties of gravity, and 2) there is a precise sense in which asymptotic safety implies a minimal length. In so doing we also discuss possible signatures of asymptotic safety in scattering experiments."
MTd2
#9
Aug23-10, 11:23 PM
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Did you notice that Eric Verlinde was acknoledged for helpful discussions?
marcus
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Aug23-10, 11:40 PM
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Sure I noticed! Why don't we list all those acknowledged?
"RP wishes to thank the Perimeter Institute for hospitality in the early stages of this work, and
D. Benedetti, S. Giddings, R. Gurau, S. Hossenfelder, T. Padmanabhan, L. Smolin, E. Verlinde
for discussions."
I can't say I would view Verlinde as especially important though, the name does not especially stand out in that list. I think he is notable primarily as a former top string theorist who got out (at least for a while) and declared "string theory is not the way to go!" I don't see him as having substantially more insight or better vision than the others. So far, I don't see the thermodynamic gravity revival as having taken off, or having gotten much past where Ted Jacobson took it in 1995. So far.
MTd2
#11
Aug24-10, 08:42 AM
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Did you also notice this?

Fig. 2 vs. http://www.canadaconnects.ca/quantumphysics/10080/

It seems the formation of transplanckian divergences is avoided in exactly the same way as the problem of ultraviolet divergence of the classical blackbody radiation problem.
qsa
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Aug24-10, 04:54 PM
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Quote Quote by MTd2 View Post
Did you also notice this?

Fig. 2 vs. http://www.canadaconnects.ca/quantumphysics/10080/

It seems the formation of transplanckian divergences is avoided in exactly the same way as the problem of ultraviolet divergence of the classical blackbody radiation problem.

this is a quote from a post I made long time ago


http://www.physicsforums.com/showthr...44#post2662244

"In my own model (my profile), something strange happens when I make position discrete, then when I almost hit 355 strange things happen to the energies of the particles( it is like fixed points). It is known that if you compute 355/113 you get PI with six figure accuracy. Moreover, as I approach 4 all the energies cap to 1 in a similar behavior to black body radiation i.e. when energies are discrete the result becomes finite. But if I make my random throws on real line then all hell breaks loose and there is no stopping to the energies. For various reasons in my model it appears that 4 could represent a length of 1 to 1/1000 times the proton diameter. I am not sure; I have to find out or may be I am just calculating the wrong thing."

I hope you find the information interesting.


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