On 12/12/2013 5:12 PM, LizR wrote:

On 13 December 2013 14:04, meekerdb <meeke...@verizon.net <mailto:meeke...@verizon.net>>wrote:The result was lack of frequency dispersion for gamma rays. So it was assuming some interaction between photons and the discrete units of spacetime. That seems pretty tight.Assuming the interpretation is correct, at least. Do you know the theory of why that wassupposed to happen, and how the quantisation was reduced so far below the Planck length?If so I'd be interested in an explanation, assuming I can understand it.

Bounds on Spectral Dispersion from Fermi-detected Gamma Ray Bursts Robert J. Nemiroff,1 Ryan Connolly,1 Justin Holmes,1 and Alexander B. Kostinski1

`1Dept. of Physics, Michigan Technological University, 1400 Townsend Dr., Houghton MI,`

`49931, USA`

Data from four Fermi-detected gamma-ray bursts (GRBs) is used to set limits on spectral dis-

`persion of electromagnetic radiation across the universe. The analysis focuses on photons`

`recorded`

above 1 GeV for Fermi detected GRB 080916C, GRB 090510A, GRB 090902B, and GRB 090926A

`because these high-energy photons yield the tightest bounds on light dispersion. It is`

`shown that`

`significant photon bunches in GRB 090510A, possibly classic GRB pulses, are remarkably`

`brief, an`

`order of magnitude shorter in duration than any previously claimed temporal feature in`

`this energy`

`range. Although conceivably a > 3 fluctuation, when taken at face value, these pulses`

`lead to an`

`order of magnitude tightening of prior limits on photon dispersion. Bound of c/c < 6.94 x`

`10-21`

`is thus obtained. Given generic dispersion relations where the time delay is proportional`

`to the`

`photon energy to the first or second power, the most stringent limits on the dispersion`

`strengths`

were k1 < 1.61 x 10-5 sec Gpc-1 GeV-1 and k2 < 3.57 x 10-7 sec Gpc-1 GeV-2 respectively. Such

`limits constrain dispersive effects created, for example, by the spacetime foam of quantum`

`gravity.`

In the context of quantum gravity, our bounds set M1c2 greater than 525 times the Planck mass, suggesting that spacetime is smooth at energies near and slightly above the Planck mass. arXiv:1109.5191v2

(Presumably if correct this result also screws up things like calculations of black holeentropy, which seem to rely on the Planck length being a "unit" in some sense - at leastaccording to the "science for dummies" books I've read.)

Does quantum mechanics tell an atomistic spacetime? Hans-Thomas Elze Dipartimento di Fisica "Enrico Fermi", Largo Pontecorvo 3, I-56127 Pisa, Italia E-mail: e...@df.unipi.it Abstract. The canonical answer to the question posed is "Yes." -- tacitly assuming that quantum theory and the concept of spacetime are to be unified by 'quantizing' a theory of gravitation. Yet, instead, one may ponder: Could quantum mechanics arise as a coarse-grained reflection of the atomistic nature of spacetime? -- We speculate that this may indeed be the

`case. We recall the similarity between evolution of classical and quantum mechanical`

`ensembles,`

according to Liouville and vonNeumann equation, respectively. The classical and quantum mechanical equations are indistinguishable for objects which are free or subject to spatially constant but possibly time dependent, or harmonic forces, if represented appropriately. This result suggests a way to incorporate anharmonic interactions, including fluctuations which are tentatively related to the underlying discreteness of spacetime. Being linear and local at the quantum mechanical level, the model offers a decoherence and natural localization mechanism. However, the relation to primordial deterministic degrees of freedom is nonlocal. arXiv:0906.1101v1 Brent -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at http://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/groups/opt_out.