Posted: Jul 26, 2010 2:50 pm
Carrying on the conversation a few posts back about DSR - some interesting remarks on the falsification of quantum spacetime from section 7 of this paper:
7 Quantum Gravity In Crisis
An important result in cosmology was obtained recently which can elucidate
the nature of spacetime down to the smallest scale. This is the observation
of the highest energy gamma rays from a gamma ray burst GRB 090510 by
the Fermi Gamma-Ray Space Telescope [21]. A single 31-GeV photon was
detected from a source at a redshift of z = 0.903 which corresponds to a
distance of 7.3 billion light years from Earth. It was the last of the seven
pulses in a short burst that lasted for 0.829 s. One of the two postulates of
Einstein’s special relativity is Lorentz invariance in that all observers mea-
sure exactly the same speed of light in vacuum, independent of the motion
of the source and of the photon energy. In certain quantum theories of grav-
ity, there is great interest in the possibility that Lorentz invariance might be
broken near the Planck scale due to quantum fluctuation of spacetime and
the notion of spacetime foam. A variation of photon speed is an indication
that Lorentz invariance is violated. This may be revealed by observing the
sharp features in the gamma ray burst light-curves. If the spread in travel
time of less than 0.9 s between the highest and lowest-energy gamma rays
in the burst GRB 090510 is all attributed to quantum effects, then a thor-
ough analysis shows that any quantum effects in which the speed is linearly
proportional to energy do not show up until the distance is down to about
0.8LPl, which is below the Planck length. This result therefore rules out a
number of quantum gravity models that predict such linear variation with
energy.
The gamma ray burst reported above is significant in that it allows for
the exploration of spacetime near Planck length by using effects accumulated
over cosmological distances since direct access to Planck energy in experi-
ments is not possible. The result indicates that there is no evidence so far
of any quantum nature of spacetime above the Planck length. Spacetime
there is smooth and continuous. The speed of light is constant and special
relativity is right. At the Planck length, quantum black holes would appear
in observation and they effectively provide a natural cutoff to spacetime. For
observable purpose, it is not necessary to consider theories below the Planck
length. Further detections using gamma ray bursts with even higher energy
photons will settle the question of quantum spacetime definitively. It would
be amazing that in effect spacetime is classical and there is no need for a
quantum theory of gravity. There would be an underlying theory for gravity
which is not gravity, just as statistical mechanics is the underlying theory of
thermodynamics. Unification would have a very different meaning from the
current understanding involving quantum gravity as a fundamental premise.