Quantum Gravity : Mathematical Models and Experimental Bounds

30 Claus Lämmerzahl
through astrophysical observation where one has access to very high energy cosmic
rays there are some advantages of low energy laboratory experiments.
While astrophysical observations one has access to ultra high energies of more
than 10 21 eV these observations are plagued with the disadvantage that there
is no systematic repeatability of these observations and that there is no unique
interpretation of the results.
Accordingly the advantages of a controlled laboratory search consists in the
repeatability of the experiment and the possibility of a systematic variation of
initial and boundary conditions. This can be used for a unique identification of
the cause of the effect as well as for an improvement of the precision of the result.
Another advantage is that certain regimes can be accessed in th laboratory only.
For example, ultra–low temperatures or ultrastable devices like optical resonators
can be build and maintained in the laboratory only.
Due to stability and repeatability of experiments, laboratory searches for QG
effects may be as promising as astrophysical observation
6. How to search for quantum gravity effects
Since QG replaces GR and/or quantum theory which are both universally valid
QG should affect all physical phenomena. However, not all phenomena are equally
sensitive to the expected QG modifications. Therefore one needs some kind of
strategy for the search for QG effects.
Standard physics is supported by all present experimental data. These data
have been obtained within some standard domain of experimental accessibility,
that is, for some energy range, some velocity, distance, temperature range, etc.
Therefore, one first attempt to search for QG effect is to explore new regimes,
that is, to go to higher energies, to lower temperatures, to longer distances, to
longer and shorter time scales, etc.: A search for new effects needs the exploration
of new non–standard experimental situations. These situations are, for example,
Extreme high energies: This regime is well suited for the search for deviations
from the standard dispersion relation for elementary particles.
Extreme low energies: With low temperatures one may search for fundamental
noise arising from space–time fluctuations, which may lead to a fundamental
decoherence of quantum systems at temperatures lower than 500 fK. Such
temperatures may be achieved in BECs.
Extreme large distances: Gravity at long distances became the subject of discussion
very recently since the unexplained phenomena dark energy, dark
matter, and the Pioneer anomaly are related to large distances. Consequently
QG induced modifications of gravity has bee proposed, see e.g. [100]. Furthermore,
the detection of ultra–low frequency gravitational waves which give
information about the very early universe where QG effects are surely more
pronounced, also need very long distances.