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304 EXACT SOLUTIONS AND SCALAR FIELDS IN GRAVITY
rival of the signals analyzed according to the two different polarization
states + and –, that is, we compare the two velocities
The difference of the time–of–arrival of the same signal is related
to the difference of the corresponding velocities according to, see Fig.l,
In ordinary Maxwell theory so that any measurement of a
connected with the same event is related to a violation of the usual
Maxwell equation. In other words, these these measurements search for
violations of universality of the freefall of photons.
The advantage of astrophysical observations can be immediately inferred
from this formula: The estimate on a violation of the Maxwell
equations, namely the estimate on is better if the event is far
away and if the monitored events are very short. The first point is obvious
for astrophysics where we have distances of up to light years.
In addition, very short events are observable even for very far sources,
e.g. for bursts which have structures of the scale of microseconds.
In addition, astrophysical sources also emit photons with very
high energies, up to see below.
The observed objects are quasars, pulsars, bursts, active galactic
nuclei and supernovae. Any anomalous form of the Maxwell equations
leads to dispersion and birefringence. We also will discuss shortly the
importance of different velocities of photons and neutrinos.