Three Roads To Quantum Gravity

POSTSCRIPT 215
Three explanations have been proposed for this effect. The
first is astrophysical, and suggests that cosmic rays, or at
least those above the threshold, are produced inside our
galaxy, close enough that the effect may not have removed
all their energy. The second solution is physical, and posits
that the particles making up the very high energy cosmic
rays are not protons, but actually much heavier particles,
which do not lose energy by interacting with the microwave
background. Instead, they decay over time, giving rise to the
protons we observe. However, their lifetime is hypothesized
to be extremely long, so that they are able to travel for many
millions of years before they decay.
Both of these explanations appear far-fetched. There is no
evidence for either nearby sources of cosmic rays or such
heavy meta-stable particles. Moreover, both theories would
require careful adjustments of parameters to unusual values
just to fit these observations.
The third explanation has to do with quantum gravity. The
atomic structure predicted by loop quantum gravity, which I
described in Chapters 9 and 10, is expected to modify the
laws that govern the interactions of elementary particles.
This modification has the effect of changing the location of
the threshold, and it is very natural that the result may be to
raise the threshold enough to explain all the observations so
far made.
This explanation leads to new predictions. First, the
threshold may be seen at higher energy, in new experiments
that will be able to detect cosmic rays at still higher energies.
This is not the case with the other two explanations. Second,
the effect must be universal, as the quantum geometry of
spacetime must affect all particles that move. Hence the
same effect must be seen in other particles.
There is in fact one case in which a similar effect may have
been observed. Very energetic busts of photons arrive on
Earth. These busts are called gamma ray busts and blazars,
and they are believed to originate far outside our galaxy and
travel for billions of years before arriving on Earth. Their origin
is controversial, but it is possible they are the result of