Three Roads To Quantum Gravity

POSTSCRIPT 213
These cosmic rays are said to originate in highly energetic
events in the centers of certain galaxies, which serve as a
kind of natural particle accelerator. The rays come from regions
of huge magnetic fields, perhaps produced by a supermassive
black hole. Such things were once the stuff of fantasy,
but we have more and more evidence for their
existence. Although there are still uncertainties in our understanding
of the origins of the cosmic rays, it seems most
probable that the most energetic ones come from far outside
our galaxy.
Consider then the most energetic cosmic ray protons observed,
traveling toward us from a distant galaxy. At the energies
they are traveling, about 10 10 times the energy of the
proton, or more than 10 million times the energy of the
largest human-made particle accelerator, they are traveling
very, very close to the speed of light. As our proton travels, it
encounters another form of radiation that fills the space between
galaxies—the cosmic microwave background.
The cosmic microwave background is a bath of microwaves
that we understand as vestiges left over from the
big bang. This radiation has been observed to come at us
equally from all directions, up to small deviations of around
a few parts in a hundred thousand. It now has a temperature
of 2.7 degrees above absolute zero, but it was once at least as
hot as the center of a star and cooled to its present temperature
as the universe expanded. Given how uniformly we observe
it to come from all directions of the universe, it is inconceivable
that this radiation does not fill all space.
As a consequence, we know that our cosmic ray proton
will encounter many photons from the microwave background
as it travels through space. Most of the time nothing
happens as a result of these interactions, because the
cosmic ray proton has so much more energy and momentum
than the photon it encounters. But if the proton has
enough energy, it sometimes produces another elementary
particle. When this happens, the cosmic ray slows down
and loses energy because it takes energy to create the new
particle.