Three Roads To Quantum Gravity

84 THREE ROADS TO QUANTUM GRAVITY
This still does not completely explain where the randomness
comes from. It turns out to have to do with another
central concept in quantum theory, which is that there are
non-local correlations between quantum systems. These
correlations can be observed in certain special situations
such as the Einstein±Podolsky±Rosen experiment. In this
experiment two photons are created together, but travel apart
at the speed of light. But when they are measured it is found
that their properties are correlated in such a way that a
complete description of either one of them involves the
other. This is true no matter how far apart they travel (Figure
17). The photons that make up the vacuum electric and
magnetic ®elds come in pairs that are correlated in exactly
this way. What is more, each photon detected by our
accelerating observer's thermometer is correlated with one
that is beyond her horizon. This means that part of the
information she would need if she wanted to give a complete
description of each photon she sees is inaccessible to her,
because it resides in a photon that is in her hidden region. As
a result, what she observes is intrinsically random. As with
the atoms in a gas, there is no way for her to predict exactly
how the photons she observes are moving. The result is that
the motion she sees is random. But random motion is, by
de®nition, heat. So the photons she sees are hot!
Let us follow this story a bit further. Physicists have a
measure of how much randomness is present in any hot
system. It is called entropy, and is a measure of exactly how
much disorder or randomness there is in the motion of the
atoms in any hot system. This measure can be applied also to
photons. For example, we can say that the photons coming
from the test pattern on my television, being random, have
more entropy than the photons that convey The X Files to my
eyes. The photons detected by the accelerating detector are
random, and so do have a ®nite amount of entropy.
Entropy is closely related to the concept of information.
Physicists and engineers have a measure of how much
information is available in any signal or pattern. The information
carried by a signal is de®ned to be equal to the number of
yes/no questions whose answers could be coded in that