Three Roads To Quantum Gravity

ACCELERATION AND HEAT
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The left photon
is measured
The right photon
is measured
time
space
Atom emits two photons
FIGURE 17
The Einstein±Podolsky±Rosen (EPR) experiment. Two photons are created
by the decay of an atom. They travel in opposite directions, and are then
measured at two events which are outside each other's light cones. This
means that no information can ¯ow to the left event about which
measurement the right observer chooses to make. Nevertheless, there are
correlations between what the left observer sees and what the right observer
chooses to measure. These correlations do not transmit information faster
than light because they can be detected only when the statistics from the
measurements on each side are compared.
signal. In our digital world, most signals are transmitted as a
sequence of bits. These are sequences of ones and zeroes,
which may also be thought of as sequences of yeses and noes.
The information content of a signal is thus equal to the
number of bits, as each bit may be coding the answer to a
yes/no question. A megabyte is then precisely a measure of
information in this sense, and a computer with a memory of,
say, 100 megabytes can store 100 million bytes of information.
As each byte contains 8 bits, and each corresponds to the
answer to a single yes/no question, this means that the 100
megabyte memory can store the answers to 800 million yes/no
questions.
In a random system such as a gas at some non-zero
temperature, a large amount of information is coded in the
random motion of the molecules. This is information about