Three Roads To Quantum Gravity

THE HOLOGRAPHIC PRINCIPLE
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general relativity and quantum theory, there is a chance that
in the end the situation will be reversed and the holographic
principle will become part of the foundations of physics, from
which quantum theory and relativity may both be deduced as
special cases.
The holographic principle was inspired ®rst of all by the
Bekenstein bound, which we discussed in Chapter 8. Here is
one way to describe the Bekenstein bound. Consider any
physical system, made of anything at all ± let us call it The
Thing. We require only that The Thing can be enclosed within
a ®nite boundary, which we shall call The Screen (Figure 39).
We would like to know as much as possible about The Thing.
But we cannot touch it directly ± we are restricted to making
measurements of it on The Screen. We may send any kind of
radiation we like through The Screen, and record whatever
changes result on The Screen. The Bekenstein bound says
that there is a general limit to how many yes/no questions we
can answer about The Thing by making observations through
The Screen
The
Thing
FIGURE 39
The argument for the Bekenstein bound. We observe The Thing through The
Screen, which limits the amount of information we can receive about The
Thing to what can be represented on The Screen.