Three Roads To Quantum Gravity

178 THREE ROADS TO QUANTUM GRAVITY
talking about all the different channels of communication that
allow information to pass from observer to observer. And
geometry, as measured in terms of area and volume, is
nothing but a measure of the capacity of these screens to
transmit information.
This more radical version of the holographic principle is
based on the ideas introduced in Chapters 2 and 3. It relies
strongly on the idea that the universe cannot be described
from the point of view of an observer who exists somehow
outside of it. Instead there are many partial viewpoints, where
observers may receive information from their pasts. According
to the holographic principle, geometrical quantities such
as the areas of surfaces have their origins in measuring the
¯ow of information to observers inside the universe.
Thus, it is not enough to say that the world is a hologram.
The world must be a network of holograms, each of which
contains coded within it information about the relationships
between the others. In short, the holographic principle is the
ultimate realization of the notion that the world is a network of
relationships. Those relationships are revealed by this new
principle to involve nothing but information. Any element in
this network is nothing but a partial realization of the
relationships between the other elements. In the end, perhaps,
the history of a universe is nothing but the ¯ow of information.
The holographic principle is still a new and very controversial
idea. But for the ®rst time in the history of quantum
gravity we have in our hands an idea which at ®rst seems too
crazy to be true, but which survives all our attempts to
disprove it. Whatever version of it ®nally turns out to be the
true one, it is an idea which seems to be required by what we
understand so far about quantum gravity. But it is also the
kind of idea which will make it quite impossible, if it is ever
accepted, to go back to any previous theory that did without
it. The uncertainty principle of quantum theory and Einstein's
equivalence principle were also ideas of this type.
They contradicted the principles of older theories and, at ®rst,
just barely seemed to make sense. Just like them, the
holographic principle is the kind of idea one hopes to run
into just as one is turning the corner to a new universe.