Three Roads To Quantum Gravity

38 THREE ROADS TO QUANTUM GRAVITY
This sounds crazy, and even thirty years after learning it I
cannot describe this situation without a feeling of misgiving.
Surely there must be a better way to understand what is going
on here! Embarrassing though it is to admit it, no one has yet
found a way to make sense of it that is both more comprehensible
and elegant. (There are alternatives, but they are either
comprehensible and inelegant, or the reverse.) However,
there is a lot of experimental evidence for the superposition
principle, including the double slit experiment and the
Einstein±Podolsky±Rosen experiment. Interested readers
can ®nd these discussed in many popular books, some of
which are included in the reading list at the end of this
book.
The problem with quantum theory is that nothing in our
experience behaves in the way the theory describes. All our
perceptions are either of one thing or another ± A or B, tasty or
yukky. We never perceive combinations of them, such as
a 6 tasty + b 6 yukky. Quantum theory takes this into
account. It says that what we observe will be tasty a certain
proportion of the time, and yukky the rest of the time. The
relative probabilities of us observing these two possibilities
are given by the relative magnitudes of a 2 and b 2 . However,
what is most crucial to take on board is that the statement that
the system is in the state aA +bB does not mean that it is
either A or B, with some probability of being A and some
other probability of being B. That is what we see if we observe
it, but that is not what it is. We know this because the
superposition aA +bB can have properties that neither tasty
nor yukky have by themselves.
There is a paradox here. Were my cat to be described in the
language of quantum theory, after tasting the mouse she
would experience either tasty or yukky. But according to
quantum mechanics she would not be in a de®nite state of
happy or displeased. She would go into a superposition of
two states which mirrors the possible states of the mouse. She
would be suspended in a superposition of a happy state and
an annoyed-for-having-bitten-into-a-yukky-mouse state.
So the cat experiences herself in a de®nite state, but in the
light of quantum theory I must see her in a superposition.