We can say a few words about quantum physics, but a good starting
explanation would take a few hours of lecture, which we can't come
close to matching in a short answer. We don't know how long a complete
explanation would take, because a satisfactory one really hasn't yet
In quantum mechanics, that state of any physical system, say a
little collection of particles, is represented by a mathematical object
that looks in many ways like a wave. Particles do not in general have
well defined positions, velocities, etc. Only in special cases do they
even have well-defined energies. They can have well-defined quantum
states, it's just that those states happen not to have the properties
which we instinctively feel that physical things ought to have. That's
our problem, not theirs.
The way in which those states change with time follows a definite
equation something like the equations governing classical waves. It has
no randomness in it- the output state is completely determined by the
input state, according to the equation. The problem is that the output
states don't look like anything we ever have seen. They have things in
them like combined live/dead cats and versions of your brain which are
combinations of having seen a live cat and having seen the cat dead.
So the best you can do with quamtum mechanics is to predict the
probabilities of seeing each of the possible outcomes. Whether there's
another process, beyond the part we can describe with an equation,
making nature choose just one outcome, or whether all the outcomes
happen and there become more and more versions of us, no one knows.
There are other possible interpretations as well, but all of them are
just as weird.
(republished on 07/21/06)