In the twentieth century, we have been greatly privileged
to witness two major revolutions in our physical picture of
the world. The first of these upturned our conceptions of
space and time, combining the two into what we now call
space-time, a space-time which is found to be subtly curved
in a way that gives rise to that long-familiar, omnipresent
but mysterious, phenomenon of gravity. The second of these
revolutions completely changed the way in which we under-
stand the nature of matter and radiation, giving us a picture
of reality in which particles behave like waves and waves like
particles, where our normal physical descriptions become
subject to essential uncertainties, and where individual ob-
jects can manifest themselves in several places at the same
time. We have come to use the term "relativity" to encom-
pass the first of these revolutions and "quantum theory" to
encompass the second. Both have now been observationally
confirmed to a precision unprecedented in scientific history.