Page 43 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 5
There is a mathematical theorem that says that any theory that obeys quantum mechanics and relativity must always
obey the combined symmetry CPT. In other words, the universe would have to behave the same if one replaced particles
by antiparticles, took the mirror image, and also reversed the direction of time. But Cronin and Fitch showed that if one
replaces particles by antiparticles and takes the mirror image, but does not reverse the direction of time, then the
universe does not behave the same. The laws of physics, therefore, must change if one reverses the direction of time –
they do not obey the symmetry T.

Certainly the early universe does not obey the symmetry T: as time runs forward the universe expands – if it ran
backward, the universe would be contracting. And since there are forces that do not obey the symmetry T, it follows that
as the universe expands, these forces could cause more antielectrons to turn into quarks than electrons into antiquarks.
Then, as the universe expanded and cooled, the antiquarks would annihilate with the quarks, but since there would be
more quarks than antiquarks, a small excess of quarks would remain. It is these that make up the matter we see today
and out of which we ourselves are made. Thus our very existence could be regarded as a confirmation of grand unified
theories, though a qualitative one only; the uncertainties are such that one cannot predict the numbers of quarks that will
be left after the annihilation, or even whether it would be quarks or antiquarks that would remain. (Had it been an excess
of antiquarks, however, we would simply have named antiquarks quarks, and quarks antiquarks.)
Grand unified theories do not include the force of gravity. This does not matter too much, because gravity is such a weak
force that its effects can usually be neglected when we are dealing with elementary particles or atoms. However, the fact
that it is both long range and always attractive means that its effects all add up. So for a sufficiently large number of
matter particles, gravitational forces can dominate over all other forces. This is why it is gravity that determines the
evolution of the universe. Even for objects the size of stars, the attractive force of gravity can win over all the other forces
and cause the star to collapse. My work in the 1970s focused on the black holes that can result from such stellar collapse
and the intense gravitational fields around them. It was this that led to the first hints of how the theories of quantum
mechanics and general relativity might affect each other – a glimpse of the shape of a quantum theory of gravity yet to
come.

PREVIOUS NEXT

file:///C|/WINDOWS/Desktop/blahh/Stephen Hawking - A brief history of time/d.html (8 of 8) [2/20/2001 3:14:54 AM]