Word
Gems
What is a
man but the sum of his thoughts?
Time:
Steven
Weinberg:
A
Certain Approximation
IT WAS GALILEO who first brought time into science 400 years ago.
Aristotle had been happy enough to describe what everyone had always seen, that earth and
water tend to fall while air and fire tend to rise... without trying to say how fast they
rose or fell. The more practical physicists working in the military-industrial complex in
Hellenistic Alexandria had been interested only in the shapes of the trajectories of
stones and arrows, not in their time of flight. Galileo for the first time took on the
task of predicting just where a falling body would be at every moment.
In the centuries that followed Galileo's pioneering work, the instruments used to
measure time became much more precise than Galileo's primitive water clock. Gradually the
idea spread that it should be possible to calculate what would happen to any sort of
system at every moment in time. Time itself was thought to be outside our control, a
relentless and objective flow unaffected by anything that happened in it.
All this has now been changed by the work of physicists in this century. Einstein
showed in 1905 that the constancy of the speed of light could be understood only if motion slows the flow of time. The changes in the rates of moving clocks had
escaped notice until then because these changes are tiny, except at speeds close to that
of light. There is, however, no doubt that Einstein was right. For instance, it has been
recently observed that the explosions of stars, known as supernovas, in galaxies that are
rushing away from us at very high speed take longer than the same sort of explosions in
our own galaxy. Observers who are moving rapidly past each other may even disagree about
which events come before and which come after. Einstein realized a few years later that
the flow of time is also slowed by gravitation.
- Time is not the background to the natural world - it is a
part of nature, like energy or matter.
The role of time in our theories has lately become even weirder. As we
understand nature better, we sometimes find that what we thought had to be key ingredients
of our deepest theories are only secondary concepts; that they are relevant only in
certain limited contexts.
For instance, through most of the 19th century, it was thought that
temperature was a universal attribute of matter. But for 100 years we have understood that
the idea of temperature makes sense only in an approximate way for collections of any
finite number of atoms or molecules, and not at all for a single atom. The same demotion
now seems to be happening to time, and to space as well.
In the most promising fundamental theories being developed these days,
time and space do not enter in the equations but only appear when we make certain
approximations to solve those equations. If these theories are right then it makes
no sense to talk about intervals of time shorter than about a
billion-billion-billion-billion-billionth of a second. We can now understand pretty well
what happened when the universe was three minutes old, and we have good hopes of
understanding what happened when it was a second old, but if we try to say what the
universe was like when it was less than a billion-billion-billion-billion-billionth of a
second old, we find ourselves dealing with a strange state in which time and space
have no meaning.
Some people have suggested that changes in the scientists' concept of time
should affect the way we look at time in human affairs. The great psychologist Jean Piaget
thought that Einstein's theory of relativity would help us understand how infants perceive
time. I have my doubts. In our daily lives we do not move around at speeds near the speed
of light, nor visit strong gravitational fields like those around black holes and neutron
stars, nor notice time intervals of a billionth of a second, much less a
billion-billion-billion-billion-billionth of a second.
As far as human affairs are concerned, time is what it was to Galileo:
- an inexorable progression of instant after instant.
As always, the most important thing about time is that, as Dylan Thomas put it, time
holds us green and dying. Each of us only gets so much time, and our fate is to watch this
stock steadily diminish.
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