A Brief History of Time - Stephen Hawking | Page 9

the gravitational attraction of a star went down faster or inc\
reased more rapidly with distance, the
orbits of the planets would not be elliptical, they would either spiral \
in to the sun or escape from the sun.
The big difference between the ideas of Aristotle and those of Galileo a\
nd Newton is that Aristotle believed in a
preferred state of rest, which any body would take up if it were not dri\
ven by some force Or impulse. In
particular, he thought that the earth was at rest. But it follows from N\
ewton’s laws that there is no unique
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standard of rest. One could equally well say that body A was at rest and body B was moving at constant speed
with respect to body A, or that body B was at rest and body A was moving. For example, if one sets aside for a
moment the rotation of the earth and its orbit round the sun, one could \
say that the earth was at rest and that a
train on it was traveling north at ninety miles per hour or that the tra\
in was at rest and the earth was moving
south at ninety miles per hour. If one carried out experiments with movi\
ng bodies on the train, all Newton’s laws
would still hold. For instance, playing Ping-Pong on the train, one woul\
d find that the ball obeyed Newton’s laws
just like a ball on a table by the track. So there is no way to tell whe\
ther it is the train or the earth that is moving.
The lack of an absolute standard of rest meant that one could not determ\
ine whether two events that took place
at different times occurred in the same position in space. For example, \
suppose our Ping-Pong ball on the train
bounces straight up and down, hitting the table twice on the same spot o\
ne second apart. To someone on the
track, the two bounces would seem to take place about forty meters apart\
, because the train would have
traveled that far down the track between the bounces. The nonexistence o\
f absolute rest therefore meant that
one could not give an event an absolute position in space, as Aristotle \
had believed. The positions of events
and the distances between them would be different for a person on the tr\
ain and one on the track, and there
would be no reason to prefer one person’s position to the other’s.\

Newton was very worried by this lack of absolute position, or absolute s\
pace, as it was called, because it did
not accord with his idea of an absolute God. In fact, he refused to acce\
pt lack of absolute space, even though it
was implied by his laws. He was severely criticized for this irrational \
belief by many people, most notably by
Bishop Berkeley, a philosopher who believed that all material objects an\
d space and time are an illusion. When
the famous Dr. Johnson was told of Berkeley’s opinion, he cried, “\
I refute it thus!” and stubbed his toe on a
large stone.
Both Aristotle and Newton believed in absolute time. That is, they belie\
ved that one could unambiguously
measure the interval of time between two events, and that this time woul\
d be the same whoever measured it,
provided they used a good clock. Time was completely separate from and i\
ndependent of space. This is what
most people would take to be the commonsense view. However, we have had \
to change our ideas about space
and time. Although our apparently commonsense notions work well when dea\
ling with things like apples, or
planets that travel comparatively slowly, they don’t work at all for \
things moving at or near the speed of light.
The fact that light travels at a finite, but very high, speed was first \
discovered in 1676 by the Danish astronomer
Ole Christensen Roemer. He observed that the times at which the moons of\
Jupiter appeared to pass behind
Jupiter were not evenly spaced, as one would expect if the moons went ro\
und Jupiter at a constant rate. As the
earth and Jupiter orbit around the sun, the distance between them varies\
. Roemer noticed that eclipses of
Jupiter’s moons appeared later the farther we were from Jupiter. He a\
rgued that this was because the light from
the moons took longer to reach us when we were farther away. His measure\
ments of the variations in the
distance of the earth from Jupiter were, however, not very accurate, and\
so his value for the speed of light was
140,000 miles per second, compared to the modern value of 186,000 miles \
per second. Nevertheless,
Roemer’s achievement, in not only proving that light travels at a fin\
ite speed, but also in measuring that speed,
was remarkable – coming as it did eleven years before Newton’s pub\
lication of Principia Mathematica. A proper
theory