Page 11 - A Brief History of Time - Stephen Hawking

A Brief History of Time - Stephen Hawking... Chapter 2
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 train was at rest and the earth was moving
south at ninety miles per hour. If one carried out experiments with moving bodies on the train, all Newton’s laws
would still hold. For instance, playing Ping-Pong on the train, one would 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 whether it is the train or the earth that is moving.

The lack of an absolute standard of rest meant that one could not determine 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 one 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 of 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 train 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 space, as it was called, because it did
not accord with his idea of an absolute God. In fact, he refused to accept 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 and 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 believed that one could unambiguously
measure the interval of time between two events, and that this time would be the same whoever measured it,
provided they used a good clock. Time was completely separate from and independent 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 dealing 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 round 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 argued that this was because the light from
the moons took longer to reach us when we were farther away. His measurements 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 finite speed, but also in measuring that speed,
was remarkable – coming as it did eleven years before Newton’s publication of Principia Mathematica. A proper
theory of the propagation of light didn’t come until 1865, when the British physicist James Clerk Maxwell
succeeded in unifying the partial theories that up to then had been used to describe the forces of electricity and
magnetism. Maxwell’s equations predicted that there could be wavelike disturbances in the combined
electromagnetic field, and that these would travel at a fixed speed, like ripples on a pond. If the wavelength of
these waves (the distance between one wave crest and the next) is a meter or more, they are what we now call
radio waves. Shorter wavelengths are known as microwaves (a few centimeters) or infrared (more than a
ten-thousandth of a centimeter). Visible light has a wavelength of between only forty and eighty millionths of a
centimeter. Even shorter wavelengths are known as ultraviolet, X rays, and gamma rays.

Maxwell’s theory predicted that radio or light waves should travel at a certain fixed speed. But Newton’s theory
had got rid of the idea of absolute rest, so if light was supposed to travel at a fixed speed, one would have to
say what that fixed speed was to be measured relative to.

It was therefore suggested that there was a substance called the "ether" that was present everywhere, even in
"empty" space. Light waves should travel through the ether as sound waves travel through air, and their speed
should therefore be relative to the ether. Different observers, moving relative to the ether, would see light

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