The two faces of light

I now
need to return to the subject of light, not because it ismerely ‘useful’
but because it underpins the whole of special relativity.
By the late nineteenth century, Thomas Young (the Englishman
who proved Newton wrong about light being made up of
particles) and James Clerk Maxwell (the Scotsman who discovered
that light was made up of electromagnetic waves) had shown
beyond any doubt that light behaves like waves. Today there
are numerous experiments that clearly and beautifully reveal the
wave nature of light. It is true that quantum mechanics has since
shown that light can, under certain circumstances, also behave
like a stream of particles, but for the following discussion it is its
wavelike nature which we require.
An important property of waves is that they need something
to move through; a medium through which the vibrations can
propagate. When you speak to someone standing next to you,
the sound waves that travel from your mouth to his ear need
the air in between to move through. Likewise, water waves on
the surface of the sea need the water, and the ‘bump’ that travels
along a length of rope when it is given a flick at one end needs the
rope. Clearly, without the medium to carry the wave along there would be no wave. This was why nineteenth century physicists
were convinced that light, having been confirmed as a wave, also
needed a medium. And since no one had seen such a medium,
they had to think of a way of proving its existence.
It was known as the luminiferous1 ether—not to be confused
with the organic chemical used as an anaesthetic—and the hunt
was on to find it. If it existed then it had to have certain properties.
For a start, it had to permeate the whole of space in order for
starlight to be able to reach us. It had to exist everywhere, even in
the empty space inside atoms. An important property of the ether
was that it could not interact with material objects and therefore
could not be dragged along with them when they moved. This
had been confirmed as long ago as 1729 due to a property of light
known as aberration.
Nothing else was known about the ether. It was hoped that
much more would become clear with advances being made in the
field of optics. However, nobody was prepared for what was to
come next.
In 1907, AA Michelson became the first American to win the
Nobel prize in physics for an experiment he had carried out in the
1880s together with E W Morley. It is probably the most famous
experiment in the whole of physics. Michelson had invented a
device known as an interferometer which relies on the wave nature
of light to measure the time it takes for a light beam to cover a fixed
distance. By clever use of his interferometer to measure how fast
light beams travel he was able to prove beyond any doubt that the
ether could not exist!
An important fact in physics is that all waves travel at a speed
that does not depend on the speed of the source of the waves.
Think of the sound of an approaching fast car. The sound waves
will reach your ear before the car since they are travelling faster, but
their speed is to do with how quickly the vibrating air molecules
can transmit them. They do not reach you any quicker by virtue
of being ‘pushed’ along by the moving car. What happens instead
is that the waves get squashed up to higher frequency and shorter wavelength in front of the car (the Doppler effect) but the speed of
the sound itself doesn’t change2.
Sound waves travel through air at a speed of 1200 kilometres
per hour. This speed is independent of how fast the car is moving.
If the car is travelling at 100 kilometres per hour then the driver
would see the sound waves (assuming sound waves could be seen)
moving ahead of him at a speed of only 1100 kilometres per hour
(1200 minus 100). The faster the car goes the slower the relative
speed of the sound waves that the driver sees. But to a stationary
observer watching the approaching car, the sound waves always
travel at 1200 kilometres per hour no matter how fast the car is
moving. If the driver and the stationary observer had an argument
about the speed of sound, the driver would have to admit that the
speed he observes the waves moving at is not their true speed
because he too is moving relative to the air molecules.
Michelson and Morley applied this principle to light waves.
They assumed that the Earth is moving through the ether as it
orbits the Sun (at about a hundred thousand kilometres per hour).
Their experiment is a little tricky to describe so I will not go into the
finer details. Suffice it to say that they measured with very high
accuracy the time it took light in a laboratory to travel along two
paths of equal distance, one in the direction of the Earth’s motion as
it orbited the Sun and the other at right angles to it. Sitting in their
laboratory on Earth and observing the speed of light they were like
the car driver who would measure the sound waves leaving the car
at different speeds depending on what direction he looked. After
all, to him the sound waves that were travelling straight upwards
would still be moving at 1200 kilometres per hour.
If the ether existed, and Michelson and Morley knew that
the Earth had to be moving freely through it, then the light
moving along the different paths would cover the two equal
distances in different times. This would indicate that, relative
to the moving Earth, the light was moving at different speeds in
the two directions. Although the speed of light is three hundred thousand kilometres per second, which is ten thousand times faster
than the speed of the Earth, Michelson’s interferometer was still
accurate enough to pick up any difference in the timing between
the two beams if there was any. None was found. Many more
precise experiments using laser beams have since then confirmed
Michelson and Morley’s result.
Their experiment had shown that light was not like other
waves. It travels at a speed that is the same whether you are
moving towards the source or away from it. It doesn’t have a
fixed background against which its speed can be measured. So
there was no need for the ether at all.
Most physicists at the time refused to believe this and tried to
modify the laws of physics to accommodate the new result but to
no avail. They tried to argue that light was behaving as a stream
of particles (since that would also explain the result) but the experiment
was set up specifically to detect the wave nature of light.
It detected intereference patterns between the waves in a manner
quite similar to Thomas Young’s original set-up which confirmed
the wave nature of light in the first place. In any case, light behaving
as particles would also do away with the need for an ether
since they would not require a medium to travel through.