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Prior to the twentieth century, when the notion that light was made up of
waves
dominated physical thought, scientists believed that there must be a medium
in which light waves were propagated. After all, water waves were carried
by water and sound waves were carried by molecules in the air (or whatever
the medium was that transmitted the sound).
Sound, for example, does not travel through a vacuum, because there are no
molecules to transmit it. Waves, it was thought, could not be transmitted
without there being some substance to carry them, so light had to have a
supporting medium.
The medium that allowed propagation of light waves was called the ether-
(The term 'ether' is derived from the Greek word for 'blazing': the ancient
Greeks used the word to describe the element from which the stars were made.)
Nobody knew exactly what the ether was: it was necessary to assume its existence
in order to accept the view that light was made of waves.
Light also has a finite speed: it takes some time for it to travel from its
source to another object. The light from distant stars that arrives on Earth
has travelled so far that what we actually detect is light that left a star
thousands, millions or even billions of years ago, depending how far away
the star is.
Of course, that light has a finite speed is not noticeable in everyday events:
it is so fast that it takes only a billionth of a second for light to travel
to our eyes from a source a few metres (or feet) away. When a light is turned
on in a room we do not notice the tiny fraction of a second that it takes
for the light to travel from the bulb to our eyes.
The first attempt at measuring the speed of light was made in 1676 by the
Danish astronomer, Ole Römer (1644-1710). More accurate measurements
were made later on, and it became clear that liaht did have a definite speed.
According to Maxwell's equations,which assumed that light consisted of waves,
light should travel more slowly in denser transparent materials such as glass
and water than it does in air.
Experiments in which the speed of light was measured in different materials
confirmed this. For example, the speed of light is reduced by about a third,
to 200000 kilometres per second (124000 miles per second) when it enters
a block of glass, largely because of the time it takes to be absorbed and
emitted by atoms in the glass during its transmission.
When more of the detailed properties of light were established, calculations
showed that the ether had to be a rigid solid that vibrates when light passes
through it
Since nobody could see or detect the ether, it had to be very fine as well
as rigid, and it had to be everywhere, even in a vacuum, because light can
travel through a vacuum. clearly, the ether, if it existed, had to be a new
kind of 'substance' compared with those previously known.
Nevertheless, if the ether existed it should be possible to devise experiments
that could detect its presence. Indeed, in 1887 two US scientists, Albert
Michelson (1852-1931) and Edward Morley (1838-1923) carried out a precise
experiment aimed at detecting the ether.
Their reasoning was that since the Earth is in motion it should be moving
through the stationary ether in a similar way to a swimmer moving through
water. This should create an 'ether pressure' that would be due to the ether
apparently 'flowing' past the Earth.
Furthermore, because the Earth is rotating about its own axis as well as
travelling around the Sun in an elliptical orbit, its direction of travel
through the ether will vary at different times, and so the direction of the
'ether pressure' at a particular point on the Earth will change with the
Earth's motion.
If the speed of light was measured from a point (a physics laboratory, for
example) on the Earth, its measured speed would depend on whether the light
was transmitted in the same or a different direction compared with that of
the Earth's motion in the ether.
calculations showed that the measured speed of light travelling parallel
to the direction of apparent flow of the ether should be slower than the
measured speed of light travelling at right angles to this direction of flow,
on a round trip.
In much the same way, a person who swims up and down a river against the
water's current will take longer to cover a certain distance than he or she
would take to travel the same distance by swimming at right angles to the
current.
Michelson and Morley used a very accurate device for simultaneously measuring
the speed of light in different directions compared with the Earth's motion
and their equipment should easily have detected any movement through the
ether. However, they failed to find any differences between their measured
values of the speed of light, even after thousands of attempts. They could
not detect any ether pressure - they could not detect the ether.
Since Michelson and Morley carried out their experiments, many more scientists
have attempted to find evidence for the ether. Some of these experiments
were carried out more recently using highly sophisticated technology. None
of them has found any evidence for the existence of an ether: the speed of
light was always the same whenever and in whatever direction it was
measured.
To the physicists of the nineteenth century, the ether was not only an essential
medium in which light waves were propagated: it was also an absolute frame
of reference that allowed the true motions of objects to be determined.
To illustrate the significance of this statement, consider a train moving
through the countryside at a certain constant speed. Passengers on the train
have no doubt that they are moving because they can see the trees and fields
go by. However, if the background trees and fields are removed, it becomes
more difficult to assess whether or not one is moving unless the train changes
its speed when a passenger can feel the
force produced by the slowing down or speeding up of the train.
If another train appears on an adjacent track, it becomes difficult to assess
which train is moving the fastest, and even whether or not one train is
stationary. If one train was travelling at 100 kilometres per hour (65 miles
per hour) and the other was moving in th same direction at 60 kilometres
per hour (40 miles per hour), the situation would look much the same to
passengers as that existing when one train travelled at 40 kilometres per
hour (25 miles per hour) with the other train at rest.
In addition, without the background as a frame of reference, it would not
be possible for passengers of either train to be sure which train was moving:
each of them could claim that they were at rest and the other was moving,
and each could claim that they were moving and the other was at rest
The point about the background scenery or a moving train is that it provides
a trame of reference from which one can say that one is in motion. It allows
one to say that the train is moving relative to the scenery (the Earth).
without some frame of reference, it is impossible to say whether anything
is moving or at rest, provided that its speed is constant.
Indeed, the Earth is revolving around the Sun at 108000 kilometres per hour
(67000 miles per hour); the Sun and its solar system are revolving around
the centre of our Milky way galaxy at 500 000 kilometres per hour (310 000
miles per hour); and our galaxy is travelling through the Universe at a speed
of 2 300 000 kilometres per hour (1 500 000 miles per hour).
There is no doubt that we are all whizzing through the cosmos at a breathtaking
speed, but we fail to perceive this in our everyday lives.
The ether provided physicists with a universal background 'scenery' against
which the absolute motions of objects could be assessed.
When we say that the Earth is revolving around the Sun, we use the Sun as
a frame of reference: the revolution of the Earth is considered relative
to the Sun. However, if the Sun is moving as well, we get no indication of
the absolute motion of the Earth and we have to consider the motion of the
Sun relative to something else, for example, another star. But if this other
star is also moving, we need another frame of reference with which to measure
its speed, and then what is the true (absolute) speed of the Earth or the
Sun?
The stationary ether provided that frame of absolute rest: it was considered
to be completely stationary and not to be moving with the Earth, the Sun,
the stars, or any other celestial object. If the speed of the Earth could
be measured with respect to the ether, then its absolute speed would be
determined, and the speed or anything measured relative to the Earth could
then be used to determine the absolute speed of that object.
Newton was well aware of the problem of relative motion. His concept of absolute
rest was a religious one. Although we cannot be sure that any of the objects
we see are absolutely still, he said that there is such a thing as absolute
rest and it is known by God.
Although faith can be a good thing under some circumstances, it does not
provide any practical way of sorting things out in the physical world. The
ether was a more concrete and tangible way of providing a frame of reference
that was at absolute rest. Unfortunately, its existence could not be detected,
and absolute rest was on shaky ground.
This was a considerable blow to classical physics. For example,
Newton's Laws of Motion
required the existence of a frame of reference that was at absolute rest,
and Maxwell's equations also needed an absolute frame of reference. without
the ether, there could be no medium for transmission of light waves and there
could be no absolute frame of reference against which to measure the speeds
of moving oblects.
In order to explain Michelson and Morley's failure to detect the ether the
Irish physicist,George FitzGerald (1851 - 1901), proposed that a moving object
shortens in length in the direction of its absolute motion.
According to FitzGerald, a ruler pointing in the same direction of the Earth's
motion would contract lengthwise, whereas a ruler pointing at right angles
to the Earth's motion would not contract lengthwise (although it would contract
widthwise, since its edge is moving in the same direction as the Earth).
FitzGerald derived a mathematical equation showing that any measurements
made or the speed of light in the same direction as the Earth's motion would
be compensated for by contraction of the measuring apparatus and would be
the same as the measurements of the speed of light made at right angles to
the Earth's movement.
In other words, the ether could still exist but it might not be detected
by measuring the speed of light as Michelson and Morley did. The idea that
objects get shorter in the direction of their motion was a rather peculiar
one, but it did provide a possible answer to the experimental data; and Einstein
used the same idea in his Theory of Relativity
The degree of contraction or an object moving under most everyday circumstances
was minuscule, according to FitzGerald, but as the speed approached that
of light, contraction became obvious.
Thus, a 30 centimetre (1 foot) ruler would contract to about 27 centimetres
(10.5 inches) when it was travelling at half of the speed of light (150 000
kilometres per second); at three quarters of the speed of light it would
become about 20 centimetres (8 inches) long; and at the speed of light it
would not have any length at all!
Subsequently, the Dutch scientist, Hendrik Antoon Lorentz (1853-1928) showed
mathematically that not only should an object contract in its direction of
absolute motion, but also its mass should increase.
For example, an object weighing a Kilogram at rest should weigh about 1.15
kilograms when it travelled at half the speed of light; its mass would increase
to 1.5 kilograms at three-quarters of the speed of light; and at the speed
of light its mass should become infinite! Lorentz described this effect for
charged particles in motion, but Einstein later showed that the mass increase
occurred with all moving bodies.
It may sound ridiculous that a moving object not only contracts but also
that it gains mass as its speed increases: the whole idea seems to be contrary
to common sense. However, common sense deals with the everyday world, where
speeds are very small compared with that of light.
A car travelling at 60 kilometres per hour (40 miles per hour) is moving
with a speed of only one eighteen-millionth of the speed of light and this
results in a contraction and mass increase that are not noticeable. As speeds
approach that of light, contraction and mass increase do become apparent,
but we never see objects moving at these speeds under most normal
circumstances.
Although FitzGerald had proposed contraction to save the existence of the
ether, neither contraction in the direction of an object's motion nor its
mass increase with speed were dependent on the existence of the ether.
Albert Einstein arrived at his special
Theory of Relativity
by ignoring the ether:
according to him, the ether was not needed. This interpretation of the motion
of objects also solved many of the problems that classical physics had failed
to explain.
©WebsterWorld Pty Ltd/contributors 2002