2 November 2019 | New Scientist | 3319th century drew to a close, though, there was
less and less reason to believe it existed. First,
the newly developed electromagnetic theory
identified light waves as vibrations of the
intensity of the electromagnetic field. This
was a new kind of wave, unlike sound, that
had the freedom to travel without the aid
of a separate medium. That kicked away
the aether’s strongest theoretical strut.
Second, experiments designed to directly
measure the aether’s existence found no effect.
The most famous, conducted by physicists
Albert Michelson and Edward Morley in 1887,
sought to see whether light travelling through
the aether was in any way slowed down by it. It
wasn’t. “People reacted in different ways,” says
Allori. They invented all sorts of theoretical
reasons why the aether could still be out there.
Then Albert Einstein got involved.
Einstein’s theory of special relativity,
published in 1905, dealt the aether its
final blow. While the theory probably
didn’t arise as a reaction to Michelson
and Morley’s results, it was published
at the ideal moment to capitalise on the
concept’s dwindling popularity.Relatively successful
The theory of relativity made a remarkable
prediction about the world. It posited that
the laws of physics should be the same to all
observers who see themselves as being at rest,
even if they are moving at a constant velocity
relative to one another. On a gut level, this
makes good sense. After all, our own constant
motion relative to the ground, on a train or
plane journey for instance, doesn’t interfere
with our physical movements. It isn’t as if
we need to compensate for the plane’s speed
when walking down the aisle.
But there is a twist. If two observers can
both claim to be perfectly still despite each
appearing to be in motion to the other, the
notion of anything ever being absolutely
“at rest” is a fiction. That meant that the
aether – a universe-spanning reference frame
that all observers were supposed to see as at
rest – would be incompatible with relativity.
From there, the aether rapidly disappeared
from serious physics. Nowadays, it is often a
symbol for discredited ideas that hang around
long past their expiry date. That’s not really a
fair judgement, says Allori: Einstein didn’t
prove the aether couldn’t exist, he merely
showed there was no need for it. His
explanation was simpler for contemporaryThere were also good reasons to believe in
the aether. For centuries, one of the most hotly
debated questions in physics concerned the
true nature of light. Were beams of light made
up of particles, firing one after the other like
bullets out of a gun, or waves, lapping on each
other’s heels like a rising tide? Today, we know
that light is capable of existing in either form,
but in the 17th century such a compromise
seemed impossible. Only one school of
thought could be right.
Many ardent believers in the wave picture,
such as Dutch luminary Christiaan Huygens,
found one particular aspect of light’s
behaviour bothersome. “They thought it
was like a sound wave,” says Valia Allori, a
philosopher of physics at Northern Illinois
University. Just as sound waves cannot
propagate without a medium to travel
through, light waves needed a medium too.
And given that we see the stars at night,
this medium must also be present in the
apparent emptiness of outer space. This
space-filling feature gave it another important
role: it could set a standard frame of reference
with respect to which all things moved.
Huygens called this the luminiferous aether,
a sort of invisible, all-pervading mist devoted
to ensuring the smooth passage of light
and defining absolute motion.
The aether rapidly became a mainstay of
physical thought, even though no evidence for
this mysterious medium ever emerged. As the>“ Einstein didn’t
prove that
the aether
couldn’t exist,
merely that
there was no
need for it”