This week, 99 years ago, Sir Arthur
Eddington arrived at his destination to test a theory of Albert Einstein.
Einstein had just four years earlier published his groundbreaking papers that
introduced the theory of general relativity, and it had not gone down well with
the scientific community, as it had contradicted the great Sir Isaac Newton’s
model of classical mechanics as formulated in his 1687 book Philosophiæ Naturalis Principia Mathematica.
It was a natural extension of Einstein’s
earlier work, the 1905 publish theory of special relativity, where he showed
that the laws of physics were the same for all non-accelerating observers. See,
in the late 19th century, physicists were searching for the ether –
the medium that light travelled through. But between April and July 1887,
Albert Michelson and Edward Morley conducted an experiment to detect the speed
of light as it travelled through the ether in perpendicular directions in order
to detect the movement of the ether.
Their experiment did not produce the
results they expected. Michelson and Morley found no difference in the speed of
light no matter what direction they used – the speed of light was the same with
or against the earth’s rotation, and with or against the earth’s orbit. This
experiment would in the years following become known as the most famous failed
experiment.
Other experiments after this continued to
provide the same results, with most physicists assuming the experiment was
wrong. A young physicist named Einstein tackled the problem from a different
direction. He assumed that the laws of physics do not change, and that the
experiments were correct – the speed of light is the same for all observers no
matter the speed they were moving.
Einstein’s special theory of relativity
removed ether from the equation, and fundamentally changed how we perceive the
world around us. Instead of an inert ‘ether’, the only frame of reference would
be the relative difference in velocity. It also introduced the concept of
space-time. Physicists had always assumed that physical space was simply the
normal three dimensions, but Einstein had shown that, due to the speed of light
being constant, time was also a part of space, and was manipulated by movement
through it.
As an example, suppose you were in a
spacecraft moving at half the speed of light. Since you’re not accelerating,
you don’t appear to be moving from your frame of reference. If you took a
laser, and shone it against a mirror on the roof to reflect to a detector, it
would appear to you as if the light travelled in a straight line up and down to
the reflector. But from an observer stationary outside the ship, and looking
into the window at the exact right time, it would look very different – the
light would appear to move at an angle up to the mirror and at an angle down to
the reflector! Since the speed of light is constant, what could account for
this difference?
The answer, according to Einstein, was
obvious. Time is slower in a fast moving object! This effect of special
relativity is known as time dilation. This is not obvious – in fact, in our
tough experiment above, moving at just half the speed of light would slow time
inside by only 13.4%. By 90% of the speed of light, time would slow by more
than half – 56.4%. By 99% of the speed of light, time would pass at 14.1% of
normal speed.
Einstein’s theory seemed to solve the
problem of the missing ether, but Einstein noticed a paradox it created.
Newton’s first law of motion stated that an object remains in uniform motion
unless acted on by a force. So you should feel a force if you’re accelerating,
correct? So then why, when you fall, and gravity acts on you, do you not feel a
force acting on you, but instead feel weightless as you accelerate?
Einstein realized that the space-time
concept in his special theory of relativity could explain this, and explain the
origin of gravity. He theorized that mass warped space-time! Earth’s mass would
warp space-time in such a way that a moving object without a force acting on it
would actually move in a straight line from its own frame of reference. In
fact, time dilation occurs in gravity – the time difference between earth
surface and its centre would be about 3.4 microseconds over 1000 years.
This
was what Einstein proposed in 1915 with his publication of the general theory
of relativity, and was met with great scepticism. How could this 34-year-old
try and upstage the tested works of the great Isaac Newton? It was also in the
middle of World War I, and Einstein’s work was isolated to the German
scientific community. Dutch physicist Willem De Sitter, however, heard of it,
and told British astronomer Sir Arthur Eddington.
Since Einstein’s theory could only be
tested by observing the light from stars bending around the sun (the only
massive object close enough that such distortions in space-time would be
visible), and the sun’s glare making that impossible, Einstein’s theory could
not be tested. Sir Arthur Eddington, however, was an astronomer, and he knew of
a way. In 1917, he discussed it with Sir Frank Watson Dyson, Astronomer Royal,
and in 1919 he was sent to the remote island of Príncipe, off the west coast of
Africa.
During January and February 1919, he had
measured the true positions of the stars in the Hyades star cluster, and on 29
May 1919, he was ready on Príncipe. The sky was clear, and then it happened. A
total solar eclipse. The Hyades were visible during the day, and right next to
the sun. The astronomer and his team took several pictures during the eclipse,
and returned to England. There they studied the pictures, and checked and
rechecked their calculations. On November 6, 1919, he published his results –
Einstein had dethroned Newton, and the stars had moved just like his theory
predicted. When the London Times published his results a day later, with the
headline “Revolution in Science: New Theory of the
Universe: Newtonian Ideas Overthrown,“ Einstein was no longer an
obscure German physicist, but a worldwide celebrity.
