Albert Einstein was right, say scientists, 100 years on

The theory of general relativity is as relevant
to us today as it was when it was formulated, as
a discovery about space-time reveals


After working for half a century and spending
£500m, scientists last week revealed that they
have detected strange fluctuations in Earth's
orbit.
Space-time
is bent and then twisted round our planet as it
rotates, announced researchers with Nasa's Gravity Probe B project.

The effect is tiny but crucial, they added – and
was predicted almost 100 years ago by
Albert
Einstein in his great theory of gravity, general
relativity. According to Einstein, an apple falls
to the ground not because it feels the force of
Earth's gravity but because the apple is
responding to the curvature of space-time near
the Earth's surface caused by the planet's huge
mass. In the same way, the Sun bends space in a
manner that allows Earth to revolve around it.

Crucially, the theory raised a host of other
predictions that scientists have been confirming
for the past century. The findings of Gravity
Probe B are the latest in a long list of these
many vindications of Einstein's genius and reveal
how his great theory touches our lives in unexpected ways.

"We have completed this landmark experiment of
testing Einstein's universe," said project leader
Francis Everitt, of Stanford University. "And Einstein survives."

Everitt began work on Gravity Probe B in 1962 and
has worked on nothing else since, although he had
many close shaves, with the satellite being
cancelled and then revived on seven occasions
before it was eventually launched in 2004. Then,
after the probe reached orbit, spurious
electrical signals were found to be distorting
data that it had sent back. It took five years'
study before scientists found how to extract clean data from it.

Results of the analyses of this data were
revealed last week. They showed that Earth does
indeed bend space-time. It was also found that,
as our planet rotates, it drags space-time with
it – a phenomenon known as frame-dragging. The
effect is like spinning a spoon in a cup of tea,
causing the liquid to start swirling round inside the cup.

These phenomena are tiny, it should be noted. In
the case of frame-dragging, space around Earth
turns at a rate of 37 one-thousandths of a second
every year because our planet pulls it round as
it revolves – a rate predicted by Einstein.

"The probe's results are a great achievement, but
we should not think of them as a new proof that
general relativity is right," said Graham
Farmelo, physicist and author. "Einstein was
shown to be correct long ago, only a few years
after he came up with the theory. However, we are
still testing out all its predictions. The
results from Gravity Probe B are just the most recent, successful outcomes."

In fact, the premise of Einstein's theory of
general relativity was proved within three years
of its publication in 1916. British astronomer
Arthur Eddington was involved in an expedition to
Príncipe island in west Africa, where he
photographed the total solar eclipse of 1919. The
photographs showed that the positions of stars
whose light rays passed near the Sun appeared to
have been slightly shifted because their light
had been curved by the Sun's huge gravitational
field. This was noticeable only during an eclipse
because the Sun's brightness would otherwise obscure the affected stars.

"Eddington presented these as a triumph for
general relativity, and Einstein, who was known
to physicists but not the public, became a star overnight," said Farmelo.

For his part, Einstein never had any doubts that
he was right. When asked how he would have
reacted if Eddington's observations had disproved
his theory, he replied: "I would have felt sorry
for the dear Lord. The theory is correct."

Since then, scientists have produced many other
confirmations. One example was provided by US
astronomers Russell Hulse and John Taylor, who
discovered two dense collapsed stars, known as
neutron stars, that were in orbit around each
other and that were losing energy that could only
be explained through the gravitational radiation,
another phenomenon predicted by Einstein. In 1993
Hulse and Taylor were awarded that year's Nobel
prize for physics.

"General relativity touches our lives in many
unexpected ways," added physicist Dr Charles
Wang, of Aberdeen University. "Another effect
that is predicted by general relativity is the
phenomenon known as gravitational time dilation.
This states that time slows down as gravitational
strength increases, a fact that has been
confirmed by GPS satellites. These carry atomic
clocks that have to run at a different speed to
those on the ground because gravity is greater
there. If you didn't, the world's GPS system would break down."

"Einstein's theory of general relativity is one
of the most beautiful pieces of scientific work
in history," added Farmelo. "But it is not the
whole story. It explains how massive objects
affect space and time, but it tells us little
about how very small sub-atomic particles behave."

This point was acknowledged by Wang. "We still
have to test how gravity behaves at a sub-atomic,
quantum level," he said. "We hope to do that with
a project called the Space-Time Explorer Quest,
which the European Space Agency is now studying."

Together with Wang's group, scientists at
Birmingham, the Rutherford-Appleton Laboratory
and the National Physical Laboratory, along with
German researchers, have proposed building a
probe that would use caesium or rubidium atoms,
cooled close to absolute zero, to test the effect
of gravity at the sub-atomic level and, they
hope, provide data that could reconcile relativity and quantum theory.

"That won't be for another 10 years or more,"
said Wang. "That means we are going to have a lot
of work to do on general relativity for quite some time."

* guardian.co.uk © Guardian News and Media Limited 2011