[IP] Einstein, the first spin doctor
Einstein, the first spin doctor
An experiment to test the theory of relativity could rock the science world
Paul Davies
Saturday April 10, 2004
The Guardian
<http://www.guardian.co.uk/comment/story/0,3604,1189322,00.html>
The birth of science as we know it arguably began with Isaac Newton's
formulation of the laws of gravitation and motion. It is no exaggeration to
say that physics was reborn in the early 20th-century with the twin
revolutions of quantum mechanics and the theory of relativity. The latter
was famously the brainchild of Albert Einstein, and attained its general
form as a theory of gravitation, motion and space-time structure.
Whereas Newton envisaged gravity as a force operating between bodies
across empty space, Einstein attributed it to a warping or distortion of
the geometry of space and time. It was a radical idea that demanded
stringent tests.. The first was already in the bag. Astronomers had long
been puzzled about a tiny but persistent deviation of the motion of the
planet Mercury from the orbit predicted by Newton's theory. Einstein's
general relativity theory not only accounted for this effect, but gave the
right answer for its magnitude.
The second test concerned the way that the sun's gravity bends starbeams.
In 1919, the astronomer Sir Arthur Eddington led an expedition to West
Africa to measure the bending of starlight during a solar eclipse, and
triumphantly vindicated general relativity. Then, in 1960, the third test
was finally conducted with reasonable accuracy. Physicists at Harvard
University measured a small shift in the frequency of gamma rays directed
vertically up a tower, confirming that gravity slows time.
All three effects are extremely small and hard to measure, for although
general relativity proceeds from a conceptual basis totally unlike Newton's
theory, the observational consequences are almost identical for weak
gravitational fields, and bodies travelling slowly compared to light.
Things changed with the discovery of neutron stars and black holes -
objects with gravitational fields so intense that dramatic space and
time-warping effects occur. These are located many light years across the
galaxy, and their behaviour is often complicated by other physical
processes. With advancing technology, physicists began to wonder whether
there might be simpler ways to investigate general relativity by conducting
new forms of highly sensitive experiments from within the solar system.
One possibility concerns the gravitational properties of spinning bodies.
In Newton's theory, the inward pull of a spinning star or planet depends
only on the distribution of matter within the body. But general relativity
predicts that spin itself should produce its own distinctive gravitational
imprint. Roughly speaking, as a body rotates, its spacewarp turns with it,
and an orbiting satellite should experience a sideways force encouraging it
to co-rotate.
In 1959 Leonard Schiff, a physicist at Stanford University in California,
devised an experiment to put a gyroscope in orbit around the Earth, and
observe its motion very carefully. According to Newton's theory, the spin
axis of the gyroscope should always point to a fixed part of the sky (this
is the basis of spacecraft navigation). But general relativity predicts a
tiny twist in the spin axis caused by the Earth's rotation tipping the
gyroscope's axis. The trouble is, the effect is almost unbelievably small.
It has taken an incredible four decades of planning and laboratory
development before Schiff's experiment is ready to fly. The payload, to be
launched from Vandenberg Air Force Base on April 19, consists of four
gyroscopes engineered to astonishing precision, cooled by a huge vat of
liquid helium to enhance stability and provide superconducting shielding
from electromagnetic disturbances.
It is very rarely that physicists get a chance to test the foundations of
a fundamental theory in a clean, make-or-break manner. General relativity
is the cornerstone of cosmology and astrophysics. It has also provided the
conceptual basis for string theory and other attempts to unify all the
forces of nature in terms of geometrical structures. But general relativity
is not the only show in town. Other theories of gravity besides Newton's
exist, some of which predict different effects of rotating bodies.
If the experiment confirms the general theory of relativity, it will be a
stunning tribute for Albert Einstein in the centenary year of his annus
mirabilis. If the results turn out to be different, then the cat will truly
be put among the pigeons. A central pillar of modern physics will have
collapsed, with consequences that can scarcely be predicted. Our
painstakingly crafted understanding of stars, black holes and the universe
would be thrown into the melting pot. The stakes are therefore very high.
To paraphrase Neil Armstrong, one tiny twist for a gyroscope would turn out
to be one giant leap for theoretical physics.
· Paul Davies is a physicist at the Australian Centre for Astrobiology. His
latest book is How to Build a Time Machine pdavies@xxxxxxxxxxxxx
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