| General
relativity is a theory of gravitation developed by Einstein in the years
1907–1915.
The development of general relativity began with the equivalence
principle, under which the states of accelerated
motion and being at rest in a gravitational
field (for example when standing of the surface of the Earth) are
physically identical. The upshot of this is that free
fall is inertial
motion: In other words an object in free fall is falling because
that is how objects move when there is no force
being exerted on them, instead of this being due to the force of gravity
as is the case in classical
mechanics. This is incompatible with classical mechanics and special
relativity because in those theories intertially moving objects
cannot accelerate with respect to each other, but objects in free fall
do so. To resolve this difficulty Einstein first proposed that spacetime
is curved.
In 1915, he
devised the Einstein
field equations which relate the curvature of spacetime with the
mass, energy, and momentum within it.
Some of the consequences of general relativity are:
Technically, general relativity is a metric
theory of gravitation
whose defining feature is its use of the Einstein field equations. The
solutions of the field equations are metric
tensors which define the topology
of the spacetime and how objects move intertially.
Einstein was right: space and time bend
Ninety years after he expounded his famous theory, a $700m Nasa probe has proved that the universe behaves as he said. Now the race is on to show that the other half of relativity also works
Anushka Asthana and David Smith
Sunday April 15, 2007
The Observer
Under his name in the Oxford English Dictionary is the simple definition: genius. Yet for decades physicists have been asking the question: did Albert Einstein get it wrong? After half a century, seven cancellations and $700m, a mission to test his theory about the universe has finally confirmed that the man was a mastermind - or at least half proved it.
The early results from Gravity Probe B, one of Nasa's most complicated satellites, confirmed yesterday 'to a precision of better than 1 per cent' the assertion Einstein made 90 years ago - that an object such as the Earth does indeed distort the fabric of space and time.
But this - what is referred to as the 'geodetic' effect - is only half of the theory. The other, 'frame-dragging', stated that as the world spins it drags the fabric of the universe behind it.
Francis Everitt, the Stanford University professor who has devoted his life to investigating Einstein's theory of relativity, told scientists at the American Physical Society it would be another eight months before he could measure the 'frame-dragging' effect precisely.
'Understanding the details is a bit like an archeological dig,' said William Bencze, programme manager for the mission. 'A scientist starts with a bulldozer, follows with a shovel, then finally uses dental picks and toothbrushes to clear the dust away. We're passing out the toothbrushes now.'
The Gravity Probe B project was conceived in the late 1950s but suffered decades of delays while other scientists ran tests corroborating Einstein's theory.
It was Everitt's determination that stopped it being cancelled. The joint mission between Nasa and Stanford University uses four of the most perfect spheres - ultra precise gyroscopes - to detect minute distortions in the fabric of the universe. Everitt's aim was to prove to the highest precision yet if Einstein was correct in the way he described gravity.
According to Einstein, in the same way that a large ball placed on a elasticized
cloth stretches the fabric and causes it to sag, so planets and stars warp space-time. A marble moving along the sagging cloth will be drawn towards the ball, as the Earth is to the Sun, but not fall into it as long as it keeps moving at speed. Gravity, argued Einstein, was not an attractive force between bodies as had been previously thought.
Few scientists need the final results, which will be revealed in December, to convince them of Einstein's genius. 'From the most esoteric aspects of time dilation through to the beautiful and simple equation, e=mc2, the vast bulk of Einstein's ideas about the universe are standing up to the test of time,' said Robert Massey, from the Royal Astronomical Society.
He said the mission was 'legitimate science' to test a theory and confirm its brilliance, but others have
criticized the costs and length of the study, claiming that what was announced had already been shown. Sir Martin Rees, the Astronomer Royal, said the announcement would 'fork no lightning'.
The theory explained
When Einstein wrote his general theory of relativity in 1915, he found a new way to describe gravity. It was not a force, as Sir Isaac Newton had supposed, but a consequence of the distortion of space and time, conceived together in his theory as 'space-time'. Any object distorts the fabric of space-time and the bigger it is, the greater the effect.
Just as a bowling ball placed on a trampoline stretches the fabric and causes it to sag, so planets and stars warp space-time - a phenomenon known as the
'geodetic effect'. A marble moving along the trampoline will be drawn inexorably towards the ball.
Thus the planets orbiting the Sun are not being pulled by the Sun; they are following the curved space-time deformation caused by the Sun. The reason the planets never fall into the Sun is because of the speed at which they are
traveling.
According to the theory, matter and energy distort space-time, curving it around themselves. 'Frame dragging' theoretically occurs when the rotation of a large body 'twists' nearby space and time. It is this second part of Einstein's theory that the Nasa mission has yet to corroborate.
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