According to general relativity, even empty space-time, devoid of
stars and galaxies, can have a life of its own.
Ripples known as
gravitational waves can propagate across space in much the same way that
ripples spread across the surface of a pond.
One of the remaining tests of general relativity is to measure
gravitational waves directly. To this end, experimental physicists have
built the Laser Interferometer Gravitational-Wave Observatory (LIGO) at
Hanford, Washington, and Livingston, Louisiana.
Each experiment consists
of laser beams that are reflected between mirrors placed up to 4
kilometres apart.
If a gravitational wave passes through, it will
slightly distort space-time, leading to a shift in the laser beams.
By
monitoring time variations in the laser beams, it is possible to search
for the effects of gravitational waves.
No one has yet detected a gravitational wave directly, but we do have
indirect evidence that they exist.
When pulsars orbit very dense stars,
we expect them to emit a steady stream of gravitational waves, losing
energy in the process so that their orbits gradually become smaller.
Measurement of the decay of binary pulsars’ orbits has confirmed that
they do indeed lose energy and the best explanation is that these
pulsars are losing energy in the form of gravitational waves.
Pulsars are not the only expected source of gravitational waves.
The
big bang should have created gravitational waves that still propagate
through the cosmos as gentle ripples in space-time.
These primordial
gravitational waves are too faint to be detectable directly, but it
should be possible to see their imprint on the relic radiation from the
big bang – the cosmic microwave background. Experiments are now under
way to search for these signatures.
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Gravitational waves should also be emitted when two black holes
collide. As they spiral in towards each other, they should emit a burst
of gravitational waves with a particular signature. Provided the
collision is sufficiently close and sufficiently violent, it may be
possible to observe them with instruments on Earth. A more ambitious project is the Laser Interferometer Space Antenna
(LISA), made up of a trio of satellites that will follow the Earth in
its orbit around the sun. They will emit precisely calibrated laser
beams towards each other, much like LIGO. Any passing gravitational wave
will slightly distort space-time and lead to a detectable shift in the
laser beams. NASA and the European Space Agency hope to launch LISA in
the next decade.
Time travel
Einstein’s theory allows for the intriguing possibility of time
travel.
One suggested way of achieving this involves the construction of
tunnels called wormholes that link different parts of space at
different times.
It is possible to build wormholes – in theory.
But
unfortunately they would require matter with negative energy, and other
unnatural physical circumstances, not only to open them up but also to
allow them to be traversed. Another possibility is to create a large
region of space that rotates, or use hypothetical objects called cosmic
strings.
“It is possible to build tunnels linking different parts of space and different parts of time – in theory, at least”
The possibility of time travel can lead to physical paradoxes, such
as the grandfather paradox in which the time traveller goes back in time
and kills her grandfather before he has met her grandmother.
As a
result, one of her parents would not have been conceived and the time
traveller herself would not exist. It has been argued, however, that
physical paradoxes such as these are, in practice, impossible to create. Read more:Instant Expert: General relativity
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