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General relativity: Gravitational waves

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.

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.

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

Pedro Ferreira is professor of astrophysics at the University of Oxford. He works on the origin of large-scale structures in the universe, on the general theory of relativity and on the nature of dark matter and dark energy

Issue 2767 of New Scientist magazine
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Gravitational waves, ringing through space (Image: Henze/NASA)



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