See gallery: 鈥�Space superlatives: The universe鈥檚 extreme performers鈥�
Speed is relative. There is no absolute standard for 鈥渟tationary鈥� in the universe. Perhaps the nearest thing is the all-pervasive cosmic microwave background radiation. Its Doppler shift across the sky 鈥� blue in one direction, red in the other 鈥� reveals that, relative to the CMB, the solar system is rattling along at 600 kilometres per second. Microwaves are rather insubstantial, though, so we don鈥檛 feel the wind in our hair.
Distant galaxies are also moving at quite a rate. Space is expanding everywhere: the more space you are looking through the faster the galaxies you see are moving away from us. Far enough off, galaxies are effectively retreating faster than light speed, which means we can never see them because their radiation can鈥檛 reach us.
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While such inaccessible extremes may have abstract appeal, speed becomes much more interesting if you are moving fast relative to some large object nearby 鈥� something you can see whoosh past your windows, or something you might just crash into.
Within our solar system, Mercury, the messenger of the gods, is the fastest-moving planet, with an orbital speed of about 48 kilometres per second; Earth manages only about 30 km/s. In 1976, Mercury was outpaced for the first time by a human artefact, the Helios 2 solar probe, which reached more than 70 km/s as it whizzed by the sun. Sun-grazing comets that swoop in from the outer solar system trump both, skimming past the solar surface at up to 600 km/s. Speed does not guarantee escape: a few hit the sun and are swallowed.
The outer reaches of the Milky Way are also home to some oddly busy bodies: 鈥�hypervelocity stars鈥� speeding past the rest of the galaxy at up to 850 km/s. The theory is that they were flung out in a close encounter with the giant black hole in our galaxy鈥檚 centre. Black holes make particularly effective cosmic slingshots because of their peerlessly powerful gravity. Some also create magnetic tornadoes that squirt out tenuous jets of matter at more than 99 per cent of the speed of light.
The spinning neutron stars we know as pulsars also perform high-speed magnetic magic. Pulsars can rotate up to 1000 times a second, which means their surfaces move at up to 20 per cent of the speed of light. Far enough away from the surface, the magnetic field projected by the pulsar can even move faster than light. That is not in conflict with the laws of physics as the magnetic field carries no energy or information. These superfast fields are perhaps the source of the powerful, regular pulses of radiation pulsars send our way. Tiny variations in the timing of those pulses could soon be used to detect gravitational waves, travelling space warps predicted by Einstein鈥檚 relativity (New Scientist, 20 March 2010, p 30).
Even solid objects can approach light speed, with the aid of a black hole鈥檚 gravity. At a hole鈥檚 event horizon, a single rock will simply disappear without a splash, but two rocks on different trajectories could collide with one another. According to calculations posted online last year by Tomohiro Harada at the University of Tokyo, Japan, and his colleague Masashi Kimura, the rotation of the black hole whips up a whirlpool in the surrounding space and increases the maximum collision speed. The upshot is that somewhere in the universe, two rocks caught in the grip of a rapidly spinning black hole could be hurtling towards one another at close to the speed of light ().
Read more: 鈥�Extreme universe: Eight cosmic record-breakers鈥�
