See gallery: “Space superlatives: The universe’s extreme performers“
In medieval cosmology, the universe was a nested series of perfect crystal spheres that carried the sun, moon, planets and stars. We now know that space is rather messier, but does it hold anything to mirror that vision of spherical perfection?
Planets themselves are pulled into fairly tidy spheroidal shapes by the force of their own gravity. The most prominent of Earth’s bumps and its deepest wrinkles, from mount Everest to the Mariana trench, point in or out by less than 0.2 per cent of the planet’s radius. If it weren’t for the slightly squashed shape caused by Earth’s daily rotation – pulled in at the poles, bulging at the midriff – our home would make a good cosmic pool ball.
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Earth is positively craggy compared with neutron stars. Their huge density (see “Heavyweight division”) results in a surface gravity something like 200 billion times as strong as Earth’s. That is enough to flatten out all but the slightest irregularity: a neutron star’s Everest would probably be no more than 5 millimetres high. As these stars are typically 10 to 15 kilometres across, that Himalayan height is less than one part in a million of the stellar radius.
For a period of 16 months during 2004 and 2005, we launched our own balls into space that rivalled neutron stars for roundness. Gravity Probe B was a satellite designed to look for distortions in space-time created by our planet’s great mass, which are predicted by Einstein’s general theory of relativity. One of these is an effect called frame-dragging, in which space is dragged around with the rotation of the Earth. Gravity Probe B used four gyroscopes based on small spheres of quartz polished so thoroughly that they have no irregularities larger than 0.4 parts in a million.
Relativity happens to offer us something rounder even than that probe’s spheres. A black hole’s event horizon marks the region from which no light can escape to reach the eye of a distant observer. It isn’t exactly a surface: you couldn’t run a hand over it and marvel at its new-shave smoothness. But soon astronomers may be able to discern images of some black hole event horizons and eventually give us a sharp picture of these pseudosurfaces which are perhaps the nearest thing in nature to perfect roundness.
Observing matter falling in to an event horizon could be a sterner test of Einstein. If we see shreds of gas on orbits slightly different from the predictions of relativity, we may need a new theory of gravity. And of course if black holes turn out not to have the expected event horizon, that would be a shocker.
Read more: “Extreme universe: Eight cosmic record-breakers“
