Einstein'stheory of general relativity has passed its toughest-ever test with flying colors, a new study reports.
General relativity, which the great physicist proposed in 1916, holds that gravity is a consequence of space-time's inherent flexibility: Massive objects distort the cosmic fabric, creating a sort of well around which other bodies orbit.
Like all scientific theories, general relativity makes testable predictions. One of the most important is the "equivalence principle" — the notion that all objects fall in the same way, no matter how big they are or what they're made of. [Einstein's Theory of Relativity Explained (Infographic)]
Researchers have confirmed the equivalence principle many times on Earth — and, famously, on the moon. In 1971, Apollo 15 astronaut David Scott dropped a feather and a hammer simultaneously; the two hit the gray lunar dirt at the same time. (On Earth, of course, the feather would flutter to the ground much later than the hammer, having been held up by our atmosphere.)
But it's tough to know if the equivalence principle applies in all situations — when the objects involved are incredibly dense or massive, for example. This wiggle room has given hope to adherents of alternative gravity theories, though such folks remain in the minority.
The new study could take some of the air out of their optimism. An international team of astronomers tested the equivalence principle under extreme conditions: a system composed of two superdense stellar corpses known as white dwarfs and an even denser neutron star.
The neutron star is a fast-spinning type known as a pulsar. These exotic objects are so named because they seem to emit radiation in regular pulses. This is just an observer effect, however; pulsars blast out radiation continuously, from their poles, but astronomers' instruments pick these beams up only when they're directed at Earth. And because pulsars spin, they can direct their poles toward Earth at regular intervals.
The system in question, known as PSR J0337 1715, is located 4,200 light-years from Earth, in the direction of the constellation Taurus. The pulsar, which rotates 366 times per second, co-orbits on the interior with one of the white dwarfs; the pair circles a common center of mass every 1.6 Earth days. This duo is in a 327-day orbit with the other white dwarf, which lies much farther away.