Little Green Men
When pulsars were first discovered in 1967, astronomers half-jokingly called them LGM for "Little Green Men," because the pulsing radio signal is so regular it seemed to be a sign of intelligent life. It is now widely accepted that pulsars are rapidly spinning neutron stars. These incredibly dense objects -- about 10 trillion times denser than a lead brick, with more mass than the sun -- produce gravity 100 billion times stronger than Earth. A powerful magnetic field traps and accelerates charged particles, which beam radio waves in a cone shape through space, like the rotating light from a lighthouse beacon. If Earth lies in the sweep of the beam, we receive the signal once each rotation.
Ticking like a cosmic metronome, pulsars are the most stable, accurate timekeepers known. "The neutron star is just sitting out there in interstellar space," says Taylor, "not touching anything, freely spinning on its axis. There's no friction of any consequence, and it makes a very good clock." For this reason, pulsars can be used to test the accuracy of the most stable Earth clocks. They also have valuable cosmological applications in measuring gravity waves and in defining a precise astronomical frame of reference, important for space navigation.
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