Gravity: Making Waves

A Rogue’s Gallery of Gravity-Makers

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● Pulsars: Pulsars are neutron stars that emit jets of electromagnetic energy that sweep past Earth during each revolution, like a lighthouse beam. Since the jet sweeps by extremely fast, we perceive it on Earth as an electromagnetic “pulse.” All pulsars have some sort of deformation—an elongated overall shape or a surface bulge, for example—that cause their spins to be asymmetric. “A pulsar thus will emit gravitational waves constantly as it rotates,” says Zucker.

● Black Holes: Black holes represent the end of the road for the most massive stars. After a star explodes as a supernova, its core can condense and become so compact that nothing, not even light, can escape its gravity. It is now a black hole. After an unlucky object disappears into its yawning cavern of gravity, the black hole’s “horizon”, or edge, wiggles as it settles down to a simple shape. This wiggle emits enormous amounts of gravitational radiation that LIGO could notice, says Zucker. Analyzing such waves could answer pressing questions about the evolution, size, and commonness of black holes.

Black holes can exist in binaries, too, either with another black hole or with a neutron star. LIGO could detect the gravitational waves purged by these pairs as they collide, just as in binary neutron stars.

● Supermassive Black Holes: Behemoth black holes the mass of a million to a billion Suns may be lurking in the center of most of the Universe’s galaxies—including ours. “We’re not exactly sure how these black holes form and grow,” says astrophysical theorist Scott Hughes. “Gravitational waves offer a window into the early development of these structures.” Because gravitational radiation interferes little with matter standing in its way, waves from supermassive black holes are able to travel unblemished for billions of years from their long-gone beginning stages. (Unfortunately, LIGO itself probably won’t be able to detect waves at this frequency. To find out what instrument might, read LIGO’s Extended Family.)

● Big Bang: Scientists are hoping to use gravitational waves to peer into the earliest moments in the development of our Universe. The Big Bang is theorized to have begun around 14 billion years ago. The microscopically small Universe then expanded dramatically. In the first split seconds, the system spewed forth gravitational waves, and after about 400,000 years, electromagnetic waves.

To this day we can still “see” the primeval Universe’s electromagnetic leftovers using radio telescopes. The prospect of LIGO seeing much, much further back into creation by detecting a constant gravitational “white noise” has scientists practically jumping out of their seats. “It would revolutionize the way we think about the Universe,” says Weiss.

● Who Knows What?: Physicists fully expect to detect gravitational waves from sources not mentioned in this Rogue’s Gallery. Some could be completely unknown to science, detectable by their gravity alone. “The rule has been,” says Weiss, “that when one opens a new channel to the Universe, there is usually a surprise in it. Why should the gravitational channel be deprived of this?” We can’t see any reason why not.