Essay: A Rogue's Gallery of Gravity-Makers

Why do all those massive, exploding outer-space events get all the fun? Why is it that only they can create a gravitational wave? The truth is, anything with an accelerating mass has a gravitational effect. Detecting it? Well, that’s another matter entirely.

Eyeball our lineup below to learn what sources produce these ripples in space, and why capturing such waves for the first time using the Laser Interferometer Gravitational-wave Observatory (LIGO) means so much to scientists.

You: “Every time you accelerate — say by jumping up and down — you’re generating gravitational waves,” says Rainer Weiss, Professor Emeritus of Physics at MIT. “There’s no doubt of it.” But just standing there won’t cut the mustard. To make a wave, your mass has to both move (have velocity) and have acceleration (change the rate of motion, direction, or both).

Still, don’t get your hopes up. No matter how fast you jump, sprint, or cartwheel, the resulting warp your waves make on space is so weak that it’s utterly unmeasurable — perhaps 100,000,000,000,000,000,000,000 times less so than the warp made by massive exploding space objects. And LIGO has a tough enough time measuring those.

Spinning Aircraft Carrier: Only enormous amounts of motion at enormous speeds from enormous masses can produce a ripple that LIGO could detect. “To rival here on Earth the strength of gravitational waves from a supernova in the center of our galaxy,” suggests Mike Zucker, the head of LIGO’s Livingston facility, “you’d need to take an aircraft carrier and spin it, end over end, a thousand times a second.” Not very likely.

Atomic Bomb: This type of acceleration — that of billions of atomic nuclei splitting and spewing energy — might launch a space warp that LIGO could notice. Scientists actually considered testing early interferometers this way. Besides the more obvious concerns, one glitch was that an atomic detonation would have to be constrained so that the explosion wasn’t spherically symmetric. Weiss explains that equal motion in all directions does not produce gravitational waves. “The waves from all the different parts of a sphere would cancel each other out,” he says. “You need motion that’s nonspherical.” In the face of these challenges, the atomic bomb idea lost steam.

Supernovas, like this explosion of star Sanduleak -69 202, emit gravitational waves. Sanduleak exploded around 167,000 BC, but the light it cast took until 1987 to reach Earth.
NASA/Harvard-Smithsonian Center for Astrophysics

Space Sources: Now we’re talking. Mere Earth objects can’t match the gargantuan, asymmetrical motions of mass produced by space phenomena like supernovas, neutron stars, and black holes. Currently, everything we know about these objects comes from telescope observations of visible light waves, radio waves, and other electromagnetic radiation. But gravitational radiation is a completely different form of energy, emitted from the objects’ obscured cores. Such data will reveal unprecedented information about the mysterious interiors of space phenomena.

For LIGO, the most anticipated space sources of gravitational waves are the billions of binary star systems in our galaxy. These are pairs of stars, in various stages of life and death, that orbit one another. But gravitational-wave detectors will also be able to catch the emissions of evolving single stars as well as other space sources, such as supermassive black holes and the Big Bang.

Supernovas: Stars have a finite lifetime in the millions to billions of years. Some very massive stars call it quits in a huge explosion called a supernova. If a nonspherical supernova occurs in our own galaxy, says Weiss, it will radiate gravitational waves in a colossal burst that LIGO could detect. “But this may happen once every 30 years,” he says. “Not such a nice source.”
Neutron Stars: The collapsed remains of a supernova can develop into a neutron star. These rapidly spinning, 10 km wide stars are so dense that a teaspoon would tip the scales at a billion tons. Neutron stars often exist in binaries. As the two stars orbit one another, they shed gravitational waves. This continuous loss of energy causes them to spiral ever-closer toward one another. Eventually they coalesce violently, releasing a spurt of additional gravitational waves that LIGO should be able to catch about once yearly at its current sensitivity, says Weiss.
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 deformationan elongated overall shape or a surface bulge, for examplethat 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.

Essay: A Rogue's Gallery of Gravity-Makers

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