Gravitational Waves

What are gravitational waves?1

Physicists Umberto Cannella and Daniel Whiteson explain what gravitational waves are and why they'll cause a big ripple in our understanding of the Universe. (Credit: PHD Comics)

Gravitational waves are a key prediction of general relativity, a theory proposed by Albert Einstein in 1915 that is still our best explanation for the force of gravity. Einstein pictured space and time as interwoven aspects of the same underlying reality, known as space-time. Objects that possess mass, such as stars and planets, warp space-time much like how a heavy ball placed on a trampoline creates a bowl-like depression around itself. This curvature in the space-time trampoline, so to speak, is experienced by all matter in the universe as the force of gravity. Whenever any mass moves, it generates gravitational waves that ripple through space-time like ripples radiating across a pond's surface. For these waves to be big enough to detect, however, extraordinarily massive, astronomical objects are required, such as accelerating black holes or neutron stars. The ability to measure the strength and frequency of gravitational waves is important because such measurements would provide vital details about the distant, exotic phenomena that produced them.

Why are gravitational waves important for science?

History of the UniverseHistory of the Universe (Credit: Wikipedia)

Virtually all of our knowledge about the universe has come to us in the form of light, or electromagnetic radiation. Gravitational waves, however, are an entirely different form of radiation, produced by some of the most violent events in the universe. The study of gravitational waves will reveal the inner workings of some of the most violent events in the universe, such as the collisions of black holes and the explosions of titanic stars. Gravity itself, as a force of nature, will be probed in new ways in these extreme astrophysical conditions that are un-reproducible on Earth in the laboratory. Gravitational waves should even let scientists see all the way back to the origin of the cosmos itself in the Big Bang. The opening of these unparalleled new vistas has a historical precedent. For nearly all of human history, we had to make do with the sunlight and starlight our eyes can see. However, visible light makes up a tiny sliver of the range of energies light can possess. In the 20th century, we learned how to detect the entire electromagnetic spectrum, from the highest-energy gamma rays on down to the lowest-energy radio waves. As a result, astronomers literally saw the universe in a whole new light. A slew of phenomena and happenings in the cosmos suddenly became evident to us, from the spasms of supermassive black holes in galaxy cores to the faint heat emanated by gas clouds where solar systems are born. Now, in the 21st century, a newfound ability to see in gravitational waves will further bolster our efforts to understand the universe.

What is the history behind the search for gravitational waves?

Albert Einstein's general theory of relativity, published in full in 1916, revolutionized the understanding of gravity that had prevailed since Isaac Newton's time in the 17th century. Rather than treating space and time as absolute, independent entities as Newton did, Einstein's equations intertwined them into a "space-time." The force of gravity, although described quite accurately by Newton's mathematics, was now conceived by Einstein to be a geometric property of space-time. According to general relativity, mass caused space-time to curve in a describable manner, manifesting as gravity. Application of Einstein's theory resolved numerous discrepancies with Newton's physics, such as tiny shifts in the orbit of the planet Mercury. Over the past century, the predictions of general relativity have been proven to extreme accuracy again and again, and have even made possible precision technologies such as the Global Positioning System.

LIGO Observatory in Hanford, WA The LIGO Laboratory operates two detector sites, one near Hanford in eastern Washington, and another near Livingston, Louisiana. This photo shows the Hanford detector site. (Credit: Caltech/MIT/LIGO Lab)

One of general relativity’s biggest unproven predictions is the existence of gravitational waves. Einstein's work held that no information could travel faster than the speed of light, including the positions of masses in the universe, which are communicated by gravity. When masses move, corresponding changes in the gravitational field move through the cosmos as gravitational waves at the speed of light, like ripples across a pond. But gravity is an extremely weak force so even the sources of the biggest gravitational waves, like the cataclysmic collisions of black holes, would only produce the tiniest of wiggles by the time they reached Earth. This movement induced by gravitational waves has been calculated to be thousands of times smaller than the nucleus of an atom. For many decades, the ability to measure on such a small scale was impossible.

Against this challenge, strong, albeit indirect support for the existence of gravitational waves emerged in 1974. The astronomers Russell Hulse and Joseph Taylor were conducting research at the Arecibo Radio Observatory in Puerto Rico. They discovered the first binary pulsar star system, consisting of two rapidly spinning, neutron star remnants of once-giant stars, orbiting each other and sending out pulses of radiation. According to general relativity, the two pulsars would draw closer together over time as they radiated gravitational waves. Indeed, over eight years of observations, the pulsars' orbits decreased at exactly the rate called for by Einstein's equations. Hulse and Taylor were awarded the 1993 Nobel Prize in Physics for their discovery.2

As exciting as these and related pulsar findings have been, no gravitational waves had ever been directly detected on Earth. LIGO was built to finally test this prediction of general relativity and made the long-awaited detection in September 2015.

What are the types of gravitational waves?3

An artist's concept of gravitational wave propagation. (Credit: R. Hurt - Caltech / JPL)

Any movement of a mass through space-time generates gravitational waves. But objects such as people or airplanes do not make waves that are anywhere near large enough to detect. Instead, scientists must look to the cosmos, where extremely massive objects capable of generating detectable waves are relatively commonplace.

LIGO scientists have come up with four different types of gravitational waves that, depending on their origin, should each leave a distinctive signature on the experiment's detection equipment.

Continuous gravitational waves

These waves should be produced by spinning, massive objects that are not perfectly spherical. An example of such an object is a neutron star—the dense, city-sized leftover of a colossal star that collapsed on itself, exploding as a supernova. As a neutron star whirls on its axis, any bumps on its surface will emit gravitational waves. Assuming the compact neutron star's spin rate remains constant, the ripples in space-time it sends out with also remain constant, or continuous.

Inspiral gravitational waves

Two massive objects in the universe can become locked gravitationally and will draw each other closer and closer in a winding, spiral-shaped dance. As the objects "inspiral" toward each other, their orbital distances decrease and their speeds ramp up, rather like how a spinning figure skater who draws in his or her outstretched arms whirls even faster. Huge gravitational waves are generated in this process, with the waves' frequency increasing until the two objects collide and merge into a single object. There are two kinds of massive astronomical bodies that should crank out detectable inspiral gravitational waves: neutron stars (described above); and even denser objects called black holes. So-called stellar black holes form in a similar manner as neutron stars, during the collapse of a giant star going supernova. But these remnant, compact masses contain so much mass, and thus generate so much gravity, that even light cannot escape their gravitational clutches. Any combination of neutron stars and black holes spiraling in toward each other—whether two neutron stars, two black holes, or one of each—should make distinctive gravitational wave signatures.

Stochastic gravitational waves

Given all the possible sources of gravitational waves in the universe, scientists expect there to be a sort of background gravitational wave "hum." The waves composing this background "noise" are expected to be small and hard to detect. Intriguingly, one of the sources of this particular kind of gravitational wave could be the Big Bang itself, 13.8 billion years ago when the universe came into existence. Stochastic gravitational waves from the Big Bang could offer scientists an unprecedented view into the earliest moments of the cosmos.

Burst gravitational waves

This last category is reserved for the "expected unexpected." Scientists think it is likely that LIGO and similar gravitational-wave detection experiments will discover types of waves that theorists have not yet completely described or anticipated. Supernovae, for example, as well as energetic, puzzling phenomena known as gamma ray bursts could each produce telltale gravitational waves.