What You Need to Know About Gravitational Waves

What are they? Why are they important? Why should we care?

UPDATE: Gravitational waves have been directly detected for the first time by the LIGO collaboration. For more on this exciting news, read the Discovery News coverage of the announcement. Read on to find out what gravitational waves are and why they are important.

On Thursday (Feb. 11) at 10:30 a.m. ET, the National Science Foundation gathered scientists from Caltech, MIT and the LIGO Scientific Collaboration in Washington D.C. to update the scientific community on the efforts being made by the Laser Interferometer Gravitational-wave Observatory (LIGO) to detect gravitational waves.

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In the wake of some very specific rumors focused on the possible discovery of these elusive ripples in spacetime, hopes were high that the international LIGO collaboration of scientists would finally put an end to the fevered speculation and announce the discovery of gravitational waves. The announcement didn't disappoint.

But why is this exciting? And what the heck are "gravitational waves" anyway?

Gravitational waves, in their most basic sense, are ripples in spacetime. Theorized by Albert Einstein just over 100 years ago, these ripples carry gravitational energy away from accelerating massive objects in the cosmos. We can imagine gravitational waves as ripples across the surface of a pond; drop a pebble into the water and ripples travel across the surface away from the rock. Gravitational waves are similar; should two black holes collide (for example), "ripples" in spacetime will carry energy away from the impact site at the speed of light. There are indirect observations of the existence of gravitational waves, but detecting them directly has been an all but impossible task... until now.

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To find out which astrophysical phenomena produce gravitational waves, click "next" at the top of this page.

Any massive cosmic object that experiences some kind of acceleration will generate gravitational waves.

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Black holes are the most massive and dense objects known to exist in the universe and are hotbeds of gravitational wave activity, especially if they collide and merge. Merging black holes are thought to be the key growth mechanism behind these gravitational behemoths -- when two galaxies merge, their central supermassive black holes will begin orbiting one another, eventually spiraling in and then colliding to form an even bigger black hole. In this scenario, gravitational waves will be emitted from the spiraling black holes long before they collide, but as the objects draw closer, gravitational wave energy will increase, sapping more and more orbital energy from the pair until they collide, ringing like a "bell" after they merge.

Another energetic phenomenon that would generate a rapid eruption of gravitational waves are supernovas. After a massive star runs out of hydrogen fuel it implodes, succumbing to massive gravitational pressure. The resulting explosion will fire a pulse of gravitational waves that will wash through spacetime.

Gravitational waves will also be generated by rapidly spinning objects, but there's a catch. Only massive spinning objects that are asymmetric (i.e. not symmetrical) will produce gravitational waves in a periodic pattern. For example, a rapidly spinning neutron star with a clump of material bulging from one hemisphere will "stir up" spacetime to generate gravitational waves. A perfectly symmetrical neutron star, however, will not generate gravitational waves. The easiest way to visualize this is to imagine spinning an oval-shaped football on the surface of a swimming pool; as the football spins, it will create ripples across the water. A spherical spinning soccer ball, on the other hand, will create very few ripples on the surface.

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The Big Bang is also theorized to have generated a powerful hum of gravitational waves when the universe began, nearly 14 billion years ago. However, these primordial gravitational waves are unlikely to be directly detected as their signal is too weak in the modern universe. But efforts are under way to detect their presence in the "background glow" of the Big Bang. Projects like the BICEP2 telescope at the South Pole are looking for a very specific type of polarization in the cosmic microwave background (CMB) that is thought to be caused by primordial gravitational waves. Despite recent announcements to the contrary, this signal has yet to be detected.

In 2002, the Laser Interferometer Gravitational-wave Observatory (LIGO) went online with a very specific task in mind: to directly detect gravitational waves traveling through our local volume of space. Gravitational waves are transmitted through spacetime and can therefore be detected from all parts of the sky, night or day, through nebulae, stars and even solid planets. These waves continually wash throughout the cosmos, traveling unimpeded.

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These waves may be ubiquitous, but their effects are astonishingly faint, and LIGO was designed to probe down the the finest distance scales possible. LIGO is composed of two observing stations located nearly 2,000 miles apart -- one in Washington and another in Louisiana. Both stations are identical and feature two long vacuum tunnels in an "L" shape. Each tunnel is 2.5 miles long. At the join of the "L" is a sophisticated optical laboratory that uses lasers to detect the tiniest of tiny fluctuations in distance caused by the passage of a gravitational wave. By bouncing the lasers along the tunnels many times and then comparing the two beams, the LIGO instrumentation is able to detect any slight change in phase. This extremely precise technique is known as interferometry. Any phase change could indicate the very slight warping of spacetime -- as tiny as a change in distance the equivalent of 1/1000th the width of a proton.

With a recent upgrade to Advanced LIGO, the system's sensitivity was boosted, allowing gravitational waves from a black hole merger to be finally detected.

Having two stations is crucial for Advanced LIGO. Should one station detect a tiny spacetime wiggle, but the other does not, scientists can rule out the propagation of a gravitational wave. These false positives could be as mundane as the vibrational effect of a passing truck or high winds during a storm. Only if the two stations detect the same event can a gravitational wave signal be confirmed.

Other ground-based gravitational wave detectors, such as Virgo (Italy) and GEO 600 (Germany), also use interferometry to hunt down these minuscule spacetime wiggles. Recently, the LISA Pathfinder mission was launched to test key technologies behind a next generation space-based interferometer called Evolved Laser Interferometer Space Antenna (eLISA) -- which is scheduled for launch by the European Space Agency in 2034.

The confirmed detection of gravitational waves is the ultimate melding of physics theory with technological sophistication. Gravitational waves are born directly from Einstein's theory of general relativity that describes the nature of space and time. It's poetic that 100 years ago, to the year, that Einstein planted the seed for these perturbations in spacetime, only for it to take a century for us to develop the technology to actually detect them. Their detection confirms yet another general relativity prediction and may also help us answer some of the most vexing puzzles facing astrophysicists and cosmologists in the future.

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The direct detection of gravitational waves is certainly a Nobel Prize-worthy feat and the scientific community is in no doubt that this achievement is up there with the discovery of the Higgs boson in 2012 and possibly even Edwin Hubble's realization, in 1929, that the universe is expanding.

Interestingly, it is theorized that different cosmic phenomena will generate different frequencies of gravitational waves. Modern astronomy focuses on the use of the electromagnetic (EM) spectrum to explore the universe. Traditionally, the visible light portion of the EM spectrum has been used by astronomers to discover the planets and even peek at nearby galaxies. As astronomical techniques developed and the technology modernized, astronomers started studying different wavelengths, such as X-rays to see energetic events around black holes and infrared radiation to see inside star-forming nebulae.

But the direct detection of gravitational waves is a paradigm shift. With enough gravitational wave detectors, we will be able to "see" objects and phenomena that would otherwise remain hidden from the electromagnetic spectrum. Two colliding black holes, for example, may not produce much in the way of electromagnetic radiation, but they would produce a huge gravitational wave signal. And, like the electromagnetic spectrum, the frequency of gravitational waves would reveal the nature of the phenomenon generating them.

Ultimately we could produce a gravitational map of the nearby universe with the locations of transient events, like supernovas, and periodic pulses from orbiting black holes. Gravitational wave astronomy would revolutionize our view of the universe.