Gravitational Waves

A blue background with a grid, with waves spiraling out on the background from a pair of circles in the center.
An artist's impression of gravitational waves generated by binary neutron stars. R. Hurt/Caltech-JPL

Gravitational waves are waves that vibrate through spacetime itself, as a result of gravitational forces.

Origin of Gravitational Waves

In 1916, Albert Einstein himself first suggested the idea of gravitational waves as a consequence of his theory of general relativity. He was able to construct a set of solutions to his formulas for general relativity (called his field equations) which allowed for gravitational waves, but the problem was that these waves - if they did in fact exist - would be so incredibly weak that they would be virtually impossible to detect.

But, lucky for us, only virtually impossible to detect ...

Finding Gravitational Waves

The 1993 Nobel Prize in Physics was awarded to Russel A. Hulse and Joseph H. Taylor Jr. "for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation." In fact, their 1974 research that earned the Nobel Prize in Physics was precisely based upon Einstein's predictions using gravitational waves.

If gravitational waves do exist, then objects emitting them would lose gravitational energy. This is weak, but the loss of energy is indirectly detectable. By studying the orbits of binary neutron stars, they were able to detect the gradual decay within these orbits that would be expected based upon the existence of gravitational waves.

On February 11, 2016, physicists working at with the LIGO program announced that they had detected gravitational waves from a binary system of black holes colliding with each other.

The LIGO observatory used the most precise measurement device ever created, able to detect with microscopic precision behavior that happens light-years away. In fact, on their website they point out that their level of precision is equivalent to measuring the distance to the nearest star to within the width of a human hair!

The gravitational waves were detected using an laser interferometer, in a fashion somewhat similar to what was used in the classic Michelson-Morley experiment, except instead of looking for the luminous ether, modern physicists at LIGO are looking for gravitational waves. Lasers moving along different arms of a 4-kilometer-long vacuum chamber come back together. If there are no gravitational waves affecting the laser light, the beams of light would be in complete phase with each other upon arriving at the detectors, and there will be no interference patterns. If, however, the gravitational waves are present and have an effect on the laser beams - even an effect within 1/10,000th of a proton's width! - then the Advanced LIGO interferometer could detect it. And, indeed, it seems to have done so with surprising speed after beginning its 2015 run of data collection!

If you're having trouble visualizing it, don't worry. This is not the most intuitive stuff. This video from LIGO does an excellent job of making it more clear how the interferometer works.

What's Next for Gravitational Waves

The major reason for excitement over the detection of gravitational waves, other than yet another confirmation that Einstein's theory of relativity is correct, is that it provides yet another means of exploring the universe.

Astronomers know as much as they do about the history of the universe today because they explore space with every tool available, but that has been confined pretty much to looking for cosmic rays and the electromagnetic spectrum. Just as the development of radio and other advanced telescopes allowed astronomers to look at the universe outside of the visual range of the electromagnetic spectrum, this advance potentially allows for whole new types of telescopes that will explore the history of the universe at an entirely new scale.

The Advanced LIGO observatory is a ground-based laser interferometer, so one obvious move will be to create a space-based gravitational wave observatory. These would, in principle, be able to detect a different range of gravitational wave frequencies than that which was found with LIGO.

In fact, even as LIGO announced its discovery, the European Space Agency (ESA) was already running the LISA Pathfinder mission to test possibilities for space-based gravitational wave detection.

Among other things. For a sense of the enthusiasm physicists feel at this, I suggest this TED talk on the subject, from TED2016, just days after the February 11 announcement.

Primordial Gravitational Waves

Though gravitational waves are allowed in theory by general relativity itself, one major reason we're interested in them is because of inflation theory, which didn't even exist back when Hulse and Taylor were doing their Nobel-winning neutron star research.

In the 1980s, the evidence for the Big Bang theory was quite extensive, but there were still questions that the theory could not sufficiently explain. In response, a group of particle physicists and cosmologists worked together - initiated by an insight by Alan Guth - to develop inflation theory. Using principles from quantum physics and particle physics, Guth and his colleagues suggested that the early, highly-compact universe would have contained many quantum fluctuations. By suggesting a rapid expansion in the very early universe - an expansion that could be explained due to the outward pressure of spacetime itself under the rules of quantum physics - these quantum fluctuations would have expanded tremendously, and the end result of this was to explain many of the open mysteries in cosmology. If only scientists could show that it were true!

A theory is proven, of course, by making unique and specific predictions that can be confirmed by experiment and observation. One of the key predictions from inflation theory was, in fact, that the early universe would have produced gravitational waves, as laid out by string theorist Michio Kaku in his 2005 book Parallel Worlds: A Journey through Creation, Higher Dimensions, and the Future of the Cosmos:

Although inflation is the theory today that has the power to explain such a wide range of mysteries about the universe, this does not prove that it is correct.[...] The "smoking gun" that would finally verify or disprove the inflationary scenario are "gravity waves" that were produced at the instant of the big bang. These gravity waves, like the microwave background, should still be reverberating throughout the universe and may actually be found by gravity wave detectors [...] Inflation makes specific predictions about the nature of these gravity waves, and these gravity wave detectors should find them.

Specifically, there would be Cosmic Gravitational-Wave Background (CGB), sometimes called primordial gravitational waves, that would permeate the very cosmic microwave background (CMB) radiation itself. The spacetime ripples caused by these gravitational waves in the early moments of the universe would leave a polarization in the CMB radiation, and in fact inflation theory made very definite predictions about the magnitude of this polarization.

On March 17, 2014, physicists working with the BICEP2 collaboration observing the cosmic microwave background radiation out of telescopes at the South Pole announced that they had discovered the CGB polarization at exactly the same level that had been predicted by the inflation theory. This report later proved to be false, since other detector were unable to find the evidence that BICEP2 had thought it found. It turned out that this error resulted from space dust in BICEP2's field of view that had not been fully accounted for by the team's original analysis.

Gravity Waves vs. Gravitational Waves

In an earlier version of this article, I referred to these waves as "gravity waves" (as did Michio Kaku, in the quote above). This is a bit sloppy with terminology though, as what we are describing is most appropriately described as gravitational waves. Gravity waves are a term for more conventional waves, in either matter or a fluid, that are caused by gravity. Both tides and tsunamis are examples of these conventional gravity waves, since they are driven by the force of gravity. The distinction is described in greater detail in a Feb. 8, 2016, Discovery Space article.