Einstein’s theory of general relativity, just recently turned 100 years old, has been lauded with accolades for a century. It has been mused over by philosophers and stoners wondering how it can be so beautiful and so simple. And in its century of hype, general relativity has been able to describe numerous phenomenon; it predicts black holes, models the orbits of planets more closely than Newton’s Laws, and is used every day with exquisite accuracy by GPS to locate devices to within meters anywhere on the surface of the Earth.
On of the great variety of things that general relativity predicts is a phenomenon known as gravitational waves. Most people are no doubt familiar with the metaphor of gravity as a bowling ball on a rubber sheet, and a smaller ball rolled near by will be affected by the presence of the large mass. This is the concept of space as malleable and flexible. Now imagine two bowling balls on the sheet, orbiting around each other. As they move the entire rubber sheet will flex. This is the basic concept behind a gravitational wave: as objects move they deform space. When massive objects interact with each other, they create massive waves propagating at the speed of light.
Gravity waves have been observed before, but not directly. In the 1970s, astronomers Russell Hulse and Joseph Taylor discovered a binary pulsar system. A pulsar is the collapsed remnant of a midsized star, not large enough to collapse into a black hole. Thanks to conservation of angular momentum, pulsars spin very quickly and reliably, on the order of milliseconds. Continuing measurements of the rate of spin of the pulsar shows that it is slowing down in a rate almost exactly consistent with the amount of energy that gravitational waves would bleed from the system. These measurements have remained consistent with predictions for decades now.
Although gravitational waves were strongly supported by pulsar evidence, their detection is still very important for a number of reasons. The first is that more evidence for their existence is always important in fields where theories like relativity are only ever supported, not proven. The second, and far more interesting reason, is that observing gravity waves is not simply testing a hypothesis; it is opening a new field of astrophysics, with just as much promise for discovery as optical or radio astronomy. Not only can gravity waves be observed, they can be interpreted and understood and used to further our knowledge.
This first direct detection of gravity waves was performed by the Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of detectors in Washington State and Louisiana. These two facilities work by shining a laser down two perpendicular vacuum-filled tunnels, each 4 km long. When the lasers come back they should perfectly cancel each other out. If they do not, it suggests that one tunnel was shorter than the other by some small amount. The cause of this is a gravity wave changing space—and therefore distance—as it passes by.
What LIGO observed was an almost unimaginably small change in the length of the 4 km long tunnel, on the order of 10 zeptometres (10-21 metre). However this observation was significant; researchers intensely studied the 20 ms signal and were able to attribute it to phenomena that had never been observed before. Their analysis shows that the gravitational waves were made by two medium sized black holes that were orbiting each other. The signal culminates in the epic merger of the two objects, then fades away to nothing. As the paper that announced this discovery rather humbly put it, “These observations demonstrate the existence of binary steller-mass black hole systems… gravitational waves and the first observation of a binary black hole merger.” That’s three remarkable observations on its first attempt. Already LIGO is a tool for astronomy.
To cement the credibility of gravitational waves as an important astronomy tool even further, the paper even goes as far as to announce that the team can even locate the source of the observation to within 60 square degrees. This is an admittedly substantial area, 300 times larger than the moon. But using more detectors—several of which exist but were not operating at the time—the location could be refined.
Gravitational waves have been found, but this is only the start. Of particular interest, gravitational waves may be able to observe the early universe farther back than our current telescope technology; whereas the hot plasma of the early universe was largely opaque to electromagnetic waves, it would have been quite transparent to gravitational waves. Keep your eye on LIGO for more exciting discoveries in the future.
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