In a paper published Wednesday in the Physical Review Letters, physicists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) describe the experiment's second gravitational wave observation—which would also be the second such observation ever.
As with the first observation, a much-celebrated astrophysics landmark announced in February, the source of the new event is likely a pair of merging black holes. Because these black holes are smaller than those seen in the first gravitational wave detection, it was possible to observe them for nearly a full second, or five times longer than the earlier observation.
As the LIGO researchers note in the current paper, this second detection may well be the start of a whole new era of gravitational wave astronomy. In March, the same LIGO group published a paper suggesting that the population of binary black hole systems may be larger than previously thought, giving rise to a gravitational-wave background also much richer than anticipated. This background should now be within reach of both LIGO and its European kin, the Virgo interferometer. (The Virgo team, which assisted in data analysis, is co-credited on the new paper.)
If you'll recall, gravitational waves are among many implications of Einstein's theory of general relativity and probably the most difficult of them to test and observe. They occur as massive objects in space accelerate, causing a rippling in space-time itself as though a pebble were dropped into a pond. This sounds simple enough, but given that gravity is the weakest of the fundamental forces by a degree that's almost impossible to imagine, even waves resulting from the biggest things in the universe are very difficult to detect.
The LIGO experiment actually consists of two detectors, one in eastern Washington state and the other in Louisiana. Both consist of two 4 kilometer "arms" joined together into an "L" shape containing an ultra-high vacuum system. The basic idea of operation is that beams of light are fired simultaneously down each arm and are left to bounce back and forth between mirrors positioned at each end of the arm. Because gravitational waves distort space-time, should they arrive at the detector, the effect will be to slightly tweak the phase of each of the beams. Hence, a detection.
The sensitivity needed for such a detection has only recently been achieved at LIGO, when it was upgraded from its original configuration to what's properly known as Advanced LIGO. With this sensitivity achieved, more detections are bound to occur.
"The first detection was so exciting because it showed that this extraordinarily hard experiment could actually be performed successfully," offered Julian Krolik, an astrophysicist at Johns Hopkins University in Baltimore, in an American Physical Society news brief. "Additional detections [like this] turn the spotlight from the experiment itself to what it teaches us about the contents of the Universe."