When extremely massive objects collide, they produce waves of energy that ripple the fabric of spacetime called gravitational waves. Think of them as widening ripples on a pond in the wake of a tossed stone. They’re a form of energy like light and like light, they undulate across space at the speed of light.
On April 25, the National Science Foundation’s Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European-based Virgo detector registered gravitational waves from what appears likely to be a crash between two neutron stars — city-sized, super-dense remnants that remain after a stars explode as supernovae. If you could scrape together a teaspoon of neutron star matter it would weigh a billion tons.
One day later, on April 26, the LIGO-Virgo network spotted another candidate source that may have been caused by a collision of a neutron star with a black hole. A black hole’s density is infinite — if you can get your head around that —and results from the collapse of a single, supergiant star. A typical black hole is just 24 miles (42 km) across.
While a collision of titans releases enormous amounts of energy, the spacetime reverberations produce only weak signals that are difficult to detect. Patrick Brady, a spokesperson for LIGO and professor of physics compared it to “listening to somebody whisper a word in a busy café; it can be difficult to make out the word or even to be sure that the person whispered at all.” LIGO (LYE-go) works by detecting incredibly tiny distortions of space from passing gravitational waves (see above).
LIGO uses two detectors — one in Washington and one in Louisiana — along with Virgo, located in Italy. Their field of views are extremely large, making it difficult to pinpoint the source if only one or two instruments happen to be online. But when all three are surveying the sky at the same time, they can narrow down the object’s location to a much smaller chunk of sky. Astronomers can then search the area for any unusual activity with telescopes.
When black holes collide, we detect only ripples, but when two neutron stars smash together you get both ripples and a fiery outburst of light. In August 2017, LIGO and Virgo spotted a neutron star merger. In the days and weeks that followed, about 70 telescopes on the ground and in space spotted the explosive aftermath that shot out everything from gamma rays to regular light to radio waves. During the two most recent bursts, hundreds of astronomers sought the sources, but have yet to find a sign. That’s no surprise — at best 3 percent or 1,100 square degrees of sky must be searched — and quickly — for a candidate.
In addition to the two new candidates involving neutron stars, the LIGO-Virgo network alos spotted three likely black hole mergers during its latest run. Since the history-making, first-ever detection of gravitational waves in 2015, the network has spotted evidence for two neutron star mergers, 13 black hole mergers and one possible black hole-neutron star merger. Wanna guess who predicted gravitational waves in the first place? Yeah, Einstein again. They arose from his General Theory of Relativity in 1916.
The April 25 neutron star smash-up, dubbed S190425z, happened about 500 million light-years away from Earth. Since Virgo and just one of the LIGO instruments picked up the signal (one was offline at the time), astronomers are having to search a quarter of the entire sky for a signal. The possible April 26 neutron star-black hole collision (called S190426c) occurred roughly 1.2 billion light-years away and was seen by all three facilities, narrowing the view to 3 percent of the sky.
If gravitational wave events move you, download the free Gravitational Wave Events app for iPhone (iPhone only at this time) for alerts of new discoveries by LIGO and Virgo. I mean, who doesn’t want to be notified when a black hole gobbles up a star? What a world.