Physicist David Reitze understandably cherishes his great-grandfather’s antique gold watch. And thanks to a blockbuster new discovery, he now knows just where that precious metal came from: the collision of two ultra-dense dead stars billions of years ago.
Reitze and other excited astronomers announced earlier this week that they had detected a similar collision between two stars that happened 130 million years ago. The merging stars created a pattern of ripples in the fabric of spacetime — gravitational waves first predicted by Albert Einstein a century ago. A trio of detectors here on Earth picked up the faint audio remnants of that long-ago merger in August. Once the source was pinpointed, a network of telescopes around the globe was able to capture the accompanying “kilonova” — a massive burst of energy that behaves a bit like a high-powered strobe light.
The result: an unprecedented recording of a major celestial event, combining light and sound.
“We’ve moved from the era of silent movies to talking movies,” Reitze, the executive director of the Laser Interferometer Gravitational-Wave Observatory (LIGO) Laboratory at the California Institute of Technology, said of this new era. The audio soundtrack comes from the chirp (it sounds a bit like the “plip” of a droplet falling into water) of the neutron stars as they spiraled together and collided.
The light-based data from the celestial merger also produced the telltale signatures of heavy elements, notably gold, platinum and uranium, created by the collision. Most lighter elements are forged in the death-throe explosions of massive stars known as supernovas, but astronomers have long theorized that the heavier elements might originate in kilonovas produced when two neutron stars collide. Now we know those astronomers were right — and that the gold in Reitze’s great-grandfather’s watch originated in the stars.
These are exciting times for gravitational wave astronomy. Just two weeks ago, the lead scientists behind the LIGO collaboration won the Nobel Prize in Physics for the first detection of a gravitational wave, the result of two black holes merging. It showed up in the data as a high-pitched “chirp” lasting just a few seconds. The collaboration picked up a second black hole merger a few months later, and last month it detected waves from a third such event, this time in conjunction with a third detector in Italy called Virgo.
Having three detectors means scientists can triangulate and better pinpoint where in the night sky any telltale chirps are coming from. So in August, when LIGO and Virgo picked up yet another signature chirp — this one markedly different in frequency and lasting a full minute — it was possible to corral 70 land and space-spaced telescopes on every continent to train their instruments on the right patch of sky in the southern hemisphere. That’s how they confirmed the high-energy gamma ray bursts accompanying the kilonova collision.
Finally, the gravitational wave signal and the gamma ray burst arrived within two seconds of each other, despite traveling an enormous distance over 130 million years across the universe. That means that both travel at the speed of light — something that Einstein had also predicted.
When gravitational waves were first detected, scientists hailed the dawn of a new age of astronomy. Every time we have looked at the sky in a new regime of light — with x-rays, or infrared, or gamma rays — we have seen things previously hidden from the naked eye. Adding gravitational waves into the mix gives astronomers an even richer data source. Ultimately that is the true significance of this latest discovery: it marks the first real result from so-called “multi-messenger” astronomy. This is only the beginning of exciting developments to come.