LIGO scientists have announced the direct detection of gravitational waves, a discovery that won't just open a new window on the cosmos — it'll smash the door wide open.
Today, physicists announced the first-ever direct detection of gravitational waves, ripples in the fabric of spacetime predicted by Einstein’s general theory of relativity. Two massive accelerating objects — in this case, a pair of stellar-mass black holes in a death-spiral — passed through spacetime like paddles sweeping through water, creating vibrations that could (barely) be felt on Earth. The results are published in Physical Review Letters.
It’s been a recurring theme in history: When scientists open a new window on the universe, they make transformative discoveries. But when LIGO, short for Laser Interferometer Gravitational-Wave Observatory, caught waves from these two colliding black holes, it didn’t just open a new window — it smashed a door wide open, promising a breathtaking new ability to study exotic and otherwise-undetectable cosmic phenomena. Don’t be surprised if LIGO's founders, Kip Thorne, Ronald Drever, and Rainer Weiss, earn free round-trip tickets to Stockholm to collect a Nobel Prize.
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LIGO consists of two L-shaped facilities, one near Hanford, Washington, and the other near Livingston, Louisiana. At 5:51 a.m. (EDT) on September 14, 2015, both labs caught the gravitational-wave signature of two colliding black holes, shortly after both facilities were turned on following five years of intensive upgrades.
A series of gravitational waves from a distant galaxy first passed through the Livingston detector, then just 7 milliseconds later it passed through the detector in Hanford. Both instruments shoot infrared lasers through 4-kilometer-long arms of near-perfect vacuum. The laser light reflects off ultrapure, superpolished, and seismically isolated quartz mirrors. The passing gravitational waves slightly altered the path lengths in the arms of both detectors by about 1/1,000 the width of a proton. That slight change created a characteristic interference pattern in the laser light, an event LIGO scientists have dubbed GW150914.
Based on the signal’s amplitude (that is, the height of the gravitational wave), team members estimate that the colliding black holes had the masses of about 36 and 29 Suns, respectively. Milliseconds before they merged, these behemoths spun around each other at nearly the speed of light. LIGO watched all three predicted phases of the collision: the black holes’ death spiral and ensuring merger, as well as the ringing of the merged object as it settled into its new form.
The merged black hole contains about 62 solar masses, so it's short three solar masses — the gravitational waves themselves carried away three solar masses worth of energy.
The minuscule difference in the waves’ arrival times at the two facilities was exactly what’s expected for gravitational waves, which travel at the speed of light. The LIGO team claims a 5.1-sigma detection, meaning the odds of the signal occurring by chance are about one in 3.5 million.
With only two detectors, LIGO can’t pinpoint the source’s exact location or host galaxy — it could come from anywhere within about 600 square degrees of sky, somewhere near the Large Magellanic Cloud in the Southern Hemisphere sky. Nor can they exactly pinpoint its distance, but measurements show the source lies between 700 million and 1.6 billion light-years away.
The beginning of this video (at 0:07) shows an all-too-brief simulation of the merging black holes and the extreme warping of spacetime around them:
A New Window on the Cosmos
The direct detection of gravitational waves opens up an entirely new spectrum that doesn’t involve any form of light. “It’s a spectrum that carries entirely new kinds of information that have so far been largely invisible,” says physicist Robert Owen (Oberlin College).
Or, as Eric Katsavounidis (MIT and LIGO team member) puts it, "This is the end of the silent-movie era in astronomy."
Previously, radio astronomers studying pairs of neutron stars, the crushed, spinning remains of massive stars, had revealed compelling indirect evidence of gravitational waves. Einstein’s general theory of relativity says that gravitational waves should carry away orbital energy, and indeed, these pulsars’ orbits spiral inward at exactly the rate relativity predicts. Joseph Taylor and Russell Hulse shared the 1993 Nobel Prize in Physics for discovering the first of these systems.
But direct detection has remained elusive because of the incredible difficulty of catching gravitational waves. Merging binaries involving black holes or neutron stars generate stupendous amounts of energy. “In terms of gravitational waves, for that one millisecond prior to merger, this binary black hole system was ‘brighter’ than all the rest of the universe combined!” Owen says. In fact, later calculations say that at its peak, the merging black was putting out 50 times more energy than the rest of the universe.
But the waves are incredibly difficult to detect because gravity is the weakest of the four known forces of nature, the strength of the waves fall off sharply as they traverse space, and because matter barely feels the presence of gravitational waves. “The gravitational waves from a distant galaxy that are detectable to LIGO are squeezing and stretching the Milky Way Galaxy by the width of your thumb,” says LIGO science team member Chad Hanna (Penn State University).
The National Science Foundation-funded $500 million LIGO experiment has been on the lookout for gravitational waves since 2002. But only recently, after a five-year rebuild and redesign to improve LIGO’s sensitivity, did the facilities have a realistic chance of catching these subtle spacetime ripples. LIGO began its first “advanced” observing run last fall, but improvements continue and future runs will have at least twice the sensitivity and enable LIGO to survey ten times the volume of space.
Theorists predict Advanced LIGO should catch an additional five binary black hole mergers in its next observing run. They also expect roughly 40 binary neutron star mergers every year it runs, and an unknown number of signals from black hole-neutron star mergers and supernovae. It's even possible that LIGO could detect exotic cosmic strings.
The direct detection of gravitational waves represents another triumph for Einstein, almost exactly 100 years after he predicted their existence — and despite the fact that he never thought they’d be detected. But as LIGO builds up a catalog of events in the coming years, and as other advanced detectors come online in Europe and Japan, physicists will be scrutinizing the waveforms in detail to see how closely they conform to general relativity’s predictions.
Though this black hole merger went entirely according to Einstein’s predictions, scientists hope to eventually see discrepancies that could provide vital clues to new physics, potentially reconciling contradictions between relativity and quantum theory.
“Gravitational-wave measurements will allow us to directly probe some of the most violent events in the universe, to directly measure the most tumultuous dynamics of spacetime geometry,” says Owen. “Gravitational waves would allow us to probe how spacetime really behaves under the most radical of circumstances.”
LIGO will prove a gold mine for astronomers: enabling them to study and build up a census of neutron stars, stellar-mass black holes, and other dim or otherwise impossible-to-detect objects in faraway galaxies. And LIGO also offers the tantalizing prospect of discovering new types of objects and phenomena hitherto unknown to science.
“We want to give ourselves plenty of opportunity to be surprised,” says Hanna. “We don’t want to open a new window to the universe and then refuse to look outside because we think we know what we’ll see. We expect the bread-and-butter sources, but we certainly hope it doesn’t stop there.”
- Get the prescient cover story of Sky & Telescope's December 2015 issue.
- LIGO scientists get their feet wet to explain the historic detection:
- What are gravitational waves? Read a primer from UniverseToday.com on the phenomenon.
- How does LIGO detect gravitational waves? Piled Higher and Deeper (PhD Comics) explains:
- What upgrades made Advanced LIGO's detection possible? Read our story on ALIGO's first first observing run last September.
- Want more? Check out an award-winning 20-minute documentary on LIGO, as well as a slew of videos describing how the European space-based gravitational-wave observatory, eLISA, will work, and more in our Beyond the Printed Page resources.
- Take part in the science with Einstein@Home. Sign up and enlist your computer's idle time to help analyze gravitational-wave data.