The Long Search for Elusive Ripples in Spacetime
This month, one of the most elusive quests in physics began … again. A century ago, Albert Einstein’s theory of general relativity predicted the existence of rippling disturbances in spacetime called gravitational waves. Physicists have been searching for them ever since. In particular, they’ve been looking with the Laser Interferometer Gravitational-Wave Observatory, which started looking for wave signatures in 2002.
But after eight years of work, LIGO still had nothing to show for itself—so instead of bailing out, engineers decided to level up LIGO’s detectors. Now it can search 1,000 times more of the sky. Because I mean, no way that Einstein got it wrong, right?
Advanced LIGO, which began its first observations this month, is now the most sensitive gravitational-wave detector in the world. Working perfectly, it can detect gravitational waves originating as many as 326 million light years away. On Earth, this means Advanced LIGO’s laser interferometers can suss out fluctuations as small as one-billionth the width of an atom.
What’s the big deal about such tiny ripples in spacetime anyway? Let’s zoom out … way way way out, since this is space. Gravitational waves ripple out from violent astronomical events such as supernovae or colliding black holes. If a detector on Earth—or someday even in space—could detect these ripples, it could open a whole new window into the “warped” side of the universe. Oh, and it would also once and for all prove that the theory of general relativity (you know, the one underlying much of modern physics) hasn’t been wrong this whole time. LIGO has a lot riding on it.
Wind, Trains, and Trees
Upgrading LIGO to Advanced LIGO meant ripping out the guts of the interferometers and starting over. LIGO has two observatories in Hanford, Washington and Livingston, Louisiana, which can crosscheck observations and narrow down where a gravitational wave, if detected, originated. But the two sites had some unique problems. The Washington detector sometimes picked up noise from the wind, as little as 20 or 25 miles per hour. Things were even worse in Louisiana, where trains and a nearby logging operation constantly shook the ground during the day. “They had these rather impressive machines to slice the trees and tear the roots out of the ground,” says Dennis Coyne, LIGO Laboratory Chief Engineer.
After the upgrade, the two instruments now have active vibration isolators: Seismometers pick up tiny ground vibrations and motors automatically push to cancel them out—like how noise-cancelling headphones get rid of background noise. In the ramp up to this month’s observational run, the Livingston site asked the logging company to fell its trees closest to the site first. Those falling trees didn’t affect the interferometer at all.
Getting rid of noise is pivotal because the new, improved LIGO is also a hell of a lot more sensitive. LIGO detects gravitational waves by splitting a laser beam down two four-kilometer long tunnels and letting them meet at a detector. Normally, the waves of the two light beams match up perfectly. But if a gravitational wave passes through—remember, it’s literally a ripple in the fabric of spacetime—one of those tunnels becomes a tiny bit shorter, and the beams no longer match up. Engineers replaced the old 10 watt laser with one that can go up to 180 watts, and other equipment helps boost the light signal at the detector.
All those changes mean that LIGO’s current observational run is three times more sensitive than it used to be. But that doesn’t mean the work is done. In three months, LIGO will shut down again for six to nine months to get everything perfectly aligned before it ramps up to full power. This is delicate stuff: “If you have mirrors separated by 4 kilometers you have to work hard to keep everything aligned,” says Matthew Evans, a physicist at MIT.
Where Are the Gravitational Waves?
Advanced LIGO has so far been a massive engineering challenge. But if it still doesn’t find any gravitational waves, it’ll no longer just be engineers on the hook. “People would have to start to wonder what’s wrong with the astrophysics,” says Fred Raab, head of the LIGO Hanford observatory.
To be clear, physicists think the chances of seeing nothing with Advanced LIGO are low. LIGO is optimized to detect the gravitational waves from binary neutron stars—in part because astrophysicists are sure they exist through radio telescope observations. Models suggest binary neutron stars are frequent enough that their gravitational waves should hit Earth about once a month (though that model has plenty of holes). If Advanced LIGO never finds any, it could mean what astrophysicists think they know about binary neutron stars is wrong.
Indeed, the unsuccessful search for gravitational waves has gotten scientists questioning other astronomical models. This week in Science, a group of researchers who spent 11 years unsuccessfully using the Parkes telescope to detect the “death spiral” from merging black holes decided the best explanation is that the death spiral does not really exist. Instead, black holes may just smack into each other really fast.
Advanced LIGO isn’t supposed to detect the frequencies of gravitational waves from merging black holes, but continued failure to detect anything at all could mean models of how stars are born and die need some revision. (Though enough uncertainty in those models exists that the theory of general relativity is probably still be safe. Whew.)
From an engineering perspective, it’s also possible that scientists could build more sensitive detectors. Coyne says instruments kept in ultracold temperature or made out of different materials could be even more sensitive. The key, though, would be money. The National Science Foundation has poured $620 million into LIGO. If it ends up detecting nothing, says Evans, “I think it would be hard to get funding to build a better detector.”
But let’s not get too far down the pessimistic rabbit hole. Now is Advanced LIGO’s time to shine. “It’s been extremely challenging, very rewarding as an engineer,” says Coyne, who has worked on LIGO for twenty years. Let’s hope LIGO is rewarding for astrophysicists as well.