Shielded under a concrete berm in Livingston, La., the Laser Interferometer Gravitational-Wave Observatory, or LIGO, is tuned in and listening for the music of the cosmos — tunes playing within the range of human hearing.
“If there was a way to take the signal and hook it into speakers, we’d be able to hear it,” says Scott Hughes, a physicist at the Massachusetts Institute of Technology.
The trick is that the signal itself is really, really hard to detect — its effect, says Hughes, is a vibration smaller than the nucleus of a hydrogen atom. This is because the noise from distant galaxies isn’t coming to us through traditional sound waves, but through a different class of energy called gravitational waves.
Gravitational waves get made by big things in the universe, like supernovae and pairs of orbiting, colliding black holes. These events send out ripples of energy in all directions, like a pebble dropped into a lake. Instead of making waves in water, however, the energy emanating from these super-dense objects produces tiny undulations in space-time itself, vibrations in the fabric of the cosmos.
These vibrations, or gravitational waves, were predicted by Albert Einstein’s general theory of relativity in 1916, but they have never been directly measured. Now, scientists hope to detect them with the help of experiments like LIGO, as though listening to a cosmic drumbeat, for the first time.
“What we want to do is turn on the soundtrack to the universe,” said theoretical physicist Janna Levin of Barnard College at Columbia University. “If we can listen to it, it’s like archaeology — we can mine it for details” about the history of the universe, she said.
Using some of Einstein’s equations, physicists have already prepared models of what gravitational waves will sound like coming from various sources, like spiraling pairs of black holes or neutron stars. Scott Hughes, whose work at MIT includes modeling gravitational waves, explains that the pitch, tone, and frequency of the waves depend on how objects interact with each other. In other words, listening to these cosmic songs could tell us a lot about how huge objects like black holes form, and provide important insight into the evolution of galaxies and our universe.
LIGO — one of five separate observatories on Earth designed to record gravitational waves — consists of two light beams, each five kilometers in length, set up in an L shape at right angles to each other. Passing gravitational waves will stretch one beam while compressing the other — an effect the highly sensitive instrument can translate into a distinct signal.
Although LIGO can only sense gravitational waves in our own cosmic neighborhood, its planned companion in space, LISA — the Laser Interferometer Space Antenna — would be much more sensitive, picking up waves traveling from the remotest regions of the universe. LISA is slated to launch in 2018.
The last major discovery in the detection of gravitational waves came in 1974, when Russell Hulse and Joseph Taylor, Jr., both of Princeton University, used a telescope to observe binary pulsars — a pair of rapidly rotating, ultra-massive neutron stars. The binary pulsars lost energy at exactly the rate predicted by general relativity — the rate they would lose energy if they were sending out gravitational waves. The work won Hulse and Taylor the 1993 Nobel Prize in Physics.
While scientists could learn a lot about the behavior of black holes and galactic evolution from listening to gravitational waves, Janna Levin, the Barnard physicist, says she’s hoping for even more. “Hopefully we’ll see something that’s not black holes,” she said, and added, “I’d like to see something that we haven’t been able to predict.”