Albert did it again! 100 years ago Einstein predicted that moving objects would emit gravitational waves, tiny ripples in the fabric of space and time produced by violent events like supernovae explosions and colliding black holes. Today, astronomers announced they finally found exactly what the wild-haired physicist had predicted so long ago.
Think of the ripples like waves from a pebble dropped in a pond that begin at their source and radiate outward across the water or in this case, space. In our modern view of space, also bequeathed to us from Einstein, time and space combine to form a single 4-dimensional entity called spacetime. Massive objects create dimples and curves in spacetime the same way standing on a trampoline makes the fabric sag. Earth revolves around the sun because it follows the curvature of the dimple created by the massive sun in space. Wild.
When a massive object accelerates through space, it produces ripples in the fabric. Everything with mass, even you and I, produces gravitational waves so long as the object is accelerating. Problem is these waves are incredibly weak and notoriously difficult to detect even when we’re talking big stuff like stars and galaxies. Like trying to hear a conversation not just across a room but across a continent. For that you need something ultra-powerful like the two black holes some 30 times more massive than the sun that merged into a single object while traveling toward one another at half the speed of light.
The ripple produced as the holes approached and merged were spotted by two identical detectors in the Laser Interferometer Gravitational-Wave Observatory (LIGO) 1,900 miles apart with one in Livingston, Louisiana and the other in Hanford, Washington. Two detectors were built a great distance apart to guarantee that any signal received would be from space and not locally generated. In this instance, both observatories picked up nearly the identical signal, guaranteeing that astronomers had found the real thing.
And get this. Because these black holes are located 1.3 billion light years from Earth, this spectacular spacetime catastrophe happened about 1.3 billion years ago, when multi-cellular life was just beginning to establish itself on our planet. Isn’t it wonderful that its descendants (us) are finally able to listen in on the event more than a billion years later?
Listen to the sound of the first gravitational waves ever detected by LIGO
And I do mean listen. Properly amplified, the waves fall within the range of human hearing. Like radio and light waves, gravitational waves travel at the speed of light. The reason their discovery is so momentous — and sure to bring Nobel Prizes to LIGO’s creators — is because the waves allow us to fathom the universe with an entirely new tool. Waves carry information about gravity, black holes, dark matter and even the Big Bang. Telescopes do a great job but gravitational wave detectors like LIGO (LYE-go) provide us with a sense of hearing when we’ve only ever used our sense of sight.
“This is an exciting time that is quite similar to when the astronomy community introduced radio astronomy,” said Denise Caldwell, NSF division director for physics. “In much the same way that radio astronomy added another dimension to how scientists could observe celestial phenomena, Advanced LIGO also offers yet another, different perspective. We have found that each time we open a new window of observation, we are able to make discoveries that lead us to a new frontier.”
A great primer on how LIGO works
So how does LIGO work? Conceived and built by researchers at MIT and the California Institute of Technology (Caltech) and funded by the National Science Foundation, each system uses two identical interferometers carefully constructed to detect incredibly tiny vibrations from passing gravitational waves. An interferometer works on the principle of light waves interfering with other light waves to either cancel each other out or reinforce each other to make a brighter wave.
A laser is aimed at a beam-splitter that sends half the light to one nearby mirror and the other half to a mirror perpendicular to the first mirror. Light reflects from both mirrors back to the beam-splitter which sends it on to a detector. The light has been split in such a way that the returning light waves exactly cancel each other out. When a gravitational wave passes by, it distorts space, changing the distance between the mirrors. One arm of the interferometer becomes a little longer for an instant and the other arm a little shorter. A moment later they switch. As the passing wave squeezes and distorts the instrument, the crests and troughs of the light waves briefly misalign and no longer cancel each other out.
You can imagine how precisely you’d have to build an instrument to detect these exceedingly tiny distortions. In the real LIGO, the beam-splitting arrangement is 2.5 miles (4 km) long, the longer the better to see any shrinkage and expansion. In the case of the first detection with the merging black holes, the amount of stretching equaled 1/1000th the diameter of a single proton in an atomic nucleus!
Here’s the kicker. The black holes weighed in at 29 and 36 times the mass of the sun. You’d think the merged holes would weigh in at 29+36 or 65 solar masses, but they don’t. From studying the waves, astronomers determined the final black hole tipped the scales at only 62 solar masses. The other three were converted into energy and emitted as gravitational waves. Three whole suns worth!
Gabriela Gonzalez, LIGO Scientific Collaboration Spokesperson, put it best at this morning’s press conference:
“Now that we know gravitational waves are there, we begin to listen to