Ligo black hole waves

Gravitational Wave Discovery Rocks The Scientific World

Diagram showing two black holes orbiting each other. The massive objects produces ripples in the fabric of spacetime. Credit: NASA
Artist view showing two black holes orbiting each other. The massive objects produce ripples in the fabric of spacetime similar to the waves a pebble makes when tossed into a pond. Credit: NASA

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.

Massive objects create pockets or dimples in the fabric of spacetime. In the modern view of gravity, Earth revolves around the sun because it follows the curvature in space created by the sun. Credit: NSF
Massive objects create pockets or dimples in the fabric of spacetime. In the modern view of gravity, Earth revolves around the sun because it follows the curvature of space created by the massive sun. Credit: NSF

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.

Animation showing gravitational waves rippling through spacetime as two massive objects revolve about one another.
Animation showing gravitational waves rippling through spacetime as two massive objects revolve about one another.

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.

An aerial view of the Laser Interferometer Gravitational-wave Observatory (LIGO) detector in Livingston, Louisiana. LIGO has two detectors: one in Livingston and the other in Hanford, Washington. LIGO is funded by NSF; Caltech and MIT conceived, built and operate the laboratories. Credit: LIGO Laboratory
An aerial view of the Laser Interferometer Gravitational-wave Observatory (LIGO) detector in Livingston, Louisiana. LIGO has two detectors: one in Livingston and the other in Hanford, Washington. Credit: LIGO Laboratory

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.

Signals of the first detection of gravitational waves picked up by both the Hanford and Louisiana LIGOs. Credit: NSF
Gravitational wave signals from the merger of two black holes picked up by both the Hanford and Livingston LIGOs. Credit: NSF

“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.

How LIGO works. A single laser beam is split into two beams, reflected off mirrors and redirected back to a detector where they cancel out. Gravitational waves cause the instrument to stretch and squeeze, throwing the beams very slightly out alignment. Credit: NSF with additions by the author
How LIGO works. A single laser beam is split into two beams, reflected off mirrors and redirected back to a detector where they cancel out. Gravitational waves cause the instrument to stretch and squeeze, throwing the beams very slightly out alignment. Scale this up to 2.5 miles on a side and you’ve got a LIGO detector. Credit: NSF with additions by the author

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!

Rainer (Rai) Weiss is professor of physics emeritus at MIT. ... gravitational wave detector, and co-founded the NSF LIGO (gravitational-wave detection) project.
Rainer Weiss, professor of physics emeritus at MIT and co-founders of the LIGO project, uses stretchable mesh to demonstrate the elasticity of spacetime and how it’s affected by the passage of gravitational waves during this morning’s press conference. Credit: NSF

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
theh universe.”

2 Responses

  1. Giorgio Rizzarelli

    Great article as always, Bob.

    A couple of additional interesting facts, which you may have already read:
    – Kip Thorne, cofounder of LIGO and famous to wide public as consultant behind the movie “Insterstellar”, was present at the press conference yesterday in Washington D.C.
    – The signal, 8 waves increasing in frequency and amplitude from 35 to 250Hz, lasted 0.2s: if it was acoustic it would sound as a bird “chirp”.

    And, about the internationality of the discovery:
    – The LIGO signal was first seen through web in Germany by Italian post-doc researcher Marco Drago.
    – The Virgo experiment in Pisa (Italy), similar to LIGO and actually in some collaboration with LIGO, could also detect the event, but was offline for upgrade.

    Also interesting to note that the same kind of detector (a Michelson interferometer), over a hundred years ago, gave of the main experimental results (constancy of light speed) on which the other theory of relativity, the special one, is based.

    Let this be the beginning of a new exciting era of astronomy.

    1. astrobob

      Giorgio,
      Very nice – thank you for all of the additional points about the discovery! It really is the beginning of a new era. I’m happy to be here.

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