Tag: General Relativity

Gravitational Waves

A long time ago, in a galaxy far far away, two black holes were orbitting each other. As time went on, they began to spiral into each other, in accord with our understanding of relativity. Eventually, they merged into a single black hole, an event an unimaginable violence. The physical laws of the universe would have been strained to their very limits. The collision was so violent, it created massive ripples in the fabric of spacetime itself.

Those ripples propagated out into the universe, growing fainter and fainter. Over 1.3 billion years after the event, they traversed an utterly insignificant little blue green planet orbiting a small unregarded yellow sun in the uncharted backwaters of the unfashionable end of the western spiral arm of the Galaxy.

Such things had probably happened billions of times in the history of the Universe. Only this time, the clever apes that occupy that planet were looking.

A team of physicists who can now count themselves as astronomers announced on Thursday that they had heard and recorded the sound of two black holes colliding a billion light-years away, a fleeting chirp that fulfilled the last prophecy of Einstein’s general theory of relativity.

That faint rising tone, physicists say, is the first direct evidence of gravitational waves, the ripples in the fabric of space-time that Einstein predicted a century ago (Listen to it here.). And it is a ringing (pun intended) confirmation of the nature of black holes, the bottomless gravitational pits from which not even light can escape, which were the most foreboding (and unwelcome) part of his theory.

More generally, it means that scientists have finally tapped into the deepest register of physical reality, where the weirdest and wildest implications of Einstein’s universe become manifest.

The search for gravitational waves has been going on for a couple of decades and through a previous version of LIGO. Each time, our sensitivity has not been quite good enough to pick up gravitational waves. This time, they were. In fact, this signal was detected very early on in the LIGO run, surprising everyone. At the time, there was a wide range of predictions for how many gravitational waves LIGO would detect. A lot of people thought it wouldn’t detect any. This was a very solid detection (and the paper is refereed, so it has been through the vetting process).


I was involved in this in a small way, helping look for the potential astronomical sources of the gravity waves. Almost a year ago, I was in Pasadena, meeting with a group of astronomers and physicists to try to figure out what to do if a gravitational wave was detected. We were cautiously pessimistic about whether one would be detected simply because detecting one is so damned hard. LIGO’s announcement of a clear detection (among four possible detections) is exciting and wonderful news. And now that we’ve found them, we’re looking ahead to the next run to see if we can find something even more spectacular. This one was very far away — 1.3 billion light years. Maybe the next one will be closer so that we can study it in depth.

(While in Pasadena, I got to see a smaller test version of LIGO. It was incredibly impressive. The real thing has two 4-kilometer long vacuum-tube laser arrays and can detect vibrations smaller than a proton. The engineering alone is worth a Nobel Prize.)

For more, you can read Phil Plait, who goes through how the experiment works. Or check out this video from PhD Comics.

Science. It works.