Physicists were all abuzz in February 2016 with a universe-shaking announcement: Scientists had detected the sound of two massive black holes smashing into one another, forming a new black hole nearly twice as big.
The sound came in the form of a brief burst of gravitational waves, vibrations that radiated from the collision-like ripples spreading on a pond, traveling more than a billion light-years before they reached Earth. This was the first time in history that anyone had directly detected gravitational waves.
The hypersensitive antenna that picked up the sound, a brief chirp that lasted a fraction of a second, is the Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO is actually two separate antennas, one in Hanford, Washington, the other in Livingston, Louisiana.
Each consists of a pair of 2.5-mile tunnels at right angles to each other, down which researchers shoot laser beams and measure their reflections in order to detect tiny movements. Boston University physicist Andrew Cohen recently talked with university writer Neil Savage and explained what LIGO found and why scientists are so excited.
WHAT DID LIGO FIND?
Einstein’s theory of gravity, called general relativity, has been around for about a hundred years and it predicts gravitational waves, which have been seen indirectly in the past but have never been directly detected until now. What’s really exciting about this observation is twofold.
First, it’s direct confirmation of the existence of gravitational waves—although I will tell you that no astrophysicist doubted the existence of them, because the theory of general relativity has been so well tested. Although it’s always exciting to see something like this confirmed, it hasn’t changed anybody’s view of anything.
What is more exciting is the fact that we can now see an astrophysical event in gravitational waves and study it in a way that we never would have had access to before.
MEANING THE MERGER OF TWO BLACK HOLES. BUT HOW DO GRAVITATIONAL WAVES TELL US ABOUT THAT?
The details of the event—exactly what was happening, the nature of the black holes, how big they were, what the size of the object is once the merger has happened—are all encoded in the details of the gravitational wave they detected.
The gravitational wave is not very different from radio waves; they differ in some important details, but generally it’s about the same. When you receive a radio transmission on your car radio, how do you know who the singer is? Well, you can recognize the sounds that are being produced.
And in exactly the same way, the black holes, when they merge, produce a characteristic wave signature. The wave has features we can hear in this detector, and by looking at the shape of that wave, we can infer all the details of the event that created it.
SO HOW IS THIS DIFFERENT FROM AN ANTENNA OR A RADIO TELESCOPE?
It’s the same in principle. Radio telescopes and optical telescopes are both seeing electromagnetic radiation, which is produced from the motion of electrically charged particles.
You shake electrically charged particles and the waves travel away from the particles that you shook, and then you need a detector to see them. The same is true of gravitational waves.
You need to shake a mass so that the gravitational forces associated with that shaking produce the gravitational wave, and then that wave, in turn, will shake another gravitational mass in the antenna for detection.
Gravitational waves require very, very strong forces to produce them—at least to produce them enough that we can see them. So that’s why the event that they saw, this merger of black holes, had to be very massive objects with very strong gravitational forces involved, in order to produce the radiation so we could see it.
The typical kind of astrophysical event that will produce these are collisions or mergers of very massive gravitating bodies—so, black hole mergers, neutron star mergers, and potentially things that we haven’t even thought of.
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