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What’s the sound of two black holes colliding? Proof that Einstein was right

February 11, 2016 at 6:35 PM EDT
Gravitational waves -- ripples in the fabric of spacetime -- aren’t just an Einstein theory any more. A team of international scientists announced Thursday that they confirmed the waves’ existence after recording feedback from a black hole collision a billion light-years from Earth. Hari Sreenivasan learns more from Dave Reitze of the California Institute of Technology.

HARI SREENIVASAN: Now some truly cosmic news.

The sound of two black holes colliding more than a billion years ago, it was recorded by a team of scientists at the LIGO Observatory, proof of gravitational waves, or ripples in time and space, first theorized by Albert Einstein.

We explore this monumental moment in physics with Dave Reitze of Caltech, executive director of the LIGO Laboratory.

Now, that was a rudimentary attempt at explaining what a gravitational wave is. But what are they, and why is it such a big deal to find one?

DAVID REITZE, California Institute of Technology: Actually, you did a pretty good job.

So, gravitational waves are fluctuations in space-time. And any time you have a mass, something that has matter in it, accelerating, all right, it produces a gravitational wave. All right? And that’s a consequence of Einstein’s theory of general relativity.

Now, these particular gravitational waves, in order to be able to detect them, you need really, truly massive objects. So, in this case, these were black holes that had about 30 times the mass of the sun in them.

So, why gravitational waves are so interesting is that they tell us something about the universe that you can’t get from any other kind of astronomy. So, if you think about optical astronomy, that looks at certain classes of light. If you look at radio astronomy — so gravitational waves are completely different.

They come from a different sector of the universe, and that’s why they’re so exciting.

HARI SREENIVASAN: OK. So Einstein is sitting at his patent clerk’s office thinking about this big thought, and what is the connection to space-time?


HARI SREENIVASAN: Does time and space bend? And if you heard or saw, so to speak, this moment, does that mean that time and space bent just a little bit at those points?

DAVE REITZE: Oh, in fact, this particular event was, as my colleague Kip Thorne calls it, a storm in space-time.

All right? As these two black holes came together and collided, they really disrupted space-time and produced this burst of gravitational radiation. It’s interesting that you mention Einstein. Gravitational waves were first predicted actually 100 years ago. And Einstein himself, all right, thought it was an interesting consequence of the theory of relativity, but didn’t think that it had any practical value, because he said that the effect is so tiny that we will never be able to measure them.

And it took 100 years from the time that he predicted them to the time we have been able to measure them.

HARI SREENIVASAN: And, also, to measure them, you have very giant antennae that most people probably haven’t heard about until today, one in Louisiana and Washington. Describe those.

DAVE REITZE: So, these are two interferometers.

Interferometer is a — basically a type of laser measuring device. A laser goes out. It goes — it gets split. It goes along the arms of the interferometers. And they’re very long. The arms are very long. They’re two-and-a-half-miles long. Comes back. We do a lot of tricks to make the interferometer very sensitive.

And as a gravitational wave passes, what it does is, it stretches and squeezes the light between the arms of the interferometer. And because light is made up of waves, we use the fact that that stretching and squeezing changes the relative relationship, the phase, if you will, between the waves.

HARI SREENIVASAN: So, do we — how is it that we got this sound? I mean, these are lasers, and you’re seeing the disruption or change in light.


HARI SREENIVASAN: What does that mean to say, we heard two black holes?


So the interferometers are very nice. They basically — the light goes on in the detector, and the frequency that these gravitational waves were generated by these two block holes is in the audio band. So, you could literally — it’s like a C.D. player. You could literally put a plug into the detector and put headphones on, after a little bit of data processing and filtering.

It’s not perfect, but after a little bit of data processing and filtering, you can actually hear the signal. And it’s a very interesting signal. It goes whoop like that. Actually, it goes a little bit lower, but that gives you the idea.


But what does this mean for science? Does this mean that there’s new branches of study that have opened up? What are things that people are going to think about now because you have this moment?


I think there are two things that are important here. First of all, it’s really a big confirmation that gravitational waves exist. Now, we knew they existed. There was a nice experiment done by Hulse, Taylor, and Weisberg early on using radio waves. But this is the first direct measurement of them, and so that’s exciting.

But I think the thing that’s more exciting about it is, this opens up a completely new way of looking at the universe. So, everything we know about the universe, we know from light, electromagnetic radiation.

This is the first time that the universe is communicating to us using gravitational waves. So, it’s like the cosmos is talking to us.

HARI SREENIVASAN: OK. This makes me think of all the sci-fi movies that are going to spin off from this. Right?


HARI SREENIVASAN: If we can measure that space and time are bending and it’s happening all around us, we just happen have to have these two little antennas on this tiny little planet, it’s probably happening all over the universe.


Oh, yes, sure, sure, sure, sure. And we hope that we will be able to hear more of more of these things as they go along. And I should point out that this is only the beginning, that we’re actually — there are projects that are undergoing in Japan. There’s one in Italy right now that is undergoing. We’re hoping to put a gravitational wave detector in India, because the more you have of these things, the more ears you have, then actually that gives you a huge advantage.

HARI SREENIVASAN: All right, Dave Reitze from Caltech, thanks so much for joining us.

DAVE REITZE: Thank you. Been my pleasure.

HARI SREENIVASAN: We have more on Einstein’s theory and why it was so revolutionary when he posed it more than 100 years ago.

Theoretical physicist Sean Carroll wrote about the twists and turns of space-time in the universe, and you can read that essay on our home page,