What happens when two black holes collide?

If two black holes collide in space more than a billion lightyears away, do they make a sound? A team of scientists recently announced that, in fact, they do. Janna Levin, physicist and author of Black Hole Blues and Other Songs from Outer Space, joins Hari Sreenivasan to tell the story behind the breakthrough detection of gravitational waves.


If two black holes collide in space, more than a billion light-years away, do they make a sound?

A team of scientists recently announced that, in fact, they do.

Here to tell us the story behind the breakthrough detection of gravitational waves is physicist Janna Levin, author of the book 'Black Hold Blues and Other Songs from Outer Space.'

So, what does it mean when black holes are colliding and we heard a sound?

Well, the black holes colliding are kind of like mallets on a drum.

So you should think of space-time itself -- the three-dimensional space we live in and the one dimension of time -- as being like a drum.

And when these mallets are moving around, they're ringing the drum.

The actual shape of space starts to ring.

And this creates gravitational waves.

Now, the gravitational waves travel throughout the universe, and it's very hard to disturb them.

So, for the 1.3 billion years since this collision, they've been on their way here.

You know, meanwhile, multi-celled organisms were fossilizing on the Earth.

You know, when they get towards a near nebula outside our solar system, Einstein starts theorizing about curved space-time.

When they enter the orbit of the solar system, they just have hours.

And this instrument, LIGO, was developed over 50 years.

During the time of the advanced runs, the hits from the Southern sky rings a machine in Louisiana.

Seven milliseconds later, it's recorded in Washington state, where the second machine is.


So, you had confirmation.

You had two sources.


But it's literally like measuring the shape of a drum, which is what the instruments do.

They record the ringing shape of the space, and they literally play it back through an amplifier and a speaker system.

So you can listen to the machine in the control room.

In your book, you say if two astronauts were floating near where this was happening -- if that was even possible.

Just a thought experiment -- that they wouldn't see anything.

But they would hear something.

That's right.

So, this collision, as far as we can tell, to the best of our knowledge, happened in complete darkness.

But it was the most energetic event we've detected since the Big Bang.

More power came out of this collision than all the power of all the light of all the stars in the universe combined.

But it came out purely in the gravitational waves.

And so if you were an astronaut floating by, it would be dark, but it could technically -- I mean, we think it's at least hypothetically possible, ring your auditory mechanism, and you would hear the black holes colliding without an instrument between you and the sound.

Of course, by that time, it'd be too late, and you'd probably be sucked into the --

You'd probably be dead.

Yeah, there's that.

There's that.

So that's a downside of the experiment.

To get to the point where we had measurement devices is a pretty amazing scientific journey in itself.

Incredible, yeah.

That humans had to do a whole lot to even figure out what to listen for.


At the time that people like Ray Weiss, one of the original architects -- Ray Weiss and Kip Thorne, Ron Drever -- started thinking about building these instruments, people weren't even sure gravitational waves were real.

Even Einstein kept changing his mind.

Are these real?

Are they not real?

It's not an easy problem.

And not only that, they didn't know black holes were real astrophysical objects.

So they were really dreaming of making a recording device, this cosmic recording device, kind of before it was plausible to do so.

It was a 50-year campaign.

And you've followed the scientists kind of the front lines of this, and they didn't really -- they had to kind of double check and triple check, like, 'Is this really a blip or is this a thing or what?'

[ Laughs ] Yes.

So, imagine their surprise the morning of this detection.

They had just installed the advanced components.

I mean, it had taken a couple of years to integrate them and to lock the machines.

Very technologically sophisticated machine.

It's measuring changes in the shape of space-time over four kilometers, which is the scale of the machines, of less than 1/10,000 of the width of a proton.

I mean, this is just a phenomenal technological achievement.

But even though they had reached that point, they weren't sure they were gonna succeed.

And even the months leading up to locking this machine, they thought it might be three years out, or maybe never.

I mean, nature doesn't always comply, right, and give you what you need.

So they really just had basically turned the machine -- the advanced machine on after all of these years.

And I think they were very surprised.

Rana Adhikari, who's one of the experimentalists on the team, said, 'Oh, come on.

We just turned the thing on.

There's no way.'

He said it even took him a day to even look at the data.

He was incredulous.


And that was the reaction, I think, of a lot of people at first.

So does that mean there are more of these explosions happening or have happened over time and we just haven't been able to sense them?

Yes, exactly.

So it's already honing in on the science that we're discovering that black-hole/black-hole collisions are maybe more frequent than we previously thought.

It's very hard to see black holes, so we don't have great numbers on two black holes colliding.

We have no numbers on that.

It's the first black-hole/black-hole merger we've ever detected.

And the black holes were also big.

They were both 30 times the mass of the sun.

One was even bigger than that.

And that was unexpected.

So there's a lot of things we've learned just from this one event.

And the anticipation is that every time the instrument is online, it will ring with black-hole/black-hole collisions, maybe neutron-star/neutron-star collisions, maybe stars exploding.

Maybe. If we're really lucky.

And if we're really lucky, it'll be something we haven't thought of yet.

Someone in the audience, they say, 'Okay, fine.

So you got these big sensors that you've built.

We've spent all this time and energy and money just figuring out how to learn something like this.

But what's the practical impact of how these technologies influence our lives?'

I mean, usually it's bleeding-edge research that ends up translating into something that we are all familiar with, we'd take for granted.


You can get a new ringtone for your phone.

[ Both laugh ]

Just that.

Well, I mean, really, what scientists will go through, what they'll do to climb that mountain, you know, is so tremendous that there's always some technological implications.

We don't maybe know it right now, but there's no question that this was a -- even if it hadn't succeeded scientifically, which would have been terribly heartbreaking, it already was a technological achievement.

And that will leak into other fields.

So there's quantum aspects of this instrument that are very sophisticated, that cross -- you know, there are crossovers with other fields.

Time will tell.

All right.

Janna Levin, professor in the Department of Astronomy and Physics at Barnard College.

Thanks for joining us.

Thank you so much.

Good to be here.