How do these super long tubes detect black holes?

Jenn Bane here. I had the pleasure of interviewing Daniel Holz, a professor of physics and astronomy at the 美国芝加哥大学 .

Daniel is also the Chair of the Science and Security Board of the Bulletin of the Atomic Scientists and also part of LIGO — read on to learn what that means.

So I’m told you “listen” to blackholes. Tell me what that means.

We've built the world's most sensitive instrument called LIGO — the Laser Interferometer Gravitational-Wave Observatory — that was built to listen to colliding black holes. We’re not listening to sound, however; we’re instead directly listening to vibrations of space and time, a phenomena we call gravitational waves. In 2015, we turned on LIGO and almost immediately we heard two giant black holes collide. We’re [continually improving] our instrument, and we now have heard a total of over 200 black hole collisions — of all shapes and sizes. We study the entire sample? and try to figure out what they can teach us about the universe.?

Do you have a favorite?

The next event is always the favorite, so we’ll see what shows up tomorrow. But there was this one — GW190521?— where the two black holes were unusually big. We have a story of how the universe makes black holes, and the story says that it shouldn't be easy to make ones that large. That is still perplexing us.

It’s perplexing me too. OK, please explain how the instrument picks up these collisions.

Yes, the basic idea is a laser Interferometer. Picture two perpendicular tunnels shaped like an “L." Each tunnel is 2.5 miles long and is completely empty tube — a vacuum. At the very end of the tunnels are really shiny mirrors. Where the two tunnels meet there are also shiny mirrors and a big building with a huge laser. The laser shines light down the tubes, which bounces off the mirrors and comes back.?

One way to think about what’s happening is that we send light down each tube, and it’s a race:? We want to know in which tunnel the light returns from the far end first? Of course light travels at the speed of light and that's a fixed speed. So you're racing your photons (particles of light) and, then, a gravitational wave passes through. It wiggles space and time. And in particular, consider the space part. Space itself is wiggling. The light will get back faster in one tunnel compared to the other because it has less distance to go, because space has wiggled and changed the distance. And you can very carefully measure that difference and then say, oh, look, a gravitational wave has gone through!

There are two of these instruments, on opposite sides of the U.S. One is in Livingston, Louisiana, and the other is in Hanford, Washington. They both wiggle when a gravitational wave comes through. It should pass through both of them at essentially the same time. That's the way we know it's a real signal.

And what happens when LIGO detects a collision? Do you get a text message or something?

Actually, yes. We have an automated system. And when something happens, I get a short text message.?

No way!

Yes, although it’s not quite that simple. These instruments? are insanely sensitive. In Livingston, the UPS truck kept showing up to the building and slamming on its brakes, causing the ground to shake. The detector picked it up, and it sort of looked like a black hole. But since that only happened in one detector, and not the other, we knew it wasn't [legitimate].

Is the instrument on right this second?

We’re listening right now, the instrument is on. My phone could beep right now and we’re off to the races.

Does all of this give you an existential crisis?

I am much more worried about what we are doing right now here on earth.?

Yeah, good point. OK, final question. The movie Interstellar…

Great movie. I’m a fan. The original screenplay was written by Kip Thorne — one of the people that came up with the LIGO instrument. The movie is quite accurate up until the end. That last part is complete gobbledygook and wild speculation. But everything up until that moment is consistent with the laws of physics.

Just Curious is sponsored by Provable, the science communication force of M. Harris & Co. and MG Strategy + Design.

This interview has been condensed and edited for clarity. Header image by DALL-E Open Ai. If you liked what you read today, here's more: