#35 - The Technical Challenges of Measuring Gravitational Waves - Rana Adhikari of LIGO

**Craig Cannon** (0:00)
Hey, this is Craig Cannon, and you're listening to Y Combinator's Podcast. Today's episode is with Rana Adhikari. He is a professor of physics at Caltech, and one of the members of the LIGO team, who are the first to measure gravitational waves. So Rana and I met at the YC Research Conference. Shout out to Michael Nielsen, thanks for the intro. And while there, Rana gave a talk about LIGO, and their effort to parse all the data they're collecting. And he was actually looking for help, and they still are looking for help. So if you're interested, you can reach out to Rana on Twitter at Rana X Adhikari. I'll link that up in the show notes.
And just two quick announcements before we get going. The first is that YC is going on a fall tour, where we're going to be doing a bunch of office hours and Q&A sessions all over the world. And that's at blog.ycombinator.com. And the second is that YC applications are open for the winter 2018 batch. So that link is ycombinator.com/apply.
All right, here we go.
What is LIGO?

**Rana Adhikari** (1:00)
LIGO is a huge project aimed at being able to take the bending of space that we think is happening all the time and turn it into some kind of a signal that we can use and measure. And yeah, it's a transducer. It's like having a voltage meter or a microphone, but it's like a microphone for space.

**Craig Cannon** (1:22)
Can you do the beginner's explanation of what it actually is, what the device actually is?

**Rana Adhikari** (1:29)
So we think, or we now know, based upon Einstein's relativity theory from 100 years ago, that there isn't a real force of gravity, like there is a force between magnets or between charges or things like that. But instead, the way that gravity works is that it curves space. And so it's a lot like imagining what happens when you're jumping up and down on your bed and somebody else is jumping out on their bed. I guess, because if this happens to you all the time, then this is something you're familiar with. But whoever's heaviest makes a big impression in the bed, and then whoever is littler has to account for... What kind of an analogy is this? Whoever's littler has to account for the depression in the middle of the bed, and just are jumping accordingly, and you tend to slide into the biggest dimple in the bed. So those who do trampolines are jumping up and down on their bed, understand how well-trained and understand how gravity works.
So to detect this on the earth is incredibly hard, and people have long ago measured the curvature of a space due to the earth and due to the moon and other planets and that sort of thing. And so we well understood that in fact, space is curved, but we had no evidence to support the idea that the curvature of space could travel through space as waves.
The detection of gravitational waves for decades had been debated in the scientific community. People thought it was just imaginary, that it was waves of thought, people called it, because they thought it was just waves of mathematics. It was just some equation you wrote down, but it didn't make any sense. It was just kind of a nonsense thing. And some decades ago, people realized it was real, and then some crazy people said, hey, let's try to measure this, even though it was millions of times, I mean, like factors of millions not possible.
But luckily, through a, this is just something I'm not capable of, but through a combination of optimism and courage and not knowing the right answers to several equations, they were able to start up the field and start to look for these things. I think if they had known how tough it would be, or that it was gonna take 50s, five years to have success, probably no one would have started.
And so now here in the modern times, the way we do it is we use the tool of laser interferometry, which is, for those of you who are interferometer-officiating on those, it is a Michelson-type interferometer with a lot of extra stuff added onto it. For those who are not, the concept is simple. It just has to do with interference. So you take a laser, like a laser pointer, but much more expensive and therefore much more stable.

**Craig Cannon** (4:36)
Is it a billion dollars now into the project?

**Rana Adhikari** (4:39)
Yeah.

**Craig Cannon** (4:40)
Roughly.

**Rana Adhikari** (4:40)
Yeah. The laser itself is cheaper. You can do, probably you could do the whole thing with a hundred thousand dollar laser. That's about the laser cost. You split it in two, and you send it in two separate directions. And then when the waves come back, they interfere with each other. And you look at differences in that interference to tell you the difference in how long it took for one beam to go one way, and the other beam to go the other way. And so this, the way I said it was really careful there because there's a lot of confusion about the idea of these are waves, and space is bending, and everything is shrinking, and how come the light's not shrinking, and so on.