In 1969, was a young MIT professor. At the time gravitational waves were a theoretical curiosity: Einstein himself took years to be convinced by his own prediction that moving cosmic bodies would send out ripples through space-time. Then physicist Joseph Weber claimed to have recorded one on a xylophone-like instrument he called a resonant bar detector. Weiss takes up the story.
The students on my course were fascinated by the idea that gravitational waves might exist. I didn’t know much about them at all, and for the life of me I could not understand how a bar interacts with a gravitational wave.
Advertisement
I kept thinking, well, there’s one way I can explain how gravitational waves interact with matter. I said, suppose you take a light – I was thinking of just light bulbs because, in those days, lasers were not yet really there – and sent a light pulse between two masses. Then you do the same when there’s a gravitational wave.
LIGO Detection: Behind the scenes of the discovery of the decade
Watch the exclusive world premiere on 7 February 2017
Lo and behold, you see that the time it takes light to go from one mass to the other changes because of the wave. If the wave is getting bigger, it causes the time to grow a little bit. If the wave is trying to contract, it reduces it a little bit. So, you can see this oscillation in time on the clock.
I was hiding in this little office in Building 20 at MIT and for about three months I thought about how you might do this. First I thought you couldn’t get clocks good enough. But we did some experiments and I learned you could do unbelievably exquisite measurements with lasers.
I wrote this up but I didn’t publish it. The people at MIT wanted to know where the hell I’d been spending my time, so I put it into the quarterly progress report for my lab. I came to the conclusion that, if you built this thing big enough, you could probably detect gravitational waves.
Based on Weiss’s idea, the US National Science Foundation (NSF) eventually began funding the development of what became the Laser Interferometer Gravitational Wave Observatory (LIGO) in 1979. But progress was slow, and when Caltech physicist took the lead on the project in 1994, questions were being asked
The NSF had lost confidence and were basically giving up on it. There was a lot of resistance. It represented the extreme of what you might call a high-risk, high-pay-off project. So we revamped it entirely. Over a period of six months we made it look like a new project. And it was hard because if you had asked me then, could we build what we now know we need to detect gravitational waves, the answer would have been no.
So the idea, and the words that I used, was that with the initial version of LIGO it would be possible to detect gravitational waves. And then we would evolve into a detector, which we called Advanced LIGO, where it would be probable. But in all honesty, we had nothing more than ideas on how to do the Advanced LIGO part. To me, the miracle in this whole thing is that we somehow got financial support for 22 years until we succeeded.
LIGO’s detectors occupy two sites in Livingston, Louisiana, and Hanford, Washington. Since joining the project in 2000, Hanford’s lead detection scientist has been ensuring the instrument is as sensitive as possible to the minuscule signals it was built to detect
Space is a stiff medium, so it doesn’t want to vibrate. The detector has to register changes that are about a thousandth the size of a proton. If you were trying to measure the distance between here and our nearest star, Proxima Centauri, it would be like watching that distance change by the width of a human hair.
It’s an ongoing battle to suppress noise in the instrument. There are ground noises like earthquakes, but a less obvious example is Earth’s ringing – at low frequencies, it rings like a bell because of ocean waves hitting against the continental shelf.
If you get storms off the coast of Alaska or the Gulf of Mexico, ground motions increase. We have to suppress that motion by registering it with seismometers and feeding it into seismic suppression systems – kind of the way noise-cancelling headphones work by sampling the ambient noise and then playing it with the right phase to cancel it out at your ear.
There are also a lot of internal noises that we have to suppress. Things like electronic noise, or quantum noise in the laser. All of that means the LIGO detectors are absolutely the most quiet and the most sensitive detectors ever built.
In September last year, just a few days after Advanced LIGO finally came on stream, Michael Landry received notice of an unexplained signal. At first he was convinced it was an “injection” – an artificial pulse used occasionally to test the instrument
On the morning of 14 September, I opened my computer and saw an email indicating this event seen in the data literally tens of minutes earlier. I thought, it’s probably an injection. It was so early in the observation phase. It wasn’t until later, in the lab, that we determined there was no such injection. We did a whole lot more investigation and it took months to validate it. But it was immediately obvious that if this thing wasn’t an injection, it was the best damn thing we had ever seen.
The gravitational wave apparently came from the collision of two black holes, formed from collapsed stars, 1.3 billion light years away. For Rainer Weiss, now 83 years old and a professor emeritus at MIT, it was a long-sought vindication
The discovery itself was spectacular. To me, it was the thing that I would have wanted most, to see the collision of two black holes. If you want to ask, what was the reason for building this thing in the first place, it’s to check up on whether Einstein’s theory works in strong gravitational fields. That was the one place where general relativity had not been tested. And here, suddenly, we have in our hands a thing that says Einstein’s field equations, the whole thing, is absolutely right.
For , an MIT physicist who has worked on LIGO for 25 years, this detection is just the beginning
I don’t think this was some master plan from nature, as in “let’s be nice to these people here on this little Earth place”. Black holes collide all the time. What was lucky was that we happened to have a detector with sufficient sensitivity to see this one at the moment it went up. If we continue, we will see more.
One of the amazing things about general relativity is you solve the equations – although that’s taken decades – and create templates for how signals should look. Nature was kind in that the very first signal we saw was so clear. Many people expected we would see really weak signals, barely poking above the noise, and that there would be a lot of discussion about whether it was a detection or not. None of that happened. That’s where we lucked out.
The discovery drives us harder because we know there is stuff out there waiting to be observed. You might imagine we would think, “OK, now we’ve seen it we can pack up and go home”. But in fact it’s just the opposite. We’ve seen the very first gravitational wave, but we have so much more to discover.
We have a lot to learn about black holes, and then there are neutron stars. Personally, my hope is that we will see something that really has us scratching our heads. Maybe we will have discovered some new object that I can’t begin to describe or name.
This article appeared in print under the headline “Wave hello”
You asked, we answered: find out more in “Gravitational waves: Your cheat sheet on the find of the decade“
