Everything Everywhere Daily: History, Science, Geography & More - How LIGO Works

Episode Date: October 19, 2021

To explore the universe humans have made any manner of telescopes. These telescopes can observe visible light, infrared light, radio waves, and even x-rays. One of the most important forces in shaping the universe is gravity. How can astronomers observe gravity? In 2002, the National Science Foundation, Caltech, and MIT managed to build a gravitational observatory. Learn more about your ad choices. Visit megaphone.fm/adchoices

To explore the heavens, humans have made many different types of telescopes. These telescopes can observe visible light, infrared light, radio waves, and even x-rays. One of the most important forces shaping the universe, however, is gravity. And how can astronomers observe gravity? Well, in 2002, the National Science Foundation, Caltech and MIT, managed to build a gravitational observatory. Learn more about the laser infomerometer gravitational wave observatory, or LIGO, the most accurate instrument ever created on this episode of Everything Everywhere Daily. Do you ever climb into bed ready to sleep only to have your mind start racing the moment your head hits the pillow?

Thoughts bouncing around, replaying the day or jumping ahead to tomorrow? That is exactly why Catherine Nikolai created Nothing Much Happens. Each episode is a gentle, cozy bedtime story where, well, nothing much happens. No drama, no tension, nothing you need to follow closely. Just soft narration, calming repetition, and soothing sensory deep. designed to help your mind slow down and your body relax. It's not about entertainment, it's about rest. And millions of listeners around the world use it every night to quiet their thoughts and

finally fall asleep. If you've ever struggled to shut your brain off at night, this might be exactly what you've been missing. You can listen to Nothing Much Happens wherever you get your podcasts. Episodes are every Monday and Thursday. This episode is sponsored by CuriosityStream. I've talked many times before about CuriosityStream, but if you still haven't gotten a subscription, you really need to consider doing it. CuriosityStream has thousands of educational and documentary programs

covering a wide range of subjects. I personally watch CuriosityStream several times a week, and I've used their programs to get ideas for this podcast, as well as doing show research. You can get an entire year of CuriosityStream for less than $20. It's so cheap, you almost can't afford to not get it. If you're even remotely curious about the world you live in, go to everything-everywhere.com slash curiosity stream to start your subscription. Once again, that's everything-dash everywhere.com slash curiosity stream. The origin of this episode comes from the Nobel Prize winning physicist Richard Feynman. The origin of the episode doesn't come from any of his theories of physics, but rather his views on education. He felt that to truly learn something, you had to be able to understand

it well enough to be able to explain it to someone else in simple terms. This episode came about from my attempt several years ago to understand how LIGO works. When it was first announced, there were parts of it that made absolutely no sense to me. It turns out there is a very good reason why LIGO is the most accurate measurement device ever created. So let's start with first things first. What is LIGO trying to observe? Ligo is trying to measure gravitational waves. Gravitational waves were predicted by Einstein. Gravity, like everything else in the universe, can't travel faster than the speed of light. Just as an astronomical observatory is detecting light that may have been sent millions or billions of years ago,

so too is LIGO trying to detect gravitational waves which were sent out billions of years ago. In particular, LIGO is trying to detect massive gravitational events such as the merger of two black holes. These events involved masses potentially many times greater than the mass of our own sun. An optical observatory collects photons of light. It can compensate for the lack of light by just fixating on a star for an extended period of time. A gravitational observatory can't do that. The thing that had me initially confused was how it was possible to detect gravity over such distances while doing so with interference from other objects.

Gravity, like light, is subject to the inverse square law. That means if you double the distance between two objects, the strength of the gravitational attraction between them is only one-fourth of what it was before. When you expand that to distances over billions of light years, no matter how powerful the initial event was, the effect when it reaches us would be minuscule. The flip side to the inverse square law is that smaller objects, which are very close, can have much larger effects. Everything that has mass exerts a gravitational attraction.

That includes you, a truck, or an airplane, or the moon. What I didn't get is, even if you could create a sensitive instrument, how could you possibly filter out the noise of a truck driving by or an airplane flying overhead or even someone standing next to the detector? Trucks and people are small in the cosmic scheme of things, but when they're very close to a detector, their influence could be just as large. To explain how it solves these problems, I need to explain just how Lego works. As I stated in the introduction,

LIGO stands for Laser Interferometer Gravitational Wave Observatory. The key to the whole operation here is lasers. The observatory doesn't have a dish or a lens or an antenna. There are two LIGO observatories. One is located in Hanford-Sight, Washington, and the other is located in Livingston, Louisiana. The fact that there are two of them is an important part to the story that I'll get to in a bit. Each observatory has two arms that are four kilometers long.

They are set at 90 degrees to each other almost as if it's half a square. Each of these four kilometer arms has a tube that is one meter in diameter, which is a near-complete vacuum. And at the end of each tube is a mirror, which is the most reflective mirror ever made by humans and cool to temperatures just above absolute zero. Here is the bit that is the whole core of the system. A laser is shot down one of the tubes. However, where the tubes intersect, it goes through a partial mirror. Half the light goes down one tube, and half the light is reflected 90 degrees to go down the other tube.

When the light bounces back, it's sent to a detector. The light from the two laser beams at this point should be perfectly out of sink. The peaks and troughs of the two light waves should cancel each other out. When a gravitational wave hits the Earth, it will cause space time to warp the mirrors ever so slightly, and change the distance between them. By ever so slightly, I really mean ever so slightly. The change in distances between the mirrors can be as small as 10 to the minus 18 meters, or 1.1,000th, the diameter of a proton.

They can measure this by measuring how out of phase the two beams of light become. The ability to measure a change of 1.1.000th the diameter of a proton is what makes LIGO the most accurate measuring device ever made. So, observing the changes in the split light beam is how they're able to measure something so small. Okay, so that answers one question. However, it doesn't answer how they're able to filter out all the random vibrations and small gravitational poles. As I mentioned before, something very small and very close can have the same effect as something massive but distant. That's why they built two of them.

Both of the observatories in Washington and Louisiana are built out in the middle of nowhere, so outside interference is minimal. For the observatories to register a detection, both observatories have to observe the same thing at roughly the same time. The theoretical basis for LIGO dates back to the 1960s, Rainier Vice of MIT and Kip Thorne of Caltech were the leaders in trying to get such an observatory built. Small prototypes were built, including a 40-meter version created in the 1980s, but nothing

close to the several kilometer version which would be required to get really accurate measurements. It wasn't until 1994 that LIGO got the go-ahead and received a grant for $395 million, making it the largest project ever funded in the history of the American National Science Foundation. LIGO was turned on in both facilities in 2002, and for the next eight years, it didn't detect anything. During this time, the National Science Foundation was preparing for what they called enhanced LIGO, In 2010, the project was shut down, and new detectors and equipment were installed over a five-year period. The new enhanced LIGO had four times the sensitivity as the previous setup, and it was turned on in September of 2015. Within two days of turning on the new system, they finally had a gravitational observation.

Named GW-150914, the observation was of two merging black holes, 1.4 billion light years away. They were 30 and 35 times the size of the sun respectively. The signal which both observatories found was almost a perfect fit for what was predicted from the theory of relativity. Since then, there have been several more gravitational observations made. A new gravitational observatory was opened in Italy in 2017 named Virgo, and it cooperates with the LIGO observatories which helps improve the accuracy of the observations. There are also plans to open a gravitational observatory in India, an observatory in space, and future plans to improve the Lego observatories in the United States.

More observatories around the world will also help us better estimate the direction of any observations made by triangulating the small differences in when the signal was registered at various observatories. In 2017, the Nobel Prize in Physics was awarded to Kip Thorne, Barry Barish, and Rainier-Vice for their efforts in making the first gravitational wave observation. Gravitational observation is really cutting-edge science. The first observation occurred just six years ago, and with the new observatories coming online and new equipment being adopted, we should be discovering even more in the years to come.

The associate producers of Everything Everywhere Daily are Peter Bennett and Thor Thompson. If you'd like to support the show, please join the list of patrons over at patreon.com. And also remember, if you leave a review or send me a question, you two can have it read on the show.