How LIGO Works

Podcast Transcript

To explore the heavens humans have made many types 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 the Laser Interferometer Gravitational-Wave Observatory or LIGO, the most accurate instrument ever created, on this episode of Everything Everywhere Daily.

The origin of this episode comes from the Nobel Prize winning physicist Richard Feynman. The origin 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 didn’t make any sense to me. It turns out that there is a very good reason why LIGO is the most accurate measurement device ever created.

So first, what is it that LIGO is 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 photos 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 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 that smaller objects which are very close can have much larger effects. Everything that has mass exerts a gravitational attraction. That includes you, or a truck, or an airplane, or the moon.

What I didn’t get is that even if you could create such a sensitive instrument, how could you possibly filter out the noise of a truck driving by, 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 are very close to the detector, their influence could be just as large.

To explain how it solves these problems, I need to explain just how LIGO works.

As I stated in the introduction, LIGO stands for Laser Interferometer Gravitational-Wave Observatory. The key to the whole operation is lasers.

The observatory doesn’t have a dish, or lenses, or mirrors.

There are two LIGO observatories. Once located in Hanford Site, Washington, and the other in Livingston, Louisiana. The fact that there are two is an important part to this story.

Each observatory has two arms which are four kilometers long. They are set at 90 degrees to each other.

Each of the arms houses a tube that is one meter in diameter which is a near-complete vacuum. At the end of each tube is a mirror which is the most reflective mirror ever made and cooled to temperatures just above absolute zero.

Here is the bit which is the core of the system. A laser is shot down one of the tubes. However, at the intersection of the tubes, it goes through a partial mirror. Half of the light goes down one tube and half of the light goes 90 degrees down the other tube.

When the light comes back, it is sent to a detector. The light from the two laser beams at this point should be perfectly out of synch. The peaks and troughs of the light wave should cancel each other out.

When a gravitational wave hits the Earth, it will cause spacetime to warp the mirrors ever so slightly and change the distance between them. By every so slightly, I really mean every so slightly. The change in distances between the mirrors can be as small as 10^-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 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 are able to measure something so small.

OK, that answers one question. However, it doesn’t answer how they are able to filter out all the random vibrations and small gravitational pulls. As I mentioned before, something small by very close can have the same effect as something massive but distant.

That is why they built two of them. Both of the observatories in Washington and Louisiana are built out in the middle of nowhere so the 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 of LIGO dates back to the 1960s. Rainer Weiss 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 large 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 funded in the history of the 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 5 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 the new system on they finally had an observation. Named GW150914, 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 which 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 improves the accuracy of observations. There are also plans to open a gravitational observatory in India, an observatory in space, and future plans to improve the LIGO 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 the various observatories.

In 2017, the Nobel Prize in Physics was awarded to Kip Thorne, Barry Barish, and Ranier Weiss 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 and new equipment being adopted, we should be discovering even more in the years to come.