Gravitational Waves, LIGO, and the Nobel Prize

Every year, the Nobel Foundation awards a prize of over a million dollars to people who have made groundbreaking impact in each of six categories: peace, literature, medicine, economics, chemistry, and physics. Each prize can be split between up to three people. On Tuesday of this week, the winners of the physics prize were announced: Rainer Weiss, Kip Thorne and Barry Barish, leaders of the LIGO Collaboration. LIGO, the Laser Interferometer Gravitational-Wave Observatory, heard humanity’s first gravitational wave two years ago. My own research involves looking through LIGO data for signals. Although I came on board after the famous first hit, I still feel awash in team spirit at the Nobel Prize announcement.

Left to right: Rainer Weiss, Kip Thorne, Barry Barish

A gravitational wave is a ripple in spacetime that travels across the universe at the speed of light. Put simply, gravitational waves are to gravity what light is to electromagnetism. But to understand it more in depth, we need to shake loose a common paradigm about space. We all start out thinking of space as flat. By flat, I of course don’t mean squished, but that parallel lines stay parallel no matter how far you go, the inner angles of all triangles add up to 180 degrees, and if you go in a straight line in any direction in space, you will have to turn around if you want to get back to where you started. Flat space may seem obvious to our minds, and maybe even the only possibility, but it is actually not true.

Curved space is not easy to comprehend, nor to explain, and adding time to the mix just makes everything crazy. Nonetheless, the universe we live in can and does bend, compress, expand, and warp. Have you ever had a dream where you were running down a hallway, only to find that the end kept getting farther away from you instead of closer? That’s what expanding space is like, except it’s cool instead of scary. If you imagine a hallway where you stay still but the end moves back and forth, closer and farther—on a solid foundation, without motors or anything—that is spatial expansion and compression. There is more to the curvature of spacetime, but for today, all that really matters is compression and expansion.

Image credit: NASA

Gravity is commonly thought of as the force that binds the universe together. It keeps planets moving around stars, and stars inside their galaxy. But technically, gravity is space that is ever so slightly bent. If you could measure triangles with extreme accuracy, you would find that a triangle that wraps around the sun will have interior angles that add up to ever so slightly more than 180 degrees. If that is difficult to understand, don’t worry, the important thing is that gravity is a distortion in spacetime.

What would happen if the sun were to suddenly disappear? We might think that all of the planets would immediately stop orbiting and fly off into interstellar space. But that is not correct. The effect of gravity is not instantaneous, but is limited to the speed of light. The sun is 8 light minutes from Earth, so it would be 8 minutes before the Earth stopped feeling the sun’s gravity. It would be longer for the outer planets.

The sun disappearing is completely unrealistic, but there are more realistic phenomena that cause gravity to change extremely quickly. There are things in the universe that are so massive and so tightly packed as to defy comprehension. Neutron stars, for instance, have masses slightly greater than the sun’s, but they are packed into a space the size of a mountain. Their gravity is so strong and it crams the matter they are made of so closely together that their atoms collapse and only neutrons are left. When two of these monsters orbit each other closely enough, they whip around in circles thousands of times every second. That’s some serious changing of the gravitational field! Because they orbit in a circle, the change in gravity is periodic, meaning it repeats itself. A periodic change in gravity traveling outward at the speed of light is a gravitational wave.

The waves actually go more up and down than sideways, and they are invisible. Probably.

Neutron stars whipping around each other at insane speeds is one source of gravitational waves. Black holes, objects more massive and even more dense than neutron stars, are another. But the real money comes when they collide, throwing off so much energy that they are the most powerful type of event in the universe next to the big bang itself. Indeed, all of LIGO’s signals so far have come from colliding black holes. At first they orbit, multiple times the sun’s mass whipping around hundreds of times every second, until, bam, the two black holes become one. At this point, several suns’ worth of mass is converted into gravitational energy at once and sent across the universe in something like a shockwave in all directions. When this front washes over the Earth, our detectors send out the alert.

When a gravitational wave passes over us, what does it look like? Imagine holding a large, rubber loop, between your hands in front of you so that you can see through it. You start squishing the loop and relaxing, getting a nice bouncy rhythm. As your hands move closer together, the sides of the loop get squished and the top and the bottom get longer, and when you move your hands apart, the sides of the loop get longer and the top and bottom get squished. The space a gravitational wave travels through is like the loop. As the gravitational wave passes through it, it expands space in one direction and compresses space in the other direction, and then the expanded direction compresses and the compressed direction expands.

After traveling billions of light years across the universe, gravitational waves become super weak and ridiculously hard to detect. We’re talking one part in a hundred quintillion (1 in 1 followed by 20 zeros). This number is so small that our brains are not equipped to deal with it. We are looking for a change as small as a thousandth the width of a proton compared to 4 kilometers, or the width of a human hair compared to the distance between the sun and the nearest star. That’s a big difference. To detect such infinitesimal changes, we are going to need some seriously high-tech machinery.

That’s where detectors like LIGO come in. LIGO is an interferometer with 4-kilometer arms, which act as moving hallways for the light that travels down them. A laser is shone through a beam splitter, which makes half the beam go down one arm and half the beam down the other. Mirrors at the ends bounce the light back, and the beam splitter reunites them and shines the recombined beam toward the light detector. Normally, the returning beams are out of phase, which means the detector sees nothing. But if a gravitational wave comes by and changes the length of the arms while the light is in them, even by a distance smaller than a proton, the light will change phase ever so slightly and suddenly the faintest shimmer will make it to the light detector. That shimmer is all we need to find out everything we want to know about the source of the gravitational wave.

And we have not even mentioned all the noise-countering technology involved. Wind, tiny earthquakes, a car driving by, a change in temperature or humidity—all these things will contaminate the data. So we need a plethora of other sensors around so that we can log every tiny thing that happens and cancel it out from LIGO's data. Despite how monumental the challenge, LIGO is successful. In its two six-month-or-so runs, it has confidently found four pairs of merging black holes.

Before LIGO, we had two windows into the universe. The first was light, not just what is visible, but the entire electromagnetic spectrum. This showed us that there are billions of stars in our galaxy, and billions of galaxies in the observable universe. The second was particles, cosmic rays, neutrinos, and more. These taught us about the magnetic fields between the stars and the chemical and nuclear processes that happen in stars and nebulae. Now, there is a third window open, of gravitational waves, which will let us hear the hidden mysteries that cannot be seen. It is right and just that those who contributed the most effort toward opening this window receive their due recognition and be remembered by history, as Galileo has been for inventing the astronomical telescope.