- Gravitational waves are a unique natural phenomenon that can affect reality
- Einstein used math to predict the existence of gravitational waves 100 years ago
- Gravitational wave detectors are “like a new sense we are acquiring of the universe.”
One lesson science repeatedly teaches us is that our own perception of reality can’t be trusted. Your eyes are wonders of evolution, but they only pick up on 0.0035% of the electromagnetic spectrum. There’s a whole world of X-rays and ultraviolet and other light that you just can’t see.
Gravitational waves are similar in this regard. They’re happening all around, but you’d never be able to detect them. These natural phenomena were first predicted by Albert Einstein in 1916 as a part of his general theory of relativity. His math indicated that objects with mass would alter space-time as they are accelerated. Interestingly, he also believed we would never be able to effectively test for them.
That may have been true of the technology in Einstein’s day, but progress moves ever forward. On September 14, 2015, scientists at the Laser Interferometer Gravitational-Wave Observatory (LIGO) found definitive proof of the existence of gravitational waves. Vicky Kalogera, an astrophysicist at Northwestern University and a member of the LIGO team, points out the multi-generational effort required for a discovery like this.
“It was a whole century from the very first theoretical prediction—when none of the current discoverers were even born—to get to the point to having indirect evidence of the existence of these waves,” says Kalogera.
The LIGO team she worked with utilized incredibly complex technology to observe what Einstein couldn’t, and it’s now opening up a whole new field of study.
Riding the wave
If you’ve got the space and time, you may want to try out this little experiment. Clench your hands into fists and swing them around like an egg beater. When you’re done, you’ll have accomplished two things:
- Look like a dummy to any and all onlookers
- Generate gravitational waves
But as Kalogera points out, the only kinds of gravitational waves that most scientists are interested in right now are the ones produced by enormously massive objects, that occupy very small volume, and can move very fast.
“Gravitational waves can be very weak or they can be a little stronger,” says Kalogera. “What makes them stronger is if they have a big amount of mass moving at very high velocities.”
The objects that Kalogera and her fellow scientists at LIGO have detected weigh between 1 and 50 solar masses. These celestial bodies are extremely compact and can therefore move in tight orbits. They can also reach accelerations up to half the speed of light. Their high-speed rotations create gravitational waves, which are more easily understood with Kalogera’s lake metaphor:
“If we think of a lake and we throw in a stone, then the stone disturbs the water and then ripples are propagated from where we threw the stone,” says Kalogera. “And what happens to the surface of the lake? It gets disturbed and you have oscillations of that surface and parts of the lake surface goes up in peaks and parts of it go down.”
Rather than disturbing water, gravitational waves distort reality itself. When a gravitational wave hits you, it stretches and compresses your body in a predictable fashion. Of course, because you’re experiencing reality as it’s being squished and pulled, you can’t see any difference. To confirm the existence of these waves, LIGO had to get clever.
“We use lasers and try to sense whether a certain distance between two reference points has changed because of the passage of a gravitational wave,” says Kalogera. “Therefore, if we can identify two reference points and we can tell what their distance is, then our goal is to monitor that distance and figure out if that distance ever changes.”
Since the first discovery in 2015, LIGO has detected 10 instances of these large gravitational waves. That may sound like a lot, but Kalogera points out that this has more to do with the universe being infinitely huge. In our own galaxy, she states that such an event happens only once in ten million years.
A whole new sense
Kalogera states that the LIGO detectors act like amplifiers for our ears. Much like how non-optical telescopes let us see events well beyond our .0035% of the electromagnetic spectrum, the LIGO detectors allow us to “hear” waves we never knew were there.
“Regular electromagnetic telescopes in observatories are like our eyes,” says Kalogera. “We have to turn the telescope, just like we turn our eyes, to look at something. But our ears can hear sounds from every direction. We can hear better if we turn our ear in some direction; but even if I don't turn my ear, I can receive sounds from every direction. Gravitational wave detectors are exactly like this.”
Kalogera describes detectors like LIGO and the Virgo detector in Europe as “like a new sense we are acquiring of the universe.” And as with anything new in science, she’s excited to see a continuation of the multi-generational collaboration that helped bring about this discovery in the first place.
“This is not my work as a person or even just my group,” says Kalogera. “I'm a member of a large collaboration in the international collaboration that includes hundreds of people. It's not an individual or three individuals or ten. It's really a huge team effort. We all worked together for decades and we're all very happy to be a part of this excitement.”