By Emily Levesque, University of Washington
Gravitational waves are density waves in the fabric of space-time, which is that grid that ties together space and time and is bent under the influence of gravity. Bending it was one thing but making waves in it might sound a bit strange or be a little hard to picture.
It’s Like Playing with a Slinky
To understand how gravitational waves work, imagine that you’re holding a Slinky between your hands. If you look at that Slinky, there are two different ways that you could cause a wave. One is the obvious way—you could take the Slinky, lift one end, and watch a curved wave propagate from one end to the other—but this isn’t quite how gravitational waves work.
Gravitational waves are more like compression waves propagating through space-time. If you instead hold the Slinky and briefly bring one hand closer to the other, you’ll make a different type of wave. This one squeezes and stretches the coils of the slinky as it travels, and this is the best way to imagine a gravitational wave.
Gravitational waves travel at the speed of light and squeeze and stretch space-time itself, along with everything in it. This includes our own planet. As a gravitational wave passes through Earth, we’ll be squeezed and stretched as well.
Where Does the Lost Energy Goes?
The challenge of detecting gravitational waves is that they’re tiny. An epic collision between two black holes, like the one discovered in September of 2015, will generate a squeeze in space-time that’s a thousand times smaller than a proton! Einstein himself described this effect as having a “practically vanishing value”.
Early work on gravitational waves was focused on detecting not the waves themselves but the energy of the waves. In 1974, two astronomers named Russell Hulse and Joseph Taylor discovered a binary star system composed of two neutron stars. One of these neutron stars was a pulsar, the same type of strange star first discovered by Jocelyn Bell that emitted perfectly timed pulses of radio emission.
By studying this system for years on end, Hulse and Taylor discovered that the binary orbit of those two stars was slowly decaying, with the two neutron stars spiraling closer and closer. This decay meant that the system was losing energy, and the decay agreed perfectly with the predictions of general relativity: the lost energy was most likely being carried away by gravitational waves.
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Experimenting with Aluminum Bars
Some scientists were still dead set on spotting the actual flames. One of these scientists was Joseph Weber. In the 1960s, following a conference that discussed gravitational waves, Weber set up a series of experiments using enormous aluminum bars. He imagined that a gravitational wave passing through one of these bars would squeeze it in one direction, stretch it in another, and set it ringing like a tuning fork struck by a hammer.
He fashioned carefully engineered bars, fitted them with detectors that would report this ringing, and placed them far apart: one in his lab at the University of Maryland, another at a golf course eight miles away, and a third hundreds of miles away at the Argonne Lab in Chicago.
The bars immediately began ringing, often and occasionally simultaneously, and Weber enthusiastically announced that the simultaneous rings were being caused by gravitational waves. Unfortunately, other teams began trying to reproduce Weber’s results and came up empty-handed.
Detecting the Gravitational Waves
The curious physicists included a small group of graduate students at MIT. They were just beginning to study general relativity and were stymied by how these bars could possibly work, so they brought their questions to their professor, Rai Weiss. Asked to teach a general relativity course in 1967, Weiss didn’t dare admit that he didn’t actually know much about general relativity, and instead scrambled to stay just ahead of his students by learning about experimental tests of gravity as quickly as he could.
When his students asked him to explain Weber’s strange ringing bar idea, Weiss puzzled over it for a weekend and returned to his class with a thought experiment. Instead of imagining a solid bar, he instead stripped the experiment down to the most basic principles: how gravitational waves affected space-time, and how that effect could be detected by Earth-bound laboratories.
Gravitational Wave Detectors
Weiss’s imagined experiment ultimately grew into the gravitational wave detectors used today, based on a deceptively simple principle. Imagine a detector made of two long, straight arms, set at a right angle to one another and connected to a central building housing a powerful laser that gets split, with half of the beam sent down each arm.
The two arms are precisely the same length, so on a normal day, the two laser beams will each travel along the arms, reflect off of mirrors placed at the end, and come back, traveling the exact same distance and arriving back at the building at precisely the same time. Inside the building the simultaneous arrival of the two laser beams will be in phase with one another and produce a null signal, showing nothing out of the ordinary.
Now, imagine doing the same thing when a gravitational wave comes by, compressing spacetime in the direction it’s traveling and squeezing the one arm of the detector as a result. Now, if you send lasers along both arms, they’ll arrive back at the central building out of phase. One laser has taken a slightly shorter trip than the other because of the different effects that the gravitational wave will have on the two arms of the observatory. The result produces a signal that we can detect.
Common Questions about Gravitational Waves and How They Were Detected
Gravitational waves should be taken as compression waves that travel through space-time. To demonstrate how gravitational waves work, one can hold a Slinky and brings one hand closer to the other, causing the coils of the Slinky to squeeze and stretch as it travels through them.
Density waves in the fabric of space-time are called gravitational waves. Traveling at the speed of light, gravitational waves can squeeze and stretch space-time along with everything in it, even our own planet.
Joseph Weber set up massive aluminum bars and fitted them with detectors so that they could detect the gravitational waves. He then placed those bars far apart from one another. He imagined that a gravitational wave that passes through one of the bars could squeeze and stretch it and make it ring like a tuning fork.
