What the Heck are Gravitational Waves?

Image courtesy of ligo.caltech.edu
Image courtesy of ligo.caltech.edu

Before I go any further, I must point out the difference between gravitational waves and gravity waves. Gravitational waves are waves that occur in the fabric of spacetime created by gravitational phenomena in a massive body in space. Gravity waves are waves that occur in some fluid medium when disturbed from a state of equilibrium, like when you jump into a swimming pool and the water moves from side to side. Today, we’re talking about the former, gravitational waves.

Gravitational waves are generated by bodies in space of great mass that are in motion. Sources of detectable gravitational waves are very massive and energetic, and include black holes, supernovae and white dwarf stars. Their existence was predicted in Albert Einstein’s general theory of relativity in 1916. Proof that they existed was discovered in 1974 when a binary pulsar was found by astronomers at the Arecibo Radio Observatory in Puerto Rico. According to Einstein’s predictions, this was precisely the kind of system that should generate gravitational waves. Astronomers then measured the changes in the stars’ orbits over time. The stars grew closer together at a rate predicted by general relativity. Arecibo has continued to monitor the system even today, and it still behaves as predicted by general relativity.

Why do we care about this? Well, the first thing is that it demonstrates that there are ripples in spacetime, like you’ve heard of in Star Trek or Doctor Who. Isn’t that cool? But seriously, using gravitational wave detection will allow us to observe bodies in space we wouldn’t otherwise detect. Gravitational waves can’t be scattered by matter in space like light can. Gravitational wave astronomy is a promising branch of astronomy, and we can get information about objects like neuron stars and black holes that we couldn’t get before. We can also get information about the early universe not long after the Big Bang.

So how are they detected? Laser interferometry. Here’s a simplified diagram of how laser interferometry works:

Laser Interferometer illustration

A laser beam is fired into a splitter, creating two beams. These beams are perpendicular (90 degrees apart). Each new beam goes into a tunnel (they are of equal length). They are perpendicular so that when waves pass through along the direction of one tunnel, it will move the corresponding weight and not the other. At the end of the tunnel is a weight of specific mass M1 or M2 (they are both equal) attached to a mirror that reflects the laser beam. The mirrored beams recombine and the combined beam is directed into a photodetector. If there is no gravitational wave present, the recombined beam is exactly the same as the original beam. When a gravitational wave passes through the detector, one of the tunnels will be slightly distorted, moving its mirror slightly farther away. The resulting change in phase between the two beams indicates the level of distortion of the tunnel (it’s small!)

Gravitational wave observatories must be located in areas without any significant tectonic movement; external vibrations render the process of laser interferometry useless.

On September 14, 2015, gravity waves were detected for the first time at LIGO (Laser Interferometer Gravitational-Wave Observatory). Their origin was 1.3 billion light years away, a merger of two black holes. Waves from other sources have been detected since then.

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