# 6: Gravitational Waves

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• 6.1: What are Gravitational Waves?
In 2016, scientists published the first paper announcing the detection of gravitational waves. This was a very big deal, not only because it verified a very old prediction of General Relativity, but also because it was an experimental feat.
• 6.2: Gravitational Wave Metric
Gravitational waves were predicted by General Relativity long before they were actually detected. How? Well, a lot of the theoretical predictions of General Relativity came about just from playing around with the Einstein Field Equations. You can propose a matter distribution and see what metric is consistent with that, or you can propose a metric and see what that implies.
• 6.3: LIGO
The first discovery of a gravitational wave occurred at the Laser Interferometer Gravitational Wave Observatory (LIGO) in 2015. LIGO actually consists of two detectors, one in Hanford, Washington, and the other in Livingston, Louisiana (see Figure 6.3.1). One of the reasons for having two detectors is because any real gravitational wave would necessarily have to travel though both detectors; if only one detector sees what it thinks is a gravitational wave, then it could just be noise.
• 6.4: Gravitational Wave Sources
In general, gravitational waves are produced by moving masses. This can include the collapse of stars or objects in orbit. One caveat, though, is that the motion can't be spherically symmetric. For this reason, the primary source that we can expect to detect gravitational waves from is objects in orbit.

Thumbnail: Two-dimensional representation of gravitational waves generated by two neutron stars orbiting each other. (Public Domain; NASA).​