A few weeks ago, talk of gravitational waves dominated most of science reporting. Gravitational waves are similar to the ripples that move out along the surface of the water after you drop a pebble into a pond, except that instead of being ripples in water they are ripples in space-time. Einstein predicted gravitational waves as part of his general theory of relativity, but believed that they were too small to be detected.
New equipment and developments has continually improved our ability to detect smaller and smaller things. LIGO is the world’s largest gravitational wave observatory, which means it is a multi-kilometer long facility designed to detect extremely small changes in space-time. There also has to be more than one LIGO that are located apart from each other. Otherwise local vibrations may be mistaken for gravitational waves.
So how does it work? LIGO’s detecter is a laser interferometer. So when light is behaving as a wave you can get interference. Consider a water wave. When two wave crests meet they add to each other to become bigger. This is how rogue waves happen. But if a wave crest meets the lowest point on another wave they will cancel each other out. Light does the same thing. When the crests meet you get a brighter light, when a crest meets a trough you get dimmer or no light (depending on how perfectly they lined up).
LIGO uses a laser that is split into two beams. Each beam travels down a different arm of the facility where it eventually is reflected by a mirror. Then the two waves will merge back into a single beam. The mirrors at LIGO are set up so that if the arms are exactly the same length the split beams of light will cancel each other out and nothing will be seen by the photodetector.
If the arms are not exactly the same length, the resulting merged beam will be brighter or dimmer than it was originally. So, in theory, a gravitational wave will affect the two arms differently bending space-time and changing the length of the arms. The change is extremely small, 1/10 000th the width of a proton. But this will cause the beam to flicker.
Now remember, that other things cause it to flicker as well such as earthquakes and traffic nearby, so you also need to filter out the noise.
But, after filtering out the noise, the LIGO scientists determined that they had detected a gravitational wave started when two black holes (29 and 36 times the mass of the sun) merged and became a single, massive black hole. When the black holes combined they released energy (think E=mc-squared) and that energy created the gravitational wave. All of this happened 1.3 billion years ago, and was actually detected on September 14, 2015 at 5:51 a.m. Eastern Daylight Time.
As always, more research is required, and we need more observatories to really study gravitational waves. But finding one piece of the puzzle usually makes it easier to find the motivation (and the funding) to find the rest of the pieces.
References:
https://www.sciencedaily.com/releases/2016/02/160211103935.htm
https://www.ligo.caltech.edu/page/what-is-interferometer
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