B How does LIGO measure gravitational waves

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LIGO measures gravitational waves using a single beam of light that is split into two paths, where interference patterns are created to detect changes in amplitude rather than frequency. The system is sensitive to changes as small as 10^-18 meters, which refers to the change in length of the beam paths due to gravitational waves. The light's wavelength remains constant during this process, and the interference patterns indicate the stretching of space caused by gravitational waves. There is confusion regarding why the light itself does not stretch while the arm lengths do, despite the light being affected by cosmic expansion. This highlights the complexities of how gravitational waves interact with space and light.
AamsterC2
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I'm aware that the LIGO system uses interferometry but I'm confused how it works in this case. Do they test both beams of light? Or do they use one beam to test the wavelength of the other and see the difference? Also it is sensitive up to 10 e-18 meters, is that for the change in wavelength or change in the length of the beam?
 
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This may help. It talks about how interferometers are used to make interference patterns:

http://www.kavlifoundation.org/how-ligo-works
 
AamsterC2 said:
I'm aware that the LIGO system uses interferometry but I'm confused how it works in this case. Do they test both beams of light? Or do they use one beam to test the wavelength of the other and see the difference? Also it is sensitive up to 10 e-18 meters, is that for the change in wavelength or change in the length of the beam?

It is one beam of light. It gets split. The wavelength of light does not change. Interference changes the amplitude measured not the frequency.

Check these two ideas:

Beam Splitter.
Destructive interference.
 
I'm puzzled that the stretching of space by a gravitational wave stretches the LIGO arm but not the light within it. Because we are told that the red shift of a distant galaxy is caused by light being stretched by the expansion of space (the universe).
 

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