How does LIGO measure gravitational waves

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Discussion Overview

The discussion focuses on the operational principles of the LIGO system, particularly its use of interferometry to measure gravitational waves. Participants express confusion regarding the specifics of how the measurement process works, including the behavior of light beams and the implications of sensitivity levels.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions whether LIGO tests both beams of light or uses one beam to measure the wavelength of the other, and seeks clarification on the sensitivity measurement of 10 e-18 meters.
  • Another participant suggests that the sensitivity refers to changes in the amplitude measured rather than frequency, asserting that the wavelength of light does not change.
  • A different participant expresses confusion about how gravitational waves stretch the LIGO arm without affecting the light traveling within it, drawing a comparison to the redshift caused by the expansion of space.

Areas of Agreement / Disagreement

Participants exhibit uncertainty and confusion regarding the mechanics of LIGO's measurement process, with no consensus reached on the specifics of how light behaves in the context of gravitational wave detection.

Contextual Notes

Participants have not fully resolved the assumptions regarding the behavior of light in relation to gravitational waves and the implications of interferometry in this context.

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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