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In last few years, I watched a lot of YouTube videos claiming that gravitational waves can be and has been heard. They also play a short audio signal. How do experiments such as LIGO replicate the pattern of a gravitational wave signal into a funny sound signal?

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Amazingly enough, gravitational waves are actually in a real sense, the closest thing you can come to having true "sound in space" in the sense of being a mechanical signal propagating through hard vacuum with no material medium present. Indeed, were one suitably close to a powerful enough gravitational wave source, they would act similar to vibrations passing through your body just like those from a sound wave in atmosphere, and hence, would likewise excite your ear drum as well, meaning you would hear the sound audibly, with no special equipment required. This is because all matter interacts with such waves and would be mechanically distorted thereby.

(This was discussed on another thread on this site somewhere; can't remember where.)

LIGO - the gravitational wave detector - effectively acts as an extraordinarily sensitive "microphone" that is capable of picking up these sounds from the distances we are at. What you are hearing is the literal, faithful conversion of the gravitational wave in essentially the same way that your ear would convert a pressure wave, to audible perception or, perhaps better, a computer microphone would: when the wave goes one way, the mic records a signal going the same way, and when it goes the other, it goes back the other way, same frequency, same wave shape, and then that is played back through your computer speakers just as a regular sound would be.

Indeed, LIGO could not only actually record "ordinary" sounds, too, but a great deal of the difficulty in building it involved trying to keep those out since the gravitational waves are effectively zillions of times fainter!

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