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Comment by mysterypie | Hacker News Reader

I figured that one tube is stretched by the gravitational waves and the perpendicular tube is unaffected, so you're measuring the difference between the two tubes. Simple and easy to understand. However, the video mentioned that both light and the detection apparatus are stretched by the same degree and the physicist says "the light does get stretched and that part doesn't do the measurement for us"[1] -- which makes me think that the simple and easy explanation is not quite right. There's some subtlety that I think I'm missing.

[1] https://m.youtube.com/watch?v=iphcyNWFD10&t=376s (6min 16sec into the video)

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The subtlety is this:

Imagine you are in the weightlessness of space, and you measure distances by taking a ruler and placing unconnected beads 10 cms apart.

Now a gravitational wave passes through, what do you see? Nothing. The beads don't move. The intervals you use to measure the distance are stretched in the same way as the space you want to measure in the first place.

So what you can do is, every milisecond or so you lay a new string of beads into empty space. And now when a gravitational wave passes through you'll see that when you lay the string of beads at the moment the gravitational wave passes it doesn't line up with the previous ones.

That you can measure. And by observing the miniscule misalignment of the beads you can see stretching that is much smaller than the 10cm intervals.

So the strings of beads are light rays, and their intervals are the wavelength. The difference is that they don't last, but if there is a light ray in there right as the tube is stretched, it get's stretched along with the tube and you would not see a difference with that, whereas the light that's coming right after it will place new beads/amplitude peaks and actually see the stretched distance.

Light takes different amounts of time to travel down the stretched vs squeezed tube.

Yes, the speed of light cannot exceed its limitation, and as one channel stretches in length, it simply requires more time to travel that length at the constant maximum speed.

The fixed channel remains the same, and covers the unstretched distance, so the light, traveling at the same constant speed reaches the end of the unchanged channel within less time than the other channel.

Or invert the same effect for cases where one channel is compressed to a shorter length, since waves oscillate. So both circumstances may be encountered. Some photons will traverse a compressed distance at a fixed speed, other photons will traverse a stretched distance. Meanwhile, the other perpedicular path is neither stretched nor squashed, and the photons in that channel demostrate the difference when compared to the effected channel.

So no matter what happens, the rate of travel is locked at the upper limit of 186,000 miles per second, whether all of space, energy and matter is squashed in a slightly smaller volume, or expanded to fill a slightly larger volume, the relativistic distances do not change, and the rate of passage of time does not change.

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