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0 pointstheophrastus10y ago0 comments

Here's my re-statement of this confusion, isn't everything we can experience embedded in time-space, including the LIGO experiment itself? So how is there any relative shift allowed to be detected when everything we know is fundamentally intrinsic to time-space? That is, I too would appreciate having this mis-conceptualizing, of mine, cleared away.

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evanpw10y ago

Another re-phrase: how can we detect that space has stretched out if all of our rulers also get stretched by exactly the same amount?

The answer is that we have a ruler that doesn't get stretched in this way: light. The speed of light is a constant dictated by the laws of physics; stretching out our flashlight to twice its normal size wouldn't make the light it emits go twice as fast. So if you just measure the time it takes for a light ray to go from one point to another, you can compute the distance that it must have traveled, and if that distance changes, then you know that the space in between must have been stretched.

theophrastusOP10y ago

Thank you. i think that settles my confusion. (it's somewhat like we witnessed a length/Lorentz–FitzGerald contraction in a situation where there was apparently no reason to witness one). Now i can go back to being simply amazed by it all.

terryf10y ago

Thanks for the nice explanation! But another question: since the expansion rate of the universe has been different at different times, does this mean that the measured speed of light would be different at different times as well?

Because if the ruler you are using to measure is expanding at rate x, the measured speed of light would be different than when the ruler is expanding at rate y?

So, during the deflationary period of the universe, the speed of light would be significantly smaller, correct? In fact, would it be "negative" due to the universe expanding faster than light?

platz10y ago

Does this mean that in earth's gravity well, there is an absolute difference in the time light takes to travel compared with light travelling in the void of space?

Can we compute the strength of a static gravity field we are inside, by measuring the time that light takes to propagate through it?

nikdaheratik10y ago

The light is constant, which means it moves at the same speed in both cases (assuming the light is in a vacuum). It won't move faster or slower based on the gravity field (other than in the case of black hole where it can't escape at all).

What happens instead is that the speed that an object moves through space-time changes dependent upon gravity. Using an atomic clock, we've actually measured the effect of gravity to show that time moves more slowly down on earth than it does in an orbiting satellite.

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tedd4u10y ago

Thanks for this explanation. The universe is a pretty freaking amazing place.

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