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Time is a dimension, but not like space

(bigthink.com)
173 points belter | 8 comments | 17 Oct 24 15:29 UTC | HN request time: 1.422s | source | bottom
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heed ◴[20 Oct 24 05:48 UTC] No.41893173[source]▶
Also consider the speed of light is also the speed of causality. If there was no such limit it means it would be possible for effects to precede causes which would lead to a very different kind of universe!
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MattPalmer1086 ◴[20 Oct 24 09:18 UTC] No.41894129[source]▶
How could an effect precede a cause if there were no speed limit to causality?

No matter how fast an effect propogates, it is always after the cause (with an infinite speed, I guess effects happen instantaneously, but not before).

Of course, this doesn't fit with a universe described by general relativity, where time can be different for different observers. But you wouldn't have a universe described by general relativity without that constraint in the first place.

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andsoitis ◴[20 Oct 24 14:30 UTC] No.41895599[source]▶
> How could an effect precede a cause if there were no speed limit to causality?

> No matter how fast an effect propogates, it is always after the cause (with an infinite speed, I guess effects happen instantaneously, but not before).

If everything happens instantaneously then there is no real cause and effect, and the universe would be over before it really got started.

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withinboredom ◴[20 Oct 24 18:29 UTC] No.41897444[source]▶
I'm having trouble with this assertion. Light travels slower in water than in air, by your assertion that light is the limit of causality; then surely we can create a paradox with ftl right in a pool.
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1. Etherlord87 ◴[21 Oct 24 10:38 UTC] No.41902684[source]▶
Light moves with `c` speed regardless of medium. Whenever we say light moves slower in a medium, we simply mean it is measured to be slower, it is macroscopically slower, it's as if having a hypothetical vehicle that, when it moves, it always moves with a constant speed, but you measure it by taking the time of departure in place A, time of arrival in place B, measure the distance |AB| on a map, and from that calculate the speed of the vehicle. Your measurement will be affected by exact path shape (which isn't a straight line), as well as the number of times the driver decided to take a break to sleep in a motel, eat something, go to a toilet on a gas station etc.
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2. jncfhnb ◴[21 Oct 24 11:48 UTC] No.41903107[source]▶
This example seems… bad to me. Are you simply saying that light moves slower from A to B through a medium like water because it takes a path that is less direct to navigate the medium?
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3. felbane ◴[21 Oct 24 12:19 UTC] No.41903354[source]▶
I agree, that answer is misleading. The way I've always understood it: light is an EM wave, and it interacts with medium that it travels through. When traveling through a vacuum, the "beam" source is the origin, but when traveling through a medium the "beam" is a propagation of emissions from the matter absorbing, oscillating, and re-emitting a photon. These interactions take (an extremely small, but nonzero) amount of time, but the light being absorbed and emitted always travels at c.
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4. withinboredom ◴[21 Oct 24 15:44 UTC] No.41905369{3}[source]▶
No. It actually goes slower. https://youtu.be/uo3ds0FVpXs?si=b7sDxNuQkTuwAkcP gives a pretty good overview.

Edit: wrong one https://youtu.be/yP1kKN3ghOU?si=hsBj0RpzOb3JZWdS the one above is the "why."

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5. dcow ◴[21 Oct 24 19:46 UTC] No.41907787{4}[source]▶
No. It propagates slower.

https://youtu.be/KTzGBJPuJwM?si=Pab2F0EFsB4oGhUo

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6. Etherlord87 ◴[23 Oct 24 08:49 UTC] No.41923123{3}[source]▶
The absorptions are the "breaks" in the analogy.
7. withinboredom ◴[24 Oct 24 11:04 UTC] No.41934306{5}[source]▶
I trust the quantum physicist more than a pop-sci YouTuber.
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8. dcow ◴[25 Oct 24 03:51 UTC] No.41942067{6}[source]▶
Both videos come to the same conclusion so your comment betrays the fact you didn't even watch the one I linked.

That or you misunderstood the physicist. You need to watch both videos to understand what's happening here.

The speed of light is not "slower in water". Light propagates more slowly through water. The subtle difference is not just pedantic semantics. It's the key to understanding how we don't have a paradox on our hands.