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Time is a dimension, but not like space

(bigthink.com)
171 points belter | 8 comments | 17 Oct 24 15:29 UTC | HN request time: 1.819s | 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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1. gavmor ◴[20 Oct 24 14:24 UTC] No.41895562[source]▶
Here's a great video explaining how, due to relativity, FTL travel can cause grandfather paradoxes: https://youtu.be/an0M-wcHw5A?si=RYFGOmQOlaC2t0bM

Edit: in short, not all reference frames can agree on the order of events, and FTL events propogate "backwards" between some reference frames.

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2. MattPalmer1086 ◴[20 Oct 24 15:54 UTC] No.41896178[source]▶
Nice video, thanks for posting.
3. david-gpu ◴[20 Oct 24 16:45 UTC] No.41896640[source]▶
That doesn't mean that light (causality) couldn't be faster, right? You could increase the speed of light (causality) as much as you want and wouldn't run into any paradox.
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4. gavmor ◴[20 Oct 24 17:33 UTC] No.41897026[source]▶
What does it mean to increase the speed of causality? This seems like asserting that we can add as many tick marks to the axis as we like, since the only universal unit of measurement is a velocity's percentage of C.

If we imagine something going faster than the speed of causality, we're simply misconcieving the properties of space.

5. MattPalmer1086 ◴[20 Oct 24 18:26 UTC] No.41897420[source]▶
Right. The speed limit itself is arbitrary.
6. coldtea ◴[20 Oct 24 21:54 UTC] No.41898803[source]▶
Mainly in the sense that you can create an amp that goes to 11.
7. layer8 ◴[21 Oct 24 00:32 UTC] No.41899682[source]▶
With respect to what, though? One light-second would still be one light-second. The sizes of atoms and elementary particles probably also are a function of that. (We don’t know that, but it seems plausible.)
8. simonh ◴[21 Oct 24 12:53 UTC] No.41903643[source]▶
You'd be able to use an FTL laser to shoot your own grandfather as a baby. Plus you'd be able to receive an FTL broadcast of what's going to happen tomorrow.