# Could we tell if time isn't constant? **URL:** \ **Category:** Factual Questions\ **Created:** [May 19, 2016, 6:00pm UTC](https://boards.straightdope.com/t/could-we-tell-if-time-isnt-constant/755176 "2016-05-19T18:00:58Z")\ **Posts on this page:** 3\ **Page:** 3
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [May 24, 2016, 3:22am UTC](https://boards.straightdope.com/t/could-we-tell-if-time-isnt-constant/755176/41 "2016-05-24T03:22:20Z")
> [@Lemur866](#): > > But if we think the lizards are experiencing time more slowly, and the lizards think we’re experiencing time more slowly, which of us are correct? > > The answer is that from our perspective they are zooming away at fantastic speed, while we stay still. From their perspective we are zooming away, and they stay still. > > But we can’t compare clocks with each other unless we both end up in the same frame of reference. Take a spaceship and travel at 99.99999% light speed for 12 billion years until you get to the home of the lizard people, and compare your clock. Ooops, now we see that your clock is slower than theirs. But that’s because of the 12 billion year near-lightspeed trip you took to visit them. I can’t agree with that, time dilation is precisely a systematic method of comparing clocks. In general relativity a systematic method of comparison may not exist as such methods depend on the symmetries of the particular spacetime. It’s also distinct from direct visual comparison of the rate of clocks which is governed by red shift/blue shift. What we would say instead is that when there is time dilation there is no consistent way of synchronizing the clocks. However in the specific situation described there is no time dilation (as normally understood - see my previous post) as both us and the lizard people can possess consistently synchronized clocks - we interpret this as they are expanding relative to us rather than moving relative to us. > [@](#): > > Time dilation is always relative. To a guy on a spaceship blasting off from Earth at 99.99% the speed of light, he’s stationary and it’s the Earth blasting off. Or to put it a third way, the Earth is traveling through space at 99.99% the speed of light, and he’s just slowing himself down. > > So which frame of reference is correct? If the Earth really is blasting off at .9999c, then the rest of the galaxy and the rest of the local galaxies and all the other nearby galaxies are doing the same. But, as you say, from 12 billion light years away it makes sense to say that the Earth is zooming away at some fantastic speed, and from even farther away it makes sense to say that the Earth is moving away faster than the speed of light, and so even if you could travel at lightspeed you’d never be able to reach Earth from that distance. I’ve said it before, but at the risk of repeating myself, it is possible (in theory) to reach galaxies that are receding from us faster than c in the standard cosmological model. Recession velocity depends only on the current value of the Hubble parameter, but whether we can reach a faraway galaxy depends on the future evolution of the Hubble parameter. ---
**Author:** ![rsa](https://avatars.discourse-cdn.com/v4/letter/r/839c29/32.png) [@rsa](https://boards.straightdope.com/u/rsa)\ **Post date:** [May 25, 2016, 3:20am UTC](https://boards.straightdope.com/t/could-we-tell-if-time-isnt-constant/755176/42 "2016-05-25T03:20:15Z")
> [@Kevbo](#): > > OK, time for Kevbo’s crackpot cosmology idea: > > The current thinking seems to be that Space and time are pretty closely tied together: spacetime is “a thing.” > > What if spacetime is conserved…as space expands, time somehow shrinks? > > Could it be the case that time isn’t constant? If time were somehow contracting as space expands, wouldn’t this be perceived as an accelerating expansion? If there were this spacetime contraction/space expansion relationship, what would happen to time if the the universe were expanding faster than the speed of light. Would time move backwards? This would seem to imply violation of causality and/or the stoppage of time at expansion equal to the speed of light? N.B. not well thought out. 🙂 ---
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [May 25, 2016, 3:48am UTC](https://boards.straightdope.com/t/could-we-tell-if-time-isnt-constant/755176/43 "2016-05-25T03:48:50Z")
Are there any processes we could create here which would react differently if time changed speed? A watch is just a mechanical process and everything inside would change the same as time changed. But what about some sort of setup with photons or something? For example, what if you had two photon colliders which were different diameters but could somehow be synchronized? Or a system with photons going through materials of different density and different lengths? Something where if time changed, the results would be slightly off because the photons in the different experiments would react slightly differently to the change in time. [Previous page](https://boards.straightdope.com/t/could-we-tell-if-time-isnt-constant/755176.md?page=2)