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What would it look like if time were expanding? (pawb.social)

submitted 11 hours ago* (last edited 11 hours ago) by OwOarchist@pawb.social to c/askscience@lemmy.world

14 comments fedilink hide all child comments

Okay, I know it sounds weird, but hear me out:

We know space is expanding, sure. That's been established science for a while now. All three spatial dimensions at the same (increasing) rate.

So ... why should we assume that the 4th dimension -- time -- is static and unchanging like we used to think space was? Could time also be expanding or contracting? (Expanding seems more likely, as it would match what the other dimensions are doing.) After all, spacetime is all one thing, really. Space and time are inextricably linked. When you think of it that way, it seems nearly impossible that space would be expanding while time is not. Spacetime is expanding, so wouldn't that include time as well?

My question here is: what would it look like, subjectively, from our perspective inside it, if time was expanding just like space? Would we be able to measure it at all? Would there be any difference? Could the acceleration of space expansion ('dark energy') actually be explained by time expansion instead?

For a moment, imagine a universe where time definitely is expanding. Even if you don't think time could really be expanding, let's think about that hypothetical universe where it definitely is. What would that universe be like? How would it be different -- if at all -- from our universe?

Does it even matter? If time is expanding, but we still experience it passing at a constant rate, why would we even care whether it's expanding or not? An observer somehow watching it happen from 'outside of time' might, say, see things happening slower and slower ... but for beings living inside of spacetime, with their subjective perceptions also dependent upon the flow of time, would it actually change anything at all?

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[–] bjoern_tantau@swg-empire.de 3 points 10 hours ago* (last edited 9 hours ago) (2 children)

I mean, we do know that time slows down in a gravitational field ~~. And it speeds up for~~ and fast objects. We have to consider that with our GPS (and similar) satellites. They are basically just atomic clocks sending down their current time. They have already drifted from clocks on the ground by several seconds.

In terms of time dilation influencing expansion you should look into timescapes. If I recall correctly it is meant to explain the apparently accelerating expansion by time moving faster in emptier regions of space.

This stuff is just hard to study on a cosmic scale because we don't know exactly where everything is in the universe. Building a 3d map of all the galaxies is super hard and in parts relies on correct assumptions about expansion.

[–] fallaciousBasis@lemmy.world 4 points 10 hours ago (1 children)

No. As an object speeds up, time dilates (slows.) Length contracts. Outside observers see this as slow mo. Internal observers see this as business as usual.

[–] bjoern_tantau@swg-empire.de 1 points 9 hours ago

Thanks, something felt off when I wrote that.

[–] NaibofTabr@infosec.pub 1 points 10 hours ago

I mean, we do know that time slows down in a gravitational field. And it speeds up for fast objects. We have to consider that with our GPS (and similar) satellites. They are basically just atomic clocks sending down their current time. They have already drifted from clocks on the ground by several seconds.

GPS is my favorite demonstration of relativity in practice. Technically, the clocks have not drifted, but are in fact self-correcting and account for the effects of special and general relativity.

Special relativity predicts that as the velocity of an object increases (in a given frame), its time slows down (as measured in that frame). For instance, the frequency of the atomic clocks moving at GPS orbital speeds will tick more slowly than stationary clocks [...] The result is an error of about -7.2 μs/day in the satellite.

According to general relativity, the presence of gravitating bodies (like Earth) curves spacetime, which makes comparing clocks not as straightforward as in special relativity. [...] In case of the GPS, the receivers are closer to the center of Earth than the satellites, causing the clocks at the altitude of the satellite to be faster by a factor of 5×10−10, or about +45.8 μs/day. This gravitational frequency shift is measurable.

Combined, these sources of time dilation cause the clocks on the satellites to gain 38.6 microseconds per day relative to the clocks on the ground. This is a difference of 4.465 parts in 1010. Without correction, errors of roughly 11.4 km/day would accumulate in the position.

https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System