Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Kozmyk on 17/08/2021 02:04:14
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Hello,
I've been asking the same question for some time... lots of smart people have provided answers but I still don't get it and I'd like to ask again:
When a photon left a galaxy, let's say 8 billion years ago, and we think our Solar System is 4 BY old, where was the spacetime that Earth occupies now (since Earth did not exist 8 BY ago) ??
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When a photon left a galaxy, let's say 8 billion years ago, and we think our Solar System is 4 BY old, where was the spacetime that Earth occupies now (since Earth did not exist 8 BY ago) ??
By definition, and in any coordinate system, the spatial location that is here has always been here. The galaxy may have moved here from elsewhere (perhaps 30 million light years if it hasn't curved much, and depending on how you measure distance), but here cannot have ever been anywhere but here. That's kind of tautological.
It is wrong to call it spacetime. Earth occupies a worldline through spacetime, and 'now' is only one event on that worldline. Each temporal coordinate (including the one you are calling 'now') along that worldline defines one location in space.
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If you used the word 'space' the question would make more sense, if it is inflation and a expanding acceleration you're thinking of? SpaceTime is SpaceTime, and it is the universe all the way back to ~13.8 billion years ago. We don't have any coordinate system that's not 'relative', possibly except the age for it.
https://news.stonybrook.edu/featuredpost/scientists-confirm-age-of-universe-is-13-8-billion-years/