this post was submitted on 29 Apr 2024
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You're just knowledgeable enough to know that Earth moves, but not intelligent enough to know that there's no absolute reference frame it moves in respect to.
If you don't continue travelling with the Earth along its path when you time travel, you could literally end up at any random point in the universe, unless you pick a different, arbitrary, body to move in reference to.
But the universe is also constantly expanding. So the frame of reference becomes obsolete because it's at an entirely different point in space now.
Throw thousands of satellites back in time but each offset. Measure when you get a broadcast from them and how far back you sent them and bam, we find out for reals.
Then there's you who forgot that we actually do have a universal reference frame with the cosmic microwave background.
The CMB is everywhere, and anywhere in the universe it's the same distance from a hypothetical observer. I fail to see how you can use it as an absolute reference frame.
I think they're trying to say, it can be considered to be a non-accelerated reference frame, where stuff like planets and stars would be accelerated.
Though I have a problem in understanding how it could be taken as a reference frame in the first place.
Indeed, it can't be a reference frame, as even if it's not accelerated, it's everywhere, so it doesn't have a position or orientation.
Not only that, it's not even a single object. It's just the name given to a group of radiation, which is ultimately just light going randomly here and there.
It isn't a single "thing" you are some distance away from. It's photons remaining from the early universe that can be found everywhere without direction. Pick "one" of them and you can track your speed relative to it. It's the closest thing we have to a universal reference frame.
Also see the later questions on https://www.astro.ubc.ca/people/scott/faq_basic.html
Edit: I'm stupid, photons move at light speed of course. But you can detect a colder and hotter side of the CMB and use that as a reference frame.
Nah, I know the thing with reference points, but that's a matter of navigation and relativity.
In reality, a point in space is a point in space, like, a specific "pixel" of the Universe (oversimplified) that might be occupied with something or not.
We just can't anchor this point since we don't know what reference is absolute and the laws of physics can be applied to every inertial reference, so this doesn't help.
It's not that we don't know which reference point is absolute, but there are still absolutely defined 'points in space', it's that there is no absolute reference point, and so there are just 'points in space' relative to whatever arbitrary body you decide to make your reference frame.
Then we have to define what body serves as a reference point. "Relative to the observer" doesn't seem to work here, since we try to decide where should the observer themselves go.
If so, then why should it be Earth? Why not the Sun, or the center of a Milky Way, or literally anything else? As you said, it's arbitrary. And how do we choose the reference frame?
Doesn't make any sense outside spacetime as a whole.