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Comment by Zenst | Hacker News Reader

One thing about time travel that I've pondered. Say you could travel in time, say back a year, well in space coordinates the Earth position in the Universe would be different, given the galaxy movement alone. Hence to master time travel, you would also need a way to lock onto a position in the time you want to arrive at.

Heck, maybe the universe was born out of failed time travel attempts as they all ended up at 0time at space coordinates 0,0,0 and the culmination of all those failed attempts gave us the big bang. Extremely unlikely, but equally, something you can't fully dismiss based upon our current understanding to the point of saying there is 0 chance of this could happen.

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If you think about it in terms of the book Time Machine, the machine itself is still affected by gravity as it travels through time. You would expect it to remain stationary relative to the the objects it started traveling on, assuming it wasn't affected by some external force.

The end of eternity by Issac Asimov touches on this. I believe the analogy is that of a bird being left behind by earth as it flies through the air because earth is moving through space. So as you travel through time you move in time with whatever it is you're traveling on, I guess is the argument the book makes.

The thing is that there exists no universal frame of reference. There is no such thing as "space coordinates 0,0,0". Those coordinates would have to be relative to some object in space.

Yes, that's kinda the crux as the relative reference of one time can and will be different in another time. Sure you can say here is a an inch cube and 10 years from now that cube will still have it's demensions relative distance to each other party upon the cube the same. But once you start adding expanding space, well, the 3 body orbit problem was bad enough, let alone at larger scales.

I still read this as a frame of reference with absolute positions.

I think the point of GP was that any time travel device you build, calculations/projections you make, will all be built around a frame of reference where the planet IS in a fixed position, as far as you are concerned. Since you don't know any other frame of reference, the machine won't either.

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