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Comment by zoratu | Hacker News Reader

I did not see mention in the link of the problem of space. You might very well move through time, either forward or backward, but the solar system is moving through the galaxy is moving through the universe is still expanding. Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.

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This assumes some fixed, absolute "background" space through which the universe is moving; the idea being that you'll stay in the same position in "background" space but all of the things you care about (e.g the earth) have moved on.

Is space not relative?

That makes no sense to me. One particle goes in one direction at a particular velocity and another particle goes in another direction at another velocity. They diverge over time. When you travel through time, which particle’s path do you follow through space?

I often think along the gp’s lines and would love an explanation of why it’s wrong?

It all depends on your point of view (literally, frame of reference). The Earth can be considered:

- stationery (geocentric observer)

- orbiting the Sun (heliocentric observer)

- spiraling about the Sun (galaxy-centric observer)

All of these are simultaneously correct and bonkers in conjunction. Hence, you need a frame of reference if you're specifying movement.

> Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.

It just means you have to move at an angle. Like when you jump onto a slowly moving train. If you want to land in a specific place on that train you have to aim ahead of it.

The Strontium Dog series in the 200AD comic had T-weapons. They're grenades that displace everything in a bubble around them a few minutes forward in time. The problem for the target is that by then planet Earth has moved on in it's orbit and they're dumped into the vacuum of space.

The article and the book it's reviewing are discussions of the philosophical issues raised by time travel, in particular the logical paradoxes and metaphysical implications. They are not really looking at the practical aspects of it or the physics, except to the degree that those might impinge on the logical and metaphysical issues.

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