[sandycheeks]

When I was younger I thought a lot about the physics of traveling back in time but I always seemed to hit a brick wall with spatial coordinates.

Where we are in the universe today is very far from where we were in the universe yesterday based on the movement of the earth alone. Add to that the movement of our solar system, galaxy, cluster, supercluster and movements I am not even aware exist and it becomes really far away. Grandpa would probably be light years away from me and my time machine.

Am I missing something here, because I've never heard this mentioned by anyone else?

[Steuard]

It's a valid issue. You wouldn't need to worry about any component of your motion that's "constant velocity" (so, inertial straight-line motion), since all inertial reference frames are equally valid. But you would need to keep track of curving motion (like the rotation of the Earth, orbit around the Sun, Sun circling around the galactic core, etc.).

You might be able to wave your hands and claim that the time machine follows paths back in time along natural world-lines in curved space-time (gravitational geodesics), which will at least get things like orbiting around the Sun right (and galactic motion as well). But I think you'd still need some way to keep the machine's path rotating on the surface of the Earth rather than falling back and forth through the center of the planet.

[sandycheeks]

I thought it might follow backwards in space as well as time but when the book The Physics of Time Reversal by Robert G. Sachs came out in 1987 it made me skeptical of that possibility. I haven't followed the theory of CP violation for many years so this may not be a pertinent point anymore.

[Steuard]

For most "time machines" you read about, you aren't really going to run into the "CPT" issues from quantum field theory.

Unless you're considering a model of time travel where you're still literally in the same room and just living in reverse (while still visible to all the forward-living people around you and interacting with them as you go), this isn't equivalent to CP symmetry reflection.

Most time machine ideas instead picture some sort of wormhole or "stitching" between different points in space-time, in such a way that the local "forward time direction" remains continuous for observers taking the trip (and then connects back up with forward-directed time at some point in the past).

(I'm teaching a low-level course on "Time Travel in Science and Literature" this term. It's fun, but a challenge without using math.)

[Hermel]

Maybe time travel needs some kind of tracks, like a railroad. That would also explain why we don't see any time travelers - they can't visit us as there are no tracks that lead back to 2014.

[rjshade]

"Perhaps! But this problem only comes up if you're not the centre of the Universe."

http://www.qwantz.com/index.php?comic=1394

[TheLoneWolfling]

Most of the time this is handwaved over, for bunches of reasons. Part of it is that there is no one valid frame of reference: you can't just say "this stays stationary" - because what does that even mean? Stationary relative to what?

If you mean "you go wherever inertia would take you in that amount of time, with no acceleration", then if you go backwards a multiple of a year you end up pretty close to Earth (quick approximation: the Sun's acceleration in its orbit in the milky way is ~2*10^-10m/s/s, which works out to ~100km in a year, ~400km in two years, etc.). In actuality you could probably get somewhat better range by letting your velocity build up to intercept Earth-that-was at the target time. (You'd need to take into account a bunch of things for best accuracy: the Earth's orbit is not "perfect", the moon's phase, the acceleration of the Sun, etc, but we know most of that.)

You probably would end up needing to make multiple jumps, pausing in-between to let Earth's (or another planet's, or even the Sun's) gravity well readjust your velocity to what is needed for your next jump.

This means that you'd need to build in re-entry capability into the machine (as you are unlikely to have the precision necessary to be able to jump into atmosphere at a sane velocity), and potentially maneuvering delta-v. Note that with a time machine you lose conservation of momentum, which makes things a lot easier. You can change the direction of your velocity vector by jumping back in time and interacting with your past self. You can potentially use this to get "free" (read: reactionless) velocity out of any gravity well.

[jdjb]

If you can travel backwards in time (or even just stop time) you can be at any spatial location you desire in the entire universe (since you have essentially infinite speed).

[stan_rogers]

That is a major plot point in Spider Robinson's Callahan's Con. (Look, his first few Callahan stories were charmingly quirky, and I got into the habit of reading them. This is not an endorsement of his larger oeuvre, okay?) In that case, the cosmic microwave background was used as the inertial reference, probabilities were the main problem to solve (we'd be well beyond the relative simplicity of the three-body problem), and little details like delta-v were not merely hand-waved away.

[kevincennis]

It's been almost 10 years, so I may be mistaken... but I think the director of Primer mentioned this in the DVD commentary.

EDIT: Right, not really part of the plot. I just think I remember him (in the commentary) talking about some of the "trickier" aspects of writing a story about time travel, and I think he mentioned the fact that nobody ever really addresses the positional aspect.

[slazaro]

I think the model of time and space in Primer is more consistent because it's a very specific kind of time machine.

If I recall correctly, you'd turn on the machine, wait N hours, go inside the machine, and in the next N hours you'd be going back in time and come out the other end at the moment the machine was turned on. So you would have spent 2*N extra hours, the machine would have always been in the same spot, so there'd be no problem with traveling through space.

There are still problems with the paradoxes, but that's part of the plot of the movie.

[sp332]

It wouldn't seem to matter in the movie, since whatever you put in the device would get dragged around with the device as it traveled through space.

[growse]

Moreover, imagine I write down some information, then wait for exactly 6 months for the earth to take me far away from that point in space. If I now travel 6 months minus 1 nanosecond back in time with that piece of paper, but preserve my spacial position, haven't I just 'transmitted' information through space faster than light can?

[TheLoneWolfling]

FTL travel is synonymous with time travel, and vice versa.

Relativity is weird.

[bengali3]

whats your reference for coordinates?

Earth Center? Solar Center? Galaxy Center? ... Universe Center?

hmm traveling backwards along the 4th dimension by itself sounds like we are NOT traveling in the other 3.... so where would the atoms (that originated from your banana at 9:25am 3 days ago) go if we merely changed their value on the 4th axis? does their xyz stay locked at the current values or revert back to their previous values.

[bithive123]

The time travel show "Seven Days" from the 90s dealt with this -- sometimes the time machine would end up stuck underground or floating out in space.

[wuliwong]

You are not alone! I've thought a lot about how much we move in space while we "sit still" on earth. :)