Time Travel Reference Frames

[Lorizael]

Okay, so there's a classic objection to time travel that goes like this: if you jump one day in the past, where are you? If you jump one day in the past, the earth is about two and a half million km behind you in its orbit. So if all you do is jump through time, then you find yourself adrift in the vacuum of space and die soon thereafter. Oops. Therefore, any time machine must really be a spacetime machine.

The problem with this objection, however, is that it doesn't take into account relativity. It implicitly assumes there is some background reference frame against which the earth moves and your position can be pinned. But we know that's not how the world works. There is no universal reference frame. The only way the above objection stands is if what stays the same is your position relative to the sun, and given the infinitude of objects out there, that seems like kind of an arbitrary choice.

The again, you only reappear in the exact same spot on earth if what stays constant is your position relative to the earth. Is that just as arbitrary? This gets us into the somewhat incoherent idea of what a time machine would even be.The kind of time travel permitted by general relativity is, by its very nature, about creating paths through spacetime. There's no confusion there as to where you end up. If you can travel faster than the speed of light, you can break causality, leading to a kind of time travel as well. But since what you're doing is traveling, again where you end up is clear.

Neither of these forms of time travel resemble the classic science fiction idea of a time machine, however. If you pop out of existence at time t and pop back into existence at t-1000, is it at all clear where you should be spatially?

[Aeson]

The time vortices that facilitate time travel are physically linked to matter with "time-ons", the result is sorta like ocean currents which flow around and eddy near islands. These time islands are actually more like strings in 4d space time, and they have their own "time gravity" and "time electromagnetic force". If you just go with the flow (either forward or backwards) you naturally follow the most significant local forces, which keeps you near to your relative location in most cases. It also ensures you don't end up stuck in a wall, as as you travel back, your course is modified by the wall and the destruction/building of the wall. The wall is also affected by your travel, and this tends to be confused with wind by construction workers who are having a hard time getting that bit of particle board up into place, and wind by those who notice a degradation of the stone work over centuries, both happening because of the unseen time traveling entity applying force to it.

[The Mad Monk]

You travel forward in time without fear of being thrown into the void, why not backward?

(if you travel to back before the creation of the local gravity well, all bets are off.)

[Lorizael]

Because we are traveling through time. A day from now, we will have traversed a continuous path through spacetime as dictated by the local spacetime geometry (mass, gravity, etc.). If I simply pop back in time one day, there's a discontinuity that needs to be accounted for. What happened to the path? If instead I am traveling back in time, then it does seem like I would move with everything around me, but it also seems like I would be affecting the things around me during my travel because I'm not going anywhere. How does that work?

[BeBMan]

The classic objection, even if erroneous, sounds more fun tho. It raises the image that time travel would be like some ejection seat and you never know where you land. Which adds a whole lot to the adventure!

[pchang]

Time travel requires large amounts of suspension of disbelief. Everyone knows that our continued existence demonstrates the impossibility of time travel to the past within the same universe. If time travel to the past within the same universe were possible, there would be continual time travel to the past in order to change things. A stable universe could only result from 2 outcomes: a timeline where time travel to the past was not created, or a timeline where all intelligent life were destroyed.

[Jon Miller]

Outside of maybe Timescape, time travel and hard science fiction go together not at all.

JM

[ZEE]

There sure is soem nerdy stuff going on in this thrade.

[Lorizael]

Outside of maybe Timescape, time travel and hard science fiction go together not at all.

JM

This book is sitting on my bookshelf. I have never read it.

[The Mad Monk]

Because we are traveling through time. A day from now, we will have traversed a continuous path through spacetime as dictated by the local spacetime geometry (mass, gravity, etc.). If I simply pop back in time one day, there's a discontinuity that needs to be accounted for. What happened to the path? If instead I am traveling back in time, then it does seem like I would move with everything around me, but it also seems like I would be affecting the things around me during my travel because I'm not going anywhere. How does that work?

What "pop"? Have you even read HG Wells' The Time Machine???

[Lorizael]

Yes, The Time Machine's time machine is moving through time, but I think popping into and out of existence is a pretty standard time travel trope.

[The Mad Monk]

...so you're siding with popular dumbed-down sci-fi writing?

[The Mad Monk]

I mean, even in classic Trek, The Enterprise used a relativistic slingshot involving a black hole (accidentally in Tomorrow is Yesterday) to get where they were going

[Dry]

The problem with this objection, however, is that it doesn't take into account relativity. It implicitly assumes there is some background reference frame against which the earth moves and your position can be pinned. But we know that's not how the world works. There is no universal reference frame. The only way the above objection stands is if what stays the same is your position relative to the sun, and given the infinitude of objects out there, that seems like kind of an arbitrary choice.

The again, you only reappear in the exact same spot on earth if what stays constant is your position relative to the earth. Is that just as arbitrary?

I don't think the choice is so arbitrary. When choosing a reference frame for position, you rather go for a inertial one. All inertial references frame are indeed equivalent.
For that mater, the earth is not a good choice as it is more subject to the sun gravitational force than the sun to all its planets. The sun is thus a much more valid (inertial) reference.
Same goes with the center of the galaxy in regard to the sun. The sun is subject to gravitational force of the whole galaxy, and but thus, the center of the galaxy is a more valid reference frame than the sun.
I think that when galaxies are close enough, they also may influence each other...
So yes, the position problem is a big one when time travel is involved.
My knowledge of (astro)physics is not sufficient to understand what the expansion of the universe in all directions might have as influence on position over time.

[Dauphin]

Because we are traveling through time. A day from now, we will have traversed a continuous path through spacetime as dictated by the local spacetime geometry (mass, gravity, etc.). If I simply pop back in time one day, there's a discontinuity that needs to be accounted for. What happened to the path? If instead I am traveling back in time, then it does seem like I would move with everything around me, but it also seems like I would be affecting the things around me during my travel because I'm not going anywhere. How does that work?

If anti-matter is matter travelling back through time, could explain why i) it's not a great idea and that ii) you do interact as you travel back in time - very badly.....