One thing I’ve never seen people mention re: time travel is that if you travel back in time six months, say, you’ll find yourself floating out in space with the Earth on the other side of the Sun. (The Sun is in a slow orbit around the Milky Way, which itself is moving, right?) So practical time travel also requires practical space travel?
What exactly determines where you will appear in the past? Because there is no absolute reference frame, so...
Those who say “on the other side of the Sun” assume that time travel follows the position of the Sun. Well, why Sun? Why not the center of the galaxy? Why not Earth?
Given these three options, Earth reference frame feels most logical to me… the intuition is, it is the gravity of Earth that impacts me most, and in the absence of absolute reference frame, the time travel should track the gravity lines instead.
Problem is, I am not a satellite orbiting Earth. I am standing on the ground, which limits my movement as the gravity of the Earth would want it to be. Should the time travel also take this into account? Sounds wrong: then it should track all interactions of my body with everything, including the air I would be passing through… does not make sense. So if I change my model into “time travel converts my body into a point-with-mass and then tracks the gravity lines”, travelling in time backwards should move me up—into such height that I will drop to the ground during the time interval.
Under this model, travelling six months in the past would move me to a place in a space, difficult to calculate precisely (chaos theory, etc.), where if I start freely falling, in exactly six months I would drop on the ground approximately on the place where the time travel started (but not exactly there, because of friction and other interactions). Sounds similar in effect, but it’s not the same.
I have a whiff of this type of literary time travel being supernatural i.e irreducibly mental: the closest model is replaying a memory differently this time: a mental thing.
That seems like a magical sort of time travel; the sort that is conceivable but unphysical; a garbage in, garbage out type-deal. I’d echo Viliam’s remark on no absolute reference frames. I think it helps to imagine how you might actually, physically perform time travel. I usually see it suggested that one create a wormhole and accelerate one of the mouths. Time dilation will cause an observer in the reference frame of the accelerated mouth to experience less subjective time. Travel through the stationary mouth, and you’ll apparently come out of the accelerated mouth at an earlier moment in time. No magic involved.
Any system of time travel in which the traveler does not teleport through time, but instead traverses all intervening time somewhat addresses this. There is an implication that the vessel is held near the Earth by the same gravity that would have held it in place ordinarily. In particular, this was true in Wells’s original story. I think that the film Primer addresses this explicitly.
One thing I’ve never seen people mention re: time travel is that if you travel back in time six months, say, you’ll find yourself floating out in space with the Earth on the other side of the Sun. (The Sun is in a slow orbit around the Milky Way, which itself is moving, right?) So practical time travel also requires practical space travel?
What exactly determines where you will appear in the past? Because there is no absolute reference frame, so...
Those who say “on the other side of the Sun” assume that time travel follows the position of the Sun. Well, why Sun? Why not the center of the galaxy? Why not Earth?
Given these three options, Earth reference frame feels most logical to me… the intuition is, it is the gravity of Earth that impacts me most, and in the absence of absolute reference frame, the time travel should track the gravity lines instead.
Problem is, I am not a satellite orbiting Earth. I am standing on the ground, which limits my movement as the gravity of the Earth would want it to be. Should the time travel also take this into account? Sounds wrong: then it should track all interactions of my body with everything, including the air I would be passing through… does not make sense. So if I change my model into “time travel converts my body into a point-with-mass and then tracks the gravity lines”, travelling in time backwards should move me up—into such height that I will drop to the ground during the time interval.
Under this model, travelling six months in the past would move me to a place in a space, difficult to calculate precisely (chaos theory, etc.), where if I start freely falling, in exactly six months I would drop on the ground approximately on the place where the time travel started (but not exactly there, because of friction and other interactions). Sounds similar in effect, but it’s not the same.
I have a whiff of this type of literary time travel being supernatural i.e irreducibly mental: the closest model is replaying a memory differently this time: a mental thing.
Charles Williams’ Many Dimensions has time travel as moving your sense of the present along your time line.
That seems like a magical sort of time travel; the sort that is conceivable but unphysical; a garbage in, garbage out type-deal. I’d echo Viliam’s remark on no absolute reference frames. I think it helps to imagine how you might actually, physically perform time travel. I usually see it suggested that one create a wormhole and accelerate one of the mouths. Time dilation will cause an observer in the reference frame of the accelerated mouth to experience less subjective time. Travel through the stationary mouth, and you’ll apparently come out of the accelerated mouth at an earlier moment in time. No magic involved.
Any system of time travel in which the traveler does not teleport through time, but instead traverses all intervening time somewhat addresses this. There is an implication that the vessel is held near the Earth by the same gravity that would have held it in place ordinarily. In particular, this was true in Wells’s original story. I think that the film Primer addresses this explicitly.
Slow orbit? More like 120 miles per second in reference to the galactic center.
Charlie Stross’s Eschaton books have a pretty good take on time travelling, light cones, and causality.