Interestingly, the boxes from the movie Primer can be made to avoid that problem.
A short recap of how they work. You switch the box on, walk away from it to avoid running into past you, come back to the box several hours later, switch it off, climb inside, sit there for several hours, and climb out at the moment the box was switched on. One reason this model is cool is that it avoids one common problem with fictional time travel, the changing location of the Earth. You don’t end up in interplanetary space because you travel back along the path of the box in spacetime.
So here’s how you make the Primer boxes obey conservation of mass as well. The idea is that a box containing a time-reversed human should weigh less than an empty box. Let’s say you weigh 70kg, and the box weighs 100kg when empty and switched off. When you switch the box on, a past version of you climbs out, and the box now weighs 30kg. Several hours later, a future you climbs in and the box now weighs 100kg, at which point the box switches off and sits there as empty as before.
At first I felt pretty smart for figuring this out because this whole issue never came up in the movie at all. And then I remembered the small detail that the boxes in the movie were an accidental invention, whose original purpose was to reduce the mass of objects. Wow.
That got me thinking about the other possible hole in the movie, namely all the abandoned timelines. Can this model of time travel be made to work correctly with not just spacetime paths and conservation of mass, but also causality and probabilistically branching timelines? For example, if you travel back in time and kill your past self, can that yield a unique consistent assignment of probabilities to timelines, where all time travelers “come from somewhere” and can’t affect their “probability weight”? The result was this comment, for which I later found a proof of consistency which this margin is too small to contain ;-)
Interestingly, the boxes from the movie Primer can be made to avoid that problem.
A short recap of how they work. You switch the box on, walk away from it to avoid running into past you, come back to the box several hours later, switch it off, climb inside, sit there for several hours, and climb out at the moment the box was switched on. One reason this model is cool is that it avoids one common problem with fictional time travel, the changing location of the Earth. You don’t end up in interplanetary space because you travel back along the path of the box in spacetime.
So here’s how you make the Primer boxes obey conservation of mass as well. The idea is that a box containing a time-reversed human should weigh less than an empty box. Let’s say you weigh 70kg, and the box weighs 100kg when empty and switched off. When you switch the box on, a past version of you climbs out, and the box now weighs 30kg. Several hours later, a future you climbs in and the box now weighs 100kg, at which point the box switches off and sits there as empty as before.
At first I felt pretty smart for figuring this out because this whole issue never came up in the movie at all. And then I remembered the small detail that the boxes in the movie were an accidental invention, whose original purpose was to reduce the mass of objects. Wow.
That got me thinking about the other possible hole in the movie, namely all the abandoned timelines. Can this model of time travel be made to work correctly with not just spacetime paths and conservation of mass, but also causality and probabilistically branching timelines? For example, if you travel back in time and kill your past self, can that yield a unique consistent assignment of probabilities to timelines, where all time travelers “come from somewhere” and can’t affect their “probability weight”? The result was this comment, for which I later found a proof of consistency which this margin is too small to contain ;-)