Suppose backward time travel is possible. If so, it’s probably of the variety where you can’t change the past (i.e. Novikov self-consistent), because that’s mathematically simpler than time travel which can modify the past. In almost all universes where people develop time travel, they’ll counterfactualize themselves by deliberately or accidentally altering the past, i.e. they’ll “cause” their universe-instance to not exist in the first place, because that universe would be inconsistent if it existed. Therefore in most universes that allow time travel and actually exist, almost all civilizations will fail to develop time travel, which might happen because those civilizations die out before they become sufficiently technologically advanced.
Perhaps this is the Great Filter. It would look like the Great Filter is nuclear war or disease or whatever, but actually time-consistency anthropics are “acausing” those things.
This assumes that either most civilizations would discover time travel before strong AI (in the absence of anthropic effects), or strong AI does not rapidly lead to a singleton. Otherwise, the resulting singleton would probably recognize that trying to modify the past is acausally risky, so the civilization would expand across space without counterfactualizing itself, so time-consistency couldn’t be the Great Filter. They would probably also seek to colonize as much of the universe as they could, to prevent less cautious civilizations from trying time-travel and causing their entire universe to evaporate in a puff of inconsistency.
This also assumes that a large fraction of universes allow time travel. Otherwise, most life would just end up concentrated in those universes that don’t allow time travel.
Good point. I may be thinking about this wrong, but I think Deutsch self-consistent time travel would still vastly concentrate measure in universes where time travel isn’t invented, because unless the measures are exactly correct then the universe is inconsistent. Whereas Novikov self-consistent time travel makes all universes with paradoxes inconsistent, Deutsch self-consistent time travel merely makes the vast majority of them inconsistent. It’s a bit like quantum suicide: creating temporal paradoxes seems to work because it concentrates your measure in universes where it does work, but it also vastly reduces your total measure.
That’s why it’s not usually called “Deutsch self-consistency.” It’s not supposed to be a filter on legal universes, but a dynamic rule that each initial condition does lead to a consistent universe. The resolution of the grandfather paradox is a 50-50 superposition of the universe where you are born and leave and the universe where you appear, kill your grandfather, and are never born. You could say that it filters out the 80-20 superposition, but that’s like saying that Newton’s self-consistency principle filters out universes that don’t obey his laws. (Well, maybe that’s Lagrange’s self-consistency principle...)
I’m not sure that comparison works. I can pick any initial universe-configuration and time-evolve it under Newtonian gravitation; a solution will always exist. But if I time-evolve initial conditions under laws that allow backwards time travel, it’s not clear to me that there necessarily exist any solutions. Maybe the Deutsch law forces the superposition to be 50-50, but the other physical laws force it to be 80-20. It may be that the Deutsch law is just a logical consequence of the other physical laws, in which case I think you’d be right. (This all with the caveat that I don’t really know physics, so I’m likely completely wrong.)
I can’t really think about this without having some idea of how it’s chosen which universes are real.
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.
Alternately, if a type time travel is invented where you can change the past, you would expect people to keep meddling with the past until they accidentally changed it so much that time travel had never been invented.
This process would continue, over and over again, and the final results is that the final “stable” timeline will be one where time travel is never invented; not because it’s not possible, but simply because every timeline where time travel is invented eventually changes it’s own past until it no longer has time travel.
Suppose backward time travel is possible. If so, it’s probably of the variety where you can’t change the past (i.e. Novikov self-consistent), because that’s mathematically simpler than time travel which can modify the past. In almost all universes where people develop time travel, they’ll counterfactualize themselves by deliberately or accidentally altering the past, i.e. they’ll “cause” their universe-instance to not exist in the first place, because that universe would be inconsistent if it existed. Therefore in most universes that allow time travel and actually exist, almost all civilizations will fail to develop time travel, which might happen because those civilizations die out before they become sufficiently technologically advanced.
Perhaps this is the Great Filter. It would look like the Great Filter is nuclear war or disease or whatever, but actually time-consistency anthropics are “acausing” those things.
This assumes that either most civilizations would discover time travel before strong AI (in the absence of anthropic effects), or strong AI does not rapidly lead to a singleton. Otherwise, the resulting singleton would probably recognize that trying to modify the past is acausally risky, so the civilization would expand across space without counterfactualizing itself, so time-consistency couldn’t be the Great Filter. They would probably also seek to colonize as much of the universe as they could, to prevent less cautious civilizations from trying time-travel and causing their entire universe to evaporate in a puff of inconsistency.
This also assumes that a large fraction of universes allow time travel. Otherwise, most life would just end up concentrated in those universes that don’t allow time travel.
Since quantum mechanics is true, Deutsch self-consistency has pretty big advantages over Novikov self-consistency.
Good point. I may be thinking about this wrong, but I think Deutsch self-consistent time travel would still vastly concentrate measure in universes where time travel isn’t invented, because unless the measures are exactly correct then the universe is inconsistent. Whereas Novikov self-consistent time travel makes all universes with paradoxes inconsistent, Deutsch self-consistent time travel merely makes the vast majority of them inconsistent. It’s a bit like quantum suicide: creating temporal paradoxes seems to work because it concentrates your measure in universes where it does work, but it also vastly reduces your total measure.
That’s why it’s not usually called “Deutsch self-consistency.” It’s not supposed to be a filter on legal universes, but a dynamic rule that each initial condition does lead to a consistent universe. The resolution of the grandfather paradox is a 50-50 superposition of the universe where you are born and leave and the universe where you appear, kill your grandfather, and are never born. You could say that it filters out the 80-20 superposition, but that’s like saying that Newton’s self-consistency principle filters out universes that don’t obey his laws. (Well, maybe that’s Lagrange’s self-consistency principle...)
I’m not sure that comparison works. I can pick any initial universe-configuration and time-evolve it under Newtonian gravitation; a solution will always exist. But if I time-evolve initial conditions under laws that allow backwards time travel, it’s not clear to me that there necessarily exist any solutions. Maybe the Deutsch law forces the superposition to be 50-50, but the other physical laws force it to be 80-20. It may be that the Deutsch law is just a logical consequence of the other physical laws, in which case I think you’d be right. (This all with the caveat that I don’t really know physics, so I’m likely completely wrong.)
I can’t really think about this without having some idea of how it’s chosen which universes are real.
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.
Some civilization would have thought of that and made sure to direct their research away from time travel and towards AI.
Alternately, if a type time travel is invented where you can change the past, you would expect people to keep meddling with the past until they accidentally changed it so much that time travel had never been invented.
This process would continue, over and over again, and the final results is that the final “stable” timeline will be one where time travel is never invented; not because it’s not possible, but simply because every timeline where time travel is invented eventually changes it’s own past until it no longer has time travel.