an effectively omnipotent far-future civilization that can just revive everyone by, e.g., scanning the planet and running physics backwards
How much physics have you studied? Scanning the planet would not be enough. You’d need something like a timeslice through the future light cone of the point you wanted. This would be a huge volume of space.
Time travel is probably impossible, or if it is possible, then our current understanding of reality is so badly wrong that this entire discussion is probably nonsense.
Quantum immortality is somewhat deceptive… if you buy into it you’ll become apathetic (“there’s always some branch of the wavefunction where what I want happens, so why should I bother?”). Similar comments apply to “all those copies of you” in an infinite universe.
How much physics have you studied? Scanning the planet would not be enough. You’d need something like a timeslice through the future light cone of the point you wanted. This would be a huge volume of space.
I haven’t studied much physics. Would scanning the planet not be enough? Humans, and human brains, have some predictable structure, so if you were smart about things, it isn’t like you’d have to be able to run backwards an arbitrary physics. You’d just to have to be able to make a good probabilistic guess about those portions of a particular state you cared about (the portions that made people “themselves”, which you could conceivably infer by, say, having a notion of the probability space “human brains/minds”, and by having footprints, bits of writing, etc. left by the person at various points in their life). I don’t know how to evaluate the odds here.
Due to QM the function is many-to-one and can’t be reversed. If you isolate a single Everett branch and look only at that branch and not all the other branches, and run that branch backward in time, it will evolve into a multiplicity of pasts (that would in the greater scheme be coherently canceled out by the past evolution of other branches). The upshot is that even if you timeslice the entire future lightcone in a single branch you cannot get the exact past. And we can’t get photons that have escaped over the horizon, so we can’t get the whole future lightcone anyway, and small divergences will amplify, and the whole thing would require more computing power than all the particles we’re trying to run back.
And a time camera using new physics violates the character of known physical law, in particular its elegant locality, that each element of reality only interacts with immediate neighbors.
It’s too early to give up hope. But as a working assumption, the dead are dead.
If you isolate a single Everett branch and look only at that branch and not all the other branches, and run that branch backward in time, it will evolve into a multiplicity of pasts (that would in the greater scheme be coherently canceled out by the past evolution of other branches). The upshot is that even if you timeslice the entire future lightcone in a single branch you cannot get the exact past. And we can’t get photons that have escaped over the horizon, so we can’t get the whole future lightcone anyway, and small divergences will amplify, and the whole thing would require more computing power than all the particles we’re trying to run back.
Then use the quantum ‘probability’ distribution over pasts to randomly pick a person to instantiate (more properly, pick a past and instantiate everyone in it). If you got the distribution right, then each resurrectee has the same relative measure they did before they died (original measure times fraction of worlds in which you do this). Obviously, you can also do this with subjective probability distributions derived from locally available information.
Just how much good this does for the dead is hard to say.
Um… I think the vast majority of false pasts you got this way would be nothing like the real past. I think they may even end up with higher entropy, i.e., arrow of time runs forward from here after a brief reversal (in the vast majority of extrapolated pasts).
Okay, that would mean you can’t just run the physics backward, but that shouldn’t stop the Bayesian method (come up with a prior over histories of Earthlike worlds or some similar class, update on memories, records, etc.) or computationally feasible approximations thereof.
For most people in the past, we have no footprints, bits of writing, etc. A crude back-of-envelope calculation suggests that maybe it takes ~ 10^15 bits to describe one person’s brain; I wouldn’t be surprised to find that wrong by a couple of orders of magnitude, but in any case it’s rather a lot. Anyone who’s (1) in the not-very-recent past and (2) not exceptional in the traces they leave behind has left only very subtle such traces—which will be tied up in computationally intractable ways with everyone else’s very subtle traces, and with all kinds of extraneous cruft. I’ve no idea what might turn out to be possible in principle, and saying “we’ll never have the technology to do X” doesn’t have a great track record of success … but I wouldn’t hold out much hope of being able to retrieve enough information to reconstruct past people even with future-lightcone-scanning technologies, never mind without.
How much physics have you studied? Scanning the planet would not be enough. You’d need something like a timeslice through the future light cone of the point you wanted. This would be a huge volume of space.
Time travel is probably impossible, or if it is possible, then our current understanding of reality is so badly wrong that this entire discussion is probably nonsense.
Quantum immortality is somewhat deceptive… if you buy into it you’ll become apathetic (“there’s always some branch of the wavefunction where what I want happens, so why should I bother?”). Similar comments apply to “all those copies of you” in an infinite universe.
I haven’t studied much physics. Would scanning the planet not be enough? Humans, and human brains, have some predictable structure, so if you were smart about things, it isn’t like you’d have to be able to run backwards an arbitrary physics. You’d just to have to be able to make a good probabilistic guess about those portions of a particular state you cared about (the portions that made people “themselves”, which you could conceivably infer by, say, having a notion of the probability space “human brains/minds”, and by having footprints, bits of writing, etc. left by the person at various points in their life). I don’t know how to evaluate the odds here.
Due to QM the function is many-to-one and can’t be reversed. If you isolate a single Everett branch and look only at that branch and not all the other branches, and run that branch backward in time, it will evolve into a multiplicity of pasts (that would in the greater scheme be coherently canceled out by the past evolution of other branches). The upshot is that even if you timeslice the entire future lightcone in a single branch you cannot get the exact past. And we can’t get photons that have escaped over the horizon, so we can’t get the whole future lightcone anyway, and small divergences will amplify, and the whole thing would require more computing power than all the particles we’re trying to run back.
And a time camera using new physics violates the character of known physical law, in particular its elegant locality, that each element of reality only interacts with immediate neighbors.
It’s too early to give up hope. But as a working assumption, the dead are dead.
Then use the quantum ‘probability’ distribution over pasts to randomly pick a person to instantiate (more properly, pick a past and instantiate everyone in it). If you got the distribution right, then each resurrectee has the same relative measure they did before they died (original measure times fraction of worlds in which you do this). Obviously, you can also do this with subjective probability distributions derived from locally available information.
Just how much good this does for the dead is hard to say.
Um… I think the vast majority of false pasts you got this way would be nothing like the real past. I think they may even end up with higher entropy, i.e., arrow of time runs forward from here after a brief reversal (in the vast majority of extrapolated pasts).
Okay, that would mean you can’t just run the physics backward, but that shouldn’t stop the Bayesian method (come up with a prior over histories of Earthlike worlds or some similar class, update on memories, records, etc.) or computationally feasible approximations thereof.
I agree but find the wording in your comment confusing… especially the part after “i.e.” (ironically)
For most people in the past, we have no footprints, bits of writing, etc. A crude back-of-envelope calculation suggests that maybe it takes ~ 10^15 bits to describe one person’s brain; I wouldn’t be surprised to find that wrong by a couple of orders of magnitude, but in any case it’s rather a lot. Anyone who’s (1) in the not-very-recent past and (2) not exceptional in the traces they leave behind has left only very subtle such traces—which will be tied up in computationally intractable ways with everyone else’s very subtle traces, and with all kinds of extraneous cruft. I’ve no idea what might turn out to be possible in principle, and saying “we’ll never have the technology to do X” doesn’t have a great track record of success … but I wouldn’t hold out much hope of being able to retrieve enough information to reconstruct past people even with future-lightcone-scanning technologies, never mind without.