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gjm comments on Rationality, Cryonics and Pascal's Wager - Less Wrong
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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.
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.