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Even back when I wasn't happy about quantum mechanics, I recognized a significant point: it provided perhaps the only way for traditional notions of free will to sneak into our description of the universe. Before quantum mechanics, the universe seemed to run like clockwork: once you'd set up the initial conditions, every moment of the future was uniquely determined.
But quantum mechanics brings fundamental uncertainty into the mix: only probabilities could be predicted, so one could hope that "free will" could somehow slip into the picture. In particular, as I've discussed before, quantum mechanics implies that every possible history "happens" and gets an equal vote to determine the probabilities of what we'll actually observe. One could imagine that what we see as free will is the branching of the wave function at each moment of decision: when we decide "Should I flip this switch or not?", both histories "happen" and both have some chance of being observed. It's not entirely comforting, but it better than clockwork.
Shifting gears, I've also at times contemplated how time travel would fit into physics if it were actually possible. The framework for thinking about questions of time is general relativity, since the ability to loop back to an earlier time would imply specific things about the structure of space-time. Relativity is a classical (i.e. pre-quantum) theory, and in it one should view all of space-time as a single four dimensional object. If "loops" in time are possible, those must be built in to the structure of space-time from the start. In practice, that means that time travel paradoxes from sci-fi simply aren't possible: "If something happened, it happened", as Sean Carroll puts it. In other words, history will automatically and by definition be self-consistent. You can't specify a single space-time object that includes a person murdering their own grandfather as a child, so that can't happen. But again, that's a classical theory, so we wouldn't necessarily expect to have free will in that context. What happens when we bring quantum mechanics back into the mix?
Sadly, that still doesn't save the day. The "quantum histories" that we're summing over in this case must be self-consistent space-times! Yes, we still have a chance of seeing any possible history actually occur, but if the switch you're considering flipping would kill Grandpa, you simply don't have that option no matter how easy the action itself might be. So the comforting notion that free will is hidden within quantum mechanics doesn't hold up in a world with time travel: quantum histories have to be defined globally, not locally.
In fact, even if time travel isn't possible, I suspect that conclusion holds: the idea that every moment and every decision spawns a branching tree of quantum histories doesn't quite capture all of physics. There are global effects that give important contributions to quantum calculations, quite apart from time travel (I study some of those in my research). So if there is free will out there, we probably shouldn't look to quantum histories to provide it. I'm not sure that we're left with much wiggle room, though: perhaps traditional notions of what free will really means just aren't right.
But quantum mechanics brings fundamental uncertainty into the mix: only probabilities could be predicted, so one could hope that "free will" could somehow slip into the picture. In particular, as I've discussed before, quantum mechanics implies that every possible history "happens" and gets an equal vote to determine the probabilities of what we'll actually observe. One could imagine that what we see as free will is the branching of the wave function at each moment of decision: when we decide "Should I flip this switch or not?", both histories "happen" and both have some chance of being observed. It's not entirely comforting, but it better than clockwork.
Shifting gears, I've also at times contemplated how time travel would fit into physics if it were actually possible. The framework for thinking about questions of time is general relativity, since the ability to loop back to an earlier time would imply specific things about the structure of space-time. Relativity is a classical (i.e. pre-quantum) theory, and in it one should view all of space-time as a single four dimensional object. If "loops" in time are possible, those must be built in to the structure of space-time from the start. In practice, that means that time travel paradoxes from sci-fi simply aren't possible: "If something happened, it happened", as Sean Carroll puts it. In other words, history will automatically and by definition be self-consistent. You can't specify a single space-time object that includes a person murdering their own grandfather as a child, so that can't happen. But again, that's a classical theory, so we wouldn't necessarily expect to have free will in that context. What happens when we bring quantum mechanics back into the mix?
Sadly, that still doesn't save the day. The "quantum histories" that we're summing over in this case must be self-consistent space-times! Yes, we still have a chance of seeing any possible history actually occur, but if the switch you're considering flipping would kill Grandpa, you simply don't have that option no matter how easy the action itself might be. So the comforting notion that free will is hidden within quantum mechanics doesn't hold up in a world with time travel: quantum histories have to be defined globally, not locally.
In fact, even if time travel isn't possible, I suspect that conclusion holds: the idea that every moment and every decision spawns a branching tree of quantum histories doesn't quite capture all of physics. There are global effects that give important contributions to quantum calculations, quite apart from time travel (I study some of those in my research). So if there is free will out there, we probably shouldn't look to quantum histories to provide it. I'm not sure that we're left with much wiggle room, though: perhaps traditional notions of what free will really means just aren't right.
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http://patrissimo.livejournal.com/1355535.html
:)
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I'm happy to provide inspiration, even if it does mean that some of your less careful readers have decided that I'm an exemplar of failed reasoning. (I still think I was coming out right back when we argued about the rationality of voting, by the way, but I doubt most of your readers followed the thread that long. To be fair, though, real life responsibilities seemed to have forced you to leave the discussion before we'd entirely established that.)