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Time Reversal In Tenet
The mew movie Tenet shows plenty of eye-candy action and charismatic characters, though it didn’t really make me care much emotionally about its characters or their problems. The movie did, however, scratch my big physics idea itch. So I’m going to talk a bit about the movie’s key physics premise. I’ll give spoilers; you are warned.
First the movie’s key social premises seem quite crazy, even if they are common in action movies. The movie postulates that future scientists figured out how to locally reverse, or “invert”, the flow of time, and so now often move and send stuff back through time that way. In addition, one of them also figured how to invert time across the entire world (or universe?), which would supposedly destroy all of us. To prevent that, this scientist broke her “algorithm” into nine physical parts and hid them back in her past, in our world. There’s now a war between a future alliance seeking to find and activate these parts and other future alliance seeking to stop them.
Our hero starts out as a CIA agent who takes what he thinks is a suicide pill, awakes, and is told he has passed a test of true devotion. He is then told about this time war and is invited to join the our-time army on the sly. But not as a U.S. representative with official approval; the U.S. isn’t to be told about it. Our hero promptly joins, quickly hurting many people in the process, yet without doing much research to check out these crazy claims.
I’m pretty sure that the vast majority of CIA agents will not drop their U.S. allegiance on the sly to join some new army, just because some folks says some crazy sounding stuff, and do a few magic tricks. Which seems on average a good thing. I’m also pretty sure there are far easier ways to destroy an algorithm, if the info about it remains sufficiently localized in spacetime. And I have strong doubts about the feasibility or wisdom of inverting all of time, even if local inversions are feasible. More on this below.
Today most of our physical laws are about rather local processes, saying how stuff here and now must relate to stuff nearby in spacetime. But there’s one big exception: the second law of thermodynamics, which in effect puts a constraint on the earliest state of the universe, saying it must have been a state of extremely low entropy. The rest of universe seems well predicted by the usual local processes plus this one early constraint. Together they imply an increasing entropy “flow of time” from this distant past state toward our future.
As we don’t know where this constraint comes from, it isn’t completely crazy to imagine that it might be possible to make some new device which adds other similar constraints at other smaller places in spacetime. Strong constraints of low entropy at points in our future would induce “inverted”, or backwards, flows toward our time. And in principle, the “initial” state of this backward flow might be set equal to that of some nearly forward moving flow, which is then ended. That is, some forward moving people and stuff might be suddenly turned and sent moving backward, or some backward moving stuff might be turned to move forward.
So far, this matches the movie. But in the middle, where forward and backward moving time flows meet, the obvious thing to expect is a gradual blending, wherein the state is predicted as the most likely state given all the constraints in its full light cone. So when people moving forward and backward in time interact a lot with each other, they should blend together into a bloody gory deadly mess.
However, what we see in the movie is that flows look far more binary and stable. For example, early in the movie the hero visits a museum full of inverted materials, including an inverted bullet. Within the bullet the flow is supposedly backwards, even though most stuff around it is moving forward, and even after many years of strong entropy interactions with other materials nearby. I find it hard to see this as the result of adding more low entropy constraints to more places in spacetime.
But we don’t understand this low entropy constraint very well. So maybe it really could be more of a setting of a binary flow direction within particular materials, a setting that follows the material around where ever it goes? This doesn’t fit physics I understand, but okay. It would apparently allow the creation of perpetual motion machines, that generate arbitrary free energy to run any machines forever. Including big bombs. So all those inverted materials shown in that museum with that bullet would actually be incredible valuable to make perpetual motion machine parts.
Even with this binary flow in a material assumption, the interaction between adjacent materials should still be set by thermodynamics, however. The state observed should still be what is most likely given the state constraints within each connected material. But I have a hard time fitting this with what we saw in that first museum, much less the rest of the movie.
Our hero is shown waving his hand above an unspent bullet, which then flies up into his hand, and pulling the trigger on an empty gun, just before which a spent bullet flies out from being stuck in a wall and enters the gun to become an unspent bullet. Neither of these seem to me at all the most likely events given the local entropy constraints. It seems to me that the entropy calculations say that the more likely events are that the hand just waves with no bullet jumping up, and that the gun makes an empty trigger click with the spent bullet staying in the wall. These may be nice vivid examples of experiencing events in reverse, but they do not seem to me how these materials should behave, even with time inversion.
The usual question in time travel movies is how grandfather paradoxes are resolved. This question is often evaded by assuming inconsistent outcomes happen in different parallel universes. But if you are going to face it head on, which this movie seems to want, the interesting thing to see is what exactly would prevent someone trying to create an inconsistency. I don’t think I saw this issue directly engaged in Tenet, but I’ve only seen the movie once; maybe I missed something.
But one big consistency issue arise with the movie’s extreme scenario of inverting the entire planet, or whatever, at a moment. Since we’ve already seen history up to that moment, all this can mean is reversing the flow of time after that moment. But that only allows future people to exist for the duration from their future date back to that date at which the flow is reversed. And if everyone is reversed at once at some future date as well, then afterward from their point of view everything around them would look exactly the same; only the distant universe might have any different relation. So if you had problems before with resources or ecological damage, those problems continue exactly as before. And now you can only continue another few years before you hit that earlier date and end. So what’s the point?