Tenet got time inversion all wrong but we can invert the flow of time

Tenet came out in 2020 which seems like a lifetime ago now. The Christopher Nolan sci-fi thriller quickly became divisive partly because the time inversion aspect made the film so confusing. Although not nearly as confusing as the low-budget time travel film Primer, Tenet contained enough switchbacks to lose anyone not paying careful attention.

I saw the movie recently while I was packing boxes for my big move to Texas, and, while I found the premise interesting, I found the physics a bit lacking. People and objects who are inverted can interact with objects in the ordinary world in an inverted way. The laws of inertia are supposedly reversed although people seem to be able to walk normally while inverted. That’s a good thing because a truly inverted object wouldn’t experience inverted inertia. It would experience inverted energy dissipation which is a different phenomenon, but I’ll get to that.

Entropy likewise is said to be reversed with heat flow being reversed. This would have caused the inverted people to overheat unless kept in a very hot environment since the body needs to be able to release heat to survive. If they were absorbing heat from the environment, they would die from heatstroke in temperate to cold weather. The colder the weather, the faster they would succumb.

What makes the inversion possible in the movie is something called a “turnstile” which is like a sort of round room. You go in one side and you come out the other inverted.

Is something like the turnstile possible?

The temporal flow of an object, whether cause preceded effect in the arrow of time we know or the opposite, depends on its entropy.

Individual particles, electrons, photons, and so on have no experience of time. The equations that govern them are the same backward and forwards.

What does an electron look like when it is moving backward through time, inverted?

It looks like a positron, a particle of antimatter. (This is mentioned in the movie.)

This is because of a symmetry of the universe called Charge, Parity, Time, or CPT. If you reverse a particle’s charge and its “handedness” which in the electron’s case is the direction it is moving, then you get that particle moving backward in time.

When an electron and a positron collide, they produce two photons that carry their energy forward in time, but from a certain point of view, which Richard Feynman proposed, you can see the electron and positron as being the same particle following a loop through time.

What happens when you bring together a collection of antimatter particles is unknown. It has been proposed that these would follow an inverse 2nd law of thermodynamics where the collection of antiparticles, in isolation, would flow backward in time relative to us.

This is true if and only if the CPT symmetry of the universe is valid for large groups of particles, not merely individual ones. If that is not true, then we can say that the CPT theorem is violated in a statistical sense if not in a fundamental sense.

That seems plausible on its face but there is a type of antimatter that we observe all the time, and it obeys the 2nd law of thermodynamics. That particle is the humble photon.

Photons are their own antiparticles. Collections of photons tend to increase in entropy until they approach a maximum which can be found in Black-Body Radiation. This is the kind of radiation that a perfectly black body emits as it absorbs photons. This can also be achieved with light in a chamber with mirrored interior. It will approach black body radiation.

Given this reality check, it seems that CPT symmetry does not apply to collections of particles. In fact, CPT symmetry and the 2nd law are not even in conflict because one is a microscopic law and the other is statistical.

The arrow of time, therefore, seems more fundamental than that and has to do not with the nature of the particles at a microscopic level but the emergent flow of increasing entropy in a universe where nothing is truly isolated.

Rather than thinking of time as something that “causes” entropy to increase. It may make more sense to think of the flow of entropy in the universe as the thing that causes time to flow. All matter in the universe shares a common past. Therefore, there was plenty of time for the direction of entropy to emerge from particle interaction.

What this means is that to invert the flow of time for something, it doesn’t have to be antimatter. There is nothing intrinsic about any kind of microscopic particle that makes it flow in one direction of time or another. Looking at individual particles and trying to find the arrow of time is like looking at the individual neurons of the brain and trying to find the mind. You won’t!