The central conceit of Tenet is the concept of “Inversion”. Obviously with this being a major plot point we can’t go any further into explaining what “Inversion” is without warning you that this will be a spoiler for the film.
Major spoilers for Tenet to follow
It’s time travel, all right? “Inversion” is what Chris Nolan calls time travel.
Except it’s actually not quite that, and the twists on how time travel is used in this film make for a very Chris Nolan experience.
Most time travel in movies relies on one of two mechanisms. The first is some version of Albert Einstein’s theories, either time dilation caused by moving at extreme speeds or playing with the edge cases invited by the “rubber sheet” model of spacetime described in Einstein’s theories. Wormholes, basically.
The other mechanism is of course, magic.
Tenet, however, is based on the idea of reversing entropy.
So rather than delving deeply into the Special Theory of Relativity, we’re looking at basic thermodynamics. Now first, awkwardly, entropy isn’t a process, so you can’t reverse it.
Entropy can be described as a measurement of how much information is needed to describe a system. So for instance in a block of ice the atoms will form relatively regular and predictable formations. In a puddle of water, those atoms will be all over the place. At the same time, the atoms in the block of ice are mostly still. The atoms in the puddle are moving all the time in different directions and at different speeds. You need a lot more information to describe the atoms in the puddle than you do in the block of ice, so the puddle has more entropy.
The second law of thermodynamics states that the entropy of a system will always increase.
This doesn’t mean it’s physically impossible to freeze things, obviously. You can freeze water, but a freezer generates heat, which radiates outwards in ways that will require yet more information to describe – while entropy might appear to be decreasing inside your fridge freezer, the overall level of entropy in the universe has still gone up.
On a small scale, this is why your perpetual motion machine will never work – it will always be giving out at least some waste heat. On a large scale, this is the process by which the universe went from being a ball of energy occupying an infinitely small amount of space, to that energy being uniformly spread out across a cold and unfeeling void some time in our future.
Basically, entropy is the pessimist’s best friend. Everything decays. Everything gets worse. Everything falls apart.
So here’s the thing – because the second law of thermodynamics states that the amount of entropy can only increase, throughout history many scientists and philosophers have used the increase of entropy as a handy by-word for the arrow of time – the idea that everything moves from the past, to the future.
Tenet posits a process for “inverting” entropy in objects, which in scientific terms would mean that the amount of information needed to describe a system would be reduced. Rather than becoming more disordered and chaotic, energy would, from a forward-moving person’s perspective, spontaneously come to together and create order.
If you do the first break in a game of pool, the balls will scatter across table. If you use an inverted pool table, the balls all rush from around the table to form a triangle in the middle, which fires the white ball at your pool cue.
This is, of course, completely impossible comic book science with absolutely no bearing on anything in real physics, but it has some fun implications.
In Tenet, this gives you devices like a gun that can suck bullets out of the walls, and building on that it also gives us time travel, but at what we would call a walking pace.
Pedestrian Time Travel
Again, with typical time travel movies the time machine is a vehicle or a magic button, and when you activate it there will some flashing lights or a big swirly space tunnel and then it will be 1356, or last November or the Space Year 3000.
With Tenet’s inversion this is impossible. If you want to travel backwards in time to last week, you’ve got to step into the inversion turnstile, then spend a week living in Backwards World, then go into the turnstile again to start moving forward a week ago.
If you want to kill Hitler before World War II breaks out, you’re going to have to live backwards for 81 years, so make sure you eat plenty of greens and get lots of exercise on the trip.
If you want to prevent the assassination of Abraham Lincoln, well, maybe your great, great, great grandchildren will be able to pull it off.
This is a rare approach to time travel. The only other time travel film I can think of that uses a similar technique is the brilliant, actually best time travel movie ever made, Primer, and even there the time travellers spend their backwards-travelling-time living in a cargo container.
Primer is also a hard to film to keep track of, with its many divergent timelines. Given our history with such things you might expect us to start going into detail about the different timelines in Tenet as well. Except for one small thing.
About that “Killing Hitler” plan you had…
There’s Only One Timeline
Yes, in Tenet there is no changing history. Honestly, this shouldn’t come as a surprise. Chris Nolan is known for creating incredibly tightly plotted clockwork masterpieces of films, which you can’t really do if everyone keeps messing up the timeline higgledy-piggledy.
The name of the film, Tenet, is a palindrome, tying into the symmetry of the film’s plot, the way that the inverted actions seen during the beginning of the film are reflected in the film’s end.
Stories that have used this model before include the book, movie, and soon the TV series of The Time Traveller’s Wife, the Spanish 2007 film Timecrimes, and the forgotten and underrated Michael French vehicle, The Crime Traveller.
But with Nolan’s “Inversion”, the “What happens stays happened” version of time travel logic is essential for the film to work. The assumption underlying the film is that the sequence of events it shows us make logical sense forwards and backwards. When the Protagonist waves his hand over an inverted bullet, summoning it into his hand, it makes as much sense as the Protagonist dropping the bullet when the footage is played in reverse.
If you allow for branching timelines while people are moving forward and backwards in time, the timeline branches in both directions and you end up with alternate pasts as well as futures, which sounds super cool, but would probably lead to a much more confusing movie.
So while Tenet is a film that demands your concentration and possible several viewings, it is still a film that can be plotted out on a single timeline, rather than an extensive tree diagram.