Before I begin, I must warn my reader that I will be discussing specific details of the film Tenet (https://en.wikipedia.org/wiki/Tenet_(film)). Thus, if you have not seen this film yet, you may want to avert your eyes and wait to read this blog post until after you have. Or, if you are like me, you can proceed; but then you will have to deal with the consequences that some aspects of this film will no longer be a pleasant surprise for you.

!!! SPOILER WARNING !!!

Do you feel sufficiently warned yet? Good. Let’s proceed with the post.

Tenet is a time travel movie, sort of. It is a time travel movie, however, it does not deal with time travel in the manner that most will be familiar with. Most stories about time travel have characters that travel instantaneously from one point in time to another point in time. At the beginning of their journey, the character ceases to exist at the particular place and at the particular time they were, and instantaneously come into existence at the new particular place and particular time of their destination. This is something of a hard break typically, where I generally expect a sonic boom as their atoms are removed from one place, causing a vacuum whereby the surrounding air quickly seeps in to fill the voided space. Furthermore, there ought to be something like a sonic boom at the destination as the atoms that were in the place where the character arrives must be pushed out of the way quickly in order to make room for the character to arrive successfully. Ironically, I would expect something similar to this in the cases of teleportation, as is utilized in stories such as Star Trek.

Of course, all of this “sonic boom” business can be alleviated if one suggests that instead of the process only occurring in one direction, it instead occurred in two. That is, the object (or character) that is travelling in time is actually swapping places with a similar object at the destination. The character might move back in time to occupy a space that was previously occupied by a volume of air, and that air may be moved forward in time to occupy the space that the character formerly occupied. A swapping of atoms. While this process would clearly be asymmetric (that is, the mass of the character is clearly much greater than the mass of the air; similarly with the number of atoms), it would at least account for the lack of vacuum found on the beginning side, and the excess of atoms on the destination side. No need for a sonic boom now.

I dwell on this issue for a reason; in Tenet, instead of instantaneous travel, the travelers are simply shifting direction of flow. That is, while under normal circumstances an object will flow “forward” in time, other objects (that the story refers to as “inverted” objects) are flowing “backward.” This is presented simply through the reversal of the film reel. That is, the film scene is played in reverse. It is a trick of the medium, and nothing terribly new or innovative. If most objects in our universe are sliding through time in one direction, what if other objects in our universe happened to be sliding in the opposite direction? It would raise big questions regarding what it means for an object (or person) to age. Instead of a rock wearing down over time through erosion, that same rock, if inverted, would somehow be slowly gaining size and mass through the ages from the very same process. It is an incredibly compelling question to ask.

If that isn’t enough to confuse the audience, there is one more thing to consider. As not all objects are sliding through time in the same direction, what happens when two objects conflict? That is, if one object is sliding forward and another backward, what if they both intend to occupy the same space at the same time, through their respective journey’s through time? The film has an answer to this, sort of. It is suggested that “the world” is sliding in the forward direction, hence why we are all familiar with the forward flow of time (one might suggest that is why we consider it the “forward” direction as well). Objects that are sliding in the opposite direction than the world are “swimming upstream,” that is, they are fighting against the “normal” flow of time against the bulk of the objects they will encounter. If all this is true, then one would expect that the forward sliding objects would overtake the backward sliding objects.

Unfortunately, this isn’t really good enough. You see, if most objects are moving with the world, flowing through time in the forward direction, then that will include all the air and other atoms that we may often take for granted. The film does touch on some of these less often considered objects by emphasizing that inverted people need to breath inverted air, but the reason given has to do with the permeability of the lungs’ membranes to absorb air that is flowing through time in the “wrong” direction. In order for the lungs to operate correctly, they must absorb air that is flowing through time in the same direction. This is touching on the idea that those objects sliding through time in the opposite direction will not behave as expected.

There is further discussion on this point as the film suggests that inverted fire is incredibly cold, instead of being incredibly hot. That the wind that would normally be at your face will instead be at your back. That friction itself will “feel weird,” especially when trying to drive an inverted car. It seems that the nature of the universe itself is operating in the opposite way we might expect while we are inverted. It would require me to spend a lot of time on each individual characteristic to discuss what makes the most sense in this circumstance, so I will leave this exercise to the reader to pursue, if they so desire. For now, I will focus on one aspect, which I’ve already been alluding to: the occupation of space.

I return to the question of objects moving through time in opposite directions. To help with this, I will talk about objects that are moving in opposite directions in space, but in the same direction in time. This is a situation that I have a lot of experience with. Anyone who has taken a high school level physics class should be familiar with the many experiments with billiard balls bouncing off each other. If one ball is moving while the other is stationary, and if they hit each other at just the right angle, the energy from the moving ball is transferred to the stationary ball completely. That is, the moving ball becomes stationary, while the stationary ball starts moving. In essence, the balls swap their motions and energies. The ball that was moving gives its energy and motion to the other ball, while the stationary ball gives its lack of motion and energy to the formerly moving ball.

Taking this example further, if both balls are moving in opposite directions, and assuming they collide with each other just right, they will again swap their respective energies and motions. It is worth noting here that the transfer is not complete in any of these cases, as there is a loss due to friction and heat and other generally ignored effects. So the two balls will bounce and then move away from each other at roughly the same speeds as we might expect. This is how objects moving in opposite directions in space, but the same direction in time, behave.

Now for a much more controversial analysis. Let us consider objects moving in the opposite directions in space and the opposite directions in time. If the two objects are moving at the same rate, both in space and in time, then we would expect them not to ever touch. That is, as they are moving in opposite directions, both in space and in time, then they are in fact moving in the same direction at the same rate relative to a single observer, in whatever direction the observer happens to be travelling through time. This example is not terribly helpful to the answering of my initial question, but helps me orient myself. I have to keep in mind that objects sliding through time in the opposite direction are in fact moving in the opposite direction than I might initially expect.

So then, the example I need to consider is of two objects moving in the SAME direction in space but opposite directions in time. These two objects are now on a collision course due to how they end up operating through their flow through time. And once again, we are struck with how they will resolve such a collision. The first, simple possibility is that the two objects are in fact the same object, simply viewed in two very different ways. If that were the case, then there would not be a collision, as the object is itself the same, and can certainly occupy the same space as itself at the same time, regardless of the direction of the flow of time. If this were the case, then problem solved. However, the issue at hand is that the two objects are not identical with each other.

Our situation, put simply, is the fluid of our world (the air in our atmosphere) flowing in the forward direction of time (as it is with and part of the world), colliding with the solid inverted objects of the film, which include the characters themselves. The characters, quite literally, are fighting against the current in order simply to occupy the space that the air has already claimed. As the movie demonstrates to us, the characters succeed in occupying the space, and thus the air must have failed in that battle. The air was either pushed out of the way, or is annihilated. Of course, in the wake of the inverted characters’ movement, there is no sonic boom, and so air must also be generated on the other side if the air that loses the former battle is annihilated. It would seem that the air has likely behaved in the fashion we might normally expect, assuming it was encountering an object that was flowing through time in the same direction as we are used to. The air, was pushed out of the way, flowing through time in its usual forward direction, but redirected through space around the inverted object. At least, this is what the film tells us.

Ultimately, the problem the audience has in the entire story, is how to understand the boundary between the normal objects and the inverted objects. When normal objects make contact with normal objects, everything behaves as we expect. And when inverted objects make contact with inverted objects, again, we understand what must happen. However, when normal and inverted objects make contact, it is not entirely clear what to expect. An inverted fire draws the heat energy out of the surrounding normal objects, while radiating energy upon the surrounding inverted objects. Inverted wind is simply air moving in the opposite direction, thus you feel it at your back if it might normally be in your face, assuming you are normally oriented and not inverted yourself. This point about wind is possibly the most telling.

Perhaps the intent is that the inertia of objects remains the same, regardless of the flow of time of the object. That is, air is air, whether it is normal or inverted. My solid body pushes air out of the way, whether the air is flowing in the same direction through time as I am, or whether the air is flowing through time in the opposite direction. In both cases, the air is displaced by myself. Of course, solid objects colliding with other solid objects becomes a bit more complicated again. When the protagonist ends up fighting his inverted self, the precise manner in which each blow is landed seems counter intuitive. The inverted fighter is not throwing punches, but is instead catching them, healing injuries that they seem to have no intention of creating. This might be true of the inverted protagonist, not actually wanting to cause harm to himself, but what about the characters in the battlefield during the film’s climax?

Tenet is a very interesting story. It raises a lot of questions regarding how time might operate by pressing our current flow of forward moving time against several objects moving through time in the opposite direction. This, of course, is where the great apocalyptic event that is foreshadowed at the beginning takes its shape from. However, if one takes all these things seriously, the story reveals its end at its beginning. After all, there is a world beyond when the final battle takes place, therefore the mission to prevent the apocalypse will definitely succeed. If it had failed, then there would be none of the future events that take place throughout most of the movie.

Finally, there is one other rather significant detail that is revealed in the story that does not seem to weigh on many people’s minds: free will. According to this story, it does not exist. At least not the sort of free will that many would have you believe makes humans unpredictable. Everything in the film that will happen, has already happened. Events are tied together in a complicated Möbius strip. The world is purely and completely deterministic. And if our world really is that way, then either Stephen Hawking is correct, and there is no time travel, or time travelers in the future are simply not interested in what we consider to be one of the most brilliant minds of our time.