TENET Is A Palindrome And Maybe Self-Driving Cars Ought To Be Reversible Too

The new sci-fi thriller Tenet has brought palindromes to the forefront of our minds, along with jolting us all to think outside of the box about time.

You’ll see in a moment how palindromes and time appear to tie together, ingeniously so.

Writer and director extraordinaire Christopher Nolan has once again decided to get us all to think carefully about time, doing so in his latest provocative blockbuster-to-be film Tenet and further extends his legacy of toying with time in his prior films including having cleverly done so in Inception, Interstellar, Dunkirk, Memento, and so on. To avoid spoiling his latest movie, let’s avert discussing any of the details about how time is utilized in Tenet, though we can certainly discuss the name of the film, which serves as an immediate and telltale clue.

In case you didn’t already know, the word “TENET” is an especially famous example of a palindrome. A palindrome is a word, name, phrase, sentence, verse, or essentially any general sequence that can be read the same when doing so both forwards and backward. For example, the word “level” is read the same in either direction, reading either from left to right (forwards), or reading from right to left (backward or perhaps more aptly, in reverse). Likewise, the word “kayak” and the word “racecar” are examples of palindromes too, as are the names “Bob” and “Anna.”

The instances of palindromes consisting of phrases or sentences can get a bit more complicated. Sometimes the reading process is straightforward in either direction, such as “step on no pets” is the same whether reading forwards or in reverse. Meanwhile, a trickier palindrome is “was it a car or a cat I saw” and for which you need to mindfully break up the letters into correspondingly aligned words when reading the phrase in reverse. Numbers can be labeled as palindromic and mathematicians seem to delight in finding interesting examples, such as the prime numbers of 101 and 353.

What makes TENET especially notable is its inclusion in the famous Sator Square. Historically, the Sator Square is a well-known exemplar of a palindrome and has stoked intense mystery and overall intrigue. There are five words in the Sator Square, each in Latin, and collected together to create a 5-by-5 square, consisting of the words SATOR, AREPO, TENET, OPERA, and ROTAS. No one is exactly sure what the message is supposed to mean, though many believe it is a story about a farmer named Arepo. Depending upon how you decide to translate the Latin words, perhaps there is a sentence composed of the five words that roughly indicates that the farmer Arepo works to rotate wheels (such as a plow), or maybe the farmer Arepo holds the wheels with difficulty, or effort.

Instances of the Sator Square have been discovered in various places and presumably must be of some significance, which has also led to a wide range and at times wild speculation about the alleged powers or magic that are imbued within this collection of five palindromes.

The Latin palindrome TENET is a word that generally is indicative of the notion to hold or keep or comprehend something. You might suggest that this is somewhat similar to the everyday word “tenet” which generally means to hold a belief or principle to be considered true.

Okay, so we have the palindrome TENET that can be read forwards and backward (in reverse), along with a meaning associated with holding or comprehending something, plus it is famously used in the Sator Square, which might or might not be somehow magical, and there is a new movie made by a director seemingly obsessed with time that has opted to name the film as Tenet.

What do you get?

Well, focus on the idea of being able to read a word or sentence both forwards and in reverse. Suppose that we could do the same thing with time. Right now, we assume that time can only flow in one direction, namely forward. Mankind has seemingly dreamt forever that it would be nifty if time could be reversed. There are gobs and gobs of science fiction stories, movies, TV shows, poetry and you name it that have sought to explore what could happen and what might be done if time could flow in reverse.

Time reversibility or sometimes referred to as time inversion is undoubtedly a fascinating topic.

Shifting gears, you might be surprised to know that time reversibility has been given some strident attention in the realm of computers and computing machines, doing so in a field of study known as reversible computing. Most people have never heard of such a thing. Indeed, even those within the computer field are often taken aback to discover that research and attention are being put toward reversible computing.

The usual reaction is in three stages. First, amazement that such a field of study exists. Second, a doubtfulness that it makes any sense to study the notion and that it might very well be an utter waste of time (a bit of a pun there). Third, curiosity about what exactly reversible computing is, and how it might be of use.

Generally, there are two major ways to categorize reversible computing facets. One is a means of performing a physical reversibility of computational activities, while the other has to do with undertaking a logical semblance of computational reversibility. These two categories tend to work hand-in-hand.

A simple example might suffice to get you started on the topic of reversible computing.

It seems that everyone nowadays knows that there are usually computer bits consisting of the binary values of 1 and 0. We could construct an electronic chip that would take as input a bit, either in the state of 1 or the state of 0 and produce as output the so-called opposite or inversion of the bit (i.e., if the input is a 1 then output a 0, while if the input is a 0 then output a 1). Those of you familiar with such matters would recognize this as the NOT operation (confusing perhaps that this is known as NOT, which might seem like not doing something, when in fact it will produce the outputs as mentioned herein). More commonly, this is referred to as an inverter.

Could you receive the output of a NOT operation and make things go in reverse, returning back to whatever you started with?

Yes, it would certainly seem straightforward to do so. If the output was a 0, you know that the input must have been a 1, while if the output is 1 then you know that the input must have been a 0 (assuming of course that the operation worked flawlessly). Now, keep in mind that most electronic chips are not made to work in reverse. We have logically identified how to go in reverse, but the electronics might not be set up to allow a reverse physical operation to occur. As such, if we wanted this to happen in a real-world sense, we might need to adapt the electronics accordingly.

A reversible electronic circuit then is a circuit that allows for bits to flow in either direction, forward or in reverse, through the circuitry. There is no longer a fixed indication of what is an input and what is output since either side of the circuitry could be considered available for input and equally available for output.

Where this can be usefully employed involves a somewhat complex understanding of entropy in computing and computers. Generally, it is posited that a reversible computing or computer system could potentially consume much less energy than a conventional forwards-only computer. There would in theory be less heat dissipated for a reversible computing system. That’s important because computers are getting smaller and smaller, and limitations are being approached involving what to do about the heat production, for which the give-off of energy can adversely affect the electronics and otherwise undermine attempts to reduce the size of chips. It is hoped that the per Joule of energy that you can squeeze out of the computer hardware would be significantly enhanced via adopting reversible computing approaches.

For those of you more interested in software than hardware, you might enjoy knowing that there is reversible computing with respect to aspects of software too. For example, most software engineers value the notion of being able to stepwise execute their computer programs when doing debugging. This is usually done in a forward motion, proceeding with each line of code to the next in sequence or the intended next line to be performed, moving from the start to the finish. Some debugging tools will allow you to also work in reverse, such that you can essentially go back to a prior line of code, retracting what has just happened, and thus invoke your code to effectively perform in a reverse direction (this is not as easy a thing to do as it might seem on the surface).

If you stop to think about it, we are seemingly conditioned to always be thinking about going forward, and less so about going in reverse. We think of computers for example as going in a forward motion, performing a series of steps in a progression from start to finish. Rarely do you think about going from the end to the start.

There’s another place that we tend to be primarily focused on going forward, entailing the driving of a car.

It is a pretty good bet that you spend most of your time driving in a forward direction. Sure, you do use the reverse capability of the car, such as when backing out of a parking spot or trying to back down your driveway. The preponderance of your time though at the wheel is likely going forward and only a tiny fraction of the time involved in reverse driving.

Some people are quite rusty at driving in reverse. You see them moving an inch at a time when backing up. They aren’t sure whether to look over their shoulder or whether to look in the rearview mirror. Their heads twist back and forth, trying to figure out what is behind them. Luckily, technology has progressed that many cars now have a back-up camera built-in, aiding the reverse driving chore. Besides the camera, there are at times sensors that scan the area behind the car as you are driving in reverse, attempting to alert you if there are any objects detected. Nonetheless, a lot of older cars still exist on our roadways and they generally lack the back-up camera and allied technologies.

Since we are discussing the notion of thinking outside the box and coupled with mulling over palindromes and going in reverse, here’s an additional twist for your day: Will AI-based true self-driving cars reconsider the act of driving in reverse, or will the task of reverse driving remain as it is today?

Let’s unpack the matter and see.

Understanding The Levels Of Self-Driving Cars

As a clarification, true self-driving cars are ones that the AI drives the car entirely on its own and there isn’t any human assistance during the driving task.

These driverless vehicles are considered a Level 4 and Level 5 (see my explanation at this link here), while a car that requires a human driver to co-share the driving effort is usually considered at a Level 2 or Level 3. The cars that co-share the driving task are described as being semi-autonomous, and typically contain a variety of automated add-on’s that are referred to as ADAS (Advanced Driver-Assistance Systems).

There is not yet a true self-driving car at Level 5, which we don’t yet even know if this will be possible to achieve, and nor how long it will take to get there.

Meanwhile, the Level 4 efforts are gradually trying to get some traction by undergoing very narrow and selective public roadway trials, though there is controversy over whether this testing should be allowed per se (we are all life-or-death guinea pigs in an experiment taking place on our highways and byways, some point out, see my indication at this link here).

Since semi-autonomous cars require a human driver, the adoption of those types of cars won’t be markedly different than driving conventional vehicles, so there’s not much new per se to cover about them on this topic (though, as you’ll see in a moment, the points next made are generally applicable).

For semi-autonomous cars, it is important that the public needs to be forewarned about a disturbing aspect that’s been arising lately, namely that despite those human drivers that keep posting videos of themselves falling asleep at the wheel of a Level 2 or Level 3 car, we all need to avoid being misled into believing that the driver can take away their attention from the driving task while driving a semi-autonomous car.

You are the responsible party for the driving actions of the vehicle, regardless of how much automation might be tossed into a Level 2 or Level 3.

Self-Driving Cars And Going In Reverse

For Level 4 and Level 5 true self-driving vehicles, there won’t be a human driver involved in the driving task.

All occupants will be passengers.

The AI is doing the driving.

Will the AI be able to drive the self-driving car in reverse?

Yes, if the AI has been set up to do so, and for which you can reasonably expect that nearly all automakers and self-driving tech firms are putting such capabilities into place.

There are some important caveats to keep in mind.

First, by-and-large, the mainstay of the sensors on a self-driving car are oriented towards going forwards, not going in reverse. There is usually not as much sensory capability installed at going in reverse as there is in going forwards, which somewhat makes sense since presumably, a self-driving car is predominantly to be used going forwards versus going backward.

Second, this means that even if the AI has been programmed or otherwise established to drive in reverse, it is doing so in a less than optimal way than it can drive going forwards. Presumably, the slow speeds usually involved in driving in reverse are sufficiently low enough that the amount of sensory gear will be good enough for reverse oriented driving.

But, this is potentially a legal liability issue that might ultimately rear its ugly head, as it were. If a self-driving car, while going in reverse, runs into say a child or harms or kills someone, you can be sure that lawyers are going to be asking pointed questions about how the reverse driving capabilities were designed and implemented. A case could be made that any shortchanges on how the reverse driving was devised are tantamount to insufficiencies that contributed to whatever regrettable incident might have occurred.

You might be tempted to assert that human drivers are relatively poor at driving in reverse, therefore if the AI is similarly insufficient at driving in reverse than we are no worse off than with human drivers. I doubt that logic will prevail. The public is likely to have higher expectations about the AI driving system, and especially so when going in reverse. In fact, people oftentimes assume that the AI to-date is safer at driving in reverse than humans are, which, debatably might be the case in comparison to rather newbies at driving, but not necessarily better than say human drivers that have honed their reverse driving skills.

Another consideration to contemplate about cars is that the design and construction of most conventional cars are entirely oriented toward forward driving. You cannot drive in reverse in any prolonged manner and nor at the same heightened speeds and fluidity as you can when driving forwards (in most cars). This is not simply due to the human driver, but also due to how the reverse driving mechanisms are devised.

Of course, it seems entirely sensible that the car is structured for forward driving, including that there are a front windshield and nothing comparable on the back (the back window is not usually the same as a windshield). Our seats for drivers are oriented toward the front of the car, and the driving controls are intended to be used while facing forward.

Here’s a mind-bender for you.

For true self-driving cars, the driving controls do not need to be available in the car per se, since the AI is doing the driving. Furthermore, the seats inside a self-driving car do not need to be facing in a fixed manner forward. The typical interior design for a self-driving car consists of seats that swivel, allowing the passengers to face in any desired direction. Plus, the seats are oftentimes designed to recline, allowing you to catch a snooze while the AI is doing the driving.

The AI doesn’t especially care that there is a so-called forward and a so-called backward (going in reverse). It can be crafted to drive in either direction.

As such, some question why we would use a conventional car design for the use in creating self-driving cars. Sure, it is easier to do things that way, since those car designs already exist. But, perhaps we ought to be reconsidering that a conventional car is oriented towards a forward driving role, and instead allow for an equal capability of self-driving cars to go in reverse as it does when proceeding forwards.

Akin to the earlier point about electronic chips that might allow for reversibility, recall that there isn’t any designated side that has exclusively the inputs or exclusively the outputs, thus, perhaps our self-driving cars should be made to go in either direction too, doing so to an equal capacity.

Some pointedly ask, why should we continue to limit self-driving cars to driving in the ways that have been structured to accommodate human drivers?

Some readers might realize that I’ve discussed this topic in prior articles, including pointing out that Zoox has had a core goal of attaining self-driving cars that can equally proceed forward and “in reverse” (see my coverage at this link here, and another article at this link here).


There is more to the debate about self-driving cars going forwards versus going in reverse.

For example, if this suggests that the sensory devices need to be doubled up to allow for reverse driving, perhaps the added cost is unwarranted (on the other hand, given the potential liability exposure, this could be considered a must in that light).

Also, some worry that since our driving world is shaped around human drivers driving forward, perhaps it is unsafe to have self-driving cars routinely aiming to drive in reverse, since our roads, parking lots, and even our mindset as pedestrians is not prepared for experiencing en masse driving in reverse.

So, which is it, does driving in reverse for self-driving cars seem revolutionary or evolutionary?

This is an open question.

Perhaps we can consult the Sator Square, given that maybe the five Latin words of SATOR, AREPO, TENET, OPERA, and ROTAS tell us the answer (per its magical powers). When loosely translated, it might mean, yes, self-driving cars should drive in both directions equally, or maybe it says do not let those self-driving cars embellish driving in reverse.

Time to hit the books on Latin and the meaning of those enigmatic palindromes.

Or, go see a movie.

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