Systems thinking fans of the movie Tenet, by Christopher Nolan, may be immediately challenged to test their understanding of entropy, as the premise for temporal inversion.
Journalists aren’t scientists, so their writing may stand up to criticism.
Here is (in my opinion) a good description.
As Nolan puts it in the film’s production notes: “Every law of physics is symmetrical – it can run forwards or backwards in time and be the same – except for entropy." Entropy, for those who’ve forgotten their teenage physics lessons, is the degree of disorder in a system. According to the second law of thermodynamics, as time moves forward, entropy can never decrease – it either increases, or remains the same. Which could mean that what we think of as time is, in fact, merely a perception governed by our observations of entropy. If we see disorder decreasing, we think we’re seeing something moving backwards in time.
“The theory being that if you could invert the flow of entropy for an object, you could reverse the flow of time for that object, so the story is grounded in credible physics," Nolan says. "I did have (Nobel Prize-theoretical physicist and consultant on Interstellar ) Kip Thorne read the script and he helped me out with some of the concepts, though we’re not going to make any case for this being scientifically accurate. But it is based roughly on actual science.”
- " How ‘Tenet’ Uses Time Inversion To Touch The Future And Flip The Past" | Olivia Ovenden | August 24, 2020, Esquire at https://www.esquire.com/uk/culture/film/a32943259/time-inversion-explained-tenet/ .
The completist might also read …
- “Black Holes, Hollywood, and Interstellar: Q&A with Kip Thorne” | December 2014 | American Physical Society at https://www.aps.org/publications/apsnews/201412/thorne.cfm
Here’s another review, where a finer appreciation of science sees a rew traps.
Tenet, however, is based on the idea of reversing entropy.
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.
- " Tenet’s ‘Inversion’ Logic Explained" | Chris Farnell | August 28, 2020 | Den of Geek at https://www.denofgeek.com/movies/tenets-inversion-logic-explained/
Firstly, if you like Ackoff’s definition of process, he reads this as “arrangement in time”. Saying that “entropy isn’t a process, so you can’t reverse it” is kind of okay, as it can be taken as physical law. If the order in a system is changed, the original arrangement might be restore with some energy (effort) … but sometimes that restoration might not be possible.
On a second point, something that I learned in big data class – related to algorithms used in data science – is that information entropy doesn’t necessarily have anything to do with entropy in the more general case. This shows up in …
- Entropy (information theory) at https://en.wikipedia.org/wiki/Entropy_(information_theory) , as compared with …
- Entropy (classical thermodynamics) at https://en.wikipedia.org/wiki/Entropy_(classical_thermodynamics)
Of course, for someone who is new to the word, there’s also …
- Introduction to entropy at https://en.wikipedia.org/wiki/Introduction_to_entropy ; and
- Entropy (order and disorder) at https://en.wikipedia.org/wiki/Entropy_(order_and_disorder)
These fine points did not deter me from enjoying the movie, Tenet!
