Tags: physics

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Tuesday, September 8th, 2020

T E N Ǝ T

Jessica and I went to cinema yesterday.

Normally this wouldn’t be a big deal, but in our current circumstances, it was something of a momentous decision that involved a lot of risk assessment and weighing of the odds. We’ve been out and about a few times, but always to outdoor locations: the beach, a park, or a pub’s beer garden. For the first time, we were evaluating whether or not to enter an indoor environment, which given what we now know about the transmission of COVID-19, is certainly riskier than being outdoors.

But this was a cinema, so in theory, nobody should be talking (or singing or shouting), and everyone would be wearing masks and keeping their distance. Time was also on our side. We were considering a Monday afternoon showing—definitely not primetime. Looking at the website for the (wonderful) Duke of York’s cinema, we could see which seats were already taken. Less than an hour before the start time for the film, there were just a handful of seats occupied. A cinema that can seat a triple-digit number of people was going to be seating a single digit number of viewers.

We got tickets for the front row. Personally, I love sitting in the front row, especially in the Duke of York’s where there’s still plenty of room between the front row and the screen. But I know that it’s generally considered an undesirable spot by most people. Sure enough, the closest people to us were many rows back. Everyone was wearing masks and we kept them on for the duration of the film.

The film was Tenet). We weren’t about to enter an enclosed space for just any ol’ film. It would have to be pretty special—a new Star Wars film, or Denis Villeneuve’s Dune …or a new Christopher Nolan film. We knew it would look good on the big screen. We also knew it was likely to be spoiled for us if we didn’t see it soon enough.

At this point I am sounding the spoiler horn. If you have not seen Tenet yet, abandon ship at this point.

I really enjoyed this film. I understand the criticism that has been levelled at it—too cold, too clinical, too confusing—but I still enjoyed it immensely. I do think you need to be able to enjoy feeling confused if this is going to be a pleasurable experience. The payoff is that there’s an equally enjoyable feeling when things start slotting into place.

The closest film in Christopher Nolan’s back catalogue to Tenet is Inception in terms of twistiness and what it asks of the audience. But in some ways, Tenet is like an inverted version of Inception. In Inception, the ideas and the plot are genuinely complex, but Nolan does a great job in making them understandable—quite a feat! In Tenet, the central conceit and even the overall plot is, in hindsight, relatively straightforward. But Nolan has made it seem more twisty and convuluted than it really is. The ten minute battle at the end, for example, is filled with hard-to-follow twists and turns, but in actuality, it literally doesn’t matter.

The pitch for the mood of this film is that it’s in the spy genre, in the same way that Inception is in the heist genre. Though there’s an argument to be made that Tenet is more of a heist movie than Inception. But in terms of tone, yeah, it’s going for James Bond.

Even at the very end of the credits, when the title of the film rolled into view, it reminded me of the Bond films that would tease “The end of (this film). But James Bond will return in (next film).” Wouldn’t it have been wonderful if the very end of Tenet’s credits finished with “The end of Tenet. But the protagonist will return in …Tenet.”

The pleasure I got from Tenet was not the same kind of pleasure I get from watching a Bond film, which is a simpler, more basic kind of enjoyment. The pleasure I got from Tenet was more like the kind of enjoyment I get from reading smart sci-fi, the kind that posits a “what if?” scenario and isn’t afraid to push your mind in all kinds of uncomfortable directions to contemplate the ramifications.

Like I said, the central conceit—objects or people travelling backwards through time (from our perspective)—isn’t actually all that complex, but the fun comes from all the compounding knock-on effects that build on that one premise.

In the film, and in interviews about the film, everyone is at pains to point out that this isn’t time travel. But that’s not true. In fact, I would argue that Tenet is one of the few examples of genuine time travel. What I mean is that most so-called time-travel stories are actually more like time teleportation. People jump from one place in time to another instaneously. There are only a few examples I can think of where people genuinely travel.

The grandaddy of all time travel stories, The Time Machine by H.G. Wells, is one example. There are vivid descriptions of the world outside the machine playing out in fast-forward. But even here, there’s an implication that from outside the machine, the world cannot perceive the time machine (which would, from that perspective, look slowed down to the point of seeming completely still).

The most internally-consistent time-travel story is Primer. I suspect that the Venn diagram of people who didn’t like Tenet and people who wouldn’t like Primer is a circle. Again, it’s a film where the enjoyment comes from feeling confused, but where your attention will be rewarded and your intelligence won’t be insulted.

In Primer, the protagonists literally travel in time. If you want to go five hours into the past, you have to spend five hours in the box (the time machine).

In Tenet, the time machine is a turnstile. If you want to travel five hours into the past, you need only enter the turnstile for a moment, but then you have to spend the next five hours travelling backwards (which, from your perspective, looks like being in a world where cause and effect are reversed). After five hours, you go in and out of a turnstile again, and voila!—you’ve time travelled five hours into the past.

Crucially, if you decide to travel five hours into the past, then you have always done so. And in the five hours prior to your decision, a version of you (apparently moving backwards) would be visible to the world. There is never a version of events where you aren’t travelling backwards in time. There is no “first loop”.

That brings us to the fundamental split in categories of time travel (or time jump) stories: many worlds vs. single timeline.

In a many-worlds story, the past can be changed. Well, technically, you spawn a different universe in which events unfold differently, but from your perspective, the effect would be as though you had altered the past.

The best example of the many-worlds category in recent years is William Gibson’s The Peripheral. It genuinely reinvents the genre of time travel. First of all, no thing travels through time. In The Peripheral only information can time travel. But given telepresence technology, that’s enough. The Peripheral is time travel for the remote worker (once again, William Gibson proves to be eerily prescient). But the moment that any information travels backwards in time, the timeline splits into a new “stub”. So the many-worlds nature of its reality is front and centre. But that doesn’t stop the characters engaging in classic time travel behaviour—using knowledge of the future to exert control over the past.

Time travel stories are always played with a stacked deck of information. The future has power over the past because of the asymmetric nature of information distribution—there’s more information in the future than in the past. Whether it’s through sports results, the stock market or technological expertise, the future can exploit the past.

Information is at the heart of the power games in Tenet too, but there’s a twist. The repeated mantra here is “ignorance is ammunition.” That flies in the face of most time travel stories where knowledge—information from the future—is vital to winning the game.

It turns out that information from the future is vital to winning the game in Tenet too, but the reason why ignorance is ammunition comes down to the fact that Tenet is not a many-worlds story. It is very much a single timeline.

Having a single timeline makes for time travel stories that are like Greek tragedies. You can try travelling into the past to change the present but in doing so you will instead cause the very thing you set out to prevent.

The meat’n’bones of a single timeline time travel story—and this is at the heart of Tenet—is the question of free will.

The most succint (and disturbing) single-timeline time-travel story that I’ve read is by Ted Chiang in his recent book Exhalation. It’s called What’s Expected Of Us. It was originally published as a single page in Nature magazine. In that single page is a distillation of the metaphysical crisis that even a limited amount of time travel would unleash in a single-timeline world…

There’s a box, the Predictor. It’s very basic, like Claude Shannon’s Ultimate Machine. It has a button and a light. The button activates the light. But this machine, like an inverted object in Tenet, is moving through time differently to us. In this case, it’s very specific and localised. The machine is just a few seconds in the future relative to us. Cause and effect seem to be reversed. With a normal machine, you press the button and then the light flashes. But with the predictor, the light flashes and then you press the button. You can try to fool it but you won’t succeed. If the light flashes, you will press the button no matter how much you tell yourself that you won’t (likewise if you try to press the button before the light flashes, you won’t succeed). That’s it. In one succinct experiment with time, it is demonstrated that free will doesn’t exist.

Tenet has a similarly simple object to explain inversion. It’s a bullet. In an exposition scene we’re shown how it travels backwards in time. The protagonist holds his hand above the bullet, expecting it to jump into his hand as has just been demonstrated to him. He is told “you have to drop it.” He makes the decision to “drop” the bullet …and the bullet flies up into his hand.

This is a brilliant bit of sleight of hand (if you’ll excuse the choice of words) on Nolan’s part. It seems to imply that free will really matters. Only by deciding to “drop” the bullet does the bullet then fly upward. But here’s the thing: the protagonist had no choice but to decide to drop the bullet. We know that he had no choice because the bullet flew up into his hand. The bullet was always going to fly up into his hand. There is no timeline where the bullet doesn’t fly up into his hand, which means there is no timeline where the protagonist doesn’t decide to “drop” the bullet. The decision is real, but it is inevitable.

The lesson in this scene is the exact opposite of what it appears. It appears to show that agency and decision-making matter. The opposite is true. Free will cannot, in any meaningful sense, exist in this world.

This means that there was never really any threat. People from the future cannot change the past (or wipe it out) because it would’ve happened already. At one point, the protagonist voices this conjecture. “Doesn’t the fact that we’re here now mean that they don’t succeed?” Neil deflects the question, not because of uncertainty (we realise later) but because of certainty. It’s absolutely true that the people in the future can’t succeed because they haven’t succeeded. But the protagonist—at this point in the story—isn’t ready to truly internalise this. He needs to still believe that he is acting with free will. As that Ted Chiang story puts it:

It’s essential that you behave as if your decisions matter, even though you know that they don’t.

That’s true for the audience watching the film. If we were to understand too early that everything will work out fine, then there would be no tension in the film.

As ever with Nolan’s films, they are themselves metaphors for films. The first time you watch Tenet, ignorance is your ammuntion. You believe there is a threat. By the end of the film you have more information. Now if you re-watch the film, you will experience it differently, armed with your prior knowledge. But the film itself hasn’t changed. It’s the same linear flow of sequential scenes being projected. Everything plays out exactly the same. It’s you who have been changed. The first time you watch the film, you are like the protagonist at the start of the movie. The second time you watch it, you are like the protagonist at the end of the movie. You see the bigger picture. You understand the inevitability.

The character of Neil has had more time to come to terms with a universe without free will. What the protagonist begins to understand at the end of the film is what Neil has known for a while. He has seen this film. He knows how it ends. It ends with his death. He knows that it must end that way. At the end of the film we see him go to meet his death. Does he make the decision to do this? Yes …but he was always going to make the decision to do this. Just as the protagonist was always going to decide to “drop” the bullet, Neil was always going to decide to go to his death. It looks like a choice. But Neil understands at this point that the choice is pre-ordained. He will go to his death because he has gone to his death.

At the end, the protagonist—and the audience—understands. Everything played out exactly as it had to. The people in the future were hoping that reality allowed for many worlds, where the past could be changed. Luckily for us, reality turns out to be a single timeline. But the price we pay is that we come to understand, truly understand, that we have no free will. This is the kind of knowledge we wish we didn’t have. Ignorance was our ammunition and by the end of the film, it is spent.

Nolan has one other piece of misdirection up his sleeve. He implies that the central question at the heart of this time-travel story is the grandfather paradox. Our descendents in the future are literally trying to kill their grandparents (us). But if they succeed, then they can never come into existence.

But that’s not the paradox that plays out in Tenet. The central paradox is the bootstrap paradox, named for the Heinlein short story, By His Bootstraps. Information in this film is transmitted forwards and backwards through time, without ever being created. Take the phrase “Tenet”. In subjective time, the protagonist first hears of this phrase—and this organisation—when he is at the start of his journey. But the people who tell him this received the information via a subjectively older version of the protagonist who has travelled to the past. The protagonist starts the Tenet organistion (and phrase) in the future because the organisation (and phrase) existed in the past. So where did the phrase come from?

This paradox—the bootstrap paradox—remains after the grandfather paradox has been dealt with. The grandfather paradox was a distraction. The bootstrap paradox can’t be resolved, no matter how many times you watch the same film.

So Tenet has three instances of misdirection in its narrative:

  • Inversion isn’t time travel (it absolutely is).
  • Decisions matter (they don’t; there is no free will).
  • The grandfather paradox is the central question (it’s not; the bootstrap paradox is the central question).

I’m looking forward to seeing Tenet again. Though it can never be the same as that first time. Ignorance can never again be my ammunition.

I’m very glad that Jessica and I decided to go to the cinema to see Tenet. But who am I kidding? Did we ever really have a choice?

Sunday, June 23rd, 2019

Rotating Space Station Numbers

Ever wondered what would happen if you threw a ball inside an orbital habitat? Well, wonder no more!

You can adjust the parameters of the space station, or choose from some pre-prepared examples: an O’Neill cylinder, a Stanford torus, a Bernal sphere, Rama, a Culture orbital

Friday, March 29th, 2019

CERN Hack days – Chiteri’s Blog

Martin gives a personal history of his time at the two CERN hack projects …and also provides a short history of the universe.

Thursday, February 28th, 2019

A Briefer History of Time

You can read all of Stephen Hawking’s 2008 book online as a web book (kind of like Resilient Web Design).

Sunday, December 9th, 2018

The Case Against Quantum Computing - IEEE Spectrum

This is the best explanation of quantum computing I’ve read. I mean, it’s not like I can judge its veracity, but I could actually understand it.

Monday, July 3rd, 2017

Greetings, E.T. (Please Don’t Murder Us.) - The New York Times

Steven Johnson dives deep into the METI project, starting with the Arecibo message and covering Lincos, the Drake equation, and the Fermi paradox.

He also wrote about what he left out of the article and mentions that he’s writing a book on long-term decision making.

In a sense, the METI debate runs parallel to other existential decisions that we will be confronting in the coming decades, as our technological and scientific powers increase. Should we create superintelligent machines that exceed our own intellectual capabilities by such a wide margin that we cease to understand how their intelligence works? Should we ‘‘cure’’ death, as many technologists are proposing? Like METI, these are potentially among the most momentous decisions human beings will ever make, and yet the number of people actively participating in those decisions — or even aware such decisions are being made — is minuscule.

Monday, November 21st, 2016

Is Dark Matter Hiding Aliens?

Here’s a fun cosmic hypothesis on the scale of an Olaf Stapeldon story. There are even implications for data storage:

By storing its essential data in photons, life could give itself a distributed backup system. And it could go further, manipulating new photons emitted by stars to dictate how they interact with matter. Fronts of electromagnetic radiation could be reaching across the cosmos to set in motion chains of interstellar or planetary chemistry with exquisite timing, exploiting wave interference and excitation energies in atoms and molecules.

Monday, October 19th, 2015

SETI: A Networked Galaxy?

A fascinating guest post by Brian McConnell on Centauri Dreams: what if there’s a galactic equivalent to the internet, allowing civilisations to communicate with a system analogous to packet switching.

Unfortunately this kind of focussed signalling would be hard to detect. But on the other hand, it could explain the Fermi paradox.

Saturday, December 6th, 2014

Asteroid Day

This is an awareness project I can get behind: a Clarke-like Project Spaceguard to protect the Earth from asteroid collisions. This campaign will focus awareness of this issue on one single day…

Now if only the front page of this website actually said when that day will be.

Update: And now it does.

Wednesday, April 30th, 2014

Staring down colliders

Craig recounts the time we visited the LHCb at CERN. It’s a lovely bit of writing. I wish it were on his own website.

Sunday, March 9th, 2014

Particle Fever

I can’t wait to see this documentary on the monumental work at CERN.

Thursday, July 18th, 2013

From Beyond the Coming Age of Networked Matter, a short story by Bruce Sterling

H.P. Lovecraft meets James Bridle in this great little story commissioned by the Institute For The Future.

Tuesday, February 28th, 2012

Prometheus Fusion Perfection

Now this is what I call science hacking: building an open source fusion reactor.

Science!

CERN | booktwo.org

James geeks out about visiting CERN. His enthusiasm is infectious.

Science!

Friday, June 24th, 2011

An Ear for Science: The Particle Physics Windchime | SLAC News Center

The story of the particle windchime—it turns subatomic particle collisions into sound—created at Science Hack Day San Francisco.

Saturday, February 6th, 2010

Our world may be a giant hologram - space - 15 January 2009 - New Scientist

This makes my brain giddy. Dizzying stuff, clearly explained.

Tuesday, August 18th, 2009

BallDroppings

Brendan Dawes pointed me to this wonderfully playful creation. It's Flash-free, believe it or not.

Sunday, June 7th, 2009

Crush the Castle | Armor Games

The game is simple, the physics are fun, the result is utterly addictive. Don't say I didn't warn you.

Thursday, February 12th, 2009

The Particle Zoo: Subatomic Particle Softies

Handmade subatomic particle plushies from the standard model of physics ...and beyond!

Sunday, November 23rd, 2008

PHYSICS INVADER (nekogames)

A brilliant take on Space Invaders where gravity does its thing.