Tag Archives: quantum mechanics

Reality as an Algebra of Observables

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Listen to a physicist talk about quantum mechanics, and you’ll hear the word “observable”. Observables are, intuitively enough, things that can be observed. They’re properties that, in principle, one could measure in an experiment, like the position of a particle or its momentum. They’re the kinds of things linked by uncertainty principles, where the better you know one, the worse you know the other.

Some physicists get frustrated by this focus on measurements alone. They think we ought to treat quantum mechanics, not like a black box that produces results, but as information about some underlying reality. Instead of just observables, they want us to look for “beables“: not just things that can be observed, but things that something can be. From their perspective, the way other physicists focus on observables feels like giving up, like those physicists are abandoning their sacred duty to understand the world. Others, like the Quantum Bayesians or QBists, disagree, arguing that quantum mechanics really is, and ought to be, a theory of how individuals get evidence about the world.

I’m not really going to weigh in on that debate, I still don’t feel like I know enough to even write a decent summary. But I do think that one of the instincts on the “beables” side is wrong. If we focus on observables in quantum mechanics, I don’t think we’re doing anything all that unusual. Even in other parts of physics, we can think about reality purely in terms of observations. Doing so isn’t a dereliction of duty: often, it’s the most useful way to understand the world.

When we try to comprehend the world, we always start alone. From our time in the womb, we have only our senses and emotions to go on. With a combination of instinct and inference we start assembling a consistent picture of reality. Philosophers called phenomenologists (not to be confused with the physicists called phenomenologists) study this process in detail, trying to characterize how different things present themselves to an individual consciousness.

For my point here, these details don’t matter so much. That’s because in practice, we aren’t alone in understanding the world. Based on what others say about the world, we conclude they perceive much like we do, and we learn by their observations just as we learn by our own. We can make things abstract: instead of the specifics of how individuals perceive, we think about groups of scientists making measurements. At the end of this train lie observables: things that we as a community could in principle learn, and share with each other, ignoring the details of how exactly we measure them.

If each of these observables was unrelated, just scattered points of data, then we couldn’t learn much. Luckily, they are related. In quantum mechanics, some of these relationships are the uncertainty principles I mentioned earlier. Others relate measurements at different places, or at different times. The fancy way to refer to all these relationships is as an algebra: loosely, it’s something you can “do algebra with”, like you did with numbers and variables in high school. When physicists and mathematicians want to do quantum mechanics or quantum field theory seriously, they often talk about an “algebra of observables”, a formal way of thinking about all of these relationships.

Focusing on those two things, observables and how they are related, isn’t just useful in the quantum world. It’s an important way to think in other areas of physics too. If you’ve heard people talk about relativity, the focus on measurement screams out, in thought experiments full of abstract clocks and abstract yardsticks. Without this discipline, you find paradoxes, only to resolve them when you carefully track what each person can observe. More recently, physicists in my field have had success computing the chance particles collide by focusing on the end result, the actual measurements people can make, ignoring what might happen in between to cause that measurement. We can then break measurements down into simpler measurements, or use the structure of simpler measurements to guess more complicated ones. While we typically have done this in quantum theories, that’s not really a limitation: the same techniques make sense for problems in classical physics, like computing the gravitational waves emitted by colliding black holes.

With this in mind, we really can think of reality in those terms: not as a set of beable objects, but as a set of observable facts, linked together in an algebra of observables. Paring things down to what we can know in this way is more honest, and it’s also more powerful and useful. Far from a betrayal of physics, it’s the best advantage we physicists have in our quest to understand the world.

The Multiverse You Can Visit Is Not the True Multiverse

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I don’t want to be the kind of science blogger who constantly complains about science fiction, but sometimes I can’t help myself.

When I blogged about zero-point energy a few weeks back, there was a particular book that set me off. Ian McDonald’s River of Gods depicts the interactions of human and AI agents in a fragmented 2047 India. One subplot deals with a power company pursuing zero-point energy, using an imagined completion of M theory called M* theory. This post contains spoilers for that subplot.

What frustrated me about River of Gods is that the physics in it almost makes sense. It isn’t just an excuse for magic, or a standard set of tropes. Even the name “M* theory” is extremely plausible, the sort of term that could get used for technical reasons in a few papers and get accidentally stuck as the name of our fundamental theory of nature. But because so much of the presentation makes sense, it’s actively frustrating when it doesn’t.

The problem is the role the landscape of M* theory plays in the story. The string theory (or M theory) landscape is the space of all consistent vacua, a list of every consistent “default” state the world could have. In the story, one of the AIs is trying to make a portal to somewhere else in the landscape, a world of pure code where AIs can live in peace without competing with humans.

The problem is that the landscape is not actually a real place in string theory. It’s a metaphorical mathematical space, a list organized by some handy coordinates. The other vacua, the other “default states”, aren’t places you can travel to, there just other ways the world could have been.

Ok, but what about the multiverse?

There are physicists out there who like to talk about multiple worlds. Some think they’re hypothetical, others argue they must exist. Sometimes they’ll talk about the string theory landscape. But to get a multiverse out of the string theory landscape, you need something else as well.

Two options for that “something else” exist. One is called eternal inflation, the other is the many-worlds interpretation of quantum mechanics. And neither lets you travel around the multiverse.

In eternal inflation, the universe is expanding faster and faster. It’s expanding so fast that, in most places, there isn’t enough time for anything complicated to form. Occasionally, though, due to quantum randomness, a small part of the universe expands a bit more slowly: slow enough for stars, planets, and maybe life. Each small part like that is its own little “Big Bang”, potentially with a different “default” state, a different vacuum from the string landscape. If eternal inflation is true then you can get multiple worlds, but they’re very far apart, and getting farther every second: not easy to visit.

The many-worlds interpretation is a way to think about quantum mechanics. One way to think about quantum mechanics is to say that quantum states are undetermined until you measure them: a particle could be spinning left or right, Schrödinger’s cat could be alive or dead, and only when measured is their state certain. The many-worlds interpretation offers a different way: by doing away with measurement, it instead keeps the universe in the initial “undetermined” state. The universe only looks determined to us because of our place in it: our states become entangled with those of particles and cats, so that our experiences only correspond to one determined outcome, the “cat alive branch” or the “cat dead branch”. Combine this with the string landscape, and our universe might have split into different “branches” for each possible stable state, each possible vacuum. But you can’t travel to those places, your experiences are still “just on one branch”. If they weren’t, many-worlds wouldn’t be an interpretation, it would just be obviously wrong.

In River of Gods, the AI manipulates a power company into using a particle accelerator to make a bubble of a different vacuum in the landscape. Surprisingly, that isn’t impossible. Making a bubble like that is a bit like what the Large Hadron Collider does, but on a much larger scale. When the Large Hadron Collider detected a Higgs boson, it had created a small ripple in the Higgs field, a small deviation from its default state. One could imagine a bigger ripple doing more: with vastly more energy, maybe you could force the Higgs all the way to a different default, a new vacuum in its landscape of possibilities.

Doing that doesn’t create a portal to another world, though. It destroys our world.

That bubble of a different vacuum isn’t another branch of quantum many-worlds, and it isn’t a far-off big bang from eternal inflation. It’s a part of our own universe, one with a different “default state” where the particles we’re made of can’t exist. And typically, a bubble like that spreads at the speed of light.

In the story, they have a way to stabilize the bubble, stop it from growing or shrinking. That’s at least vaguely believable. But it means that their “portal to another world” is just a little bubble in the middle of a big expensive device. Maybe the AI can live there happily…until the humans pull the plug.

Or maybe they can’t stabilize it, and the bubble spreads and spreads at the speed of light destroying everything. That would certainly be another way for the AI to live without human interference. It’s a bit less peaceful than advertised, though.

Particles vs Waves, Particles vs Strings

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On my “Who Am I?” page, I open with my background, calling myself a string theorist, then clarify: “in practice I’m more of a Particle Theorist, describing the world not in terms of short lengths of string but rather with particles that each occupy a single point in space”.

When I wrote that I didn’t think it would confuse people. Now that I’m older and wiser, I know people can be confused in a variety of ways. And since I recently saw someone confused about this particular phrase (yes I’m vagueblogging, but I suspect you’re reading this and know who you are 😉 ), I figured I’d explain it.

If you’ve learned a few things about quantum mechanics, maybe you have this slogan in mind:

“What we used to think of as particles are really waves. They spread out over an area, with peaks and troughs that interfere, and you never know exactly where you will measure them.”

With that in mind, my talk of “particles that each occupy a single point” doesn’t make sense. Doesn’t the slogan mean that particles don’t exist?

Here’s the thing: that’s the wrong slogan. The right slogan is just a bit different:

“What we used to think of as particles are ALSO waves. They spread out over an area, with peaks and troughs that interfere, and you never know exactly where you will measure them.”

The principle you were remembering is often called “wave-particle duality“. That doesn’t mean “particles don’t exist”. It means “waves and particles are the same thing”.

This matters, because just as wave-like properties are important, particle-like properties are important. And while it’s true that you can never know exactly where you will measure a particle, it’s also true that it’s useful, and even necessary, to think of it as occupying a single point.

That’s because particles can only affect each other when they’re at the same point. Physicists call this the principle of locality, the idea that there is no real “action at a distance”, everything happens because of something traveling from point A to point B. Wave-particle duality doesn’t change that, it just makes the specific point uncertain. It means you have to add up over every specific point where the particles could have interacted, but each term in your sum has to still involve a specific point: quantum mechanics doesn’t let particles affect each other non-locally.

Strings, in turn, are a little bit different. Strings have length, particles don’t. Particles interact at a point, strings can interact anywhere along the string. Strings introduce a teeny bit of non-locality.

When you compare particles and waves, you’re thinking pre-quantum mechanics, two classical things neither of which is the full picture. When you compare particles and strings, both are quantum, both are also waves. But in a meaningful sense one occupies a single point, and the other doesn’t.

The Wolfram Physics Project Makes Me Queasy

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Stephen Wolfram is…Stephen Wolfram.

Once a wunderkind student of Feynman, Wolfram is now best known for his software, Mathematica, a tool used by everyone from scientists to lazy college students. Almost all of my work is coded in Mathematica, and while it has some flaws (can someone please speed up the linear solver? Maple’s is so much better!) it still tends to be the best tool for the job.

Wolfram is also known for being a very strange person. There’s his tendency to name, or rename, things after himself. (There’s a type of Mathematica file that used to be called “.m”. Now by default they’re “.wl”, “Wolfram Language” files.) There’s his live-streamed meetings. And then there’s his physics.

In 2002, Wolfram wrote a book, “A New Kind of Science”, arguing that computational systems called cellular automata were going to revolutionize science. A few days ago, he released an update: a sprawling website for “The Wolfram Physics Project”. In it, he claims to have found a potential “theory of everything”, unifying general relativity and quantum physics in a cellular automata-like form.

If that gets your crackpot klaxons blaring, yeah, me too. But Wolfram was once a very promising physicist. And he has collaborators this time, who are currently promising physicists. So I should probably give him a fair reading.

On the other hand, his introduction for a technical audience is 448 pages long. I may have more time now due to COVID-19, but I still have a job, and it isn’t reading that.

So I compromised. I didn’t read his 448-page technical introduction. I read his 90-ish page blog post. The post is written for a non-technical audience, so I know it isn’t 100% accurate. But by seeing how someone chooses to promote their work, I can at least get an idea of what they value.

I started out optimistic, or at least trying to be. Wolfram starts with simple mathematical rules, and sees what kinds of structures they create. That’s not an unheard of strategy in theoretical physics, including in my own field. And the specific structures he’s looking at look weirdly familiar, a bit like a generalization of cluster algebras.

Reading along, though, I got more and more uneasy. That unease peaked when I saw him describe how his structures give rise to mass.

Wolfram had already argued that his structures obey special relativity. (For a critique of this claim, see this twitter thread.) He found a way to define energy and momentum in his system, as “fluxes of causal edges”. He picks out a particular “flux of causal edges”, one that corresponds to “just going forward in time”, and defines it as mass. Then he “derives” E=mc^2, saying,

Sometimes in the standard formalism of physics, this relation by now seems more like a definition than something to derive. But in our model, it’s not just a definition, and in fact we can successfully derive it.

In “the standard formalism of physics”, E=mc^2 means “mass is the energy of an object at rest”. It means “mass is the energy of an object just going forward in time”. If the “standard formalism of physics” “just defines” E=mc^2, so does Wolfram.

I haven’t read his technical summary. Maybe this isn’t really how his “derivation” works, maybe it’s just how he decided to summarize it. But it’s a pretty misleading summary, one that gives the reader entirely the wrong idea about some rather basic physics. It worries me, because both as a physicist and a blogger, he really should know better. I’m left wondering whether he meant to mislead, or whether instead he’s misleading himself.

That feeling kept recurring as I kept reading. There was nothing else as extreme as that passage, but a lot of pieces that felt like they were making a big deal about the wrong things, and ignoring what a physicist would find the most important questions.

I was tempted to get snarkier in this post, to throw in a reference to Lewis’s trilemma or some variant of the old quip that “what is new is not good; and what is good is not new”. For now, I’ll just say that I probably shouldn’t have read a 90 page pop physics treatise before lunch, and end the post with that.

What I Was Not Saying in My Last Post

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Science communication is a gradual process. Anything we say is incomplete, prone to cause misunderstanding. Luckily, we can keep talking, give a new explanation that corrects those misunderstandings. This of course will lead to new misunderstandings. We then explain again, and so on. It sounds fruitless, but in practice our audience nevertheless gets closer and closer to the truth.

Last week, I tried to explain physicists’ notion of a fundamental particle. In particular, I wanted to explain what these particles aren’t: tiny, indestructible spheres, like Democritus imagined. Instead, I emphasized the idea of fields, interacting and exchanging energy, with particles as just the tip of the field iceberg.

I’ve given this kind of explanation before. And when I do, there are two things people often misunderstand. These correspond to two topics which use very similar language, but talk about different things. So this week, I thought I’d get ahead of the game and correct those misunderstandings.

The first misunderstanding: None of that post was quantum.

If you’ve heard physicists explain quantum mechanics, you’ve probably heard about wave-particle duality. Things we thought were waves, like light, also behave like particles, things we thought were particles, like electrons, also behave like waves.

If that’s on your mind, and you see me say particles don’t exist, maybe you think I mean waves exist instead. Maybe when I say “fields”, you think I’m talking about waves. Maybe you think I’m choosing one side of the duality, saying that waves exist and particles don’t.

To be 100% clear: I am not saying that.

Particles and waves, in quantum physics, are both manifestations of fields. Is your field just at one specific point? Then it’s a particle. Is it spread out, with a fixed wavelength and frequency? Then it’s a wave. These are the two concepts connected by wave-particle duality, where the same object can behave differently depending on what you measure. And both of them, to be clear, come from fields. Neither is the kind of thing Democritus imagined.

The second misunderstanding: This isn’t about on-shell vs. off-shell.

Some of you have seen some more “advanced” science popularization. In particular, you might have listened to Nima Arkani-Hamed, of amplituhedron fame, talk about his perspective on particle physics. Nima thinks we need to reformulate particle physics, as much as possible, “on-shell”. “On-shell” means that particles obey their equations of motion, normally quantum calculations involve “off-shell” particles that violate those equations.

To again be clear: I’m not arguing with Nima here.

Nima (and other people in our field) will sometimes talk about on-shell vs off-shell as if it was about particles vs. fields. Normal physicists will write down a general field, and let it be off-shell, we try to do calculations with particles that are on-shell. But once again, on-shell doesn’t mean Democritus-style. We still don’t know what a fully on-shell picture of physics will look like. Chances are it won’t look like the picture of sloshing, omnipresent fields we started with, at least not exactly. But it won’t bring back indivisible, unchangeable atoms. Those are gone, and we have no reason to bring them back.

The Teaching Heuristic for Non-Empirical Science

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Science is by definition empirical. We discover how the world works not by sitting and thinking, but by going out and observing the world. But sometimes, all the observing we can do can’t possibly answer a question. In those situations, we might need “non-empirical science”.

The blog Slate Star Codex had a series of posts on this topic recently. He hangs out with a crowd that supports the many-worlds interpretation of quantum mechanics: the idea that quantum events are not truly random, but instead that all outcomes happen, the universe metaphorically splitting into different possible worlds. These metaphorical universes can’t be observed, so no empirical test can tell the difference between this and other interpretations of quantum mechanics: if we could ever know the difference, it would have to be for “non-empirical” reasons.

What reasons are those? Slate Star Codex teases out a few possible intuitions. He points out that we reject theories that have “unnecessary” ideas. He imagines a world where chemists believe that mixing an acid and a base also causes a distant star to go supernova, and a creationist world where paleontologists believe fossils are placed by the devil. In both cases, there might be no observable difference between their theories and ours, but because their theories have “extra pieces” (the distant star, the devil), we reject them for non-empirical reasons. Slate Star Codex asks if this supports many-worlds: without the extra assumption that quantum events randomly choose one outcome, isn’t quantum mechanics simpler?

I agree with some of this. Science really does use non-empirical reasoning. Without it, there’s no reason not to treat the world as a black box, a series of experiments with no mechanism behind it. But while we do reject theories with unnecessary ideas, that isn’t our only standard. We also need our theories to teach us about the world.

Ultimately, we trust science because it allows us to do things. If we understand the world, we can interact with it: we can build technology, design new experiments, and propose new theories. With this in mind, we can judge scientific theories by how well they help us do these things. A good scientific theory is one that gives us more power to interact with the world. It can do this by making correct predictions, but it can also do this by explaining things, making it easier for us to reason about them. Beyond empiricism, we can judge science by how well it teaches us.

This gives us an objection to the “supernova theory” of Slate Star Codex’s imagined chemists: it’s much more confusing to teach. To teach chemistry in that world you also have to teach the entire life cycle of stars, a subject that students won’t use in any other part of the course. The creationists’ “devil theory” of paleontology has the same problem: if their theory really makes the right predictions they’d have to teach students everything our paleontologists do: every era of geologic history, every theory of dinosaur evolution, plus an extra course in devil psychology. They end up with a mix that only makes it harder to understand the subject.

Many-worlds may seem simpler than other interpretations of quantum mechanics, but that doesn’t make it more useful, or easier to teach. You still need to teach students how to predict the results of experiments, and those results will still be random. If you teach them many-worlds, you need to add more discussion much earlier on, advanced topics like self-localizing uncertainty and decoherence. You need a quite extensive set of ideas, many of which won’t be used again, to justify rules another interpretation could have introduced much more simply. This would be fine if those ideas made additional predictions, but they don’t: like every interpretation of quantum mechanics, you end up doing the same experiments and building the same technology in the end.

I’m not saying I know many-worlds is false, or that I know another interpretation is true. All I’m saying is that, when physicists criticize many-worlds, they’re not just blindly insisting on empiricism. They’re rejecting many-worlds, in part, because all it does is make their work harder. And that, more than elegance or simplicity, is how we judge theories.

Facts About Our Capabilities Are Facts About the World

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A paper leaked from Google last week claimed that their researchers had achieved “quantum supremacy”, the milestone at which a quantum computer performs a calculation faster than any existing classical computer. Scott Aaronson has a great explainer about this. The upshot is that Google’s computer is much too small to crack all our encryptions (only 53 qubits, the equivalent of bits for quantum computers), but it still appears to be a genuine quantum computer doing a genuine quantum computation that is genuinely not feasible otherwise.

How impressed should we be about this?

On one hand, the practical benefits of a 53-qubit computer are pretty minimal. Scott discusses some applications: you can generate random numbers, distributed in a way that will let others verify that they are truly random, the kind of thing it’s occasionally handy to do in cryptography. Still, by itself this won’t change the world, and compared to the quantum computing hype I can understand if people find this underwhelming.

On the other hand, as Scott says, this falsifies the Extended Church-Turing Thesis! And that sounds pretty impressive, right?

Ok, I’m actually just re-phrasing what I said before. The Extended Church-Turing Thesis proposes that a classical computer (more specifically, a probabilistic Turing machine) can efficiently simulate any reasonable computation. Falsifying it means finding something that a classical computer cannot compute efficiently but another sort of computer (say, a quantum computer) can. If the calculation Google did truly can’t be done efficiently on a classical computer (this is not proven, though experts seem to expect it to be true) then yes, that’s what Google claims to have done.

So we get back to the real question: should we be impressed by quantum supremacy?

Well, should we have been impressed by the Higgs?

The detection of the Higgs boson in 2012 hasn’t led to any new Higgs-based technology. No-one expected it to. It did teach us something about the world: that the Higgs boson exists, and that it has a particular mass. I think most people accept that that’s important: that it’s worth knowing how the world works on a fundamental level.

Google may have detected the first-known violation of the Extended Church-Turing Thesis. This could eventually lead to some revolutionary technology. For now, though, it hasn’t. Instead, it teaches us something about the world.

It may not seem like it, at first. Unlike the Higgs boson, “Extended Church-Turing is false” isn’t a law of physics. Instead, it’s a fact about our capabilities. It’s a statement about the kinds of computers we can and cannot build, about the kinds of algorithms we can and cannot implement, the calculations we can and cannot do.

Facts about our capabilities are still facts about the world. They’re still worth knowing, for the same reasons that facts about the world are still worth knowing. They still give us a clearer picture of how the world works, which tells us in turn what we can and cannot do. According to the leaked paper, Google has taught us a new fact about the world, a deep fact about our capabilities. If that’s true we should be impressed, even without new technology.

Why I Wasn’t Bothered by the “Science” in Avengers: Endgame

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Avengers: Endgame has been out for a while, so I don’t have to worry about spoilers right? Right?

Right?

Anyway, time travel. The spoiler is time travel. They bring back everyone who was eliminated in the previous movie, using time travel.

They also attempt to justify the time travel, using Ant Man-flavored quantum mechanics. This works about as plausibly as you’d expect for a superhero whose shrinking powers not only let him talk to ants, but also go to a “place” called “The Quantum Realm”. Along the way, they manage to throw in splintered references to a half-dozen almost-relevant scientific concepts. It’s the kind of thing that makes some physicists squirm.

And I enjoyed it.

Movies tend to treat time travel in one of two ways. The most reckless, and most common, let their characters rewrite history as they go, like Marty McFly almost erasing himself from existence in Back to the Future. This never makes much sense, and the characters in Avengers: Endgame make fun of it, listing a series of movies that do time travel this way (inexplicably including Wrinkle In Time, which has no time travel at all).

In the other common model, time travel has to happen in self-consistent loops: you can’t change the past, but you can go back and be part of it. This is the model used, for example, in Harry Potter, where Potter is saved by a mysterious spell only to travel back in time and cast it himself. This at least makes logical sense, whether it’s possible physically is an open question.

Avengers: Endgame uses the model of self-consistent loops, but with a twist: if you don’t manage to make your loop self-consistent you instead spawn a parallel universe, doomed to suffer the consequences of your mistakes. This is a rarer setup, but not a unique one, though the only other example I can think of at the moment is Homestuck.

Is there any physics justification for the Avengers: Endgame model? Maybe not. But you can at least guess what they were thinking.

The key clue is a quote from Tony Stark, rattling off a stream of movie-grade scientific gibberish:

“ Quantum fluctuation messes with the Planck scale, which then triggers the Deutsch Proposition. Can we agree on that? ”

From this quote, one can guess not only what scientific results inspired the writers of Avengers: Endgame, but possibly also which Wikipedia entry. David Deutsch is a physicist, and an advocate for the many-worlds interpretation of quantum mechanics. In 1991 he wrote a paper discussing what happens to quantum mechanics in the environment of a wormhole. In it he pointed out that you can make a self-consistent time travel loop, not just in classical physics, but out of a quantum superposition. This offers a weird solution to the classic grandfather paradox of time travel: instead of causing a paradox, you can form a superposition. As Scott Aaronson explains here, “you’re born with probability 1/2, therefore you kill your grandfather with probability 1/2, therefore you’re born with probability 1/2, and so on—everything is consistent.” If you believe in the many-worlds interpretation of quantum mechanics, a time traveler in this picture is traveling between two different branches of the wave-function of the universe: you start out in the branch where you were born, kill your grandfather, and end up in the branch where you weren’t born. This isn’t exactly how Avengers: Endgame handles time travel, but it’s close enough that it seems like a likely explanation.

David Deutsch’s argument uses a wormhole, but how do the Avengers make a wormhole in the first place? There we have less information, just vague references to quantum fluctuations at the Planck scale, the scale at which quantum gravity becomes important. There are a few things they could have had in mind, but one of them might have been physicists Leonard Susskind and Juan Maldacena’s conjecture that quantum entanglement is related to wormholes, a conjecture known as ER=EPR.

Long-time readers of the blog might remember I got annoyed a while back, when Caltech promoted ER=EPR using a different Disney franchise. The key difference here is that Avengers: Endgame isn’t pretending to be educational. Unlike Caltech’s ER=EPR piece, or even the movie Interstellar, Avengers: Endgame isn’t really about physics. It’s a superhero story, one that pairs the occasional scientific term with a character goofily bouncing around from childhood to old age while another character exclaims “you’re supposed to send him through time, not time through him!” The audience isn’t there to learn science, so they won’t come away with any incorrect assumptions.

The a movie like Avengers: Endgame doesn’t teach science, or even advertise it. It does celebrate it though.

That’s why, despite the silly half-correct science, I enjoyed Avengers: Endgame. It’s also why I don’t think it’s inappropriate, as some people do, to classify movies like Star Wars as science fiction. Star Wars and Avengers aren’t really about exploring the consequences of science or technology, they aren’t science fiction in that sense. But they do build off science’s role in the wider culture. They take our world and look at the advances on the horizon, robots and space travel and quantum speculations, and they let their optimism inform their storytelling. That’s not going to be scientifically accurate, and it doesn’t need to be, any more than the comic Abstruse Goose really believes Witten is from Mars. It’s about noticing we live in a scientific world, and having fun with it.

Things I’d Like to Know More About

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This is an accountability post, of sorts.

As a kid, I wanted to know everything. Eventually, I realized this was a little unrealistic. Doomed to know some things and not others, I picked physics as a kind of triage. Other fields I could learn as an outsider: not well enough to compete with the experts, but enough to at least appreciate what they were doing. After watching a few string theory documentaries, I realized this wasn’t the case for physics: if I was going to ever understand what those string theorists were up to, I would have to go to grad school in string theory.

Over time, this goal lost focus. I’ve become a very specialized creature, an “amplitudeologist”. I didn’t have time or energy for my old questions. In an irony that will surprise no-one, a career as a physicist doesn’t leave much time for curiosity about physics.

One of the great things about this blog is how you guys remind me of those old questions, bringing me out of my overspecialized comfort zone. In that spirit, in this post I’m going to list a few things in physics that I really want to understand better. The idea is to make a public commitment: within a year, I want to understand one of these topics at least well enough to write a decent blog post on it.

Wilsonian Quantum Field Theory:

When you first learn quantum field theory as a physicist, you learn how unsightly infinite results get covered up via an ad-hoc-looking process called renormalization. Eventually you learn a more modern perspective, that these infinite results show up because we’re ignorant of the complete theory at high energies. You learn that you can think of theories at a particular scale, and characterize them by what happens when you “zoom” in and out, in an approach codified by the physicist Kenneth Wilson.

While I understand the basics of Wilson’s approach, the courses I took in grad school skipped the deeper implications. This includes the idea of theories that are defined at all energies, “flowing” from an otherwise scale-invariant theory perturbed with extra pieces. Other physicists are much more comfortable thinking in these terms, and the topic is important for quite a few deep questions, including what it means to properly define a theory and where laws of nature “live”. If I’m going to have an informed opinion on any of those topics, I’ll need to go back and learn the Wilsonian approach properly.

Wormholes:

If you’re a fan of science fiction, you probably know that wormholes are the most realistic option for faster-than-light travel, something that is at least allowed by the equations of general relativity. “Most realistic” isn’t the same as “realistic”, though. Opening a wormhole and keeping it stable requires some kind of “exotic matter”, and that matter needs to violate a set of restrictions, called “energy conditions”, that normal matter obeys. Some of these energy conditions are just conjectures, some we even know how to violate, while others are proven to hold for certain types of theories. Some energy conditions don’t rule out wormholes, but instead restrict their usefulness: you can have non-traversable wormholes (basically, two inescapable black holes that happen to meet in the middle), or traversable wormholes where the distance through the wormhole is always longer than the distance outside.

I’ve seen a few talks on this topic, but I’m still confused about the big picture: which conditions have been proven, what assumptions were needed, and what do they all imply? I haven’t found a publicly-accessible account that covers everything. I owe it to myself as a kid, not to mention everyone who’s a kid now, to get a satisfactory answer.

Quantum Foundations:

Quantum Foundations is a field that many physicists think is a waste of time. It deals with the questions that troubled Einstein and Bohr, questions about what quantum mechanics really means, or why the rules of quantum mechanics are the way they are. These tend to be quite philosophical questions, where it’s hard to tell if people are making progress or just arguing in circles.

I’m more optimistic about philosophy than most physicists, at least when it’s pursued with enough analytic rigor. I’d like to at least understand the leading arguments for different interpretations, what the constraints on interpretations are and the main loopholes. That way, if I end up concluding the field is a waste of time at least I’d be making an informed decision.

Book Review: Thirty Years That Shook Physics and Mr Tompkins in Paperback

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George Gamow was one of the “quantum kids” who got their start at the Niels Bohr Institute in the 30’s. He’s probably best known for the Alpher, Bethe, Gamow paper, which managed to combine one of the best sources of evidence we have for the Big Bang with a gratuitous Greek alphabet pun. He was the group jester in a lot of ways: the historians here have archives full of his cartoons and in-jokes.

Naturally, he also did science popularization.

I recently read two of Gamow’s science popularization books, “Mr Tompkins” and “Thirty Years That Shook Physics”. Reading them was a trip back in time, to when people thought about physics in surprisingly different ways.

“Mr. Tompkins” started as a series of articles in Discovery, a popular science magazine. They were published as a book in 1940, with a sequel in 1945 and an update in 1965. Apparently they were quite popular among a certain generation: the edition I’m reading has a foreword by Roger Penrose.

(As an aside: Gamow mentions that the editor of Discovery was C. P. Snow…that C. P. Snow?)

Mr Tompkins himself is a bank clerk who decides on a whim to go to a lecture on relativity. Unable to keep up, he falls asleep, and dreams of a world in which the speed of light is much slower than it is in our world. Bicyclists visibly redshift, and travelers lead much longer lives than those who stay at home. As the book goes on he meets the same professor again and again (eventually marrying his daughter) and sits through frequent lectures on physics, inevitably falling asleep and experiencing it first-hand: jungles where Planck’s constant is so large that tigers appear as probability clouds, micro-universes that expand and collapse in minutes, and electron societies kept strictly monogamous by “Father Paulini”.

The structure definitely feels dated, and not just because these days people don’t often go to physics lectures for fun. Gamow actually includes the full text of the lectures that send Mr Tompkins to sleep, and while they’re not quite boring enough to send the reader to sleep they are written on a higher level than the rest of the text, with more technical terms assumed. In the later additions to the book the “lecture” aspect grows: the last two chapters involve a dream of Dirac explaining antiparticles to a dolphin in basically the same way he would explain them to a human, and a discussion of mesons in a Japanese restaurant where the only fantastical element is a trio of geishas acting out pion exchange.

Some aspects of the physics will also feel strange to a modern audience. Gamow presents quantum mechanics in a way that I don’t think I’ve seen in a modern text: while modern treatments start with uncertainty and think of quantization as a consequence, Gamow starts with the idea that there is a minimum unit of action, and derives uncertainty from that. Some of the rest is simply limited by timing: quarks weren’t fully understood even by the 1965 printing, in 1945 they weren’t even a gleam in a theorist’s eye. Thus Tompkins’ professor says that protons and neutrons are really two states of the same particle and goes on to claim that “in my opinion, it is quite safe to bet your last dollar that the elementary particles of modern physics [electrons, protons/neutrons, and neutrinos] will live up to their name.” Neutrinos also have an amusing status: they hadn’t been detected when the earlier chapters were written, and they come across rather like some people write about dark matter today, as a silly theorist hypothesis that is all-too-conveniently impossible to observe.

“Thirty Years That Shook Physics”, published in 1966, is a more usual sort of popular science book, describing the history of the quantum revolution. While mostly focused on the scientific concepts, Gamow does spend some time on anecdotes about the people involved. If you’ve read much about the time period, you’ll probably recognize many of the anecdotes (for example, the Pauli Principle that a theorist can break experimental equipment just by walking in to the room, or Dirac’s “discovery” of purling), even the ones specific to Gamow have by now been spread far and wide.

Like Mr Tompkins, the level in this book is not particularly uniform. Gamow will spend a paragraph carefully defining an average, and then drop the word “electroscope” as if everyone should know what it is. The historical perspective taught me a few things I perhaps should have already known, but found surprising anyway. (The plum-pudding model was an actual mathematical model, and people calculated its consequences! Muons were originally thought to be mesons!)

Both books are filled with Gamow’s whimsical illustrations, something he was very much known for. Apparently he liked to imitate other art styles as well, which is visible in the portraits of physicists at the front of each chapter.

Pictured: the electromagnetic spectrum as an infinite piano

1966 was late enough that this book doesn’t have the complacency of the earlier chapters in Mr Tompkins: Gamow knew that there were more particles than just electrons, nucleons, and neutrinos. It was still early enough, though, that the new particles were not fully understood. It’s interesting seeing how Gamow reacts to this: his expectation was that physics was on the cusp of another massive change, a new theory built on new fundamental principles. He speculates that there might be a minimum length scale (although oddly enough he didn’t expect it to be related to gravity).

It’s only natural that someone who lived through the dawn of quantum mechanics should expect a similar revolution to follow. Instead, the revolution of the late 60’s and early 70’s was in our understanding: not new laws of nature so much as new comprehension of just how much quantum field theory can actually do. I wonder if the generation who lived through that later revolution left it with the reverse expectation: that the next crisis should be solved in a similar way, that the world is quantum field theory (or close cousins, like string theory) all the way down and our goal should be to understand the capabilities of these theories as well as possible.

The final section of the book is well worth waiting for. In 1932, Gamow directed Bohr’s students in staging a play, the “Blegdamsvej Faust”. A parody of Faust, it features Bohr as god, Pauli as Mephistopheles, and Ehrenfest as the “erring Faust” (Gamow’s pun, not mine) that he tempts to sin with the promise of the neutrino, Gretchen. The piece, translated to English by Gamow’s wife Barbara, is filled with in-jokes on topics as obscure as Bohr’s habitual mistakes when speaking German. It’s gloriously weird and well worth a read. If you’ve ever seen someone do a revival performance, let me know!