Tag Archives: physics

The Way You Think Everything Is Connected Isn’t the Way Everything Is Connected

I hear it from older people, mostly.

“Oh, I know about quantum physics, it’s about how everything is connected!”

“String theory: that’s the one that says everything is connected, right?”

“Carl Sagan said we are all stardust. So really, everything is connected.”

connect_four

It makes Connect Four a lot easier anyway

I always cringe a little when I hear this. There’s a misunderstanding here, but it’s not a nice clean one I can clear up in a few sentences. It’s a bunch of interconnected misunderstandings, mixing some real science with a lot of confusion.

To get it out of the way first, no, string theory is not about how “everything is connected”. String theory describes the world in terms of strings, yes, but don’t picture those strings as links connecting distant places: string theory’s proposed strings are very, very short, much smaller than the scales we can investigate with today’s experiments. The reason they’re thought to be strings isn’t because they connect distant things, it’s because it lets them wiggle (counteracting some troublesome wiggles in quantum gravity) and wind (curling up in six extra dimensions in a multitude of ways, giving us what looks like a lot of different particles).

(Also, for technical readers: yes, strings also connect branes, but that’s not the sort of connection these people are talking about.)

What about quantum mechanics?

Here’s where it gets trickier. In quantum mechanics, there’s a phenomenon called entanglement. Entanglement really does connect things in different places…for a very specific definition of “connect”. And there’s a real (but complicated) sense in which these connections end up connecting everything, which you can read about here. There’s even speculation that these sorts of “connections” in some sense give rise to space and time.

You really have to be careful here, though. These are connections of a very specific sort. Specifically, they’re the sort that you can’t do anything through.

Connect two cans with a length of string, and you can send messages between them. Connect two particles with entanglement, though, and you can’t send messages between them…at least not any faster than between two non-entangled particles. Even in a quantum world, physics still respects locality: the principle that you can only affect the world where you are, and that any changes you make can’t travel faster than the speed of light. Ansibles, science-fiction devices that communicate faster than light, can’t actually exist according to our current knowledge.

What kind of connection is entanglement, then? That’s a bit tricky to describe in a short post. One way to think about entanglement is as a connection of logic.

Imagine someone takes a coin and cuts it along the rim into a heads half and a tails half. They put the two halves in two envelopes, and randomly give you one. You don’t know whether you have heads or tails…but you know that if you open your envelope and it shows heads, the other envelope must have tails.

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Unless they’re a spy. Then it could contain something else.

Entanglement starts out with connections like that. Instead of a coin, take a particle that isn’t spinning and “split” it into two particles spinning in different directions, “spin up” and “spin down”. Like the coin, the two particles are “logically connected”: you know if one of them is “spin up” the other is “spin down”.

What makes a quantum coin different from a classical coin is that there’s no way to figure out the result in advance. If you watch carefully, you can see which coin gets put in to which envelope, but no matter how carefully you look you can’t predict which particle will be spin up and which will be spin down. There’s no “hidden information” in the quantum case, nowhere nearby you can look to figure it out.

That makes the connection seem a lot weirder than a regular logical connection. It also has slightly different implications, weirdness in how it interacts with the rest of quantum mechanics, things you can exploit in various ways. But none of those ways, none of those connections, allow you to change the world faster than the speed of light. In a way, they’re connecting things in the same sense that “we are all stardust” is connecting things: tied together by logic and cause.

So as long as this is all you mean by “everything is connected” then sure, everything is connected. But often, people seem to mean something else.

Sometimes, they mean something explicitly mystical. They’re people who believe in dowsing rods and astrology, in sympathetic magic, rituals you can do in one place to affect another. There is no support for any of this in physics. Nothing in quantum mechanics, in string theory, or in big bang cosmology has any support for altering the world with the power of your mind alone, or the stars influencing your day to day life. That’s just not the sort of connection we’re talking about.

Sometimes, “everything is connected” means something a bit more loose, the idea that someone’s desires guide their fate, that you could “know” something happened to your kids the instant it happens from miles away. This has the same problem, though, in that it’s imagining connections that let you act faster than light, where people play a special role. And once again, these just aren’t that sort of connection.

Sometimes, finally, it’s entirely poetic. “Everything is connected” might just mean a sense of awe at the deep physics in mundane matter, or a feeling that everyone in the world should get along. That’s fine: if you find inspiration in physics then I’m glad it brings you happiness. But poetry is personal, so don’t expect others to find the same inspiration. Your “everyone is connected” might not be someone else’s.

Movie Review: The Truth is in the Stars

Recently, Perimeter aired a showing of The Truth is in the Stars, a documentary about the influence of Star Trek on science and culture, with a panel discussion afterwards. The documentary follows William Shatner as he wanders around the world interviewing scientists and film industry people about how Star Trek inspired them. Along the way he learns a bit about physics, and collects questions to ask Steven Hawking at the end.

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I’ll start with the good: the piece is cute. They managed to capture some fun interactions with the interviewees, there are good (if occasionally silly) visuals, and the whole thing seems fairly well edited. If you’re looking for an hour of Star Trek nostalgia and platitudes about physics, this is the documentary for you.

That said, it doesn’t go much beyond cute, and it dances between topics in a way that felt unsatisfying.

The piece has a heavy focus on Shatner, especially early on, beginning with a clumsily shoehorned-in visit to his ranch to hear his thoughts on horses. For a while, the interviews are all about him: his jokes, his awkward questions, his worries about getting old. He has a habit of asking the scientists he talks to whether “everything is connected”, which to the scientists’ credit is usually met by a deft change of subject. All of this fades somewhat as the movie progresses, though: whether by a trick of editing, or because after talking to so many scientists he begins to pick up some humility.

(Incidentally, I really ought to have a blog post debunking the whole “everything is connected” thing. The tricky part is that it involves so many different misunderstandings, from confusion around entanglement to the role of strings to “we are all star-stuff” that it’s hard to be comprehensive.)

Most of the scientific discussions are quite superficial, to the point that they’re more likely to confuse inexperienced viewers than to tell them something new (especially the people who hinted at dark energy-based technology…no, just no). While I don’t expect a documentary like this to cover the science in-depth, trying to touch on so many topics in this short a time mostly just fuels the “everything is connected” misunderstanding. One surprising element of the science coverage was the choice to have both Michio Kaku giving a passionate description of string theory and Neil Turok bluntly calling string theory “a mess”. While giving the public “both sides” like that isn’t unusual in other contexts, for some reason most science documentaries I’ve seen take one side or the other.

Of course, the point of the documentary isn’t really to teach science, it’s to show how Star Trek influenced science. Here too, though, the piece was disappointing. Most of the scientists interviewed could tell their usual story about the power of science fiction in their childhood, but didn’t have much to say about Star Trek specifically. It was the actors and producers who had the most to say about Star Trek, from Ben Stiller showing off his Gorn mask to Seth MacFarlane admiring the design of the Enterprise. The best of these was probably Whoopi Goldberg’s story of being inspired by Uhura, which has been covered better elsewhere (and might have been better as Mae Jemison’s similar story, which would at least have involved an astronaut rather than another actor). I did enjoy Neil deGrasse Tyson’s explanation of how as a kid he thought everything on Star Trek was plausible…except for the automatic doors.

Shatner’s meeting with Hawking is the finale, and is the documentary’s strongest section. Shatner is humbled, even devout, in Hawking’s presence, while Hawking seems to show genuine joy swapping jokes with Captain Kirk.

Overall, the piece felt more than a little disjointed. It’s not really about the science, but it didn’t have enough content to be really about Star Trek either. If it was “about” anything, it was Shatner’s journey: an aging actor getting to hang out and chat with interesting people around the world. If that sounds fun, you should watch it: but don’t expect much deeper than that.

Shades of Translation

I was playing Codenames with some friends, a game about giving one-word clues to multi-word answers. I wanted to hint at “undertaker” and “march”, so I figured I’d do “funeral march”. Since that’s two words, I needed one word that meant something similar. I went with dirge, then immediately regretted it as my teammates spent the better part of two minutes trying to figure out what it meant. In the end they went with “slug”.

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A dirge in its natural habitat.

If I had gone for requiem instead, we would have won. Heck, if I had just used “funeral”, we would have had a fighting chance. I had assumed my team knew the same words I did: they were also native English speakers, also nerds, etc. But the words they knew were still a shade different from the words I knew, and that made the difference.

When communicating science, you have to adapt to your audience. Knowing this, it’s still tempting to go for a shortcut. You list a few possible audiences, like “physicists”, or “children”, and then just make a standard explanation for each. This works pretty well…until it doesn’t, and your audience assumes a “dirge” is a type of slug.

In reality, each audience is different. Rather than just memorizing “translations” for a few specific groups, you need to pay attention to the shades of understanding in between.

On Wednesdays, Perimeter holds an Interdisciplinary Lunch. They cover a table with brown paper (for writing on) and impose one rule: you can’t sit next to someone in the same field.

This week, I sat next to an older fellow I hadn’t met before. He asked me what I did, and I gave my “standard physicist explanation”. This tends to be pretty heavy on jargon: while I don’t go too deep into my sub-field’s lingo, I don’t want to risk “talking down” to a physicist I don’t know. The end result is that I have to notice those “shades” of understanding as I go, hoping to get enough questions to change course if I need to.

Then I asked him what he did, and he patiently walked me through it. His explanation was more gradual: less worried about talking down to me, he was able to build up the background around his work, and the history of who worked on what. It was a bit humbling, to see the sort of honed explanation a person can build after telling variations on the same story for years.

In the end, we both had to adapt to what the other understood, to change course when our story wasn’t getting through. Neither of us could stick with the “standard physicist explanation” all the way to the end. Both of us had to shift from one shade to another, improving our translation.

What Makes Light Move?

Light always moves at the speed of light.

It’s not alone in this: anything that lacks mass moves at the speed of light. Gluons, if they weren’t constantly interacting with each other, would move at the speed of light. Neutrinos, back when we thought they were massless, were thought to move at the speed of light. Gravitational waves, and by extension gravitons, move at the speed of light.

This is, on the face of it, a weird thing to say. If I say a jet moves at the speed of sound, I don’t mean that it always moves at the speed of sound. Find it in its hangar and hopefully it won’t be moving at all.

And so, people occasionally ask me, why can’t we find light in its hangar? Why does light never stand still? What makes light move?

(For the record, you can make light “stand still” in a material, but that’s because the material is absorbing and reflecting it, so it’s not the “same” light traveling through. Compare the speed of a wave of hands in a stadium versus the speed you could run past the seats.)

This is surprisingly tricky to explain without math. Some people point out that if you want to see light at rest you need to speed up to catch it, but you can’t accelerate enough unless you too are massless. This probably sounds a bit circular. Some people talk about how, from light’s perspective, no time passes at all. This is true, but it seems to confuse more than it helps. Some people say that light is “made of energy”, but I don’t like that metaphor. Nothing is “made of energy”, nor is anything “made of mass” either. Mass and energy are properties things can have.

I do like game metaphors though. So, imagine that each particle (including photons, particles of light) is a character in an RPG.

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For bonus points, play Light in an RPG.

You can think of energy as the particle’s “character points”. When the particle builds its character it gets a number of points determined by its energy. It can spend those points increasing its “stats”: mass and momentum, via the lesser-known big brother of E=mc^2, E^2=p^2c^2+m^2c^4.

Maybe the particle chooses to play something heavy, like a Higgs boson. Then they spend a lot of points on mass, and don’t have as much to spend on momentum. If they picked something lighter, like an electron, they’d have more to spend, so they could go faster. And if they spent nothing at all on mass, like light does, they could use all of their energy “points” boosting their speed.

Now, it turns out that these “energy points” don’t boost speed one for one, which is why low-energy light isn’t any slower than high-energy light. Instead, speed is determined by the ratio between energy and momentum. When they’re proportional to each other, when E^2=p^2c^2, then a particle is moving at the speed of light.

(Why this is is trickier to explain. You’ll have to trust me or wikipedia that the math works out.)

Some of you may be happy with this explanation, but others will accuse me of passing the buck. Ok, a photon with any energy will move at the speed of light. But why do photons have any energy at all? And even if they must move at the speed of light, what determines which direction?

Here I think part of the problem is an old physics metaphor, probably dating back to Newton, of a pool table.

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A pool table is a decent metaphor for classical physics. You have moving objects following predictable paths, colliding off each other and the walls of the table.

Where people go wrong is in projecting this metaphor back to the beginning of the game. At the beginning of a game of pool, the balls are at rest, racked in the center. Then one of them is hit with the pool cue, and they’re set into motion.

In physics, we don’t tend to have such neat and tidy starting conditions. In particular, things don’t have to start at rest before something whacks them into motion.

A photon’s “start” might come from an unstable Higgs boson produced by the LHC. The Higgs decays, and turns into two photons. Since energy is conserved, these two each must have half of the energy of the original Higgs, including the energy that was “spent” on its mass. This process is quantum mechanical, and with no preferred direction the photons will emerge in a random one.

Photons in the LHC may seem like an artificial example, but in general whenever light is produced it’s due to particles interacting, and conservation of energy and momentum will send the light off in one direction or another.

(For the experts, there is of course the possibility of very low energy soft photons, but that’s a story for another day.)

Not even the beginning of the universe resembles that racked set of billiard balls. The question of what “initial conditions” make sense for the whole universe is a tricky one, but there isn’t a way to set it up where you start with light at rest. It’s not just that it’s not the default option: it isn’t even an available option.

Light moves at the speed of light, no matter what. That isn’t because light started at rest, and something pushed it. It’s because light has energy, and a particle has to spend its “character points” on something.

 

arXiv vs. snarXiv: Can You Tell the Difference?

Have you ever played arXiv vs snarXiv?

arXiv is a preprint repository: it’s where we physicists put our papers before they’re published to journals.

snarXiv is…well..sound it out.

A creation of David Simmons-Duffin, snarXiv randomly generates titles and abstracts out of trendy arXiv buzzwords. It’s designed so that the papers on it look almost plausible…until you take a closer look, anyway.

Hence the game, arXiv vs snarXiv. Given just the titles of two papers, can you figure out which one is real, and which is fake?

I played arXiv vs snarXiv for a bit today, waiting for some code to run. Out of twenty questions, I only got two wrong.

Sometimes, it was fairly clear which paper was fake because snarXiv overreached. By trying to pile on too many buzzwords, it ended up with a title that repeated itself, or didn’t quite work grammatically.

Other times, I had to use some actual physics knowledge. Usually, this meant noticing when a title tied together unrelated areas in an implausible way. When a title claims to tie obscure mathematical concepts from string theory to a concrete problem in astronomy, it’s pretty clearly snarXiv talking.

The toughest questions, including the ones I got wrong, were when snarXiv went for something subtle. For short enough titles, the telltale signs of snarXiv were suppressed. There just weren’t enough buzzwords for a mistake to show up. I’m not sure there’s a way to distinguish titles like that, even for people in the relevant sub-field.

How well do you do at arXiv vs snarXiv? Any tips?

Jury-Rigging: The Many Uses of Dropbox

I’ll be behind the Great Firewall of China next week, so I’ve been thinking about various sites I won’t be able to access. Prominent among them is Dropbox, a service that hosts files online.

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A helpful box to drop things in

What do physicists do with Dropbox? Quite a lot.

For us, Dropbox is a great way to keep collaborations on the same page. By sharing a Dropbox folder, we can share research programs, mathematical expressions, and paper drafts. It makes it a lot easier to keep one consistent version of a document between different people, and it’s a lot simpler than emailing files back and forth.

All that said, Dropbox has its drawbacks. You still need to be careful not to have two people editing the same thing at the same time, lest one overwrite the other’s work. You’ve got the choice between editing in place, making everyone else receive notifications whenever the files change, or editing in a separate folder, and having to be careful to keep it coordinated with the shared one.

Programmers will know there are cleaner solutions to these problems. GitHub is designed to share code, and you can work together on a paper with ShareLaTeX. So why do we use Dropbox?

Sometimes, it’s more important for a tool to be easy and universal, even if it doesn’t do everything you want. GitHub and ShareLaTeX might solve some of the problems we have with Dropbox, but they introduce extra work too. Because no one disadvantage of Dropbox takes up too much time, it’s simpler to stick with it than to introduce a variety of new services to fill the same role.

This is the source of a lot of jury-rigging in science. Our projects aren’t often big enough to justify more professional approaches: usually, something hacked together out of what’s available really is the best choice.

For one, it’s why I use wordpress. WordPress.com is not a great platform for professional blogging: it doesn’t give you a lot of control without charging, and surprise updates can make using it confusing. However, it takes a lot less effort than switching to something more professional, and for the moment at least I’m not really in a position that justifies the extra work.

Thought Experiments, Minus the Thought

My second-favorite Newton fact is that, despite inventing calculus, he refused to use it for his most famous work of physics, the Principia. Instead, he used geometrical proofs, tweaked to smuggle in calculus without admitting it.

Essentially, these proofs were thought experiments. Newton would start with a standard geometry argument, one that would have been acceptable to mathematicians centuries earlier. Then, he’d imagine taking it further, pushing a line or angle to some infinite point. He’d argue that, if the proof worked for every finite choice, then it should work in the infinite limit as well.

These thought experiments let Newton argue on the basis of something that looked more rigorous than calculus. However, they also held science back. At the time, only a few people in the world could understand what Newton was doing. It was only later, when Newton’s laws were reformulated in calculus terms, that a wider group of researchers could start doing serious physics.

What changed? If Newton could describe his physics with geometrical thought experiments, why couldn’t everyone else?

The trouble with thought experiments is that they require careful setup, setup that has to be thought through for each new thought experiment. Calculus took Newton’s geometrical thought experiments, and took out the need for thought: the setup was automatically a part of calculus, and each new researcher could build on their predecessors without having to set everything up again.

This sort of thing happens a lot in science. An example from my field is the scattering matrix, or S-matrix.

The S-matrix, deep down, is a thought experiment. Take some particles, and put them infinitely far away from each other, off in the infinite past. Then, let them approach, close enough to collide. If they do, new particles can form, and these new particles will travel out again, infinite far away in the infinite future. The S-matrix then is a metaphorical matrix that tells you, for each possible set of incoming particles, what the probability is to get each possible set of outgoing particles.

In a real collider, the particles don’t come from infinitely far away, and they don’t travel infinitely far before they’re stopped. But the distances are long enough, compared to the sizes relevant for particle physics, that the S-matrix is the right idea for the job.

Like calculus, the S-matrix is a thought experiment minus the thought. When we want to calculate the probability of particles scattering, we don’t need to set up the whole thought experiment all over again. Instead, we can start by calculating, and over time we’ve gotten very good at it.

In general, sub-fields in physics can be divided into those that have found their S-matrices, their thought experiments minus thought, and those that have not. When a topic has to rely on thought experiments, progress is much slower: people argue over the details of each setup, and it’s difficult to build something that can last. It’s only when a field turns the corner, removing the thought from its thought experiments, that people can start making real collaborative progress.