Monthly Archives: February 2013

Why a Quantum Field Theorist is the wrong person to ask about Quantum Mechanics

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Quantum Mechanics is quite possibly the sexiest, most mysterious thing to come out of 20th century physics. Almost a century of evidence has confirmed that the world is fundamentally ambiguous and yet deeply predictable, that physics is best described probabilistically, and that however alien this seems the world wouldn’t work without it. Quantum Mechanics raises deep philosophical questions about the nature of reality, some of the most interesting of which are still unanswered to this day.

And I am (for the moment, at least) not the best person to ask about these questions. Because while I specialize in Quantum Field Theory, that actually means I pay very little attention to the paradoxes of Quantum Mechanics.

It all boils down to the way calculations in quantum field theory work. As I described in a previous post, quantum field theory involves adding up progressively more complicated Feynman Diagrams. There are methods that don’t involve Feynman Diagrams, but in one way or another they work on the same basic principle: to take quantum mechanics into account, add up all possible outcomes, either literally or through shortcuts.

That may sound profound, but in many ways it’s quite mundane. Yes, you’re adding up all possibilities, but each possibility is essentially a mundane possibility. There are a few caveats, but essentially each element you add in, each Feynman Diagram for example, looks roughly like the sort of thing you could get without quantum mechanics.

In a typical quantum field theory calculation, you don’t see the mysterious parts of quantum mechanics: you don’t see entanglement, or measurements collapsing the wavefunction, and you don’t have to think about whether reality is really real. Because of that, I’m not the best person to ask about quantum paradoxes, as I’ve got little more than an undergraduate’s knowledge of these things.

There are people whose work focuses much more on quantum paradoxes. Generally these people focus on systems closer to everyday experiments, atoms rather than more fundamental particles. Because the experimentalists they cooperate with have much more ability to manipulate the systems they study, they are able to probe much more intricate quantum properties. People interested in the possibility of a quantum computer are often at the forefront of this, so if you’ve got a question about a quantum paradox, don’t ask me, ask people like WLOG blog.

A final note: there are many people (often very experienced and elite researchers) who, though they might primarily be described as quantum field theorists, have weighed in on the subject of quantum paradoxes. If you’ve heard of the black hole firewall debate, that is a recent high-profile example of this. The important thing to remember is that these people are masters of many areas of physics. They have taken the time to study the foundations of quantum mechanics, and have broadened their horizons to the tools more commonly used in other subfields. So while your average grad student quantum field theorist won’t know an awful lot about quantum paradoxes, these guys do.

Valentine’s Day Physics Poem

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In honor of Valentine’s Day, a physics-themed poem I wrote a few years ago, about unrequited love.

Measurement:

 

I once took a measurement

It was a simple, two-body problem,

Solvable. Not Poisson’s mess.

Two particles, drifting, perhaps entangled.

I wanted to know two things:

Position, and momentum:

Where they were, and where they might go.

 

I perturbed the system

Like a good scientist, I interacted, and observed,

Added input, caused change.

Then I knew their positions.

They became tightly entangled,

Bound together,

And there was no way of knowing

Any way they could change.

 

I should have remembered:

In quantum systems

The observer is always involved;

And a three-body problem

Has no solution.

Black Holes and a Superluminal River of Glass

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If I told you that scientists have been able to make black holes in their labs for years, you probably either wouldn’t believe me, or would suddenly get exceptionally paranoid. Turns out it’s true, provided you understand a little bit about black holes.

A black hole is, at its most basic, an object that light cannot escape. That’s why it’s “black”: it absorbs all colors of light. That’s really, deep down, all you need in order to have a black hole.

Black holes out in space, as you are likely aware, are the result of collapsed stars. Gather enough mass into a small enough space and, according to general relativity, space and time begin to bend. Bend space and time enough and the paths that light would follow curve in on themselves, until inside the event horizon (the “point of no return”) the only way light can go is down, into the center of the black hole.

That’s not the only way to get a “point of no return” though. Imagine flying a glider above a fast-moving river. If the plane is slower than the river, then any object placed in the river is like a “point of no return”:  once the object passes you, you can never fly back and find it again.

Of course, trying to apply this to light runs into a difficulty: you can have a river faster than a plane, but it’s pretty hard to have a river faster than light. You might even say it’s impossible: nothing can travel faster than light, after all, right?

The idea that nothing can travel faster than light is actually a common misconception, held because it makes a better buzzword than the truth: nothing can travel faster than light in a vacuum. Light in a vacuum goes straight to its target, the fastest thing in the universe. But light in a substance, moving through air or water or glass, gets deflected: it runs into atoms, gets absorbed, gets released, and overall moves slower. So in order to make a black hole, all we need is some substance moving faster than light moves in that substance: a superluminal river of glass.

(By the way, is that not an amazingly evocative phrase? Sounds like the title of a Gibson novel.)

Now it turns out that literally making glass move faster than light moves inside it is still well beyond modern science. But scientists can get around that. Instead of making the glass move, they  make the properties of the glass change, using lasers to alter the glass so that the altered area moves faster than the light around it. With this sort of setup, they can test all sorts of theoretical black hole properties up close, in the comfort of a university basement.

That’s just one example of how to create an artificial black hole. There are several others, and all of them rely on various ingenious manipulations of the properties of matter. You live in a world in which artificial black holes are routine and diverse. Inspiring, no?

Wormholes and Donut Holes

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I’ve heard people claim that in order to understand wormholes, you need to understand five-dimensional space.

Well that’s just silly.

A wormhole is often described as a hole in space-time. It can be imagined as a black hole where instead of getting crushed when you fall in to the center, you emerge somewhere else (or even some-when else: wormholes are possibly the only way to get time travel). They’re a staple of science fiction, even if they aren’t always portrayed accurately.

Probably not what a wormhole looks like

How does this work? Well like many things in physics, it’s helpful to imagine it with fewer dimensions first:

Suppose that you live on the surface of a donut. You can’t get up off the surface; you’re stuck to its gooey sugary coating. All you can do is slide around it.

It’s a simple life

Let’s say that one day you’re sitting on the pink side of the donut, near the center. Your friend lives on the non-frosted side, and you want to go see her. You could go all the way back to the outside edge of the donut, around the side, and down to the bottom, but you’re tired and the frosting is sticky. Luckily, you can use your futuristic pastry technology, the donut hole! Instead of going around the outside, you dive in through the inside hole, getting to your friend’s house much faster.

That’s really all a wormhole is. Instead of living on a two-dimensional donut surface, you live in a world with three space dimensions and one time dimension. A wormhole is still just like a donut hole: a shortcut, made possible by space being a non-obvious shape.

Now earlier I said that you don’t need to understand five-dimensional space to understand wormholes, and that’s true. Yes, real donuts exist in three dimensions…but if you live on the surface only, you only see two: inward versus outward, and around the center. It’s like a 2D video game with a limited map: the world looks flat, but if you go up past the top edge you find yourself on the bottom. Going from the top edge directly to the bottom is easier than going all the way down the screen: it’s just the same as a wormhole. You don’t need extra dimensions to have wormholes, just rules: when you go up far enough, you come back down. Go to the center of the wormhole, and come out the other side. And as one finds in physics, it’s the rules, not naïve intuitions, that determine how the world works. Just like a video game.