# Physics for Dragons

One of our poetry contest winners has been reading his proof copy of How to Teach Physics to Your Dog, and posted some comments. Actually, it was posted last week, but I was at Worldcon, and not dealing with much of anything else.

The comments are pretty positive, but he asks a couple of questions along the way, and I figured I should respond to those here. So, below the fold are Adrian's questions and my responses:

I don't get what's so weird about the Quantum Eraser, other than things that are already weird in simpler double-slit experiments in any case. Once you've accepted that a photon can interfere with itself, are the results of the quantum eraser experiment not simply what you'd expect? Even in classical physics, surely, light only interferes with light that's similarly polarised.

The quantum eraser experiment is used as an example in Chapter 3, which talks about superposition states and the Copenhagen Interpretation (you can watch a video version of the dog dialogue that goes with Chapter 3). The idea of the quantum eraser, for those who aren't familiar with it is that you take a double slit experiment and "tag" the light passing through each slit by putting a horizontal polarizer over one slit and a vertical polarizer over the other. This destroys the interference pattern that you get with a normal double slit. TYou can recover the pattern, though, by putting a polarizer after the slits that is aligned at 45 degrees from vertical, passing half of the light from each slit.

(You can do this yourself with a laser pointer and some cheap polarizing film.)

Now, it's true that, as Adrian notes, this is what you expect in the classical wave model of light: the waves from the two slits are orthogonally polarized, and do not produce a visible interference pattern. What's weird about the quantum eraser is that it does exactly the same thing when you send the light in one photon at a time: a single photon interferes with itself or not depending on the presence or absence of a polarizer after the slits. If you think of light as a particle, this is extremely odd: the particle either decides to take both paths and interfere, or to take only one path and not interfere, depending on something that it doesn't encounter until after it has passed the slits.

That's strange on a few levels, and highlights the fact that photons aren't really particles in the same way that squeaky chew toys are particles. They're a new kind of object, with some particle and some wave properties at the same time. And that's why the quantum eraser is weird.

I've just read the chapter on the Quantum Zeno Effect, and I have some problems with it, primarily with the bit that begins "Here's a simplified version of this quantum interrogation experiment". The experiment described strikes me as too simplified (as in probably a complete fiction). It involves mirrors, but I found it unclear whether it was saying you can actually get a QZ effect using mirrors or whether they're every bit as analogical as the bunny.

The bunny-and-mirrors example in that section is adapted from the bomb-and-mirrors example in this article about quantum interrogation by Paul Kwiat (way down near the end). He also (co-)wrote a Scientific American article (PDF) on the subject.

The answer to the question is yes and no. The experiment with great big mirrors and actual rabbits is impossible for technical reasons, but the effect is real, and something like that system would work just fine. And Kwiat's group has done a number of experiments demonstrating quantum Zeno effects with photons bouncing around in interferometers, as described in those articles.

So, it's an analogy in the sense that the actual experiments use a different technique, but the physics involved is real, and experiments using the same principles have been done.

There are more comments in Adrian's post, but those are the questions raised. If you'd like more information, check out DogPhysics.com, or look at the draft version of chapter 1 that I posted a while back, which will get you the basic idea if not the exact text.

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the particle either decides

I really, really hate that terminology. I've wasted several hours listening to profs try and explain that even though they use the word decide, nothing/no one is actually deciding while people in the lecture hall keep failing to get it.

Unfortunately, I don't have an alternative.

I tend to find the double-slit experiment and analogues easier to understand when I think of things in terms of a 4-dimensional spacetime instead of a 3-dimensional space through which particles move over time.

This way, the particle isn't "deciding" which path to take based on information it shouldn't have at the time (as horribly anthropomorphized as that sounds), it's more like the existence of the post-slit polarizer modifies the 4D spacetime in a way that "blocks" the photon from "having taken" only one slit.

To me, the quantum eraser, (along with Bell's theorem), is a pretty convincing argument that our standard notions of unidirectional causality break down on very short time scales.

Thanks for the response - I need more time to read the linked articles about quantum interrogation, but as for the quantum eraser, the bit that still puzzles me is this:

"the particle either decides to take both paths and interfere, or to take only one path and not interfere"

I would say, the particle either takes both paths and interferes, or the particle takes both paths and doesn't interfere. Either way, it takes both paths. The only difference is that in the latter case we can't tell that it took both paths because (as we'd expect) there's no interference pattern. Am I missing something?

I'm currently reading the teleportation chapter. I think I get the gist of the chapter on Bell's theorem, though I'm definitely going to nut out the details by scribbling on a piece of paper after I finished the book. (It helps to think of a polaroid filter as a really, really militant missionary: it tries to convert photons, and any photons it can't convert, it kills.)