What Counts as "Quantum Physics?"

In comments to yesterday's post about precision measurements, Bjoern objected to the use of "quantum mechanics" as a term encompassing QED:

IMO, one should say "quantum theory" here instead of "quantum mechanics". After all, what is usually known as quantum mechanics (the stuff one learns in basic courses) is essentially the quantization of classical mechanics, whereas QED is the quantization of classical electrodynamics, and quantum field theories in general are quantizations of classical field theories. I think saying "quantum mechanics" when one talks about something which essentially has nothing to do with mechanics is quite misleading. "Quantum theory", on the other hand, is simply a generic term which encompasses all the quantum "things" mentioned above.

This is a somewhat idiosyncratic usage, as I pointed out in a comment of my own, but it's a reasonable argument, and I'll try to be a little more careful about using "quantum mechanics" and "quantum physics" as interchangeable terms.

I wouldn't do a full post on this, except that later in the day, I was reading a popular-audience book on quantum physics that I was sent to review, and found myself getting annoyed at the way all the examples were drawn from particle and nuclear physics. I found myself saying "This is nuclear physics, not quantum physics." Which, on reflection, is kind of a strange thing to say-- the Standard Model is unquestionably an outgrowth of quantum mechanics-- but the fact is, when I think "quantum physics," I don't think of particle physics.

When I think of "quantum physics" in the abstract, I think of low-energy phenomena: quantum optics, quantum computing, atomic physics. This is largely because this is my training, and as they say, you write what you know-- I suspect that people who write pop-science books drawing all their examples from nuclear and particle physics do so because they were trained as nuclear or particle physicists.

(For a variety of reasons-- some statistical, some structural-- particle and nuclear physicists seem to write most of the pop-science books in physics. It's to the point where unless I know an author's background is in some other field, I just assume they're a particle physicist. As do the reviewers who have referred to me as a particle physicist when talking about How to Teach Physics to Your Dog...)

I do think there's a sense in which it's better not to go directly to the Standard Model when talking about quantum physics, though, which is that you want to limit the number of unfamiliar elements you throw at the reader. If your first example of a quantum superposition involves neutrino oscillation, you're making the lay reader work way too hard-- they need to know that there are particles called neutrinos, that they come in three flavors, that they have different masses, and that the difference in mass makes them oscillate between the different flavors, and thus a randomly chosen neutrino exists as a quantum superposition of all three flavors at once. That's a lot of unfamiliar physics to get across in one example.

And the fact is, you don't need all of that to get the idea of a superposition state across. There are beautiful, clean demonstrations of superposition states using more familiar objects: the double-slit experiment with electrons, for example, or a variety of "Schrödinger cat" experiments done with trapped ions. Those don't require you to juggle unfamiliar particle types along with the weird phenomena.

That's one of the things I tried to do with How to Teach Physics to Your Dog-- the vast majority of the examples in that are low-energy phenomena using atoms and molecules and photons. Mostly because I was trained in the AMO field, and those are the systems and examples I'm most familiar with, but also because I think they require a bit less work for the reader. Pretty much everybody knows that matter is made of atoms, so it's not hard for them to accept them as a component of a demonstration of something else. Using something bizarre to demonstrate a bizarre phenomenon, though, makes it harder to get the key idea across.

So, if you ask me to talk about "quantum physics," I'm not going to start off with quarks and leptons and the Higgs boson, and I'm always a little surprised when somebody does. Even though you couldn't possibly have the Higgs without quantum physics.

(I also think that the issues you get at with low-energy quantum experiments-- things like quantum measurement and the transition from microscopic quantum phenomena to macroscopic classical ones, and quantum non-locality, and quantum information processing-- are at least as important as particle physics and quantum gravity. To say nothing of being orders of magnitude cheaper. But that's a rant for a different post...)


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As someone who is in physics but not AMO, particle, or nuclear, I definitely agree that it makes more sense to use examples from AMO to explain basic quantum concepts to non-experts, including undergraduate physics majors. There are many cases where you can draw a straight line from material covered in an undergraduate QM text (or a popularization) to some basic property of atoms or molecules. Particle physics depends to a large extent on material students don't see until grad school.

By Eric Lund (not verified) on 06 May 2011 #permalink

When I was a graduate student a million years ago, the term quantum mechanics referred to non-relativistic domains while the term quantum field theory referred to relativistic domains (e.g. quantum electrodynamics).

As someone who went through physics undergrad and grad school within the past 10 years, I have to agree with SLC. Quantum Mechanics was always talked about as being the non-relativistic.

In class, you will definitely want to differentiate so as to keep things as clear as possible for the students.

It's also a good idea when you are writing for other scientists.

The general public, on the other hand, will probably mix the terms up.

In a book meant for the general public, you could afford to devote a chapter to the differences and set up your preferred terminology.

But in a lecture to the general public, I say just go with your preferred terminology. If someone asks during the Q&A why you use a certain terminology, then answer the question. Otherwise you're probably OK not to dwell on it while you are making other points (again, to avoid confusion and overwhelm for the listeners).