I Do Not Think That Means What You Think It Means

A couple of physics stories in the last few days have caught my attention for reasons that can be lumped together under the Vizzini Effect– that is, they say things that involve unconventional uses of common words. Take, for example, the Physics World story Physicists distinguish between the indistinguishable, which starts off:

Spurred on by their work on building one of the world’s most accurate atomic clocks from strontium-87 atoms, researchers in the US have now discovered that “forbidden” collisions can occur between these atoms.

Strontium-87 atoms belong to a class of objects known as fermions and according to quantum physics; fermions cannot occupy the same energy state and location in space at the same time. Fermions in identical energy states are therefore not meant to collide. Collisions perturb the internal energy levels of the atoms, and therefore such fermions should have very stable energy levels.

This is up my alley, having studied such collisions as a grad student. A violation of the prohibition against spin-polarized collisions in fermions would be big news.

The explanation, when it comes, is nothing so dramatic:

Gretchen Campbell of JILA explains that at the beginning of the measurement, all of the atoms are in the same atomic state. However, during the transition from the ground state to the excited state, using a laser pulse, light-atom interactions are not uniform across the entire atomic sample. This means that different atoms are excited at slightly different rates. These atoms are therefore no longer identical.

There’s nothing here that goes against known phyisics– in fact, what they measured is exactly what I saw in my thesis, namely, that if your state preparation isn’t perfect, you can have collisions between atoms because they’re not in exactly the same state. Each atom is in a superposition of two states, and the part of one atom that is in one state can collide with the part of another atom that is in the other state.

(It’s also slightly misleading to say that the atoms are “no longer identical.” In fact, they are still identical particles, and still subject to the requirements of Fermi statistics, that they be in an overall antisymmetric wavefunction. The effect is somewhat similar to what you would get if the particles were distinguishable, so this is a common shorthand used in the field, but there are some subtle but important differences if you want to get technical.)

The other article is from the Arxiv Blog, Physicists propose new kind of quantum tunneling, subtitled “Quantum tunnelling of a new, third kind could finally put string theory to the test.” This jumped out at me because I wasn’t aware that there were two kinds of tunneling, let alone a third type.

When I refer to “quantum tunneling,” I mean the process whereby a quantum particle– an electron, say– headed toward a barrier passes through the barrier as if it weren’t there, appearing on the other side with no loss of energy. So what’s the second type? It turns out to be a completely different process:

In recent years, physicists have explored the possibility of a second type of tunneling which happens in an entirely different way. This relies on the idea that a quantum particle can change into another quantum particle and back again with a certain probability. The tunnelling occurs when the particle changes from one that interacts strongly with a barrier and so cannot pass though it, into a particle that does not interact with the barrier and so passes through with ease.

For example, quantum particles that change briefly into particles of dark matter should pass through barriers that should otherwise be impassable. This kind of “shining-a-light-through-a-wall” experiment is a serious contender in the search for dark matter.

This is superficially similar to the normal process of tunneling, in that both involve particles showing up on the far side of a barrier, but the rest of the process is completely different. They’re different enough, in fact, that I’m not sure there’s any benefit to giving them such similar names. And the “third type” mentioned in the post seems to be a straightforward variant of the “second,” with the incident particle turning into a pair of exotic particles, rather than just one.

This strikes me as fundamentally the same sort of thing as Max Tegmark’s numbering of “multiverses,” in which he lumps together a bunch of unrelated phenomena into a large meta-category. I don’t think there’s any significant gain to lumping these things together, and there’s a significant chance of introducing confusion about what’s really going on.

I should note that I’m not complaining about the physics content, here, just the presentation. In fact, the strontium experiments described in the first story sound way cool, and my complaints about the phrasing should not be taken as questioning Jun Ye or Gretchen Campbell, both of whom are scary smart. I haven’t read their paper (it’s paywalled), but I’d be shocked if it had any physics mistakes.

This is also not intended as a look-at-how-worthless-journalists-are post (something I’ve complained about other people doing). In fact, the Physics World piece is about as good a write-up as you can get of such an experiment. My complaints are highly technical in nature, and wouldn’t occur to a lot of other physicists, let alone lay readers.