Sunday, October 15, 2006

Wave/Particle Duality Living Large

Wave/particle duality is usually a quantum phenomenon confined to photons, electrons, protons, and other ultra-tiny objects.

Quantum mechanics shows that such objects sometimes behave like particles, sometimes behave like waves, and sometimes like a little of both. All objects exhibit wave/particle duality to some extent, but the larger the object the harder it is to observe. Even individual molecules are often too large to show the quantum mechanical behavior.

Now physicists at the Université de Paris have demonstrated a classical version of wave/particle duality with a droplet made of trillions of molecules.

The experiment involved an oil droplet bouncing on the surface of layer of oil that was vibrated vertically. (Check out some pretty pictures of a bouncing oil droplet from a previous, related experiment by the same researchers in a press release put out by the French National Center for Experimental Science last year.) The droplet created waves on the surface, which in turn affected the motion of the droplet. As a result, the droplet and waves formed a single entity that consisted of a hybrid of wave-like and particle-like characteristics.

When the wave/droplet bounced its way through a slit, the waves allowed it to interfere with its own motion, much as a single photon can interfere with itself via quantum mechanics. Although the wave/droplet is clearly a denizen of the classical world, the experiment provides a clever analog of quantum weirdness at a scale that is much easier to study and visualize than is typical of many true quantum experiments.

A paper describing the research, Single-Particle Diffraction and Interference at a Macroscopic Scale, was published in the October 13, 2006 issue of Physical Review Letters.


  1. Actually, that's a pretty misleading post - particularly above the cut! If you check out the original article, the drop+surface syste produces effects which *look* quite similar to the wave/particle duality effects. Because of the interaction with the surface, we get effects propagating through both slits resulting in interference, but there's no QM here.

    In fact, as the article states, it's most equivalent to the "pilot wave" theory of QM, which as I understand it is no longer considered correct. Although your post does say the drop is classical in one paragraph, you explcitly say "demonstrated wave/particle duality with a droplet", which is explicitly wrong :)

    I'm also hesitant to push the analogy argument - while this does reproduce those effects, it has no physics in common with a QM double slit experiment, and so you're not going to learn much fundamental physics from this!

    Of course, it's fascinating nevertheless!

  2. I disagree. It demonstrates wave/particle duality because the particle and the wave form a combined thing that is neither simply a particle nor a wave.

    You are absolutely correct that it is not at all quantum mechanical. But the really cool thing here is that it shows that there is a classical way to make a wave/particle system.

    That is, QM leads to wave/particle duality but wave/particle duality does not require QM.

    That's the cool thing, IMHO. I could try to clarify it in the post.

    Thanks for the comment, though. I can see that I need to work on this one.

  3. That's a good point, but wave/particle duality is now effectively jargon that refers specifically to quantum mechanical phenomena. At least as we understand it, a single entity (electron, photon, etc) is really both a particle and a wave, in some intrinsic sense.

    In this case, however, the wave is just vibrations in the medium produced by the driving force - the connections between this and QM are strictly mathematical. I could easily argue that a tennis ball being pushed by a wind on a pond is a wave/particle system, just one where the dynamics don't correspond to QM.

    I guess I totally agree that it's a cool experiment and a neat connection, but perhaps somewhat misleading to read too much into it. :)

  4. I think you are begin very uncharitable. The wave is created by the oil droplet, so the wave is an intrinsic property of the droplet. The size of the droplet effect the properties of the wave by changing the period of oscillation of the droplet, etc.