illuminating science

So what inspired yesterday’s post on wave-particle duality (recommended reading before this one ) was a post on Physics Buzz about “wave/particle duality with a droplet made of trillions of molecules”! Recall that the best double slit experiment looking for wave-particle duality clocks in at about 100 atoms, and while experiments have demonstrated various other quantum phenomena at large scales, I think trillions of molecules would be a record for any type of experiment.

Intrigued, I read the actual article (which requires a subscription to read more than the abstract). The experiment describes an oil droplet that’s bouncing on a pool of oil. When you vibrate everything at the right speed, you create a “walker” - the waves created by the bouncing droplet actually propel the droplet forward. Damp the waves, and the drop stops moving; get rid of the droplet, and the waves vanish. It’s a symbiotic relationship

What’s neat is that if you fire off this “walker” (droplet+wave) towards a pair of double slits, you get a sort of interference effect, much like you do in quantum mechanics. The wave part of the walker goes through both slits, interferes with itself, and the various reflected waves eventually push the droplet into one direction or another. The actual droplet only goes through one slit, but if you repeat the experiment many times and look at what directions the droplet goes after going through the slits, you see the same pattern as from QM interference.

The article points out this connection, and notes that it’s quite similar to the pilot wave interpretation of quantum mechanics, where the wave function is actually a separate wave that accompanies a particle and guides it to its destination. But the physics is completely different here - although there’s connection with the mathematics, there’s no fundamental relationship between this oil drop experiment and quantum mechanics.

I thought it was a fascinating experiment, and it was an interesting connection to the quantum mechanics formulae. However, the Physics Buzz post was quite misleading, mainly because of terminology. Although wave/particle duality to a non-physicist probably means just that (something which has both a wave and a particle component), inside physics it has very definite meaning - it refers to the quantum mechanical phenomena. So to say the experimenters “demonstrated wave/particle duality with a droplet” is a quite misleading statement! If you’re not a physicist, this probably sounds a bit pedantic, but the reality is that physics (like any other profession - think of law, or medicine!) assigns very special meaning to common words - “measurement”, “field”, “interference”, all these words have a lot of meaning besides the dictionary definition, just the same way that the language used in a legal contract is very important. Use the wrong language, and the meaning is skewed or lost. At best, there’s a lot of head scratching. At worst, you end up with a movie like What the Bleep.

All that said, Physics Buzz has some good stuff, and is worth a read.