The Casimir Effect - A Water Wave Analog [video]

Abstract

The Casimir Effect: The demonstration shows that traveling water waves carry momentum. Here is a container of liquid. It is ethyl alcohol with the dye flourescein so that the waves can be seen. Water works, too, but alcohol is better. Two parallel plates are suspended from clamps attached to a support. The container rests on a shaker, which is off right now. The shaker will be driven with noise in a band of frequencies from 10 to 20 Hz, and will excite surface waves. What happens to the plates? This is an analog system of the famous Casimir effect, which was predicted by Casimir in 1948. Surprisingly, two conducting uncharged parallel plates in a vacuum attract each other. The reason is that there are always virtual photons (particles of light) that come into and out of existence, even though no real photons may be present. Virtual photons cannot be directly observed, but they have consequences that can be observed. Because the photons carry momentum, and because there are fewer photons between the plates, there is an imbalance of light pressure which produces an attractive force on the plates. This force is extremely small, and was not accurately confirmed until half a century after its prediction. We can greatly enhance the effect by using real surface waves on a liquid. The idea is the same: There will less wave motion between the plates, so there will be a net inward force on the plates due to the waves reflecting off the outside of the plates. I'm now slowly increasing the drive amplitude. Eventually we see surface waves. Nothing appears to be happening to the plates. However, when the amplitude of the waves is higher, the plates come noticeably a little closer together. This is tricky here because we are pushing the shaker and its amplifier nearly to their limit, where they automatically shut off. I'm now turning off the drive. Note that the plates are slowly moving farther apart to their original locations. A student and a colleague and I have published this effect. A pdf is available on the NPS Lecture Demonstrations website.