A coral reef looks a bit blurry to a scuba diver, in part because of light bouncing off of the water’s continual density fluctuations, which come from random motions of the warm water molecules. But some of the blurring comes from a constant quantum jiggling of molecules that occurs even close to absolute zero temperature. This so-called zero point motion–which is connected with the uncertainty principle–was thought to be too small to be noticed. But a new calculation, described in the 23 January Physical Review Letters, shows that it could account for several percent of the light scattering in cold liquids, making it detectable with current technology.
Quantum objects like atoms can never be completely pinned down–there is always some uncertainty in their positions, as German physicist Werner Heisenberg famously described in the uncertainty principle. At room temperature this effect is small compared with the ordinary thermal motion of all molecules, but at low temperatures, zero point motion has more influence. For example, it prevents liquid helium from freezing except at very high pressures.
The energy that arises from quantum motion of the molecules is somewhat like the vacuum energy (or “zero point energy”) that arises from virtual photons popping in and out of existence in empty space. Both of these quantum effects lead to a so-called Casimir attraction between two parallel plates, because there are more virtual particles outside the plates than inside. But the “acoustic Casimir” effect based on density fluctuations is far weaker and hasn’t yet been seen in the lab, despite some recent experimental proposals [1, 2].
However, Lawrence Ford of Tufts University in Massachusetts and Nami Svaiter of the Brazilian Center for Physics Research in Rio de Janeiro thought there might be a more direct way to see zero point motion in liquids. Using quantum field theory, they derived the probability for light to be deflected by density fluctuations associated with zero point motion in a fluid. Their results show that zero-point scattering increases as the fifth power of the light frequency. In contrast, classical scattering by thermal fluctuations (a phenomenon responsible for the sky’s blue color) goes up as the fourth power. The researchers calculated the amount of inelastic scattering–light coming in at one frequency and going away at another–for ultraviolet light (wavelength of 350 nanometers) in room temperature water. They were surprised that the zero point motion accounted for roughly half a percent of the total for this type of scattering. “It may seem like a small number, but for a quantum mechanical effect, we think this is quite impressive,” Ford says.
The relative contribution is even bigger at low temperatures where the thermal fluctuations become smaller. The researchers compared a handful of fluids and found that the biggest effect might come from liquid neon at around 25 degrees Kelvin. Here, the scattering off zero point fluctuations should be 13 percent of inelastic thermal scattering. Ford thinks an experiment could isolate the quantum signal by varying the temperature as well as the frequency.
Kimball Milton of the University of Oklahoma in Norman thinks the proposed scattering experiment is interesting because it will focus on the local effects of density fluctuations, rather than their global Casimir-like effects. But it won’t be easy, says Steve Lamoreaux of Yale University. “To discriminate this from the thermal effect appears difficult to me, but in principle it should be possible,” says Lamoreaux. Milton agrees but is optimistic: “This will generate great interest among experimentalists, and someone will think of a clever way to access this effect.”
- D. C. Roberts and Y. Pomeau, “Casimir-Like Force Arising from Quantum Fluctuations in a Slowly Moving Dilute Bose-Einstein Condensate,” Phys. Rev. Lett. 95, 145303 (2005).
- A. Recati, J. N. Fuchs, C. S. Peça, and W. Zwerger, “Casimir Forces between Defects in One-Dimensional Quantum Liquids,” Phys. Rev. A 72, 023616 (2005).