Saturday, September 17, 2005

Zero point energy and the Casimir effect

The all too human hope of getting "something for nothing" unavoidably affects inventors, engineers, and physicists as much as anyone else. Hence the perennial popularity of the futile quest for "perpetual motion". Akin to this, but not quite so hopeless, is the pursuit of limitless energy in the form of "zero point energy" to avert the world's looming energy crisis.

Quantum mechanics suggests that ZPE must be real for a simple reason. The uncertainty principle implies that the kinetic energy of a particle can never be precisely determined, and in particular it cannot be precisely zero. So every particle must have some nonzero kinetic energy, however small. Furthermore, there is no such thing as an absolute vacuum, since "virtual particles" can also come into existence for very short but nonzero periods of time. And so even a "perfect vacuum" can contain energy, which is called the zero point energy.

However, strangely, there is still no experimental evidence that ZPE -- or "energy of the vacuum" is actually "real". It is often supposed that ZPE accounts for the cosmological constant, also known as dark energy. Although there is now good evidence that the cosmological constant is nonzero, it's only a guess that it has something to do with ZPE.

Indeed, it hasn't been possible to actually calculate a value for ZPE. Naive calculations predict a value that is as much as a factor of 10120 larger than what it should be if it is responsible for the estimated value of the cosmological constant. Still, various arguments for the existence of ZPE are so good that few physicists actually doubt it.

Assuming that ZPE is real, many physicists, engineers, and would-be inventors have invested countless hours, in a bid for undying fame, hoping to find a way to capture ZPE in some economically useful way. Here is one of the lastest in this tradition:

Magnetic energy? Perhaps
The nation's energy industry is struggling to recover from Hurricane Katrina. Gas prices are soaring as a result of the catastrophic storm. America's reliance on overseas oil increases every year.

And from his office in the North Bay city of Sebastopol, Mark Goldes envisions a day -- perhaps not so far off -- when none of this will be a problem.

Goldes, 73, is chief executive of a small company called Magnetic Power Inc., which has spent years researching ways to, yes, generate power using magnets. ...

What Goldes believes he's done is produce power from what physicists call zero-point energy. In simple terms, zero-point energy results from the infinitesimal motion of molecules even when seemingly at rest.
Unfortunately, as already noted, there is as yet no experimental evidence that ZPE actually exists. Interestingly enough, most physicists think there is such evidence, in the form of a phenomenon known as the Casimir effect. In a nutshell, the effect is a very small but measurable force between two very flat metal plates that are very close together. Here's one of numerous references from a usually reliable source, Physics World: The Casimir effect: a force from nothing.

But just this year, in March, R. L. Jaffe came out with a paper demonstrating that the Casimir effect can be explained without ZPE:

The Casimir Effect and the Quantum Vacuum
In discussions of the cosmological constant, the Casimir effect is often invoked as decisive evidence that the zero point energies of quantum fields are "real''. On the contrary, Casimir effects can be formulated and Casimir forces can be computed without reference to zero point energies. They are relativistic, quantum forces between charges and currents.
Note that Jaffe isn't claiming that the Casimir effect isn't a result of ZPE. Instead, he's just claiming a way to derive the Casimir effect from "the forces between charged particles in the metal plates." In this view, "The Casimir force is simply the (relativistic, retarded) van der Waals force between the metal plates."

Is this nothing but an overly fastidious academic quibble? We should be careful about supposing that. If Jaffe is right, we still lack any experimental evidence for ZPE, however right it seems theoretically -- it's still a challenge to experimental physicists. And ever if ZPE is real, how it may relate to dark energy, if at all, is very mysterious.


I'm indebted to Phil Gossett for bringing the Jaffe paper to my attention in postings to a private mailing list.


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