Slashdot Mirror
MEMS Researchers Hope To Exploit Casimir Effect
smartalix writes "Researchers at Purdue University's School of Science are examining the Casimir effect (a phenomenon that explains Max Planck's and Werner Heisenberg's quantum vacuum fluctuation theory) and its impact on nanostructures in MEMS devices. At the distances these structures such as gear teeth, actuators, and such) will be operating from one another, the Casimir force may become something to reckon with, potentially forcing a limit to the level of miniaturization possible. The Purdue team is not only confirming Casimir's original theory, it is exploring possible ways to harness the effect in micromachines."
the Casimir effect (a phenomenon that explains Max Planck's and Werner Heisenberg's quantum vacuum fluctuation theory)
:) Quantum theory is full of such unhelpful infinities -- it was working out how to get rid of them ("renormalisation") that won Feynman his Nobel prize.
Whoa there, you've got it all backwards. The Casimir effect is EXPLAINED BY quantum vacuum fluctuations, though the description of the effect in the original article is so bad that I can forgive your misunderstanding.
First, let's get the names right. It was Heisenberg and Schrodinger (not Planck) who came up with the first quantum theory to predict vacuum energy. However the idea of this energy coming from virtual particles (or "spontaneously appearing and disappearing particles and photons" as the article puts it) comes from Dirac's theory of quantum electrodynamics, as perfected by Feynman, Tomonaga and Schwinger. There's no independent "quantum vacuum fluctuation theory".
Second, let's have a closer look at the physics. The article gets the basic idea right: two parallel plates close together are pushed together because there are less virtual particles between the plates than outside them. The detail, though, is wrong - photons do not "pile up" outside the plates. It's much simpler than that. In an (infinite) vacuum, photons can exist with any wavelength. But between two plates, photons can only exist with wavelengths that are simple multiples of the distance between the plates -- just like vibrations on a finite string. (So it's not simply a case of only longer wavelenths being excluded--shorter ones are too, unless they're the right length) Both inside and outside, each permitted wavelength will on average be occupied by the same number of "virtual" photons caused by vacuum fluctuations. Because there are less wavelengths permissible between the plates than outside them, there's overall a greater energy density outside, which translates into a higher pressure.
The more perspicacious reader will have noted that there's an infinite number of possible wavelengths outside, and a (smaller) infinity of permitted wavelengths inside, with the difference between the two being infinite. Since each wavelength carries the same (finite) amount of vacuum energy, doesn't this mean that the energy density of the vacuum is infinite and that the force between the two plates is infinite... Well, yes and no. It depends what you mean by infinity
One interested but little-known point about the Casimir effect is that it's not always attractive -- depending on the geometry of the components involved it can also be repulsive. However working out the result except in the most simple geometries is a VERY difficult problem...
The author of this post asserts his moral rights.
IANAPhysicist. Got that out of the way.
Do you happen to know whether these people measuring the forces factored gravity into their measurements?
I know it is not normally a problem but the plates were very close together and so the force of gravity might be measurable. This has been niggling at me since I first heard about the casmir affect. Can anyone point me to a paper that explains the set up and the calculations.
AFAIK The forces measured are of the correct order of magnitude to fit the Casimir effect. I believe there has been significant error in the experiments but getting the computation exactly right is not easy. But they're way off for gravity.
Doesn't it make you feel good to know that our freedoms are protected by politicans, lawyers and journalists.
virtual photons are the quantum-field-theoretic explanation for the zero-point field. You get the zero-point energy from summing all the Feynman diagrams which have no incoming/outgoing particles - ie those consisting wholly of virtual particles. For the simple Casimir effect between two parallel plates, it's not necessary to invoke them in the calculation -- but that doesn't mean they're not there.
It's true that virtual particles used to be primarily considered to be a mathematical construct to aid calculation (and still are by some), but they're now generally accepted to be an accurate picture of reality. Eg Hawking radiation given off by a black hole is explained via the invocation of virtual particle pairs.
Many cornerstones of modern physics started off as mathematical construct introduced to aid calculations. Quantum mechanics itself is a good example -- Planck didn't believe that energy was quantised, he just found it was a good way of explaining the black-body radiation curve. It was only when Einstein explained the photoelectric effect by assuming that radiation could only exist as quantized photons that it was generally accepted this was the case. In the same year - 1905 - Einstein did the same thing for the constant speed of light and the existence of atoms. Until then neither concept was accepted as more than a convenient explanation for some odd experimental results -- he took them to assume reality, worked through the results, and gave us relativity and the theory of brownian motion.
The author of this post asserts his moral rights.
The only Casimir effect i've heard of nearly lobotomized a whole generation of french kids 25 years ago !
There are some nice write ups on it at everything2. Just take a look there.
-><- no
Doesn't it make you feel good to know that our freedoms are protected by politicans, lawyers and journalists.
You are correct, but I doubt many people will realize it unless you explain it (or they think long and hard). For the non-physics majors out there:
Electrons have mass, and thus move very slowly compared to photons, which don't have mass and thus move at the speed of light. Among their other duties, photons carry the replusive and attractive forces we associate with charged objects such as ellections (as most of us had to memorize at some point, opposites attract and like-charges repel).
So how does this move data? To grossly oversimplify, when an electron moves down a wire (or through a semiconductor, or whatever) it emits a photon that goes rushing on ahead, and eventually encounters an other electron, which (because of the repulsive force of the electron coming towards it) starts moving in the same direction. The process continues all the way down the wire, with almost all of the distance being covered by the travel of photons. Thus we see the signal moving (via the photons) at a significant fraction of the speed of light even though the electrons themselves are poking along much more slowly.
-- MarkusQ
It comes from the fact that in curved spacetime there is no natural choice of time coordinate and so you can't distinguish between positive and negative energy modes meaning that you can't distinguish properly between creation and annihilation operators.
The answer to that's not so clear: it's true the number operator in different (curved) reference frames is different, since the annihilation/creation operators are different. However, saying that has nothing to do with pair-production isn't exactly correct - after all, the zero-point energy, in some cases, generates from normal-ordering (a adagger + adagger a) (in others from symmetry breaking, but I think the same argument applies). You could in fact, define ZPE = integral (adagger a - a adagger), which gives you something like ZPE = integral (N - Ndagger). ZPE density would just be N - Ndagger, and could be interpreted as the number of virtual particle pairs created per unit space per unit time.
Bleh, that's hard to understand without writing things down. Anyway, the point is that if you consider "ZPE" to be "all of the virtual particle pairs in the quantum vacuum", then, for instance, Unruh radiation (and likely Hawking radiation as well) can be explained 'somehow' via virtual particle pairs, as Unruh radiation is just going to be the difference between N in one reference frame and N in another reference frame, hence ZPE in one reference frame, and ZPE in another reference frame. I mention Unruh radiation because it'd be harder to explain 'canonically' than Hawking radiation, though I can somewhat see a way to explain it.
What the actual 'canonical explanation' is, that's a different story. It doesn't tremendously matter, as virtual particle pairs don't appear as two little balls zipping through space at all. It's just a picture for the minds' eye. However, basically saying that the curved spacetime produces a change in particle pair production/destruction rate from one reference frame to another, resulting in a generation of particles, that's perfectly valid, and so, the pair-production argument isn't totally bunk.