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MEMS Researchers Hope To Exploit Casimir Effect 39

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."
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MEMS Researchers Hope To Exploit Casimir Effect

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  • Well, someone had to say it!
  • Did anyone else go back throught the story description and read it like John Moshitta used to do in the Micro Machines commercials?

  • by misterpies ( 632880 ) on Wednesday August 27, 2003 @05:25PM (#6808492)
    the Casimir effect (a phenomenon that explains Max Planck's and Werner Heisenberg's quantum vacuum fluctuation theory)

    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 :) 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.

    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...
    • This talk of "virtual" photons is pretty fictitious. The Casimir effect is computed from the zero-point field ie. the lowest energy state for each wavelength. Ie. it's computed assuming no real photons. But I don't see where "virtual" photons come in either. You don't need to do any Feynman diagram computations to get the result. "Virtual" photons are just labels given to edges in Feynman graphs. No perturbation, no need to talk of "virtual" photons.
      • Re:Fiction (Score:3, Informative)

        by misterpies ( 632880 )
        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 calcul
        • Re:Fiction (Score:3, Interesting)

          Eg Hawking radiation given off by a black hole is explained via the invocation of virtual particle pairs.

          No it isn't. Sounds like you're competent to do the calculations yourself. Pick up a book or paper on Hawking radiation. No need to use that nonsense about pair-production with one half falling past the event horizon. That's just a story made up for the popular science press. It comes from the fact that in curved spacetime there is no natural choice of time coordinate and so you can't distinguish betw

          • Re:Fiction (Score:3, Insightful)

            by barawn ( 25691 )
            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 correc
            • Yes, that's exactly what it is. A pretty powerful picture too. Unfortunately that's never said in popular science writing and so people take it literally, get very confused, and are liable to consider the whole thing to be voodoo.

              It's worth noting that many problems can be solved perturbatively in different ways leading to completely different sets of Feynman diagrams and hence different virtual particles. A good example is the use of ghost particles in gauge theory.

              • Argh, Slashdot just lost my reply. Grr.

                Anyway, the whole situation isn't THAT different than the whole explanation for quark confinement (compressed flux tubes, and all that), the explanation for particle production by an inflating universe (relaxation of lowest-energy modes), or even Feynman diagrams as "particle graphs" - none of them are entirely correct, but they're all 'suggestive', and are easy to understand intuitively. I will agree that the Hawking radiation example is a little too specific for me.
                • caused by the disparity in particle/antiparticle generation

                  I don't see it that way at all. But...

                  ...this discussion reminds me of a letter I once saw in an electronics magazine. The writer was convinced that Fourier theory was all hogwash because something like a single square pulse wasn't really an infinite sum of sinusoidal waves. When I use 'is' in "the electromagnetic force is mediated by exchange of virtual photons" I'm using it the way I use 'is' in "a pulse is a sum of sine waves".

                  And I didn't

  • No thanks ! (Score:3, Funny)

    by HawkingMattress ( 588824 ) on Wednesday August 27, 2003 @06:41PM (#6809042)
    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 [].
  • Interestingly, the Casimir effect also occurs in biological systems. Proteins embedded in a lipid membrane restrict the fluctuations of the membrane and therefore decrease the entropy in the region between the proteins. Since thermal systems evolve toward maximum entropy, the proteins move toward each other. Here the fluctations are thermal instead of quantum mechanical, and the medium is the membrane instead of the vacuum, but the principle is the same.

    If you google for "casimir membrane" you'll find a

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