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MEMS Researchers Hope To Exploit Casimir Effect

Posted by timothy on from the just-call-it-cas-for-short dept.

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."

25 of 39 comments (clear)

  1. three words: quantum flux capacitor by clowe · · Score: 1


    Well, someone had to say it!

  2. here's how it really works by misterpies · · Score: 5, Informative

    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...

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  3. Fiction by exp(pi*sqrt(163)) · · Score: 1

    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.

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    1. Re:Fiction by misterpies · · Score: 2, Informative

      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.

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    2. Re:Fiction by exp(pi*sqrt(163)) · · Score: 2, 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 between positive and negative energy modes meaning that you can't distinguish properly between creation and annihilation operators.

      Many cornerstones of modern physics started off as mathematical construct introduced to aid calculations
      And if they can't be eliminated from the computation there comes a point where you need to treat them as real. You can compute the Casimir force strength without them so there's no need to invoke them.
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    3. Re:Fiction by barawn · · Score: 2, Insightful

      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.

  4. Re:The Casimir Effect? by loadquo · · Score: 2, Interesting

    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.

  5. Re:The Casimir Effect? by exp(pi*sqrt(163)) · · Score: 2, Informative
    Gravity is absolutely definitely not an issue. Measuring gravity at that level is well beyond present technology. Unless you're suggesting the presence of a deviation from the inverse square law at short distances, something that has been proposed before.

    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.

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  6. No thanks ! by HawkingMattress · · Score: 3, Funny

    The only Casimir effect i've heard of nearly lobotomized a whole generation of french kids 25 years ago !

  7. Don't know what the casimir effect is? by Pope+Raymond+Lama · · Score: 3, Informative

    There are some nice write ups on it at everything2. Just take a look there.

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  8. Correct by MarkusQ · · Score: 4, Informative

    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

    1. Re:Correct by rpresser · · Score: 1

      The photon is the exchange particle for the electromagnetic field. When two electrons approach each other, "virtual" photons carry the field between them.

  9. Re:The Casimir Effect? by loadquo · · Score: 1
    Sorry I should have said I was a bit off topic, I was referring to the macroscale experiments with large sheets of alluminium. I wasn't aware of the methodology used for the nanoscale.

    A cool picture of one of the earlier experiments can be found here.

    I would still like to know the weight of the substrate and the ball to put my mind at rest. For example if the ball weighed 1*10^-20 kg and the plate 1 g then the force would be in the order of 6.6*10-15 N at the distance of 100nM they measured at which is detectable with the AFM referenced in the article. So either the ball is a lot lighter than my uniformed guess or the gravity between a ball and plate is vastly different between two point sources. OR I have mucked up my calculations somewhere.
    Probably all of the above :)
  10. It's just a picture for the minds' eye by exp(pi*sqrt(163)) · · Score: 1
    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.

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    1. Re:It's just a picture for the minds' eye by barawn · · Score: 1

      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. I think I'd rather say something like

      "caused by the disparity in particle/antiparticle generation near the event horizon, due to the formation of distinct regions in a previously smooth particle/antiparticle sea" - worded more eloquently, of course, but I think that's the issue.

      The problem is that the Hawking radiation description actually does contain some real physics there. They're talking about a particle and an antiparticle being generated (some combination of a adagger and adagger a), and one of them being caught in the region of curved spacetime (a adagger/adagger a are different in two different reference frames), resulting in particle generation (because N = adagger a - a adagger is now different).

      One caveat, as well: ghost particles aren't virtual particles. They're ghost particles. A virtual particle is a normal particle which is off-mass-shell - it still has all of its correct quantum numbers, just bad energy. Ghost particles don't have correct spin/statistics relation, and therefore aren't particles at all. If memory serves, ghost particles are never external legs in a complete Feynman diagram, which means they're always virtual ghost particles, but they're not virtual particles. Just a bit of a nitpick. (And I didn't think that there -was- a way to solve NAGTs perturbatively without the use of Fadeev-Popov ghosts).

    2. Re:It's just a picture for the minds' eye by exp(pi*sqrt(163)) · · Score: 1
      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 think that there -was- a way to solve NAGTs perturbatively without...
      Not NAGTs necessarily but there are lots of other models that can be solved in other ways. Just changing variables in your Lagrangian is probably enough to result in a completely different set if Feynman diagrams even though the physics should be unchanged, but don't ask me to construct examples right now!
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  11. Re:What have you been smoking? by psyconaut · · Score: 1

    The electrons are used in a similar manner to those bouncing ball 'executive toys'....you push one of them, a photon shoots out, pushes the next electron if finds, and os on and so forth.

    I'm pretty sure there's actually a name for this bucket brigade type action.....

    -psy

  12. Re:The Casimir Effect? by Theaetetus · · Score: 1
    More specifically, gravity varies according to the inverse square law, whereas the Casimir effect (and the coolest thing about it, IMO) varies in an inverse 4th law! I.e. half the distance between the plates and the attractive force goes up sixteen-fold!

    This lets them easily factor gravity out of the measurements by measuring the differences in attractive force as the distances are changed.

    Incidentally, if 1/x^2 is inverse-square, and 1/x^3 is inverse-cube, is 1/x^4 inverse-hypercube?

    -T

  13. I wonder who the monkeys were... by exp(pi*sqrt(163)) · · Score: 1

    ...who modded that down to -1, Interesting.

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  14. Casimir effect in biological systems by boojum.cat · · Score: 1

    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 lot of papers on the subject.

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  15. Re:The Casimir Effect? by Misanthropic+Lycanth · · Score: 1


    16 times is 4-fold, if I'm not mistaken.
    </nitpick>

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  16. Never mind... by Misanthropic+Lycanth · · Score: 1

    IAAI (I am an idiot)

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  17. Re:The Casimir Effect? by Theaetetus · · Score: 1
    No, it actually has more to do with acoustic theory, and QED. The huge increase with the shortening of distance has to do with what frequencies are rejected as the plates get closer and closer together.

    If the distance is x then the frequencies that will fit in have to have wavelengths of x or even divisors of x (x/2, x/3, x/4, etc.) As x gets smaller, the frequences have to get higher and higher, and fewer of them can fit... and correspondingly, more are hitting against the outside of the plates. Each of them is imparting a force with an x^2 component, and the number of them is increasing at x^2 as the plates get closer, so you end up with a term like x^2^2, or x^4 for the energy increase.

    -T

  18. Re:Perpetual Motion Machines, Zero Point Energy by julesh · · Score: 1

    Err, I don't think Zero Point Energy is heat. It still exists at absolute zero, that's where the name comes from. So I don't suspect thermodynamics applies to it... that is one of the reasons a lot of people have been reluctant to accept that (a) it exists and (b) you can actually achieve a net effect with it [which I guess is just a corrollary to (a)], but you can't really use one theory to disprove another. That's what observation is for...

  19. Re:What have you been smoking? by RetsamYthgimla · · Score: 1

    That's just an over-simplification of the fact that all "everyday" interactions are controlled by photons. When I type, the electrons in the atoms on the surface of my skin push against the electrons in the atoms on the surface of the keys, and this interaction, being electrostatic in nature (with a hint of the exclusion principle thrown in for fun), is controlled by photons. To say that the information is carried in photons is like saying the information in a book is carried by photons. The information on a hard drive is carried by photons. Braille information is carried by photons.

    Get the point. It's useless to speak of information being carried by photons in a passive manner. Now, your television signal being carried on radio waves... There's an active example.

    But in computer chips, the information is carried by the electrons. Sorry, you lose this round.