Slashdot Mirror
The Casimir Effect
HobbySpacer writes "A recent article in Physics World provides a lucid description of the the Casimir effect, which is an attractive force between two surfaces caused by electromagnetic fluctuations in the vacuum. The article discusses some practical application such as the nanotech machines mentioned here earlier."
A while ago, I read Neal Stephenson's The Big U . The protagonist (well, one of them) was named Casimir, and, if I remember correctly, he built a rail gun.
;-)
Did Stephenson have a special reason for naming his protagonist after this strange force? Was it a metaphor for how he was mysteriously attracted to his female friend? Or am I just trying too hard to explain an author's choice of names?
all objects exert gravity.
The Casimir effect is something beyond that.
There's this other thing called a Casimir, when you have a Hamiltonian on an odd-dimensional space, you're guaranteed (by anti-symmetry) to have a null direction for the flow at any point, and this is called the "Casimir" for the flow. Does anyone know if this is the same thing?
Come on, give it up, that's
In simplistic terms this works because photons of certain wavelengths are excluded from the space between the plates. This doesn't happen on the outer faces of the plates, and the difference in the vacuum energy inside versus outside leads to an "attracting" force.
It only works on uncharged plates.
Physicists are able to take into account all the forces interacting between two objects. It is exactly this precise accounting that led Casimir to investigate the experimental results that were contrary to their predictions.
Gravity is proportonal 1/d^2, while the Casimir force is proportional 1/d^4. Therefore, the Casimir force is much stronger at smaller distances, but practically non-existant at larger distances. As you halve the distance between two objects, the gravity increases by 4 times, but the Casimir force increases by 16 times.
The other force they mentioned is the Van der Waals force, which is really an electric force caused by the polarization of atoms and molecules at very small distances.
At the scale they were dealing with, the mass of the objects was so small and the distances so short that the Van der Waals force and the Casimir force was much much greater (>>) than the force of gravity.
The radical sect of Islam would either see you dead or "reverted" to Islam.
I'm pretty sure that the Casimir force was originally "discovered" as a natural consequence of quantum field theory. It was only recently (something like two years ago) that it was first measured by an experiment at Los Alamos. It was very exciting when it was measured because this aspect of quantum field theory predicts that the vacuum of space contains an infinite background energy and that the Casimir force is actually due to the charged plates restricting the wavelenghts of photons which could be created and anhillated by the vacuum. By imposing this boundary condition, the pressure from these photons on the inner surfaces of the plates is less that the pressure on the outer surfaces of the plates and thus the force pushing the plates together.
The forces between two plates in a non-vacuum are difficult to predict, at best. At that scale, the laws of thermodynamics begin to become inapplicable.
However, you can use the ideal gas equation to get an idea of what is happening:
PV = nRT
P is the Pressure, V is the Volume, n the number of molecules of gas, R the Rhydberg constant, and T the temperature. If you hold the temperature and the number of molecules constant and then decrease the volume (the case you mentioned above, where a single hydrogen atom has gotten in between the plates), the pressure will begin to grow linearly.
Because you are moving the plates together, the volume reduces in proportion to the distance between the plates. This means the pressure rises in proportion to the distance between the plates. I'll also make an assumption (a dangerous thing) that the temperature will remain constant because we'll give it time to cool off, and the number of particles remain the same. The last assumption is due to the plates being much larger than the distance between them, so only a very few number of particles on the edges will ever get to escape.
The force on the plates due to pressure is only the area of the plates times the pressure. As the pressure rises, the force rises proportionally.
Because the plates are attracted by the Casimir effect, and the casimir force grows quadratically as the plates draw nearer, eventually, even the pressure will be insignificant compared to the Casimir force. I think that's a pretty good conclusion using rough estimates.
In reality, you can't control the number of particles in the system unless you build some sort of box to hold stuff in between the plates. Also, the laws of thermodynamics depend on there being a large number (millions) of particles, so you can't use them with nay degree of reliability in these kind of situations. My conclusion is suspect, because it is really only an educational guess.
It is much easier to make a vacuum, thus making the problem a lot easier.
The radical sect of Islam would either see you dead or "reverted" to Islam.
Yes. That is exactly what is happening. However gathering energy from such a source is going to be almost impossible. Just like thermodynamics breaks down on a very small level, so does quantum mechanics. You get to a point where nothing can be predicted, things are being created and destroyed, conservation of energy is violated constantly, and all the laws of physics are turned upside down and become meaningless traffic laws. But if you scoot back and look at the larger picture, you'll see that there is a pattern to this madness. You see how a photon was created and destroyed, but how it really was "moving" from one localized area to the next, and it really was a "real" photon, transferring energy from one spot to the next. It has a life beyond the small localized areas. While at this level, things aren't as predictable as you hope (which hole did the photon travel through?), it is still predictable to a large degree. Then you scoot back some more, and the probablities begin to add up, and things begin to become predictable with high accuracy. The laser beam doesn't change course, the people around you don't vanish and reappear, and you can be sure that there is going to be a sunrise tomorrow. The point is, even in thermodynamics, you can get violations of energy conservation -- or so it seems. The problem is that you were only looking too close at what was happening, and by looking to close, things don't make "sense" anymore. Things are truly random. However, the rolling of those millions and billions of dice end up with predictable results. Gathering energy from these statistical anomolies is going to be as possible as predicting with high accuracy the roll of a die. Until you can sit on the throne of Einstein's God who doesn't roll dice, it will be impossible.
The radical sect of Islam would either see you dead or "reverted" to Islam.
Maybe something based on this force could help out with this.
It would seem to me that both surfaces would want to become the leg on the T on the other (if both sides of the leg are mirrored). Would the absence of protons cause collapse into the shape of a V on the connected (or close to connected) side? I would think not since the 90 degree would be broken as soon as the force was observed, I'm assuming it would not be the only force because if so wouldn't momentum eventually make the plates paralell and the effect would complete the movement. Sounds like nanites will look like modern art or sports cars, composed of paralellagrams and curves.
Perhaps structures could use several atom mirrors like velcro in construction? Does the force disappear when the distance is 0?
Novel theory: Modern Man evolved from psychopath
hmm... is "importances" a word?
at any rate... I think one of the "very important" aspect of this has to do with multi (>3) dimentional physics. I can't remember where I read it, but physicists are theorizing that at millimeter (or less) distances, we will see gravity suddenly becomes disobedient of the inverse square law (which would prove the "we live in half dozen dimentions) theory. Now -- measuring gravity on a millimetre scale is hard enough, but when you throw in all these "fluctuation forces" (Van der Waal, Casmir, whatever), you will seriously screw up any chance you have on the Nobel prize... so people are trying to get all these things sorted out and verify some gravity, etc.
My life in the land of the rising sun.
You're absolutely right. It's a good thing our atmosphere is only made up of macroscopic objects.
~Idarubicin
I dunno whether it was because the author of the article doesnt understand physics and such, or was talking for the layperson, but a vacuum has no energy, no matter, no vacuum fluctuations, etc.
Your opening sentence is incorrect, and that makes hash of the rest of your post. You seem to be discussing classical vacuum, and in fact seem to be stuck on classical physics in general. For instace, a complete absense of everything would not have a temperature of zero, it would have no temperature.
I'm not a big physicist myself, but you are criticizing things that you don't even have the faintest conception of. Vacuum fluctations result from virtual particles, which is a concept that some view of is necessary to rationalize certain other quantum effects that occur in particle interaction. You need to learn more.
I tell ya, man, this quatum physics stuff is almost as hard to comprehand as women.
Table-ized A.I.
Confusion would be an excellent description. What in Heisenberg's name are you driving at here? The London force or dipole dispersion force varies as 1/r^6, where r is the separation radius.
The Casimir effect between two parallel uncharged plates in a vacuum at zero kelvin is given by:
pi^2 h-bar c
F = ----------- A
240 r^4
A = area of the plates and r is the separation distance.
The Van der Waals interaction can be considered and computed as a semi-classical electrostatic effect. The Casimir effect, although sometimes referred to as a "long-range Van der Waals effect" is fundamentally a quantum concept as shown by the appearance of h-bar in the equation above.
As you indicate, both forces follow a power law, but the Casimir effect will appear, again, between "neutral, conducting" plates at absolute zero, i.e. in a configuration where electrostatic forces due to charge distribution are not present, and the power relationship differs by an order of magnitude.
So the two forces are fundamentally different conceptually and similar in that they both describe attractive forces between objects. However the latter similarity would also allow you to say that casimir == ferromagnetism if that is the only metric.
Well I guess so, but of course it wouldn't really be zero, just very very close. Atomic forces would insure that.
They weren't really clear on how a perpendicular casimir force works either. Does it twist like a right hand rule(the balls and plate), does fold at an angle, or is it held in place. The story said the force was observed, but what movement?
It's possible even likely I misunderstood, I was jsut wondering if anyone else gets it.
Novel theory: Modern Man evolved from psychopath
Imagine a "vacuum" with two metallic (reflecting) plates in it, sitting near each other. The vacuum isn't pure. Photons could exist in it, momentarily, as governed by Heisenberg's Uncertainty Principle, wherein the energy of the photon is inversely proportional to the time it exists. Gamma rays would exist for a short period of time, radio waves much longer. Eventually, the Universe checks its books and corrects the accounting error, the photons go back to non-existence - conservation of mass-energy is upheld.
Here's where it gets weird. The photons kinda-sorta existed (virtually), so they kinda sorta could exert an influence. Whoa. Strange.
Next, only certain frequencies are allowed. The frequencies allowed to "kinda sorta exist" are modal, that is, they have to terminate with a node on one of the plates. So, clearly, you can't have gigantic radio waves between these two plates - radio waves are meters in length, they're too big to fit between the plates. You can have some blue photons, and all the gammas you can handle.
Meanwhile, on the OTHER side of the plate, you get all of the radio waves you want - you have an entire universe to stuff them in! And the blue photons, and all the gammas you can handle. So ... there's just a few more potential electromagnetic waves (virtual photons) on the OUTSIDES of the plates than there are on the INSIDE of the two plates - this leads to a net push of the plates together.
But that's not all - the force experienced by those two plates depends on a lot of things. In the symposium in question, it was demonstrated that, with the right geometry (concentric shells, weird flower-like arrangements) that the Casimir effect can be repulsive.
In short, it isn't always 1/r^4, and it isn't always attractive.
Now, for those of you who would like a free lunch out of this effect, it's not going to happen. Why? 'cause you have to push the plates back apart to complete a full cycle for any "free energy" machine you would like to devise. It's like a waterfall - you only get that energy from something falling down ONCE.
No, it doesn't have anything to do with the Higgs boson. No, it doesn't come from some folded-up dimension. No, it doesn't have to do anything with gravity at small ranges. C.E. results entirely from QM + EM.
On an aside, correcting other bits of non-information in these posts: no, photons are not influenced by charge. No, photons do not constantly form electron-positron pairs - most photons do not have enough energy to form an electron-positron pair - do the math if you don't believe me. The refractivity of materials comes from something entirely different. Photons don't have a non-zero rest mass - they never "rest" (discussion of the Bose-Einstein condensate usually ignores that the information about the photon was stored) - and many experiments have placed upper limits on the proper mass of a photon as being no greater than 10^-50 grams.
I am far more well versed in the field than you would be had you dedicated your college career to it.
Uh-huh. I'm counting at least 20 points scored on the crackpot index, ten from that sentence alone. Unusually low score, perhaps, but in the absense of substance that's all I've got to go on.
The humourous part is that I'd lay money your theories, assuming you actually HAVE any as opposed to merely disagreeing with thousands of very smart people, have no coherent explanation for the casimir effect or other similar phenomena.
My response to the whole italisized bullshit paragraph is that once again, you make it clear that you do not have much understanding of any physics that I've ever seen, and persist in mixing classical and quantum physics without regards for the very carefully defined boundaries placed on those definitions. Your attacks would have much more substance if you weren't attacking strawman physics. (Basically, nobody claims the classical definition of vacuum is anything but an approximation useful in certain circumstances anymore.)
I won't be replying any further, so please take the crackpot's great joy of having the last word and milk it for all it's worth.
Just wondering, but are you perhaps a proponent of electromagnetic theories of cosmology and somewhat disappointed in the standard model?
------ The only greater hazard to your liberty than n politicians is n+1 politicians.
Look at the time stamps on the replies on the same level as mine. Several people had already nicely explained the situation. I chose to vent my spleen about the that either eleven-year-olds are posting on slashdot or somebody had been the victim of a lobotomized science education.
I'm aware that the Casimir effect can appear as a repulsive force, J. Ambjørn and Stephen Wolfram wrote a detailed paper that can be found at:
c le s/physics/83-properties1/
6 .p df
;-) So stating a bald equivalence between these two well known forces is a little misleading.
;)
http://www.stephenwolfram.com/publications/arti
Originally in Annals of Physics 147 (1983). I also came across an interesting paper in the June Physical Review Letters, by P. Bruno at the Max Planck Institute, available at:
http://www.mpi-halle.mpg.de/~bruno/publis/2002_
on calculating a magnetic Casimir effect for parallel ferromagnetic plates, which shows the resultant effect as antiferromagnetic.
As classical electromagnetism derives from an underlying quantum formulation, I concede that that Van Der Waals interactions are essentially classical manifestations of an underlying quantum explanation. However the "classical" vacuum parallel plate Casimir effect is one of those quantum manifestations, like superfluidity, that would not be expected or predicted from "common sense" physics. Van Der Waals forces, calculated without recourse to quantum effects, serve to explain all manner of chemical interactions and phase transitions, and, apparently, the adhesion of geckos to walls
(Note: I am not a physicist, I only play one on Slashdot
I was wondering. As an archaeologist, I usually only pay attention to physics as entertainment. I have a friend however, who is a physicist, who has frequently expressed dissatisfaction with the standard model and what he refers as the "epicycle" problem of adding complexity in order to continue ironing out the contradictions that keep intruding. He recommended a book on electromagnetic cosmological theory.
I have been wondering how on one hand a Bose-Einstein condensate can be produced, thanks to the operation of the uncertainty principal, and yet IBM can still apparently herd individual atoms into patterns of smiley-faces and other interesting patterns, which seems to violate the uncertainty principal, since the location and momentum of the atoms employed must both be pretty well fixed in order to produce a readable image as claimed. Anyway, thanks for the reply.
------ The only greater hazard to your liberty than n politicians is n+1 politicians.