Scale Models Can "Compute" Casimir Forces

KentuckyFC writes "Place two conducting parallel plates a few nanometres apart and the well-known but difficult-to-measure Casimir force will push them together. The force depends crucially on the shape of the plates but nobody is exactly sure how. That's because calculations with anything other than flat plates are fiendishly difficult and measurements are even harder. Now a group at MIT has come up with an ingenious new way to investigate Casimir forces. What the team has noticed is a mathematical analogy between the Casimir force acting on microscopic bodies in a vacuum and the electromagnetic behavior of macroscopic bodies floating in a conducting fluid. Their idea is to build a centimeter-scale metal model of the system they want to investigate, place it in salt water, and bombard it with microwaves and see what happens. The team says the experiment does not measure the force on the scale model but instead a quantity that is mathematically related to the force. So the experiment is not a simulator but actually an analog computer that calculates the force (abstract). What's exciting is that the method should for the first time give researchers a way of testing nano-machines designed to exploit the Casimir force."

Comments

Casimir Force

[manekineko2]

Could someone provide a comprehensible description for non-physicists of what the Casimir Force is? I looked it up on Wikipedia (and like all math and physics related articles there) came up with a borderline unintelligible "summary".

It's overview is:

The Casimir effect can be understood by the idea that the presence of conducting metals and dielectrics alter the vacuum expectation value of the energy of the second quantized electromagnetic field. Since the value of this energy depends on the shapes and positions of the conductors and dielectrics, the Casimir effect makes itself manifest as a force between such objects.

It's intro is similar:

In physics, the Casimir effect and the Casimir-Polder force are physical forces arising from a quantized field. The typical example is of two uncharged metallic plates in a vacuum, placed a few micrometers apart, without any external electromagnetic field. In a classical description, the lack of an external field also means that there is no field between the plates, and no force would be measured between them. When this field is instead studied using quantum electrodynamics, it is seen that the plates do affect the virtual photons which constitute the field, and generate a net force[1]â"either an attraction or a repulsion depending on the specific arrangement of the two plates. This force has been measured, and is a striking example of an effect purely due to second quantization.

Re:Casimir Force

[Anonymous Coward]

The way the Casimir force works is that when you put smooth plates very very close together, they are pulled closer.

This is posited to be caused by pairs of virtual photons which spring into existence and annihilate constantly.

When you put the plates close enough together, there's not enough room for photons to appear between them. Therefore there is theoretically more of a vacuum between the plates than outside. As we all know, vacuum's suck so we get a force pulling the plates together.

Re:Casimir Force

[Hordeking]

<pedantic>Vacuums do not suck! They are areas of lower potential, and everything has a tendency to move from a higher potential to a lower potential. Things in the non-vacuum area are blown into the vacuum area.>/pedantic<

FYI: Black holes do not suck, either. They're pretty cool.

Re:Casimir Force

[JustinOpinion]

Good explanation.

An oft-used classical analogy is of boats on a wavy sea. It's been reported that two ships sitting on a wavy sea (but windless day) will slowly move closer together, as if they are 'attracted' to one another. The origin of the force is the waves of various wavelengths that form on the water surface. The sea surface has waves of all different sizes. In between the two ships, however, some wavelengths can't 'fit' and so those modes are suppressed. The end result is that there are fewer wave between the ships, so the greater pressure from the (more) waves on the other sides of the ships pushes them closer together. (I'm glossing over the details, e.g. that you have to take into account how the waves on the surface of the sea reflect off the ship's hulls... but hopefully you get the idea.)

The Casimir force is like the quantum version of this. According to quantum mechanics, the vacuum is constantly churning with the creation and annihilation of virtual particles. Thus there are quantum waves of all kinds of different wavelengths. In between two plates, some quantized modes can't exist, and are suppressed. The end result is that there is more pressure from the vacuum on the outside of the plates than in the gap between them. Hence the plates are pushed together by the vacuum pressure.

Note that in both cases the magnitude of the force is quite small, and so you have to be quite careful to observe the force and measure it properly.

Re:Casimir Force

[Eternauta3k]

That contradicts other explanations in this discussion, where did you read that?

Re:Casimir Force

[Wyrd01]

The way the Casimir force works is that when you put smooth plates very very close together, they are pulled closer... we get a force pulling the plates together.

I assume there is some reason this wouldn't work, but could we harness this "force" and convert it into useable energy?

Just attach something to the back of both these plates that will be pulled on by the plates as they try to move together. The "something" would not allow the plates to get together, but as far as my understanding goes, the plates would "perpetually" try to move together and you'd have a constant generation of energy.

But current physics laws don't allow endless energy for free, so what's the catch with this one? Is the amount of pull so small it would take ages to build up enough energy to be usable?

Re:Casimir Force

[Cup-of-Tea]

If the plates don't move, no work is done and you get no energy.

Re:Casimir Force

[Alinabi]

Except that it is somewhat inaccurate. The correct explanation is that only photons of certain frequencies can exist in the space between the plates, while elsewhere you can have photons of any frequency.

Re:Casimir Force

[gardyloo]

Just attach something to the back of both these plates that will be pulled on by the plates as they try to move together. The "something" would not allow the plates to get together, but as far as my understanding goes, the plates would "perpetually" try to move together and you'd have a constant generation of energy.

All you'd have in that case is a constant (and very, _very_ small, even for large plates) force. To actually do useful work, that force has to move something through a distance (which itself would have to be very small, because the plates have to be close together). Even if that were done, you'd then have to pull the plates apart to repeat the process, and to pull them apart takes just as much work as you'd get from letting them be pulled together.

      Also note that people have predicted (I'll go to a talk next week on this) that the Casimir force might be able to be reversed (that is, there's a repellent force between the plates) if the plates have certain materials properties (in this case, probably a "left-handed" electromagnetic coupling -- that is, their permeability should be negative).

Re:Casimir Force

[Joehonkie]

That helps even further, as does the "boat" analogy below. I was hoping for the car analogy, but I will take what I can get.

Re:Casimir Force

[fifedrum]

same reason you can't harvest energy when you hang a bowling ball on a string. Like Earth and the bowling ball, they attract each other, but potential energy is just potential energy

Re:Casimir Force

[orclevegam]

Someone else already gave a pretty good explanation of why this wouldn't work, but I'll see if I can give a slightly better explanation. Consider this, anytime you have unequal forces objects move, and we can in most cases convert that movement (which is simply excess energy) into power. When however forces are balanced no movement occurs, and no excess energy is released that can be harnessed. As a related example things are constantly attracted to the Earth via gravity, which is a force (rather well understood and easily quantified at that), however there is an opposing force in the collision of two solid masses, that is the force of gravity attracting an object towards the center of the Earth is opposed by the force of the ground pushing back at the object and preventing it from moving closer to the center of the Earth. In the example you give even though there is a force attempting to bring the plates together it is being canceled out by your supposed "something", thus resulting in a net zero for the forces involved, and therefore no way to harvest any energy out of the configuration. You could of course attach those "somethings" to some kind of generator that generates electricity when they move and allow the Casimir force to bring the plates together thereby harvesting that energy (a very very tiny amount of energy), but in order to generate more energy from that effect you would need to separate the plates again which would take as much or more energy than was harvested from the effect in the first place.

Time an again it has been shown there is no free energy no matter how you slice it. We had our peak during the big bang, and its all been going down hill since then. The most we can hope for is to harness as much of the already existing energy as we can.

Re:Casimir Force

[Chih]

She's went from suck to blow!

Re:Casimir Force

[electrosoccertux]

In these experiments, they ground the plates to account for this.

Re:Casimir Force

[Profane MuthaFucka]

And in the process of fine-tuning the explanation, you've managed to completely lose everyone. For your misjudgement, I sentence you to a year of hard labor, teaching kindergartners why you cannot pound a square Bessel function through a round hole.

Re:Casimir Force

[Profane MuthaFucka]

No, they don't.

Re:Casimir Force

[orclevegam]

Nope, still not possible. By the very nature of it it would take more energy to move the plate out of the way than would ever be generated by it moving in the first place. Essentially all the Casimiar force is is the quantum version of a pressure differential. It's a very interesting phenomenon and has some possible uses in the design of nano scale generators and parts, but it will never be a energy source on its own. To be clear what I mean is that the effective could be incorporated into the design of a generator for extra gains on efficiency or as part of a large principle, but the force itself is not enough to build a generator around and it will never be a primary motive force.

Re:Casimir Force

[mysticgoat]

No car analogy here, sorry to say, but I can provide a somewhat more visual description than any I have seen on this thread so far. And I would very much appreciate any critiques by those who know QM (I know of it, and use it in my fiction, but I certainly don't know it. Dammit, I'm a writer, not a quantum mechanic).

First, visualize "quantum foam": virtual particles are constantly springing into existence in "empty" space, mostly to disappear again in very short intervals of time. When working at very small distances and units of time, space is full of these virtual particles, winking in and out. Some are more common than others, but all are present. The total population of these over an interval of time will exhibit QM statistical properties: that is virtual neutrinos will be much more common than virtual electron - positron pairs, which in turn will be very much more common than virtual protons and antiprotons. The sum of all this activity has been called quantum foam [John Wheeler gets credit for this, back in 1955].

Particles are waves, and waves have wavelengths. If you can put a constraint on a location in space so that a particular wavelength cannot exist at that location, then the particle associated with that wavelength cannot exist. The quantum foam in that location is less rich than in other locations. There is now a kind of "pressure gradient" between the quantum foam in the constrained region and the unconstrained regions around it.

Placing two sheets of metal closer together than the longer wavelengths of light prevent some of those virtual photons from manifesting. (My understanding is that this would only block the ones whose wavelengths are constrained by the plates, which suggests a kind of polarizing effect, but for now we can ignore that.) The Casimir effect is the force exerted on each of these plates by the pressure gradient of the quantum foam from one side of the plate to the other.

I'm thinking that we are going to have an increased need to develop effective ways of visualizing this as we start doing more with nanomaterials. For instance, I'm guessing that some of the transmission properties of buckytubes are related to constraints on the quantum foam in the inside of the tube. That a tube of the right diameter would prevent any real electron or real photon introduced at one end from doing anything other than exiting at the other end; that the geometry would force the wave to propagate only down the center of the tube.

I'm also thinking that Casimir effects might explain the attachment and release of neurotransmitters in the synaptic gap (not that the gap is necessarily involved: the synaptic cleft is around 20 nm across and that is an order of magnitude too large I think). However the surface geometries of the binding proteins are definitely in the range of Casimir effects, and it is possible that these are changing shapes in ways that release or attach the neurotransmitters. Also, I just now came across some stuff on electrical synapses where the gap is less than 4 nm and there are transmission structures with lumens of 1.2 nm diameter, which I think does mean that Casimir effects are going to be present. (But that does not mean that they are being used. Then again, as a rule, life takes advantage of every condition and edge case it can.)

I'm hoping to see some useful comments from the QM guys. Also materials engineers, anesthesiologists, and neurologists.

Cool jobs

[zappepcs]

Don't you wish you had a job where some very important work you're doing can be described thus:

Their idea is to build a centimeter-scale metal model of the system they want to investigate, place it in salt water, and bombard it with microwaves and see what happens.

This sounds like a Saturday afternoon in the garage with just a couple too many beers, an old tube tv, a broken microwave, and a friend that is just a little too happy to be 'experimenting' with stuff at your place because of the garage fire he had last year.

Props to Myth Busters for making 'blowing shit up' cool again...

A computer?

[DriedClexler]

How does this setup possibly count as a "computer"? It's not. It's just a physical process whose input/output, under one interpretation, is isomorphic to that of a computation its user wants to know the result of ... oh, I see. Never mind!

Re:A computer?

[MoellerPlesset2]

How does this setup possibly count as a "computer"? It's not. It's just a physical process whose input/output, under one interpretation, is isomorphic to that of a computation its user wants to know the result of ... oh, I see. Never mind!

I don't have a problem with that definition. But it also means 'quantum computers' shouldn't be called 'computers' either.

Re:A computer?

[Ungrounded Lightning]

Yep.

Just like more typical "analog computers" where a voltage or current represents a value in the computation.

i am not happy with this story summary

[circletimessquare]

until it can be rephrased in such a way that it asserts the ascendency of physicists over mathematicians. or the ascendency of mathematicians over physcists. i need to keep score. joke form is acceptable

Re:i am not happy with this story summary

[circletimessquare]

at a bus stop, twelve passengers get on a bus. at the next stop, thirteen get off

theologian: "a miracle! a miracle!"
biologist: "reproduction in action"
physicist: "measurement error. roughly nine percent statistical deviation is within acceptable tolerance ranges"
mathematician: "if one person gets in, the bus is empty again"

Re:i am not happy with this story summary

[NudeAvenger]

logic puzzle: "it was the bus' last stop - the bus driver got off"

Re:Is this really the scientific method at work?

[jmv]

They understand the Casimir effect and the related equations, they just can't solve them. So what they do is they find another problem that has the same equations and they measure on that system. If both systems behave using the same equations, then the result should be the same.

Re:Is this really the scientific method at work?

[u38cg]

No. They know the mathematics behind the system, however, they cannot solve the equations directly. What they have done is taken a system that works according to the same equations. Knowing how this system responds means that you can also work out how the first system responds. Easy.

Eureka!

[moteyalpha]

I like it as it reminds me of Archimedes. If you can't compute the volume , stick it in a tub of water and do an atomic integral of the volume.
Also I didn't see the meme so I have to do this,
But will it run Linux?

to the contrary - important for nanotech

[peter303]

Nano-machines may not work as predicted unless you take into account the vacuum energy fluctuations. The sign of the force appears to be shape and material dependent.

Question: Uncertainty Principle

[A. B3ttik]

I have a question: Does the Uncertainty Principle play into this at all?

Years ago, I read "A Brief History of Time" and Stephen Hawking asserts that the reason that Particles randomly pop into existence and annihilate again is because of the uncertainty principle. You can never know the exact momentum and position of a particle with complete certainty, and the more you know of one, the less you know of the other. Then, he says, you can never have a true vacuum. The position and momentum of this "vacuum" would _both_ be zero and since that simply can't be, there must be fluctuations.

WTF??!?!

I've read the passage over and over again, and I swear that _that_ is his line of reasoning, but it makes NO sense to me. I thought that the uncertainty principle was all about measurements, and altering things whenever you try and look at it... not about whether some random hypothetical area of space can exist as a vacuum or not.

Am I missing something, or did Stephen Hawking take some particularly potent Valium that day?

Re:Question: Uncertainty Principle

[JustinOpinion]

Does the Uncertainty Principle play into this at all?

The uncertainty principle (or quantum indeterminacy, if you prefer) is fundamental to quantum mechanics, so it plays a role in ... well in just about everything.

Hawking's explanation is one way of looking at virtual particles, which are indeed the origin of vacuum fluctuations.

If his explanation seems wrong, it is because the uncertainty principle is usually misrepresented in mainstream media. It is usually described as an a measurement imprecision: as if a particle has a definite position and velocity, but there is some law that prevents us from measuring it properly. That's (if I may be so bold) a very antiquated interpretation. The more modern interpretation is that a particle is inherently fuzzy: wavelike and indeterminate in its properties. The wavefunction for a particle inherently is 'spread out': it specifies a spread in various variables (e.g. position or momentum).

The Heisenberg uncertainty principles (there are actually many such relations; there is one between position and momentum; one between time and energy; etc.) describe how these indeterminacies evolve. Certain kinds of interactions (which you can call 'measurements' if you like) will reduce one kind of indeterminacy, but there will be a corresponding 'spread out' in another quantity.

Now back to virtual particles. The time-energy Heisenberg uncertainty says that deviations in energy are allowed as long as they don't exist for 'too long' (I'm being loose with language, the actual equations of course set rigorous bounds on all these things). So a vacuum can suddenly have 'more energy' as long as that energy disappears in a short amount of time. This is what virtual particles are: particles that are created 'out of nowhere', exist for a short time, then disappear. The interesting thing is that though these short-lived particles cannot be directly measured, their effects are very real. In fact if you think about a charged particle that emits a static electric field which exerts a force on some other particle, it is in fact virtual particles which are being exchanged between the two particles which explains the origin of the force between them (and explains the seeming 'action at a distance'). A time-varying electric field would instead generate 'real' photons, which are the light and radio waves we are all familiar with.

Some people think that virtual particles sound 'silly and made up' or somesuch. But they are a natural prediction of modern quantum theory, and they happen to nicely explain a wide variety of experimental results.

So Hawking is right that vacuum fluctuations arise because of quantum indeterminacy (which you can call 'Heisenberg uncertainty' if you prefer). The vacuum has particles appearing and disappearing all the time, and they produce real, measurable effects (like the Casimir force), even if they cannot be directly measured. (Just like a static electric field.)

(Disclaimer: I'm not a quantum physicist, so I've probably made a few mistakes. Corrections and clarifications are welcome.)

Re:ah, a true geek

[poena.dare]

Everyone knows that empty buses are constantly creating and destroying virtual passengers. You obviously were asleep in class the day they covered Kramden diagrams.

Hawking radiation

[AlecC]

No, that is what Hawking, and a considerable number of other scientists believe. Essentially, nature is allowed to "borrow" energy from nowhere provided the product of the energy and time the energy exists does not exceed Planks constant. When it does so, a particle and its matching antiparticle (to keep all the charges, baryon numbers etc. matched) spring into existence for a very short time, then cancel out again, "repaying" the borrowed energy.

Except that if this happens really close to the event horizon of a black hole, one of the two particles can fall into the hole and the other doesn't, resulting in the net creation of a particle outside the event horizon. The energy needed to "balance the books" and create the particle comes from the black hole. This means that black holes are continuously emitting particles, which are called Hawking Radiation, and losing energy. However, to maximise the chance of one particle falling in and the other escaping, the gravitational field has to be very non-linear, which means that the hole has to be small. The smaller the hole, the faster it evaporates, so the faster it shrinks which eventually leads to a runaway; tiny black holes explode. However, stellar mass black holes evaporate so slowly that it takes a bucket load of exponents to measure the time until they explode.