Monday, January 31, 2011

Something from "nothing" - the Casimir Effect.

As promised, I'm taking a small break from introductory quantum mechanics, and discussing a topic that has some very real consequences in real life. This is one of the more strange predictions of quantum theory, so hold on. Ever heard of zero point energy? If you have it's related to the Casimir effect.



One of the most foundational rules of physics is that energy cannot be created or destroyed. This is known as the first law of thermodynamics, and was first stated by Rudolf Clausius in 1850. Essentially, you can change the form of energy, (e.g., from kinetic energy to potential energy), but you can't add more energy into the universe. Albert Einstein showed that this goes even farther than was originally though when he formulated his special theory of relativity. Einstein postulated that matter (what makes up everything we see) can be equated to energy - that the two are inescapably linked. Now, everything in the universe can be described in one way or another as energy. Einstein's theory was proven with the advent of nuclear theory, and since then, time and time again we have observed the conversion of mass into energy. The strangest part of this story starts back to the Heisenberg uncertainty principle once more, this time written slightly different way.
The delta E stands for the uncertainty in energy, and the delta t roughly corresponds to the time it takes for a system to change [1]. How can we interpret this? Well, it roughly means the given a process that lasts a short enough time, energy conservation can be briefly violated. Surely, you say, this must be incorrect. As it turns out, this is not incorrect and has been experimentally verified. Back in the 1940s, several physicists were trying to formulate the laws of quantum mechanics that were consistent with the strange predictions of traditional quantum mechanics, and with Albert Einstein's theory of relativity. This theory, now know as quantum electrodynamics, was finally able to make real predictions after years of formulation. The interesting part of the theory, and why I bring this up in the first place, is that calculations can be made in this theory by first drawing little diagrams of what happens.

This is known as a Feynman diagram. This diagram shows an electron interacting with an anti-electron. When this happens, a photon (light) is created. After this the photon turns back into an electron and an anti-electron once more. This is a process that has been at least partially observed. The truth is, all we can actually see in an experiment is what goes in, and what comes out. In the middle, where the squiggly line is (the photon), many other things could have happened, and must be taken into account in the calculations.

Here's the same thing, but different stuff happens in the center. In quantum electrodynamics, the contributions from each of these little pictures must be added up to get the complete picture. How do we know this? We get the wrong answer if we don't. The particles in the middle of the diagram are called virtual particles. The strange part about them is they are allowed to, and very option do violate conservation of energy! We cannot directly observe them, but we can prove their existence. How? Through the Casimir effect.

Several years after this theory was formulated, Hendrik Casimir calculated that given two parallel metal plates that are very close together and that have nothing but empty space (vacuum) between them, there would be an attractive force between them. This sounds very strange, but it was experimentally verified in 1997. What does this mean? The significance of this discover is that even in the vacuum of space, tiny virtual particles are popping in and out of existence, thumbing their noses at the 1st law of thermodynamics.

Well, I think all of this is cool, but I'm a physicist, why should you care? Well, the more technology advances, the smaller it becomes, delving into the nano-scale! Speaking of nano-scale, you should visit Nano Nook and It's a Small World for more information on nano technology! Say I'm creating a nano-scale device. It's constructed of tiny pieces of metal. It start to bring two pieces close together, but oh no! They start attracting! This has started to pose a problem for nano technology  manufacturers. Not only has it been a problem, but engineers are trying to figure out ways of using the Casimir effect to benefit nano technology. Read this article posted by The Economist on the Casimir effect for more information!

Another cool part, though it is perhaps a dead end, is that the Casimir effect admits the existence of NEGATIVE energy. Why do I find this interesting? As it turns out, negative energy density is an essential ingredient for wormholes, at least, according to Einstein's theory of relativity. I don't know about you, but the possibility of something like a wormhole really fascinates me.


Some people have postulated that the Casimir effect could be used to extract energy from empty space, but thus far, no one has been able to devise a way to do this. The energy is there, but whether or not it can ever be extracted has yet to be proven.

If you have any input on the Casimir effect, or wish to point out mistakes (which I surely have made) in this post, please feel free to comment! I have one more surprise post, then we'll return to our regular study of quantum mechanics.

6 comments:

  1. The idea of zero point energy is fascinating, but to me just seems impossible to extract. I know that the Casimir Effect is way above my head, but wouldn't taking energy out of the unknown middle part of the reaction alter the end of the product of the reaction. I would think that this would just counter our efforts in some way or another. As always though, I hope I am wrong (but zero point energy would probably put me out of work).

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  2. Well, Hawking radiation is related to the Casimir effect. That's when a pair of virtual particles are created near a black hole with one of them inside. In order for the uncertainty principle to be non-violated, the other particle must exist, so the black hole loses mass. This is a method of extracting zero point energy. Now if only we had a black hole...

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  3. Dan,

    Another great post! I like that you're taking such incredibly complex ideas (for a layperson like me) and making them accessible.

    I supposed I was going to write, though, that the bulk of your post is rather deficit model-ish. You're trying to just impart information, and it's not clear why. It becomes clearer toward the end when you write about nano, but even then, a reader has to really care about nano. Perhaps inverting your organization to talk application (or problem--the problem nanotechnologists are having) first would help with this.

    Or perhaps you're aiming at a very narrow audience. It's not clear yet. I think you'll have to continue to grapple with this problem given your subject matter.

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  4. Thanks Jen,

    My intended audience is people who are interested in quantum mechanics, but may not have the background, time, or machinery to study it in depth. A secondary goal is to try to get people interested in it. My plan is to have some of my content serve as introductory material, some on current research, and some only on applications are connections to other fields.

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  5. So does this violation of the first law of thermo only happen on an extremely small scale with your system being electrons? If that is the case then would changing the system to something larger, i.e. an entire atom follow the first law of thermo again. I know you mention that this has to occur over a small period of time but does is have to be a small amount of space as well?

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  6. Technically, it can happen with anything, but the more massive the particle, the less likely it is to happen. Electrons are light, so it commonly happens with them. It really has to happen at small scales because if a virtual pair were two far apart, they couldn't annihilate each other quickly, which would mean the time period would have to be longer.

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