The Casimir effect is a way to produce a negative curvature of spacetime by creating a negative energy density, and has been successfully done in labratory tests. The basic idea is to take two large metal plates and put them facing each other. When you do this the virtual particles between them will bounce around between the two. The particles that have a wavelength a multiple of the distance between the two plates will get stuck and simply resonate between the plates. Other virtual particles in between will eventually cancel out since their wavelengths are not a whole number multiple, after some time two particles will meet and will cancel out (remember the wholewave/particle duality thing).

So, after a bit of time what you have is only virtual particles inside the plates whose wavelength is a multiple of the distance between the plates. This means that there will be fewer particles between the plates than outside. This means the energy density in the region between the plates is less than that far away from the plates. Now, since the total energy in the grand scheme of things must be zero, and the energy density inside the plates is less that that far away, the energy density inside the plates must be negative.

So there we basically have the Casimir effect, which proves that virtual particles exist, that there can be negative energy densities, and that theoretically wormholes can be created. Bitchin', eh?

First proposed by the Dutch scientist Hendrick Casimir in 1948, the Casimir force supports the quantum electrodynamics portion of the vacuum fluctuation theory by Max Planck and Werner Heisenberg. That's the theory that proposes that space is not empty, but full of virtual particles that are constantly appearing and disappearing. (The theory has been used to explain things from how tiny black holes evaporate (Steven Hawking) to zero-point energy, although the effect is not the mechanism to extract such energy.)

Casimir theorized that if two perfectly parallel plates could be brought closely enough together, the tiny space created would be free from all long-wavelength particles, as the spontaneously-occurring particles that are constantly being created would appear on the outsides of the plates, and not in the too-small space between them. This would create a measurable pressure against the outsides of the plates, proving the existence of spontaneously-created particles from "empty space".

In 1997, Researchers led by Steve Lamoreaux at the Los Alamos National Labs were able to create an experiment accurate and sensitive enough to measure the Casimir force. Recently, experiments at Purdue University, led by Professor Ephraim Fischbach, have verified the effect to within 5% of Casimir's original predictions. They team used a MEMS torsional oscillator plate and a gold-plated aluminum sphere (easier than two plates to align properly) to measure the force, varying the distance between the two structures from 200 nanometers to 2 microns.

A Casimir Vacuum (in case this term hasn't been used before, I claim coinage) is more than just an empty space, it can be seen as a literal hole in spacetime. The stronger the force, the greater the void. UPDATE 16 Oct. '04 - I found multiple articles predating this writeup (darn!) referring to the "Casimir Vaccum", each dealing the Casimir Effect itself and the energy involved.

The effect is becoming more than just cool physics due to the progress being made in microelectromechanical systems (MEMS) and nanotechnology. When structures within nanobots and other nanodevices are close enough to one another, "photonic pressure" from the Casimir effect gives them an extra push that could cause the teeny-tiny gears and such to seize up. Research is currently ongoing to potentially harness the Casimir force to help nanomachines operate better.

The distance between any two surfaces such that the Casimir effect is manifested should be called (another I claim coinage on if there are no earlier references)the Casimir Horizon, similar to the Event Horizon of a Black Hole in that it is a boundary saying "here endeth reality" in large letters. The Casimir horizon delineates a flaw in spacetime of a nature exactly opposite to the Event Horizon created by the overly-massive Black Hole. That makes sense, as there had to be an opposite end to the mass/energy/reality spectrum, and a Casimir Vacuum is it.

An excellent paper on the effect can be found at:
http://plato.phy.ohiou.edu/~ulloa/611-612/612papers/Trang%20Nguyen--Casimir%20Effect.pdf

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