# Vacumn energy

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I read somewhere that energy also exists when there is no matter present. Furthermore, the lower the vacumn energy of a given medium, the faster light can pass through. Could someone please explain this concept and the Casimir effect? Thank you.

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When I read your post I thought you might be talking about virtual particles, and when you asked about the Casimir effect you pretty much confirmed my virtual particles thought.

In a "vacuum" there are things called virtual particles. What these are is particles which pop in and out of existence.

First nothing is there, then these virtual particles just come into existence for a very short amount of time and then they disappear again.

The uncertainty principle applies to time/energy (as well as position/momentum). The less uncertainty about energy, the more uncertainty about time.

The thing with virtual particles is that they exist for such a short amount of time that the world doesn't "notice" that they've existed and in doing so have violated the conservation of energy law.

They exist for about 10^-15 seconds, but they are continually popping in and out of existence.

Casimir effect: When I first heard about this I was just like "wow", I think it's pretty neat.

Basically some physicist (surname Casimir) predicted that two uncharged parallel metal plates will have a force pushing them together. (The attraction is only noticeable when the two plates are very close to each other. By that I mean a few atomic diameters away from each other.)

The reason this force exists is because of virtual particles. These virtual particles are everywhere. The area between the 2 plates is very small and so limits the wavelength of virtual particles that can exist in that area. Consequently fewer virtual particles exist in that area.

Now we have (relatively) a lot of virtual particles on the outside and few on the inside. This creates a pressure difference and an energy density difference. This pressure is pushing the plates together (bigger pressure on outside of plates due to lots of virtual particles... little pressure between plates due to lack of virtual particles) so the plates are pushed together.

The Casimir effect was observed in 1997 and shown to exist.

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• 2 weeks later...
Furthermore, the lower the vacumn energy of a given medium, the faster light can pass through. Could someone please explain this concept and the Casimir effect? Thank you.

This is known as Scharnhorst effect. Roughly, the propagation of a photon at the distance scale of quantum electrodynamics in vacuum can be shown to be a series of transformations involving virtual particles (e.g. the photon may be transformed into a virtual electron-positron pair). If one changes the conditions of the vacuum (generating a Casimir vacuum), the propagation of photons is affected.

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