# A query regarding Casimir Effect

As I understand the Casimir effect is the effect that when two uncharged conducting plates are placed appropriately close to each other, the net pressure b/w the plates due to virtual photons (or particle anti-particle pairs) is less than that on the outside of the plates. Thus, the two plates are pushed together.

So, if it is due to the virtual photon pressure:

1. Wouldn't Casimir effect be equally valid for any non-conducting materials too.
2. Would the uncharged non-conducting plates generate an electromagnetic field/charge/attraction in the plates? (I don't think so as the plates are uncharged, but a confirmation would be nice)

So, if it is due to the virtual photon pressure

The Casimir effect is not due to virtual photon pressure. There is no such thing as virtual photon pressure and indeed there is no such thing as a virtual particle.

Electrons and photons are described by quantum fields. There is an electron quantum field and a photon quantum field and the two fields interact with each other in a way described by the theory of quantum electrodynamics. The fields have a lowest energy state that is called the vacuum state but surprisingly when the fields are in the vacuum state that does not mean the numbers of electrons and photons present are zero. Instead what actually happens is that the numbers of electrons and photons present are not defined.

This vagueness about how many electrons and photons are present is commonly described using vacuum fluctuations, but this is a rather misleading way of looking at it. If you're interested I attempt to describe exactly what is going on in my answer to Are vacuum fluctuations really happening all the time?.

Anyhow, we can assign an energy to the vacuum state called the zero point energy, and in a vacuum far from any matter this energy has a certain value. When we introduce other matter, like a conducting or insulating plate, then this changes the vacuum state because now we have to worry about the interaction with that other matter. And the change in the vacuum state changes the zero point energy.

So what is really going on in the Casimir experiment is that the vacuum state in between the two plates is different to the vacuum state outside the plates and has a different energy. Calculating what happens turns out to be a real struggle so it was done initially for the simplest system possible and this was an ideal conductor, and it turns out we get an attraction between the plates. Real metals are actually pretty close to ideal conductors so for real metals we also predict an attraction and indeed that's what we measure.

Doing the calculation for anything other than an ideal conductor is even harder, but it can and has been done and the result is that you can have no force or even a repulsive force. Indeed this has been measured experimentally and the results were published in Nature in 2009.

• I used the term Virtual Photon Pressure analogously to 'Radiation Pressure' and it might have not been correct. (My background is Computer Science so please accept the non-standard terminology :)). Thanks for the link. If there are no virtual particles and the above interpretation is correct then I assume 2 uncharged conducting plates most definitely would not get electromagnetically charged or show any current flow when they move due to Casimir Effect in vacuum ? – TheoryQuest1 Aug 1 '17 at 16:28
• Secondly, (If I am correct) we have (directly or indirectly) observed the creation of Photons in the Dynamic Casimir Effect (the accelerating mirror experiment). All the references I have mention the actual physical creation of particles/photons in terms of 'virtual photons/particles' that did not get a chance to annihilate. I see no other way how do we explain the creation of photons out of vacuum? – TheoryQuest1 Aug 1 '17 at 16:32
• @TheoryQuest1: as the question I linked explains, virtual particles are a computational device i.e. the virtual particle in a Feynman diagram is actually the representation of an integral. Photons can be created in lots of circumstances, e.g. Hawking radiation as well as the accelerating mirror, and the calculations are done using virtual particles. However it is not correct to say that a virtual particle turns into a real particle. – John Rennie Aug 1 '17 at 16:40