When you learn about quantum mechanics and particle physics you will often hear about these weird things called virtual particles.
Virtual particles are often described as a transient fluctuation in a field, it could be an electric field or a gravitational field or any other field that is known to exist. These particles pop in and out of existence and do not violate the conservation of mass and energy as long as they do this magic trick within the constraints laid out by the Heisenberg Uncertainty Principle.
It all kind of sounds made up and fictional, that is, until you find out that there is a force that arises solely because these virtual particles actually exist.
This force is called the Casimir Effect (after the Dutch physicist Hendrik Casimir who predicted the effect in 1948) and it is explained below the figure.
It turns out that the vacuum is far from empty. In fact, it is full of fluctuating waves represented as virtual particles. It implies that the vacuum is also filled with energy but it is an energy that we can observe but cannot tap.
The effect is generated if you place two conductive plates in a vacuum very close to one another. These plates will affect the virtual photons that spontaneously pop into existence.
Since the distance between the plates is fixed this will encourage the existence only of virtual photons whose wavelengths are a multiple of the separation distance. All other photons will essentially be eliminated via destructive interference (analogous to the concept of standing waves).
Outside of the two plates this destructive interference effect will not be in effect so virtual photons of any wavelength are able to pop into existence.
We also know that photons, even though they have no rest mass, do possess momentum. It's weird but true and is the basis behind the idea for solar sail space ships (but that is a topic for another post).
This means that the virtual photons inside the plates will push out and the virtual photons on the outside of the plates will push in. Since there are many more virtual photons allowed to exist on the outside than the inside the total force will therefore be inwards.
The force is very small but it was finally measured to exist in 1997 by Steve Lamoreaux of Los Alamos National Laboratory and by Umar Mohideen and Anushree Roy of the University of California, Riverside.
Closing Words
The Casimir effect is an idea that goes against the logic of the ordinary classical world: a force that arises out of nothing. This is a very odd effect indeed.
Nevertheless it has definitely been measured and even more, some practical applications have been suggested such as potentially being used to drive nano-machines.
I think that it is worth mentioning that Stephen Hawking has applied a similar idea to this effect to the behaviour of virtual particles at the boundary of black holes and figured out that black holes can actually radiate energy (Hawking Radiation).
I created the images in this post. Feel free to use them as you will.
Thank you for reading my post.