Thread by @anderssandberg

#FridayPhysicsFun - One of the weirdest physical effects I know about is the Casimir effect (in my mental ranking it is just a step down from the Aharanov-Bohm effect). physicsworld.com/a/the-casimir-…

If you place two uncharged conductive surfaces close together in vacuum, there is an attractive force between them. Why? Because the vacuum between them has less energy than the vacuum outside them. en.wikipedia.org/wiki/Casimir_e… scholarpedia.org/article/Casimi…

Empty space, according to quantum field theory, is full of possible electromagnetic waves and they all have a finite zero-point energy. However, normally the only thing that matters is differences between this energy and fields with actual waves.

When you put the two surfaces close to each other only waves with wavelengths shorter than the distance are possible. That means that the energy in the vacuum between them is less than the energy in the vacuum outside. There is an inward pressure on the plates.

The force is weak and falls off with the fourth power of distance. It is also short range, typically measurable over a few tens of nanometers. But it is measurable. arxiv.org/abs/quant-ph/0…

It doesn't have to always be attractive. By coating one of the plates with a thin film it can become repulsive when they are too close and attractive beyond it, enabling a stable equilibrium.
phys.org/news/2019-06-c…
science.sciencemag.org/content/364/64…

It doesn't have to be plane plates. For microstructures all sorts of interactions can happen. nature.com/articles/nphot…

If the plates are moved together fast the "dynamical Casimir effect" even allows them to conjure up particles from the field (although they need to move very rapidly). technologyreview.com/2011/05/26/194… arxiv.org/abs/1105.4714

The idea that the quantum vacuum has a zero-point energy is not strange in itself and would seem to be a nice way of explaining the dark energy... except that so far all calculations produce *way* too much energy to work.

Negative energy densities are just the thing to hold wormholes open, so the Casimir effect gives some hope despite being very wimpy. It shows that the energy conditions in relativity are not quite iron-clad. link.springer.com/article/10.114…

There is also the Scharnhorst effect, that the index of refraction is predicted to be below 1 and allow light to move faster than outside. Unfortunately the effect is is tiny, a factor of 10^-32, and pretty debated.
en.wikipedia.org/wiki/Scharnhor…

If nothing else it is a nice handwave for how your fictional time machine works. imdb.com/title/tt164747…