The famous one-way mirror used in police interrogations isn't really one-way - it's just a trick of clever lighting. But physics could produce true one-way mirrors, and even walls that are only soundproofed in one direction.
The one-way mirrors we have now only work because of a lighting imbalance - the people watching have to be in a far darker room than those being watched, else the effect would vanish and both sides could see each other. But now two Italian researchers have figured out the theoretical basics for new materials that would only transmit waves in one direction, be they light waves or sound waves.
Their idea revolves around so-called nonlinear materials, which are actually able to break with reciprocity theory. This theory holds that waves passing through a material should be treated in the same way no matter which direction they come from. Here's how to get around it:
By stacking layers of nonlinear materials along with ordinary linear layers in an asymmetric fashion, the researchers have calculated, a wave would be able to pass through in one direction but would almost completely bounce off when it arrives from the other direction. The one-way bias isn't universal, however-the researchers note that each particular implementation would have a sweet spot of wave amplitudes and frequencies for which it would work best.
Right now, this is all the stuff of computer simulations, but experiments on these ideas should be possible shortly. The big question is whether these hypothetical materials required by the theories actually match any real world materials. If they can find some real counterparts, then true one-way mirrors and soundproofing could be possible, sealed and approved by the laws of physics. I feel like this could all affect privacy in some way, but honestly this is all just too damn cool right now.