About quantum correlations, which are stronger than any correlations allowed by classical physics, and about the CHSH inequality which demonstrates this fact, and which is a variant of Bell’s theorem.
Every now and then, it is nice to put down your lecture notes and go and see how things actually work in the real world. What is wonderful (and surprising) about quantum theory is that it turns up in many places that we might not expect it to, such as in the polarisation of light, where we stumble across an intriguing paradox.
If we take two polarising filters, and place them on top of each other with their polarisations oriented at
This is intrinsically linked to Bell’s theorem, which proves the technical sounding statement that “any local real hidden variable theory must satisfy certain statistical properties”, which is not satisfied in reality, as many quantum mechanical experiments (such as the above) show!