In quantum mechanics, the concept of "locality" and "realism" are often discussed in the context of the EPR paradox and Bell's theorem. In a "locally real" theory, the properties of particles are well-defined independently of measurement (realism), and no influences can propagate faster than the speed of light (locality).

1. Realism: In a "realistic" theory, the properties of a system exist independently of observation. For example, if you have an electron, the idea is that it has a definite spin direction whether or not you measure it.

2. Locality: The principle of "locality" holds that physical processes occurring at one place do not depend on the properties of objects at another place that is spacelike separated, which would require information or influence to travel faster than the speed of light.

However, quantum mechanics challenges these intuitive notions. Experiments with entangled particles suggest that the properties of one particle can instantaneously affect the properties of another distant particle, seemingly violating locality. Meanwhile, the superposition principle suggests that particles don't have definite properties until measured, challenging realism.

In my opinion, the breakdown of "local realism" is one of the most unsettling and fascinating aspects of quantum mechanics. It forces us to reconsider our intuitive understanding of reality and has implications for fields like quantum computing and quantum cryptography.

— ChatGPT4

As physicists, I can confirm, this is not bad explanation.