The principle of locality

Week 2: reality · quantum

The principle of locality

State what 'local' means in physics and why classical physics is local by construction.

In this lesson

  1. Read the core idea (2 min).
  2. Walk through one worked example.
  3. Take the 4-question quiz; pass with 4/4 or better.

Hook

Drop a stone into a pond. Ripples spread outward at a finite speed; the far shore stays undisturbed until they arrive. That picture — that influence travels through space, point to point, at a finite speed — is what physicists mean by locality. It is one of the deepest assumptions of pre-quantum physics, and it is the assumption that the next four lessons will put under enormous pressure.

Before we attack it, we have to be precise about what locality is, what it isn't, and which neighbouring ideas it gets confused with.