Activity

Light is a quantum field. Its energy comes in quanta, photons, each taken up whole at one place. But the chance of finding a photon here or there spreads and interferes like a wave: each photon interferes only with itself. So light makes interference stripes and knocks out electrons alike.

Quantum theory applies to everything, observers included, and nothing ever collapses. An observer measuring something branches: each branch sees one definite outcome, and every outcome the equations allow is seen in some branch. All the branches are actual, none more real than the rest.