Before the wavefunction, we used to explain the double slit experiment (the version without detectors at a slit) as light being an EM wave in physical space, es

Before the wavefunction, we used to explain the double slit experiment (the version without detectors at a slit) as light being an EM wave in physical space, essentially equivalent to a sound wave propagating through the EM field, which breaks on the wall and essentially transforms into two separate waves, each originating from one slit, which are then in phase and so they constructively interfere, forming the final pattern on the screen. Lots of people think that this is the same picture that the wavefunction gives, but this is wrong. In the QM picture, the emitter emits one photon, which is a quantum of energy described by a four-dimensional wavefunction which assigns some probability of a detection event at the slits, at the screen, etc. In this picture, there is no physical EM wave, any interaction with the light will happen at a single localized point in space. Of course, if you add more particles, especially those carrying charges, the picture changes, and you'll see probabilities that roughly correspond to a picture of an oscillating EM field. But the wavefunction, which is the "bedrock" physical theory, is separate from those waves in the EM field, which are just an approximate picture of the probabilities dictated by the wavefunciton.