>[!abstract] >Abstract. ## My understanding The double-slit experiment is often depicted incorrectly in popular science. It is usually presented as demonstrating the dual wave-particle nature of light (or matter). In reality, particles propagate like waves in *both* the single-slit *and* the double-slit setups. In the double-slit setup, the particles form an *interference pattern* on the screen, which indicates that they behave like waves propagating through both slits at once, and which constructively or destructively interfere with each other upon recombining to form that distinctive pattern. Importantly, this is also true if only a single particle is sent at once; over time, they will build up to form that same interference pattern, which shows that even a single particle propagates as a wave entering both slits at once. >[!note]- >The last sentence is a simplification whose more rigorous definition depends on the chosen interpretation: >- In both Copenhagen and Many Worlds, the wavefunction propagates through both slits. >- In Bohmian mechanics, the particle follows one definite trajectory and it is the guiding wave that passes through both slits. >- In QBism, the wavefunction represents an observer’s ontological knowledge about the particle’s path, which at this point is ambivalent (both paths are equally plausible). >- In Superdeterminism, which is a family of models, the particle may go through one or both slits, depending on the particular flavor of the model. But the common denominator is that the path of the wavefunction is determined by hidden variables (thus rejecting Bell’s theorem). However, the single-slit setup is what is often depicted incorrectly. The particles don’t just form a classically expected cluster on the screen behind the open slit; instead, they form a *diffraction pattern*, which is exactly what is expected of a wave passing through that single slit. To reiterate, this proves that particles *always* propagate like waves; they don’t exhibit any duality in this regard. The real weirdness of the experiment manifests once a detector is placed between the source and the screen. In this setup, each individual particle will be observed to pass through either one slit, or the other, but not both; and on the screen, two diffraction patterns form, which is consistent with a setup where both slits take turn being open and closed. That diffraction pattern is *not* the same as the interference pattern from earlier. The act of measurement (not necessarily by a conscious observer) changes the outcome. In quantum terms, the measurement has caused a collapse of the wavefunction. Each path still diffracts like a wave, but the acquisition of which-path information through detection destroys the coherence between the two paths that is responsible for the interference. >[!note]- >It is sometimes said that the particle is “disturbed” by the measurement process in some way, which could explain the de coherence of both paths. However: >1. Even extremely gentle measurements remove interference if they acquire which-path information. >2. Conversely, if the which-path information is erased *before* it becomes irreversibly available, interference can, under the appropriate experimental conditions, be recovered. > >So it really looks like the underlying mechanism is the preservation or loss of quantum coherence, not mechanical disturbance. There is something even more astonishing, though: if we measure: - before the slits, early enough that the trajectory can’t be predicted -> coherence preserved - Before or at the slits and which-path info is acquired -> decoherence, interference disappears - After the slits -> decoherence Switching on the detector after the particle has already passed the slits Now things become much more interesting. If the particle has already gone through the slits but has not yet reached the screen, and you now activate a which-path detector that can still become entangled with the particle’s path, interference is still lost. This is closely related to John Wheeler’s delayed-choice experiment. The particle does not appear to have “decided” earlier whether to behave as a wave or a particle. Instead, the quantum state evolves until the entire experimental context determines whether interference is possible. >[!related] >- **North** (upstream): — >- **West** (similar): — >- **East** (different): — >- **South** (downstream): —