# Why does quantum physics use "observation" rather than "interaction" as the term for the trigger which collapses the wave function?

So I understand that when talking about quantum mechanics, quantum entities are assumed to exist as a probability wave, until they are "observed", which collapses the wave function into an observable particle.

The classic example would be the double-slit experiment, where the interference pattern tells us that the photon or electron must have been behaving as a wave until it is "observed" or "measured" by the detector, which collapses the wave to a single position.

As I have been reading more about this, what I don't understand is, why is "observation" or "measurement" the verb used here? It seems to me that "interaction" could be substituted as a functionally isomorphic term: e.g. the photon exists as a wave until it "interacts" with something which makes it collapse as a wave. Or in other words, the interaction is what collapses the wave function, and measurement is just one such type of interaction.

It seems to me this term change would preserve all the important causal relationships required to understand how quantum systems work, but would demystify a lot of the "spooky" interpretations of quantum mechanics: i.e. that consciousness is somehow involved in the makeup of the universe, and quantum particles somehow know when they're being watched.

Is there something I am missing which means that the term "observation" is actually carrying relevant semantic information which "interaction" would not with respect to quantum systems?

• You are missing the fact that quantum entities interact all the time without collapsing a wave function. For instance, electrons interact in a solid to produce metallic, insulative, or superconductive behavior. Bosonic atoms interact near zero temperature to produce Bose-Einstein condensates, and so on. Interaction is a much more general term than one that collapses a wave function. A common modern, but hand-wavy explanation of "observation" is "an interaction that leads to a macroscopic change in the 'detector' subsystem". Nov 7 '21 at 9:02
• Wave function collapse is not a physical phenomenon. What's happening is that the wave function of the physical system and the detector are entangled. Nov 7 '21 at 12:23
• Leonard Susskind in one of his lectures said that "observation" and "observable" should really be named "measurement" and "measurable." The point is that a collapse of the wavefunction does not just come from an interaction, but rather it comes from an interaction that "leaves an imprint" in the universe from which we can extract info about the particles. Nov 7 '21 at 13:01

A single,one, photon is a point particle , and does not exist as a wave. Here is an experiment of single photons at a time, little dots on the screen.

1. Single-photon camera recording of photons from a double slit illuminated by very weak laser light. Left to right: single frame, superposition of 200, 1’000, and 500’000 frames.

The little dots on the left are the footprints of photons, certainly not a wave. What waves? The probability of hitting the (x,y) of the screen as seen in the progress to the right.

The "collapse" is the same as when throwing a dice with six faces , before the throw the probability function is 1/6 for any face to come up, a constant function. After the throw the probability function collapses, but an accumulation of throws will show the probability function for dice throws.

Observation of one photon dot follows the probability function of the quantum mechanical solution, a complicated function , not a constant as in the dice, but the logic is the same.

Observation and measurement are expressions that describe how real numbers are gathered and tabulated from experiments . In the quantum mechanical models fitting these data there can be interior interactions before the final state is reached. Look at this tau decay:

the tau interacts with the off shell $$W^-$$ which interacts with the the various quantum field theory fields of neutrinos ...to manifest the various measurable final states. Interactions that cannot be separately measured, their effect only in the final probability amplitude for the specific final state.

So observation needs at least an interaction, but interaction can exist without observation.

• There are two metaphors for thinking about photons. Neither is strictly correct, and neither is strictly wrong, They are ways for our brain to think about something that it's not designed to understand. Use whichever makes life easier. Also: a single photon certainly "is" a wave. It can be understood as an excitation of the EM mode (wave). (It exists in the zero-point excitation.) The photos presented here can be understood as such. The mode is set up by the geometry and forms the pattern. The excitation excites the entire mode, so the photon is in both slits at the same time. ... Nov 7 '21 at 13:50
• ... The interaction with the detector occurs at discrete points forming dots. The interaction at discrete points is understood as the excitation of a particular atom with the simultaneous de-excitation of the EM field. I think we've gone off-topic, and I apologize for that. Nov 7 '21 at 13:52
• @garyp the model used for calculating interaction crossections depends on a wavefunction, the mathematics. To attribute a reality to mathematics for me is analogous to thinking in classical mechanics that because one can model a trajectory of a particle or an orbit for a planet, these functions exist in space. Nov 7 '21 at 14:02

The only disadvantage of the the word interaction is that it is rather more general than measurement; its use does make the point that there is nothing spooky or special about the act of measuring or observing. Of the three words you mention, observe seems to offer the most scope for being loaded with nonsensical implications about consciousness having a role in QM.