In the context of the double-slit interference experiment, what exactly does “observation” mean? Specifically, does the act of visual observation by intelligent creatures, such as humans, play a role in determining whether light behaves as a particle or a wave?
I’m trying to understand how the concept of observation impacts the behavior of light in this experiment. How is observation defined in the quantum mechanical sense, and what role does it play in the interference pattern observed on the screen? Does merely looking at the experiment change the outcome, or is there a more technical aspect to what “observation” entails in this scenario?
Any detailed explanations or references to relevant quantum mechanics principles would be greatly appreciated!
The essential property of an observation is that some process has a macroscopic (non quantum-level) effect.
As long as a quantum-level process (say the motion of a photon) is initiating merely another quantum-level process (say the motion of a single electron), it makes sense to say that nothing has actually been observed yet, and the overall state is still progressing under the probabilistic rules of quantum mechanics. In some crude sense, anything remains possible.
But when such an effect is amplified to determine the combined state of quadrillions of quadrillions of … quadrillions atoms, any inherent randomness in that overall state is driven asymptotically to zero: based on an observation of the location of some photon, the battleship turns left or turns right, and the part of its state function accommodating the other option is driven effectively to zero.
But that of course is a continuum. When the state of only one particle is involved, it makes sense to say that no observation has been made. When a kilogram of matter has been altered, an observation clearly has been made. But when 100 atoms, or 10, or 3 have been altered, it’s kind of a gray area as to whether that is an observation or not — there’s no hard and fast line between the quantum and macroscopic realms. Quantum rules work at the smallest scale, and traditional physical laws work at macroscopic scales, but the area in-between is notoriously difficult to model, or even describe.