Why does the wave function collapse after observing a quantum particle simply by observation?
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Why does the wave function collapse after observing a quantum particle simply by observation?
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The problem is that the “wavefunction” is itself an inference. Therefore, the collapse of the wavefunction is a phenomenon that can only be inferred, and not directly observed. This really opens the gates to the various interpretations that strive to propose various mechanisms underlying the observed phenomena.
Quantum mechanics was developed to model certain observations that seemed to defy any classical explanation. In classical physics, particles and waves are considered distinct and separate phenomena. However, such distinctions no longer seem to hold in the quantum realm.
The first step was to propose that the wave-like nature is fundamental to the evolution of a quantum system. This explains the observation of interference. However, the observation of localised discrete absorption events required the idea that energy is packaged as discrete portions according to Planck's equation, E=hf. It is when you combine both of these ideas that you get the quantum formalism. However, you haven't actually specified any mechanism. Both ideas are purely phenomenological, yet they seem incompatible. Nevertheless, they were combined into a single working theory using the Born rule, which derives a probability from the wavefunction, and the collapse hypothesis, which resets the wavefunction on detection.
Given the Born rule derives a probability of detection from the wavefunction, an actual detection updates that probability to a certainty. It is this updating of the probability to a certainty that corresponds to the collapse. That's because, once you have a detection, you are certain that the probabilities of detecting at any other location are now zero. That's why observation is said to “collapse” the wavefunction. Many people now think about observation as actually updating information about a system, rather than collapsing something physical. Yet that latter interpretation is still not wholly satisfying, as the observation of quantum interference suggests that there must be something physical about the wavefunction.
Overall, there is no resolution to the mechanism behind wavefunction collapse. There are many different interpretations of quantum mechanics, each proposing a different “mechanism”. Moreover, as this debate has raged for over a century, I sincerely doubt that there will ever be a resolution. I suspect that the best we can do is define the properties of “reality” using tests such as Bell's inequalities and other derivatives. To date, such tests have all indicated that reality is inconsistent with certain classical properties. Thus, while quantum theory is a descriptive framework that doesn't seem to make sense, it seems that reality itself also doesn't make sense, which means that quantum theory cannot be invalidated purely based on classical reasoning.