Popper's experiment

Popper's experiment is an experiment proposed by the philosopher Karl Popper. As early as 1934 he was suspicious of, and was proposing experiments to test, the Copenhagen interpretation, a popular subjectivist interpretation of quantum mechanics. Popper's experiment is a realization of an argument similar in spirit to the thought experiment of Einstein, Podolsky and Rosen (the EPR paradox) although not as well known.

There are various interpretations of quantum mechanics that do not agree with each other. Despite their differences, they are experimentally nearly indistinguishable from each other. The most widely known interpretation of quantum mechanics is the Copenhagen interpretation put forward by Niels Bohr. It says that observations lead to a wavefunction collapse, thereby suggesting the counter-intuitive result that two well separated, non-interacting systems require action-at-a-distance. Popper argued that such non-locality conflicts with common sense, and also with what was known at the time from astronomy and the "technical success of physics." "[T]hey all suggest the reality of time and the exclusion of action at a distance." While Einstein's EPR argument involved a thought experiment, Popper proposed a physical experiment to test for such action-at-a-distance.

Popper first proposed an experiment that would test indeterminacy in quantum mechanics in two works of 1934. However, Einstein wrote a letter to Popper about the experiment in which he raised some crucial objections, causing Popper to admit that his initial idea was "based on a mistake". In the 1950s he returned to the subject and formulated this later experiment, which was finally published in 1982.

Popper wrote:

I wish to suggest a crucial experiment to test whether knowledge alone is sufficient to create 'uncertainty' and, with it, scatter (as is contended under the Copenhagen interpretation), or whether it is the physical situation that is responsible for the scatter.

Popper's proposed experiment consists of a low-intensity source of particles that can generate pairs of particles traveling to the left and to the right along the x-axis. The beam's low intensity is "so that the probability is high that two particles recorded at the same time on the left and on the right are those which have actually interacted before emission."

...
Wikipedia