Wikipedia: Copenhagen interpretation

1. The probability statements made by quantum mechanics are irreducible in the sense that they don't just reflect our limited knowledge of some hidden variables. In classical physics, probabilities were used to describe the outcome of rolling a die, even though the process was thought to be deterministic. Probabilities were used to substitute for complete knowledge. By contrast, the Copenhagen interpretation holds that in quantum mechanics, measurement outcomes are fundamentally indeterministic.
2. Physics is the science of outcomes of measurement processes. Speculation beyond that cannot be justified. The Copenhagen interpretation rejects questions like "where was the particle before I measured its position" as meaningless.
3. The act of observing causes an instantaneous "collapse of the wave function". This means that the measurement process randomly picks out exactly one of the many possibilities allowed for by the state's wave function, and the wave function instantaneously changes to reflect that pick.

The completeness of quantum mechanics (thesis 1) has been attacked by the Einstein-Podolsky-Rosen? thought experiment which was intended to show that there have to be hidden variables in order to avoid non-local, instantaneous "effects at a distance". [Bell's inequality]? about the measurement outcomes of such an experiment was derived on the assumption that these hidden variables exist and no non-local effects are present. In 1982, Aspect finally carried out the experiment and found Bell's inequality to be violated, thus rejecting interpretations that postulate hidden variables and local interaction. This experiment has since been criticized and improved experiments have been described by Weihs and Rowe, confirming Aspect's results.

Of the three theses above, the third is maybe the most problematic from a physist's standpoint because it gives a special status to observation processes without cleanly defining them nor explaining their peculiar effects.

Many notable physicists and philosophers have objected to the Copenhagen interpretation, both on the grounds that it is non-deterministic and that it relies on a non-physical observation process to create reality. Einstein's quotes "God does not play dice" and "Do you really think the moon isn't there if you aren't looking at it?" exemplify this. Schrödinger's cat was originally intended as an example to show how absurd the model was

The main alternative to the Copenhagen interpretation is the Everett many-worlds interpretation.

• [Physics FAQ section about Bell's inequality]
• G. Weihs et al., Phys. Rev. Lett. 81 (1998) 5039
• M. Rowe et al., Nature 409 (2001) 791.