Philosophical implications

Since its inception, the many counter-intuitive conditions and results of quantum mechanics have enraged strong philosophical contest  and many interpretations. The arguments centered  on the probabilistic nature of quantum mechanics, the difficulties with wave function crumple and the related measurement problem, and quantum nonlocality. Perhaps the only concurrency that exists about these issues is that there is no concurrency. Richard Feynman once said, "I think I can safely say that nobody understands quantum mechanics." According to Steven Weinberg, "There is now in my opinion no entirely satisfactory interpretation of quantum mechanics."

 

The opinions of Niels Bohr, Werner Heisenberg and other physicists are often grouped together as the "Copenhagen interpretation". According to these opinions, the probabilistic nature of quantum mechanics is not a temporary feature which will ultimately  be replaced by a deterministic theory, but is instead a final renunciation of the classical idea of "causality". Bohr also  in  a very particular way  affirmed  that any well-defined application of the quantum mechanical formalism must always make reference to the experimental arrangement, due to the corelative nature of evidence obtained under different experimental situations. Copenhagen-type interpretations  remain popular in the 21st century.

 

Albert Einstein, himself one of the founders of quantum theory, was troubled by its apparent failure to respect some cherished metaphysical principles, such determinism and locality. Einstein's long-running exchanges with Bohr about the meaning and status of quantum mechanics are now known as the Bohr–Einstein debates. Einstein have believed that underlying quantum mechanics must be a theory which  purposely forbids action at a distance. He argued that quantum mechanics was incomplete, a theory that was valid but not fundamental, analogous to how thermodynamics is valid, but the fundamental theory behind it is statistical mechanics. In 1935, Einstein and his collaborators Boris Podolsky and Nathan Rosen published an argument that the principle of locality implies the incompleteness of quantum mechanics, a thought experiment later termed the Einstein–Podolsky–Rosen paradox.[note 6] In 1964, John Bell showed that EPR's principle of locality, together with determinism, was actually inconsistent  with quantum mechanics: they implied constraints on the correlations produced by distance systems, now known as Bell inequalities, that can be violated by entangled particles. Since then several experiments have been performed to obtain these correlations, with the outcome that they do in fact violate Bell inequalities, and thus falsify the conjunction of locality with determinism.

 

Bohmian mechanics shows that it is definately possible to reformulate the quantum mechanics to make it more deterministic, at the price of making it explicitly nonlocal. It attributes not only in wave function but also to a physical system, but in addition a real position, that evolves deterministically under a nonlocal guiding equation. The evolution of a physical system is given at all times by the Schrödinger equation together with the guiding equation; there is never a collapse of the wave function. This solves the measurement problem.

 

Everett's many-worlds interpretation, formulated in 1956, holds that all the possibilities described by quantum theory concurrently occur in a multiverse composed of mostly independent parallel universes.[53] This is not achieved  by introducing a "new axiom" to quantum mechanics, but by removing the axiom of the collapse of the wave packet. All possible states of the measured system and the measuring apparatus, together with the observer, are present in a real physical quantum superposition. While the multiverse is deterministic, we perceive non-deterministic behavior governed by probabilities, because we don't have to  observe the multiverse as a whole, but only one parallel universe at a time. Exactly how this is supposed to work has been the subject of much debate. Why we should assign probabilities at all to results that are certain to occur in some worlds, and why should the probabilities be given by the Born rule? Everett tried to answer both questions in the paper that introduced many-worlds; his derivation of the Born rule has been censured as relying on unconcerned assumptions. Since then several other derivations of the Born rule in the many-worlds framework have been proposed. There is no consensus on whether this has been successful.

Relational quantum mechanics came across  in the late 1990s as a modern derivative of Copenhagen-type ideas, and QBism was developed some years later.

Advances in nuclear and subatomic physics:

The 1920s observed further advances in  the nuclear physics with Rutherford’s discovery of induced radioactivity. Bombardment of light nuclei by alpha particles produced new radioactive nuclei. In 1928 Russian-born American physicist George Gamow explained the lifetimes in alpha radioactivity using the Schrödinger equation. His explanation also used the property of quantum mechanics which  allows particles to “tunnel” through regions where classical physics would forbid them to be.