“The statistical predictions of quantum mechanics are incompatible with separable predetermination.” ~ John Stewart Bell
Quantum mechanics has an obvious deficiency: its mechanics. Quantifying quantum phenomena is the elephant in the room of interpreting quantum theory.
Measuring fundamental particles is an existential oxymoron. Watching a wave function collapse is a probabilistic event. The math itself is nontrivial, and the appropriateness of the bandied equations contentious.
But some quantum field phenomena have been seen. The most inexplicable is nonlocality: what Einstein called “spooky action at a distance.”
Our world works on the principle of locality: that an object can only be affected by its immediate surroundings. In contrast, nonlocality is the notion that distance is ultimately an illusion.
A 1935 paper by Einstein and 2 other physicists posited a paradox over quantum uncertainty: that either locality or uncertainty must be true. Empirically minded Einstein opted for locality (and certainty), thereby concluding that the wave function must be an incomplete description.
Despite upsetting the apple cart of classical physics with his relativity theories, Einstein still preferred the cosmos as comfortably commonplace. After all, relativity only applied when traveling near the speed of light, or at the scale of galactic expanse. These realms were practically abstract.
In response to Einstein’s 1935 paper, Irish physicist John Stewart Bell tackled the quantum measurement problem in 1964; whence arose Bell’s theorem.
Science in general, and physics in particular, long assumed that both locality and objectivity were both true.
Locality means that distance affects the probability of interactions. Locality is colloquially codified in the everyday concept of cause and effect.
Objectivity insists that reality is ultimately objective, and therefore independent of observation. With special relativity, Einstein suggested that existence was subjective.
Bell’s theorem stated that either locality or objectivity had to go. In opting for the uniformity of objective reality, Bell pitched locality.
Ironically striving to spite his own theory of special relativity, Einstein struggled to the end of his days for a theory to uphold causality, protesting the view that there is no objective physicality other than that which is revealed through quantum-mechanical formalism.
In squaring off the principle of locality against counterfactual definiteness, Bell’s theorem went the other way: stating that some quantum effects travel faster than light ever can, thus violating locality.
Bell’s theorem painted special relativity into a corner; rendering it applicable only at the macro scale, and irrelevant at the quantum level. Then even that corner was painted over in the 21st century, by nonlocality showing up in the ambient world.
◊ ◊ ◊
Cause and effect is how we understand physics in the everyday (ambient) sense. In physics, causality as predictable is called counterfactual definiteness (CFD).
CFD goes to measurement repeatability: whether what has happened in the past is a statistical indicator of the future. Locality goes along with sequential causality: cause resulting in effect.
At the quantum level, CFD butts heads with locality, by stating that past probability as indicative of the future is a chimera. Instead, uncertainty always reigns.
Here we have a basic conflict. In the physics of existence, either certainty or uncertainty is true. The two are mutually exclusive.
Bell’s theorem showed that quantum uncertainty was a certainty: the principle of locality breaks down at the quantum level. Einstein was appalled: “No reasonable definition of reality could be expected to permit this.”
Later findings demonstrated that nonlocality functions at the macroscopic level too. With spooky-action-at-a-distance a reality, superluminal (faster-than-light) effects exist. Bell’s theorem of nonlocality/entanglement is considered a fundamental principle of quantum mechanics, having been supported by a substantial body of evidence.
“Nonlocality is so fundamental and so important for our worldview of quantum mechanics.” ~ Swiss quantum physicist Nicolas Gisin
The supposed trade-off between locality and objective reality is a false one. While strict quantum locality has been disproven, there is no proof that existence is actually objective. It just appears that way as a social convention: we consider the world “objective” when others agree with us; and so, objectivity is taken axiomatically, just as locality was for so long.
“If quantum physics hasn’t profoundly shocked you, you haven’t understood it yet.” ~ Niels Bohr