- Bell’s theorem states that no theory that satisfies the property of local realism can reproduce all the predictions of quantum theory. This is important because it means that certain properties of reality have testable consequences. Bell’s inequality was then developed based on the property of local realism, setting a bound for correlations observed assuming this property. A violation of Bell’s inequality means that reality cannot have all the assumed properties.
Let’s review the property of local realism. That means that you can learn everything about an object by making measurements of the object alone. Quantum theory has a nonlocal element in the wave function. In particular, entangled particles are a perfect example of such a nonlocal wave function.
- A violation of Bell’s inequalities by entangled particles then tells us that one of the reality properties that we’re assumed is incompatible with observation. The obvious one is that of local realism because we already know that quantum theory is nonlocal.
- Ultimately, a realist assumes that there is a reality. Most people are realists. It’s a good assumption to make. However, the actual properties of reality are disputable. Tests of reality are useful in helping us refine exactly what that reality is. Here, realism is not synonymous with local realism. There must be a nonlocal element to realism.
- If you had an atom with two protons only, and they were both in a ground (lowest energy) state, their spins must be opposite because of the Pauli exclusion principle, so the nuclear force is weaker—too weak to overcome the electrostatic repulsion between the protons. In contrast, if you had an atom with a proton and a neutron, their spins could be aligned, resulting in much stronger binding by the nuclear force (and, of course, no electrostatic repulsion).
- Which is why 2H (a proton and a neutron) is a stable isotope of hydrogen (deuterium), but 2He (two protons) is not a stable isotope of helium.
