I presume the MIT literature knows more than i do here, but i'm unsure what i am missing. Anyone have an idea?
A lot of terms are reused for what are technically compound particles, especially since it isn't always too obvious which properties of a particle are intrinsic and which are caused by interactions.
That is, why do atoms normally obey the Pauli exclusion principle?
What everyday objects obey is electromagnetism. The electric bond between things we think of as objects is much much much stronger than the force you can apply by normal means. So objects which come in contact are excluded from the same space not by Pauli principle but because the force to combine them is astronomical in human terms
Another consequence is that, while black holes can have charge, we don't expect them to in nature—they'll very quickly suck up whatever it takes to balance them from surrounding space.[2]
[1] One of these: https://www.youtube.com/playlist?list=PLQrxduI9Pds1fm91Dmn8x...
[2] Via one of Sean Carroll's "Biggest Ideas in the Universe" videos
> That is, why do atoms normally obey the Pauli exclusion principle?
Whole atoms can be fermions (the Wikipedia article gives the example of ³He), and the Pauli exclusion principle applies to fermions within a given quantum system, not to bosons (though sometimes the quantum system itself is a composite boson, though it could also be a composite fermion—the most common set of systems in which it is discussed, atoms, can be either.)
Why is the situation where an whole atom doesn't obey the Pauli exclusion principle much more exotic and unusual?
I mean, take a look at the wikipedia pages https://en.wikipedia.org/wiki/Composite_boson and https://en.wikipedia.org/wiki/Composite_fermion - when one talks about composite bosons, it's exotic materials like superfluid helium, bose-einstein condensates, cooper pairs.
Since it's only a even/odd difference, I would expect that roughly half materials form composite bosons, and the other half form composite fermions, but this doesn't seem to be the case.
In the everyday case, this is pretty much always in reference to electrons (which are fermions). Even when we're talking about atoms or solids, the effects of the PEP are due to electrons: https://en.wikipedia.org/wiki/Pauli_exclusion_principle#Appl...
The quantum-mechanical wavelength of everyday whole atoms is much smaller than their physical size, so they behave as classical particles (and the PEP doesn't really apply). In contrast, electrons have wavelengths large enough that they exhibit macroscopic quantum mechanical effects in everyday scenarios.
> Since it's only a even/odd difference, I would expect that roughly half materials form composite bosons, and the other half form composite fermions, but this doesn't seem to be the case.
Every element has bosonic and fermionic isotopes. Neutral atoms have equal numbers of protons and electrons, so any neutral atom with an odd number of neutrons is a composite fermion, and any neutral atom with an even number of neutrons is a composite boson.
Atoms with both even atomic and even mass numbers, nuclei with an even mass number.
I mean, both wikipedia articles you point to focus on exotic examples, but there are far more common mundane examples of both, too.
In quantum mechanics you can also form composite objects (in the most common formalism it will be a "wavefunction") and you still can determine a total fermion/boson-ness as a function of the elementary objects that form it. So yes, an atom, a proton, a molecule, a table all can be bosons or fermions once you decide to ignore some internal degrees of freedom.
In fact even those subatomic particles you mention could be composite objects for all we know.