Looking at various particles regarding being fermions or bosons, it seems to me that fermions are something fundamentally different from bosons.
What I mean by "fundamentally different" is "as different as the electromagnetic force is to the strong nuclear force". The opposite, similar, is like "energy versus mass", or "photon particle versus wave".
Most notably, electrons, protons and neutrons are fermions, while photons are bosons.
Now, a difference that seems to me as fundamental as it gets is that the Pauli exclusion applies to fermions, but not to bosons.
On the other hand, there are very similar particles of which one is a fermion, and the other is a boson:
- A hydrogen ion – a single proton – is, as noted above, a fermion.
in contrast
- A deuterium ion – a nucleus consisting of a proton and a neutron – is a boson.
To me, and to chemists, deuterium is just a hydrogen isotope, nothing special. Or in other words, they are fundamentally similar.
Now, are fermions and bosons fundamentally different, or "not really different"?
Maybe the cases above are in some way unsuitable to be compared to each other?
Best Answer
If two atoms differ only by the spin of their nuclei, then their individual properties will be almost identical, but their collective properties will be extremely different.
Chemists often consider individual atoms (or, more often, molecules). In the case of individual hydrogen and deuterium atoms, their electronic properties are identical (except for the hyperfine spitting of their electronic energy levels due to spin-spin coupling between the electrons and the nuclei). Practically speaking, the only important difference is the mass difference due to the extra neutron.
But once you put a bunch of them together and lower the temperature enough that quantum effects (specifically multiple-occupancy of energy levels) become significant, their extremely different many-body properties become manifest. For example, neutral hydrogen atoms are bosons and can in principle Bose-Einstein condense, while neutral deuterium atoms are fermions and will instead form a free Fermi gas (to a decent approximation), which has extremely different properties. That's why it's extremely important that cold-atom experimentalists trying to form Bose-Einstein condensates get the right isotopes of the atoms that they're trying to condense.