# B Pauli's exclusion principle

#### riz

So my question is why cant 2 object be at the exact same potion, (i.e. overlap). Why cant a +ve quark and electron just merge. In an universe where there is no force caused due to charge, why cant we just walk through a solid wall.

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#### andrewkirk

Homework Helper
Gold Member
So my question is why cant 2 object be at the exact same potion, (i.e. overlap). Why cant a +ve quark and electron just merge.
The Pauli Exclusion Principle says that no two fermions (a type of particle) can have the same quantum state. You can think of that very (very!) loosely as saying that no two identical fermions can occupy the same position.

Since a quark and an electron are not identical particles, the exclusion principle has nothing to say (so far as I know) about their ability to merge.

Also, the exclusion principle only applies to fermions. There are other particles called bosons, that can occupy the same position. You might be interested to read about Bose-Einstein condensates, that are an instance of this sort of thing.
In an universe where there is no force caused due to charge, why cant we just walk through a solid wall.
A universe without charge would be different from this one, and operate under different laws. So this question is about science fiction. I imagine one could make up a set of laws for a universe in which it is possible to walk through a wall.

#### nitsuj

The Pauli Exclusion Principle says that no two fermions (a type of particle) can have the same quantum state. You can think of that very (very!) loosely as saying that no two identical fermions can occupy the same position.

Since a quark and an electron are not identical particles, the exclusion principle has nothing to say (so far as I know) about their ability to merge.
isn't the ability "to merge" the very different behavior of bosons? Why do you say "loosely as saying that no two identical fermions can occupy the same position." I thought that was the whole other side of physics we can see the effects of. Bosons can, and Fermions can't isn't "loose".

You seem to suggest non-composite fermions can be in the same place at the same time; if they are say, a quark and a lepton...??

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#### Henryk

Gold Member
Pauli exclusion principle says 'no two fermions can be in the same quantum state'. Quantum state is not the same as position.
Take helium atom, for instance. It has two electrons in the 1s state. That is, their spatial wave functions are identical and truly, they are in the same space at the same time but not in the same quantum state: their spins are different.
Pauli exclusion principle applies to the fermions of the same species, regardless if they are non-composite or not.
Take again helium as an example. It has two isotopes: Helium 3 and Helium 4. Helium 4 has 2 neutrons, 2 protons and 2 electrons - an even number of 1/2 spin particle, so the total spin has to be a multiple of 1 - hence it is a boson and does condense into a superfluid.
Helium 3 had only 1 neutron, hence an odd number of 1/2 spin particle and it is a fermion. It does not become superfluid.

"Pauli's exclusion principle"

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