Pauli Exclusion Principle

1. Pauli Exclusion Principle by Robert Nemiroff Michigan Tech

2. Physics X: About This Course • Officially "Extraordinary Concepts in Physics" • Being taught for credit at Michigan Tech • Light on math, heavy on concepts • Anyone anywhere is welcome • No textbook required • Wikipedia, web links, and lectures only • Find all the lectures with Google at: • "Starship Asterisk" then "Physics X"  • http://bb.nightskylive.net/asterisk/viewforum.php?f=39

3. Fundamental and Non-Fundamental Forces:  What keeps an electron from merging with a proton in a hydrogen atom? 1.  Electrostatic repulsion. 2.  The Pauli Exclusion Principle. 3.  The Uncertainty Principle. 4.  A bouncer.

4. Fundamental andNon-Fundamental Forces:  3.  The Uncertainty Principle. The space near the proton is small, so that the electron's Δx decreased in Δx Δp > h.  Therefore, p ~ Δp increases, keeping the electron away from the nucleus. Hydrogen is held up by the uncertainty principle! 

5. Fundamental and Non-Fundamental Forces:  Why don't you fall through the floor.  In other words, what is the primary reason that two atoms keep apart? 1.  Electrostatic repulsion. 2.  The Pauli Exclusion Principle. 3.  The Uncertainty Principle. 4.  Atoms just don't like each other.  

6. Fundamental and Non-Fundamental Forces:  1.  Electrostatic repulsion.   On the outside of all of the atoms on your feet and the floor are negatively charged electrons.  Two negative charges repel.  Therefore, the electrons in your feet repel the electrons on the floor so that you do not fall through.

7. Fundamental and Non-Fundamental Forces:  Why are atoms of higher elements fluffy?  In other words, why don't their electrons all hover about the nucleus like in a hydrogen atom? 1.  Electrostatic repulsion. 2.  The Pauli Exclusion Principle. 3.  The Uncertainty Principle. 4.  De-fluffer-izer.

8. Fundamental and Non-Fundamental Forces:  2.  The Pauli Exclusion Principle.   As quantum states fill up, electrons are kept further and further from the nucleus.  This is balanced by the attractive electric force between the electron and the positively charged nucleus. Your body would be much denser if not for the PEP!

9. Pauli Exclusion Principle The PEP says that two fermions cannot occupy the same quantum state.  Crudely, this could mean that two identical fermions cannot occupy the same place at the same time with the same orientation.   The PEP is derivable only in relativistic quantum mechanics using the spin statistic theorem.

10. Pauli Exclusion Principle Which force causes the PEP?   1.  The weak nuclear force. 2.  The strong nuclear force. 3.  Electromagnetic force. 4.  Gravitation. 5.  It is unknown.

11. Pauli Exclusion Principle 5.  It is unknown. Some hypothesize that the PEP repulsion comes from an extremely strong virtual particle exchange on very short distance scales. Some theorists say that it is caused by an "anti-symmetric wave function", where A(x,y) - A(y,x) = 0.  In my opinion, this math was created to explain known observations of the PEP, and cannot be used, by itself, to explore the limits and cause of the PEP.

12. Pauli Exclusion Principle Examples of particles that are known as "fermions": Electrons, protons, and neutrons. Bosons such as photons do NOT obey the Pauli Exclusion Principle and can all "pile into" the same state.

13. Pauli Exclusion Principle To my eye, electrons need not be completely fermions, and could, in practice, have a very small bosonic component. Experimental results that "prove" electrons are only fermions really only put a limit on the magnitude of their bosonic component.

14. Pauli Exclusion Principle Experiment shows that fermions repel by the PEP as temperature drops, but bosons don't.

15. Pauli Exclusion Principle Some stars are held up by degenerate pressure which results from the Pauli Exclusion Principle.   White dwarf stars are held up against gravity by electron degenerate pressure.  When gravity gets large enough, the electrons are pushed into protons to form neutrons, which have a more compact state. Neutron stars are held up against gravity by neutron degenerate pressure.  Neutrons are 1020 times more rigid than diamonds.