What is Pauli exclusion principle: Definition and 91 Discussions

The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formulated by Austrian physicist Wolfgang Pauli in 1925 for electrons, and later extended to all fermions with his spin–statistics theorem of 1940.
In the case of electrons in atoms, it can be stated as follows: it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers: n, the principal quantum number; ℓ, the azimuthal quantum number; mℓ, the magnetic quantum number; and ms, the spin quantum number. For example, if two electrons reside in the same orbital, then their n, ℓ, and mℓ values are the same; therefore their ms must be different, and thus the electrons must have opposite half-integer spin projections of 1/2 and −1/2.
Particles with an integer spin, or bosons, are not subject to the Pauli exclusion principle: any number of identical bosons can occupy the same quantum state, as with, for instance, photons produced by a laser or atoms in a Bose–Einstein condensate.
A more rigorous statement is that, concerning the exchange of two identical particles, the total (many-particle) wave function is antisymmetric for fermions, and symmetric for bosons. This means that if the space and spin coordinates of two identical particles are interchanged, then the total wave function changes its sign for fermions and does not change for bosons.
If two fermions were in the same state (for example the same orbital with the same spin in the same atom), interchanging them would change nothing and the total wave function would be unchanged. The only way the total wave function can both change sign as required for fermions and also remain unchanged is that this function must be zero everywhere, which means that the state cannot exist. This reasoning does not apply to bosons because the sign does not change.

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  1. Frigorifico9

    I In what chapter do Mehra and Rechenberg discuss Pauli matrices?

    I am very interested in how Pauli found the Pauli matrices, so I read his original paper, but it didn't give me the perspective I wanted, so I went to Mehra and Rechenberg, but here's the thing, after reading Volumes 1, 2 and most of volume 3, I can't find any mention of Pauli matrices anywhere...
  2. J

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  3. L

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  4. C

    Electromagnetism and the Pauli Exclusion Principle

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  5. A

    A Spin dynamics and the Pauli exclusion principle

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  6. R

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  7. B

    I How does the Pauli exclusion principle work?

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  8. J

    I Pauli exclusion principle and Hermitian operators

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  10. P

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  11. S

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  12. S

    I Pauli exclusion principle explained

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  13. Isaac0427

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  14. W

    How does the Pauli Exclusion Principle in this problem?

    Homework Statement This is not a homework problem. It's an example in a textbook. 3 electrons. For ##S=3/2##, we have that $$ m_{s_1} = m_{s_2} = m_{s_3} = 1/2 $$ Therefore by the Pauli Exclusion principle, $$ m_{l_1} \neq m_{l_2} \neq m_{l_3} $$ and they take the values ##-1,0,1##...
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  16. Kaushik_KS

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  17. Ranku

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  19. H

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  20. A

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  21. F

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  22. Mr Wolf

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  23. Lance Fernandes

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  24. Chemer

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  25. M

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  26. W

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  27. H

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  28. R

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  29. M

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  30. A

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  31. E

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  33. D

    2 Atomic Physics questions - Pauli exclusion principle

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  34. I

    Pauli exclusion principle

    I was told that the pauli exclusion principle states that no two electrons in an atom can have the same quantum numbers is it ture need answers!
  35. G

    Pauli Exclusion Principle and Entanglement

    How do fermions, which have vast amounts of empty space, know not to occupy the same space as another fermion? Do physicists say that the two fermions become entangled and that is what enables them to be "aware" of the "existence" of the other fermion? Is entanglement used as an explanation...
  36. D

    I Why is the Pauli Exclusion Principle not a force?

    This is a question I've had for some time now. Why is the exchange interaction not considered a force, like the other 4 fundamental forces? When reading solid-state physics texts, for example, I come across explanations of this kind: the atoms cannot get too close together because of the...
  37. L

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    Hey all, I have what I think (hope) is a relatively quick pair of questions regarding entanglement of fermions and bosons. First, am I right in saying that if two fermions are in the same position-state, they will necessarily be entangled? My reasoning here is just that if their...
  38. E

    Pauli exclusion principle and fermions, bosons and quarks

    Homework Statement Which of these particles don't follow Pauli exclusion principle and thus have a symmetric wave function? a) Bosons b) Fermions c) Quarks d) All particles follow Pauli exclusion principle Homework Equations None. The Attempt at a Solution I think that...
  39. M

    How was pauli exclusion principle derived ?

    How was Pauli exclusion principle derived ? Experimental ? From some other physical formula ? Which ? From some other principle ? thanx
  40. J

    Electron spin and the Pauli Exclusion Principle.

    How is it that only 1 spin up and 1 spin down electron are allowed in an atom even though there is no measurement to collapse the state function?
  41. S

    Does the Pauli Exclusion Principle Apply to All Atoms in the Universe?

    The way I understand Pauli exclusion principle is: no two electrons can be of the same quantum state in an atom. But electrons from two atoms of the same element, let’s say hydrogen for simplicity, are in the same quantum state, is that right? That is what distinguished it from helium for...
  42. C

    Pauli Exclusion Principle in Black Holes.

    Good evening. I have been reading that the repulsion generated by the Pauli exclusion principle barely prevents neutrons in neutron stars from occupying the same quantum states (after all, they are fermions). However, the principle seems to be violated in a black hole, given that fermions are...
  43. Rapier

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    Homework Statement How would the world change if there were no Pauli exclusion principle? Specifically, how would the band theory of metals change? a) Assume you had a small copper coin of mass 3.12 grams. How much energy would be released from the coin if the Pauli exclusion principle were...
  44. A

    Why isn't Pauli Exclusion Principle a force?

    In the same way we could create "principles" for the other forces which would not make them not forces. Is it a misunderstanding of the meaning of a force or principle? Could someone clarify this for me.
  45. B

    How Does the Pauli Exclusion Principle Explain the Empty Space Inside a Proton?

    This question might be beyond our current knowledge but i want to make sure. The reason why fermions do not overlap is due to the pauli exclusion principle which states that one fermion cannot occupy the same state as another fermion. a quark is 10-18m and a proton is 10-15m, 3 orders of...
  46. H

    Can Slater Determinant Explain the Difference Between Bosons and Fermions?

    According to Slater determinant, can one say that two bosons are able to place in the same position X , but two fermions can not, no matter what their states are?
  47. C

    Pauli Exclusion Principle and Black Holes

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  48. G

    Does Pauli's Exclusion Principle Apply to the Entire Universe?

    I discovered the existence of Pauli's exclusion principle from this section of this video here: http://www.youtube.com/watch?v=4f9wcSLs8ZQ&t=35m30s If you watch it for a bit, he explains that it applies to the whole universe. Is this true? On Wikipedia it says otherwise...
  49. H

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    Watching Brian Cox "BBC - Professor.Brian.Cox.A.Night.With.The.Stars - 2011" After 34 min he starts to talk about Pauli exclusion principle No electron can be in the same energy state he says. This is not just true for an atom a molecule or the diamond he is showing. No this goes for...
  50. S

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