# 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. ### 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. ### I Three Plus Anti-symmetric Particles

So I understand that fermions are anti-symmetric under exchange, but in the contexts I've seen this explained they were always talking about two particles, or at least two wavefunctions. I'm curious how this works when there are three or more particles. Is any two given pairs of those 3+...
3. ### I Fermi Gas Model: Energetic Degeneration & the Pauli Exclusion Principle

In the fermi gas model, there is assumption that there is a 3D potential well, but there is "energetic degeneration" for each three index "nx, ny, nz". Now the problem is with that image, if there is degeration, for some level En there may be 10 distinctive state with same energy, so there is 20...
4. ### Electromagnetism and the Pauli Exclusion Principle

Electromagnetism in the atoms is why we can't pass through a bank vault. But supposed electromagnetism were canceled for an object, what would happen to the residual or remaining Pauli Exclusion principle? Would it still cause resistance to passing through the vault? On a second scenerio, what...
5. ### A Spin dynamics and the Pauli exclusion principle

For high temperature superconductivity, people usually say two quasifree electrons are pairing, one is spin up and the other one is spin down. So, if that is the case, each two electrons will have zero spin angular momentum. Since the superconductivity is the magnetic properties and spin is the...
6. ### I Hund's rules and Pauli's principle

So as I can see from the literature there are two "methods" on how to apply Hund's rules to determine the ground state of an electron configuration. Method 1: One determines all possible states due to Pauli's principle (wave function must be totally antisymmetric) using angular momentum...
7. ### I How does the Pauli exclusion principle work?

Hello! I am a bit confused about the mechanism behind the Pauli exclusion principle. From what I read, it is motivated based on QFT arguments (for example if you don't impose antisymmetry of the fermionic wavefunction you get non-locality, or infinitely negative energies etc.) so mathematically...
8. ### I Pauli exclusion principle and Hermitian operators

http://vergil.chemistry.gatech.edu/notes/quantrev/node20.html "Postulate 2. To every observable in classical mechanics there corresponds a linear, Hermitian operator in quantum mechanics. " "Postulate 6. The total wavefunction must be antisymmetric with respect to the interchange of all...
9. ### A Non interacting Fermions satisfy the Pauli exclusion principle

This question is more a question I'd ask in a chat rather than formally on paper/forum. If we take the free electron model, the electrons are considered as non interacting. It is essentially a 1 particle problem where the potential is constant through space. The electrons are not perturbed at...
10. ### I Electrons and the Pauli exclusion principle

electrons are repelled using the Pauli exclusion principle?
11. ### I Difference between parallel and antiparallel spins?

What is the difference between parallel and antiparallel spins for a pair of nucleons? My understanding is that nucleons have a strong tendency to pair - proton with proton, neutron with neutron, proton with neutron. When they pair their spins either: cancel (spins pair antiparallel) pairing...
12. ### I Pauli exclusion principle explained

Hello! I am a bit confused about the Pauli exclusion principle. Let's say I have 3 electrons. Due to energy considerations the first 2 go to the ground state, and they can be only 2 electrons there, because the position wavefunction has only one option ##\psi_{100}## (and again due to energy...
13. ### I 2-particle Wavefunction and the Pauli Exclusion Principle

Say you have two particles a and b with respective positions ##x_a## and ##x_b##. Particle a is in the state ##\psi_a##, and particle b is in the state ##\psi_b##. If they are distinguishable, the wavefunction is $$\psi=\psi_a(x_a)\psi_b(x_b)$$ However, if they are identical fermions, the...
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##...
15. ### A Virtual Fermions and Pauli Principle

Hi guys, Do virtual particles, when they are fermions, obey Pauli exclusion principle as real fermions do? More specifically, what I am wondering is the following: Fermion fields would have some energy at every point in spacetime due to the uncertainty principle. Now, is it possible for the...
16. ### I Who will win -- Pauli or Einstein?

I met with a little conflict between Pauli and Einstein? Can you please help. Its a thought experiment. Consider a single crystal which is 1km long. During its formation, due to Pauli’s exclusion principle, no two electron will have same quantum state. Now consider two electron, one with E and...
17. ### I Can negative pions replace electrons in atoms?

Is Pauli Exclusion Principle alone responsible for electron degeneracy pressure?
18. ### B Bypassing Pauli Exclusion principle.

Suppose that somehow we could artificially bypass Pauli exclusion principle, and make electrons or any fermions for that matter occupy more than one state at the same time? What consequences in nature will we see? what phenomenons will occur? Suppose this mechanism for bypassing is limited in...
19. ### I What is the reason behind fermions being unable to share the same quantum state?

What is the explanation for the case that fermion's can't occupy the same quantum state? Is there some logic behind this or is it just observation?
20. ### A Exploring the Pauli Exclusion Principle and Black Holes

The event horizon of a black hole appears to be plastered with 'afterimages' of everything that ever fell into it. (Because gravitational time dilation makes every such object appear to stop at the event horizon.) Now, suppose an event horizon is 'full' as defined by the Pauli exclusion...
21. ### How does the Pauli Exclusion principle explain ionization E?

How does the Pauli-exclusion principle explain ionization energy trends? Is it just that as you move down the periodic table, the electrons experience repulsion between each other, and thus the atoms get bigger?
22. ### Pauli Exclusion Principle: how does an electron know its state?

This is one of those question you won't find the answer in any book. From Wikipedia: it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers (n, ℓ, mℓ and ms). But how can an electron know the state (the quantum numbers) of the other...
23. ### Confused about the Pauli Exclusion Principle....

The principle states that two electrons cannot have the same quantum numbers. And I've read that this applies to "fermions"- protons, neutrons, 1/2 spin particles. But how exactly does this apply to, say, a proton? Sorry if I sound stupid...I've got all my knowledge about this through the...
24. ### How Does the Pauli Exclusion Principle Limit Electron Spin Directions?

Hi, Can someone please explain this to me? "The axis of rotation for a non-quantum-mechanical object can point any way it likes. The Earth could rotate around an axis ninety degrees from the current one, so that the North Pole always faces the sun and the South Pole always faces away from it...
25. ### Pauli exclusion principle question (again)

I've seen it stated in many places that the reason why atoms don't collapse is due to the pauli exclusion principle. The exclusion principle is given as a required anti-symmetry in the wavefunction of electrons. I don't understand how this principle was derived, or where it comes from. (I've...
26. ### Navigating Wikipedia for Scientific Answers: Tips and Tricks for Laymen

I have a question: can the mechanism behind Pauli's exclusion principle be considered a fundamental force, like gravitational, electromagnetic, nuclear weak or strong? Why? Thx.
27. ### Pauli exclusion principle and position-state restriction

Can one deduce from Pauli's exclusion principle (through the Slater Determinant) that two electrons with different spins in the same energy level, can't have the same position?
28. ### Exploring Pauli Exclusion Principle: SP3 Orbitals & Electron Spin

The 3s and 3p orbitals are filled by 4 electrons.A single atom has [Ne]3s2 3p2.But when multiple atoms get together they do so in order to minimize the overall energy.And to minimize the overall energy,the 3s and 3p orbitals hybridize to form 4 tetrahedral SP3 orbitals.And the Si atoms get...
29. ### Brian Cox and the Pauli Exclusion Principle

Hi, I know this is old news at this stage, but I was watching his public lecture on quantum mechanics, and he says the energy levels of all the electrons in the universe shift to adjust when he adds energy to electrons in a diamond. I understand that he should have used the phrase quantum...
30. ### Pauli exclusion principle at distance

Hi. New member to this Physics forum and not a physicist, although have an interest in physics from a layman's position. I saw a series of threads on a Twitter discussion posted about a year ago concerning Brian Cox and some other physicists concerning a statement made by Cox that the...
31. ### The Pauli Exclusion Principle and Repulsive Forces in Atoms

Some time ago I asked about contact forces and was directed to read about the Pauli exclusion principle and the resulting L-J potential before a detailed explanation was given explaining why the electrons start interacting significantly, electromagnetically, even if the atom as a whole is...
32. ### Understanding the Pauli Exclusion Principle

When a fermion x approaches another fermion y does x send out bosons to y which tell it to get out of the way? In short, how does y know to get out of the way of x?
33. ### 2 Atomic Physics questions - Pauli exclusion principle

Homework Statement Q1. Briefly explain the relevance of the Pauli exclusion principle for the structure of the periodic table of the elements. Q2. What is the maximum number of electrons that can be located in an atomic subshell with quantum numbers n and L? Briefly expain your answer...
34. ### 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. ### 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. ### 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. ### Entanglement and the Pauli exclusion principle

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. ### 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. ### 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. ### 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. ### 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. ### 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. ### Turning Off Pauli Exclusion Principle

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. ### 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. ### 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. ### 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. ### Pauli Exclusion Principle and Black Holes

I understand that there may be no answer to the question "Why is Pauli Exclusion Principle not applied beyond a Neutron Star's mass?" since there may not be a full quantum gravity theory yet, however, I'm thinking, what if Pauli Exclusion Principle is not really a principle, but an indication...
48. ### 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. ### Brian Cox explain Pauli exclusion principle

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. ### Pauli exclusion principle and parallel spin.

Homework Statement It's not a homework question. It's a piece of my textbook I don't understand. Here's what it says In a two electron atom, taking the orbital states of two electrons to be the same, then the antisymmetric wavefunction tends to zero, as well as the quantum numbers n, l and...