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. T

    What is the physical system that follows the Pauli exclusion principle?

    Hello guys, here's something that's been bothering me for a while now: We know the Pauli exclusion principle states that 2 Fermions cannot be in an identical state. So then we have systems like solids or free electron gases, and we calculate and form Fermi-Surfaces, based on the fact that...
  2. H

    Question about applying the Pauli Exclusion Principle

    The principle states that no two identical fermions in a system can be in the same quantum state, but what I don't fully understand is how you define a "system". For example when you apply statistical thermodynamics to a gas of non-interacting fermions you say that a maximum of one can occupy...
  3. J

    How Does the Pauli Exclusion Principle Influence Orbital Stability in Atoms?

    The Pauli Exclusion Principle is the reason more than two electrons cannot occupy the same orbital. This is easy for me to grasp. Why, then, does the Pauli Exclusion Principle make an orbital MORE stable with two electrons as opposed to one? (This is in the interest of knowledge. I'm not...
  4. P

    Bloch wavepackets and the Pauli exclusion principle

    Hello, I have a question concerning the use of wavepackets to justify the semiclassical approach in solid state physics. In Ashcroft/Mermin, the authors briefly mention that we can construct wave packets and then use them to describe the motion of the center which can be interpreted as what...
  5. V

    What Happens When Two Up-Up Fermions Collide?

    This morning I've been thinking about a simple problem whose I can't find out a satisfying solution. We all know that two "close" fermions can't exist in a up-up or down-down spin configuration because of vanishing state function as expressed by the exclusion principle. But what does...
  6. R

    Definition of indistinct in pauli exclusion principle

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  7. J

    Pauli exclusion principle, quantum states, and lasers?

    As I understand it, the Pauli exclusion principle states that no two like fermions can be in identical quantum states. I also understand that the quantum states are thus: n, which is the electron shell, l, which is the subshell, m_{l}, which is orbital, and m_{s}, which is spin. However, it...
  8. alemsalem

    Question about the Pauli exclusion principle.

    Suppose there are only two states, and that only two electrons could fit in them (spin states for example), but wouldn't these two states form a basis and so generate an infinite number of states that are linear combinations of these two, so three electrons could be in three different states...
  9. S

    Pauli exclusion principle between 2 identical fermions

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  10. T

    Does pauli exclusion principle

    does Pauli exclusion principle applied outside the atom?let say 1 electron on Earth and the other on the moon.
  11. haael

    Superconductivity and Pauli exclusion principle

    I have the following problem understanding Pauli exclusion principle. Two identical fermions can't share the same quantum state. Two bosons can. Now Cooper pairs are bosons made up from fermions. Everything clear up to this point. Now several Cooper pairs can share the same quantum state...
  12. S

    Bosons and the Pauli Exclusion Principle

    Do bosons tend not to obey Pauli Exclusion Principle? I would appreciate if someone would send me some material about this question, and answer it as well.
  13. Z

    Pauli exclusion principle: a Force or not?

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  14. L

    Question about Pauli exclusion principle

    "More generally, no two identical fermions (particles with half-integer spin) may occupy the same quantum state simultaneously." Now a quantum state can be setup to describe a collection of atoms, or molecules, or the entire universe in one state. So my question is, two electrons in...
  15. Y

    Pauli Exclusion Principle: Finite Creativity or Philosophy?

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  16. Z

    Question regarding the Pauli Exclusion Principle?

    Okay, I think everyone here knows what the principle states, so I am not even going over that. Is a proton not a fermion with is +1/2 spin? It has an half integral, hence it must be. However, how is this possible for a proton to be fermion when elements like gold have a lot of protons in the...
  17. J

    A quick(?) question about the Pauli exclusion principle

    Hi, This might be an ignorant question but I have to ask it. The exclusion principle forbids particles from existing with the same quantum numbers, like, if you had 2 electrons, they have quantum numbers n, l, ml and ms, and one out of those 4 has to be different, right? What I was wondering...
  18. PrincePhoenix

    Questions about Quantum state, Pauli exclusion principle and chemistry.

    According to this website, http://www.particleadventure.org/pauli.html "At one time, physicists thought that no two particles in the same quantum state could exist in the same place at the same time. This is called the Pauli Exclusion Principle, and it explains why there is chemistry." 1-What...
  19. R

    Black hole - contradiction to the Pauli exclusion principle?

    Hello. I was thinking. The collapse of a stellar nucleus into a black hole is an apparent contradiction to the Pauli exclusion principle, right? So which one of those theories fails at that point - quantum mechanics or the theory of relativity? I used to think that it's the theory of...
  20. L

    Collapse of Pauli exclusion principle

    What happens when a neutron star collapses into a black hole and it's no longer obeying the Pauli exclusion principle? In terms of quantum mechanics? Say it collapses because it gets more massive. A "neutron degeneracy pressure" can be calculated, which is what keeps the neutron star from...
  21. D

    Higgs, Fermi-Dirac distribution, and Pauli exclusion principle

    hi, I am studying the Higgs Mechanism these days. And I get two questions. I hope some ones could help me. 1>We know that due to the non-zero VEV, SSB takes place and higgs condensates give masses to bosons and fermions. I wonder that after the SSB and before the universe became as cool...
  22. K

    Older formulation of Pauli exclusion principle

    "older formulation of Pauli exclusion principle" Is there any old formulation of pauli's principle? If so, is it explain that "forbiding the presence of two electrons in the same quantum state"? I need that explanation.
  23. D

    Pauli exclusion principle question

    I have a question in my book where five electrons are placed in a infinite square well and I am supposed to calculate the lowest energies. My problem is with the electron configuration. I think that the first two shall be placed in n=1 and then the rest shall be placed in n=2. This since l=0,1...
  24. S

    What is the Pauli exclusion principle?

    hi i just wanted to ask if anyone could help me understand this principle i have read around and still seem to be getting nowhere. i found this example but its confusing and does not give explanations http://hyperphysics.phy-astr.gsu.edu/hbase/pauli.html#c1 thanks
  25. B

    What would happen to pauli exclusion principle

    Pauli exclusion principle say that no two electrons can occupy the same state. But what would happen if we force them to occupy the same states?
  26. D

    Is pauli exclusion principle a force

    If it takes energy to squeeze matter, then its a force at work right? And what is spin? I can put the quantized nature of spin in the back of my mind for a moment, but what the heck does spin do?
  27. V

    Charge Independence and Pauli Exclusion Principle

    I remember reading about charge independence; about how the energy levels of mirror nuclei (correcting for differences in the colomb term) are identical… I think this suggests that the force between any two nucleons is the same, so the attraction of neutron-proton=proton-proton=neutron-neutron...
  28. W

    Can High Energy Levels Allow Neutrons to Overlap in Black Holes?

    So two identical fermions can't occupy the same quantum state. But if one is same except higher in energy then the quantum wave pattern is different so can occupy the same space. Are there any values on the amount of energy needed to make two neutrons exist in the same space? I'm doing a...
  29. M

    Does the Pauli Exclusion Principle Apply to All Quantum Particles?

    Having looked into neutrinos and the process in which they were found I've started looking more in Wolfgang Pauli himself. I've read into this principle but there are a few things I would like to clear up. I have picked out the information I am interested in learning abou. "The Pauli exclusion...
  30. M

    Is the Pauli Exclusion Principle Universally Applicable?

    I've just been reading my notes on quantum mechanics and got to the Pauli exclusion principle. The way it was explained makes no sense to me. It says "no two identical fermions can be in the same quantum state". Surely there are some restrictions to the applicability of this statement? It...
  31. S

    How far does the Pauli Exclusion Principle apply?

    We know that two particles can't exist in identical quantum states in the same place, fair enough. However, no particle can be sitting directly on top of another to infinite precision. Therefore, you can always say they're some minimum distance away from one another. Now suppose you have...
  32. R

    Understanding the Pauli Exclusion Principle and Its Consequences in an Atom

    hey all, posted this before but no one managed to get it? question off a past paper that it can't do.../ state pauli exclusion principle (can do that) and explain its consequence in an atom (can do that). Use it to determine the values of n, l ml and ms of 10 neon atoms placed in a box at...
  33. A

    Does the Pauli Exclusion Principle Span Distance?

    sometimes i think, would the pauli exclusion principle still apply if the fermions span a very large spatial distance? for example if i have a very very loooong chain of condensed electrons, wouldn't it be possible that the electrons at both ends occasionaly occupy the same quantum state?
  34. A

    Pauli exclusion principle, fermions, bosons

    Hi, I am having trouble understanding these concepts. I checked out some websites but it still doesn't help. First of all what's the main postulate? That there exist 2 different kind of particles: bosons and fermions? What are their fundamental definitions which lead to the fact that an integer...
  35. U

    Super fluids and pauli exclusion principle

    I don't understand how can atoms like heilium-4 which has same amounts of fermions behave like bosons and do not obey pauli's exclusion principle. Do the heilium physically stack up together like laser light, or does it just seem to do that? could someone please explain how superfluids work.
  36. C

    The Pauli exclusion principle catastrophe.

    The "Pauli exclusion principle" catastrophe. :!) Recent works with MOTTS have proved the inability of Pauli's work to account for experimental results on the field of high-temp superconductors. Could anyone add his or her own opinion about this? By the way, have any of you guys received...
  37. K

    Does the Pauli Exclusion Principle Apply to All Types of Particles and Atoms?

    Does it include all types of particles? All atoms? Thank you.
  38. I

    Pauli Exclusion Principle, what does it say?

    I know I already posted this question, but it seems to have gotten "lost" among the other questions in the same thread. This is really confusing me now, so I'll ask it again. The pauli exclusion principle says that no two fermions can be in the same quantum state. But if we have three...
  39. S

    Pauli exclusion principle - does it apply to everything

    The Pauli exclusion principle states that no two fermions can have the same quantum state. My question is, if you could conceivable solve the wave equation for every electron in a large body, perhaps even the entire universe, does this mean that no electrons in that body can have exactly the...
  40. R

    What is the derivation of the Pauli Exclusion Principle?

    I just finished a unit in statistical mechanics, and i have done some second year quantum mechanics and particle physics at university, however we have only been given the Pauli Exclusion principle and told that fermions obey it, but that bosons do not. My question is, does the Exclusion...
  41. R

    Exploring the Possible Mechanisms Behind the Pauli Exclusion Principle

    Could the Pauli exclusion principle be due to a force that has a 1 / r dependency where r is the distance between two electrons. Then in the case of electron degeneracy pressure in neutron stars could we say that uncertainty in momentum x uncertainty in position arises from a repulsive...
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