What is Pauli's exclusion principle: Definition and 26 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. J

    I Two hydrogen atoms and Pauli's exclusion principle

    Hello, I recently came across the following (apparent, I hope) paradox: suppose we have two H atoms. Now, a hydrogen atom is made up of one proton and one electron (fermions), so it is a boson. Then one could have two hydrogen atoms which are in the exact same state (including position). This...
  2. R

    B Pauli's exclusion principle

    So my question is why can't 2 object be at the exact same potion, (i.e. overlap). Why can't a +ve quark and electron just merge. In an universe where there is no force caused due to charge, why can't we just walk through a solid wall.
  3. J

    B Collision of identical fermions

    What happens if I shoot a fermion at another identical fermion at rest? For example, do the fermions stick together, or do they bounce? Let's ignore gravity, electro-magnetism, weak force and strong force. Edit: We are considering the Pauli exclusion principle.
  4. gennarakis

    I Quantum Kinetic Energy of Neutrons, Protons and Electrons

    Hi there, I have a problem to solve in Cosmology which says: "Write the formulas for the quantum kinetic energy of neutrons, protons and electrons as well as the formula for the gravitational energy for a neutron star that is comprised of free neutrons, protons and electrons in a ratio of Nn ...
  5. J

    Electrons, Muons and Pauli Exclusion

    Lepton Universality and Pauli Exclusion Put in a possibly oversimplified way, lepton universality says that electrons, muons, and taus all behave in the same way except for mass effects. The question is “Does this apply to Pauli exclusion?” Due to the Pauli exclusion principle, only two...
  6. N

    Photon excitation to full orbitals

    In introductory physics and chemistry, photon excitation is usually illstrated with a simple hydrogen molecule. I am wondering what happens if an electron is excited to an orbital that is already full. Would the orbital split up into different energy levels as hybridisation, so as not to violate...
  7. A

    Violation of Pauli's Exclusion Principle possible?

    I was reading some simplistic explanations of Pauli's Exclusion Principle (PEP) to explain a group of non-science people, and I came across this: For Fermions, even as pressure builds, no two can be located in the same energy state. This causes them to "stack up" in effect. Only under great...
  8. avito009

    Can the Pauli Exclusion Principle be explained through quantum states?

    I was reading about the Pauli Exclusion Principle and I had a doubt. This principle tells us that a maximum of two electrons can be present in an energy level and the spin of the electrons has to be in the opposite direction. But S orbital can hold a maximum of two electrons. This is fine but...
  9. H

    Fundamental forces and Pauli's exclusion principle

    To which type of Fundamental forces below the Pauli's exclusion belong? Strong nuclear forces Coulomb forces Weak nuclear forces Gravitational forces
  10. Ravi Mohan

    Pauli's exclusion principle

    How do we prove Pauli's exclusion principle? My professor makes a Slater determinant and then merrily shows how it disappears when two columns or rows are same. That is not Pauli's principle, is it? It is based on an assumption that certain particles are described by certain states. So my...
  11. A

    Scope of Pauli's Exclusion Principle

    What is the scope of Pauli's exclusion principle? When we say two particles in a system cannot have the same quantum state, how do we choose the system?
  12. P

    Using Wolfgang Pauli's Exclusion Principle for communication

    has anyone worked on using the spin of certain sister pairs of subatomic particles as digital communication that is that to have a device that spins one sister particle one way or another, that spins its sister particle in a receiver on the other side of the world or universe where a sensors...
  13. E

    Can Pauli's Exclusion Principle be Explained? A Decade Later

    Hi, I have been reading Bill Bryson's A Short History of Nearly Everything and have got to the bit about Pauli's Exclusion Principle. It states that 'certain pairs of subatomic particles, even when separated by the most considerable distances, can instantly 'know' what the other is doing.'...
  14. S

    Me not falling through the ground and Pauli's Exclusion Principle

    I explained to myself that I don't fall through the ground due to electrons repelling. Using classical electrostatic repulsion. Once in a while I hear it explained through Pauli's exclusion principle (PEP). Do we need PEP to explain this, or is classical electrostatics enough?
  15. S

    Pauli's exclusion principle and quantum entanglement

    The Pauli exclusion principle: is the quantum mechanical principle that no two identical fermions (particles with half-integer spin) may occupy the same quantum state simultaneously Quantum entanglement: the type of interaction is such that each resulting member of a pair is properly...
  16. T

    Pauli's exclusion principle Intrpretation

    Hi I spent my last couple hours reading books and browsing this forum and web to get answer on my question, obviously in vain: I know that wave function for fermions should be antisymmetric, I know fermions spin should be half integer otherwise causality will broken (but I don't why yet)...
  17. G

    What is Pauli's Exclusion Principle and How Does It Apply to Fermions?

    I know the principle says that no two fermions canNot have the same atomic number, but could you explain that in detail?
  18. A

    Pauli's Exclusion Principle - arbitrary?

    Pauli's Exclusion Principle - arbitrary?? I have a puzzle about Pauli's Exclusion Principle, which, as far as I understand it, (being neither mathematical nor a physicist), states that fermions (as opposed to bosons) can never occupy the same space at the same time - well something along those...
  19. N

    Pauli's exclusion principle and cooper pairs

    Pauli exclusion principles states and I paraphrase; No two fermions can occupy the same state; That being said, how can cooper pairs exist? Cooper pairs are when two fermions(electrons in this case) bound together ; If they are bound together, then they must occupy the same state;
  20. S

    Exploring the Implications of Pauli's Exclusion Principle in Astrophysics

    Does the exclusion principle mean that no atom and sub atomic particles can exist at the same place? Can I safely say that in stars the gravity is opposed by this aspect or implication of the principle?
  21. P

    Pauli's exclusion principle?

    There must be a force repulsing the electrons for no more than two in the 1s state for example. What force is that? How is this principle connected to the HUP?
  22. C

    Help me understand how does Pauli's exclusion principle work

    It's the second time I've posted this message, and my english simply sux, so excuse me if I will not be able to explain my misunderstanding quite clear. Let's imagine a verry long metal cable, which is heated at one head. The quasi-free electrons at that head, as their energy increases, must...
  23. J

    Pauli's exclusion principle and fine structures

    Can anybody help me understand how pauli's exclusion principle helps explain the existence of fine line structures in the emmission spectra for atoms, which could not be explained by Bohr's model.
  24. T

    Why Pauli's Exclusion Principle?

    Hi. I'm a sophomore undergrad studying chemistry at Rice University. I recently took quantum mechanics... and I now realize I should have gone into physics rather than chemistry. Ok. Whatever. Bosons are particles with integer spins (helium-4 atoms). Fermions are particles with half-odd...
  25. M

    Pauli's exclusion principle bosons

    I've heard quite abit of things about bosons and am quite confused. The biggest thing which distinguishes fermions from bosons, would be Pauli's exclusion principle. But I've also heard things about bosons having half- integer, while fermions have interger spin, among many others. I've also...
  26. D

    What is the basis for Pauli's exclusion principle?

    Pauli's exclusion principle states that no two fermions may occupy the same quantum state. But why is this so? Is there a "why" explanation, as opposed to merely saying that this is the way it is?