Talk:Pauli exclusion principle
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Conceptually, it is simple to understand the correspondence between fermions/matter and bosons/[fields; energy; ??]. It's an important point for this article too, I think - it's one of the most important consequences of the Pauli principle. What's the best way to describe it?
On one hand, describing bosons as "fields" is a little misleading, because fermions are also described as fields in QFT. The reason light is classically thought of as a "field" instead of a particle has as much to do with the fact that the photon is massless (hence long-range, hence classically detectable as a field) as the fact that it is a boson. So the distinction isn't too clear.
On the other hand, describing bosons as "energy" is also misleading, because obviously fermions carry energy just as well as bosons.
Thoughts? CYD
I changed "fields" to "non-matter".
There's a hint in Quantum Physics by S. Gasiorowicz that fermions do not require a totally antisymmetric wavefunction if there is sufficient separation. From memory, it said something like: "the reader might expect that if we have one electron on earth and one on the moon, they won't require antisymmetrization... Indeed, even at lattice spacing distances of 5-6 angstroms, antisymmetrization is usually unnecessary".
Unfortunately, the only mathematics presented to support this was a calculation of the amount of overlap in probability densities between distant electrons.
That's as much as I know - I couldn't write an authoritative summary on the matter. If true, it would impact on not only this article, but also identical particles and fermions, and perhaps others.
-- Tim Starling 11 Oct. 2002
I believe he's saying that, under certain circumstances, you can make an approximation of ignoring antisymmetrization. -- CYD
Maybe... I have the book here now, and I can quote the most suggestive statement:
"The question arises whether we really have to worry about this when we consider a hydrogen atom on earth and another one on the moon. If they are both in the ground state, do they necessarily have to have opposite spin states? What then happens when we consider a third hydrogen atom in its ground state?"
I'll try to find some more authoritative information on this. -- Tim
Okay, CYD is right. Sorry everyone. -- Tim
Is the Pauli exclusion principle a complicated way of saying that two things can't be in the same place at the same time?
Pauli exclusion principle seems to be an ADDITIONAL assumption to the quantummechanical principles, since it seems (to me) that there is no proof WHY spin-half particles have anti-symmetric wavefunction and integer spin are symmetric. If this is indeed true, can someone edit the text in this respect? The exclusion principle is explained a thousand times on the web, but (almost) no one mentions this aspect. -- John
One of the results of quantum mechanics is that spin is quantized - the spin of a particle is either an integer or a half-integer times hbar. There is a theorem in relativistic quantum mechanics, called the spin-statistics theorem, which says that particles with integer spin obey Bose-Einstein statistics, whereas particles with half-integer spin ovey Fermi-Dirac statistics, and therefore obey the Pauli exclusion principle. In non-relativistic quantum mechanics, however, the Pauli principle must be postulated (and there is certainly enough experimental evidence to call it an empirical fact.) See identical particles for a little discussion of this. -- CYD
for CYD: do you mean the exclusion principle holds only at short distances? because i just dont fully understand it, if no two 1/2 spin particles can occupy the same quantum state every atom of the same element will be different, and (i dunno much, just a guess) worse since the energy levels are quantized we wont have that many hydrogen atoms in the universe, but we do.
also, forgot to add, (remember i'm only a beginner at this stuff, so dont laugh at my questions), when the electrons in a lithum are not observed, so they remain in "waves", their spin is in superposition, so how can the exclusion principle apply to them??? i mean, doesnt it only work when you have an eigenvalue of the obserable? i know atomes will collapse that way but can you tell me why it doesnt?
thanks
-protecter
A note to questions posed above: Each of those millions and billions of Hydrogen atoms or electrons in various Lithium atoms are in different quantum states. An electron in its ground state in one Hydrogen atom is in an entirely different quantum state (i.e. posesses a different Hamiltonian or energy state) than another electron in a different Hydrogen atom some distance away. In fact, if you were to push two Hydrogen atoms close together, the Pauli exclusion principle predicts that there will arise some pressure between the two as the sates begin to overlap, in order to resist that overlap, and indeed, this pressure is detectable experimentally. The Pep holds - no two fermions can exist in the same state.
--zipz0p
- I am puzzled by this point as well. You (zipz0p) state that two ground-state electrons in two different H atoms are in a different quantum state. But: the article states that for electrons, PEP is equivalent to saying the four quantum numbers cannot all be the same. Which of the four quantum numbers is always different for two ground state electrons in different H atoms? It seems to me (non-physicist) that some implied qualification is being left out. Clarification would be much appreciated! Mrhsj 01:56, 19 January 2007 (UTC)
- The answer to that question is that you need an additional label for which atom the electron is in. In a many-atom situation, the PEP holds for the four quantum numbers plus atom label taken together. The statement in the article about the four quantum numbers only holds for a single atom. Another way of thinking about the atom label I talk about above is to introduce a continuous position variable that marks the center of mass of the atom the electron is in. In summary, the PEP holds for the complete set of quantum or classical coordinates required to uniquely identify a particle. I hope this helps. -- Custos0 01:19, 2 March 2007 (UTC)
- Yes - that's what I was looking for. Thanks! Mrhsj 05:08, 2 March 2007 (UTC)
- The answer to that question is that you need an additional label for which atom the electron is in. In a many-atom situation, the PEP holds for the four quantum numbers plus atom label taken together. The statement in the article about the four quantum numbers only holds for a single atom. Another way of thinking about the atom label I talk about above is to introduce a continuous position variable that marks the center of mass of the atom the electron is in. In summary, the PEP holds for the complete set of quantum or classical coordinates required to uniquely identify a particle. I hope this helps. -- Custos0 01:19, 2 March 2007 (UTC)
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[edit] Pep vs. PEP
I think since the Exclusion Principle is a recognised title, then this article should reside under Pauli Exclusion Principle, rather than Pauli exclusion principle. I have been reading many published texts recently on the subject and all seem to use the capitalised version. What does everyone think? - Drrngrvy 16:26, 6 January 2006 (UTC)
Since neither David J. Griffiths nor Richard L. Liboff capitalize the first letters of the whole Pauli exclusion principle, but rather write it in the form already in use in this article, I am inclined to say: leave it as is, if only for consistency. --zipz0p
[edit] Other effects of the Pep
I think the article should perhaps say something about the fact that the PEP is the primary reason that material objects collide macroscopically and that we can stand on the ground, etc. It is commonly held that this is due to electromagnetic forces, but in fact, the dominant force is a product of the PEP: electrons in the atoms of the separate surfaces will effectively repel one another as the surfaces approach and the electrons come closer to occupying the same state (which is forbidden by the Pep). --zipz0p
[edit] 1924 or 1925?
The article can't make up its mind. Which is it? --Michael C. Price talk 20:04, 17 October 2006 (UTC)
[edit] Fermi pressure?
How about discussing "matter occupies space exclusively for itself and does not allow other material objects to pass through it" in terms of Fermi pressure keeping matter apart? It might be useful to point out how much denser a Neutron Star is, where gravity overcomes to Fermi pressure of the electrons to give a star of the density of an atom's nucleus. Custos0 01:28, 2 March 2007 (UTC)
