Talk:Fermion

From Wikipedia, the free encyclopedia

	Physics Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
Start	This article has been rated as Start-Class on the assessment scale.
High	This article is on a subject of High importance within physics.
Help with this template Comments: edit – history – watch – refresh I gave the article the start rating as it is very short, but still bigger than a stub. The importance rating was set to high as fermions are very important within the field of particle physics! Snailwalker | talk 00:51, 15 October 2006 (UTC)

Contents 1 bose einstein condensate 2 Pauli exclusion principle 3 definition of fermions as particles requiring anti-symmetric states 4 Fermions with odd integer spin 5 Bosonic field and Fermionic field articles 6 Elementary particles?? 7 Please Explain 8 Confusing paragraph

[edit] bose einstein condensate

So a grain of sand prevents another grain of sand from occupying the same location.

Unless it's cold enough to form a bose einstein condensate!

BECs are formed out of bosons! Hence "bose." However, fermions can form condensates -- but they are condensates in momentum space only, not physical space. (i.e., the condensate has lots of fermions with the same momentum, but they are not all in the same place), so the Pauli ex. principle is not violated.

Can you explain this or point to a reference? As far as I'm aware, there's no difference in this respect between position space and momentum space -- antisymmetry in both cases requires the wavefunction to vanish at equal position/equal momentum. Fpahl 06:15, 8 Oct 2004 (UTC)

[edit] Pauli exclusion principle

The Pauli exclusion principle doesn't always work, according to my quantum textbook. See my discussion in talk:Pauli exclusion principle. -- Tim Starling

The result of that discussion was that the textbook meant not that it doesn't always work, but that in some circumstances it's a safe approximation to ignore it in calculations. Tim agreed, so I guess the comment above is just a leftover. Fpahl 06:15, 8 Oct 2004 (UTC)

[edit] definition of fermions as particles requiring anti-symmetric states

I'm wondering whether the article's definition of fermions as particles requiring anti-symmetric states is the best approach. According to the spin-statistics theorem, this is equivalent with having half-integer spin. But if a particle were found to violate that theorem, say a particle with integer spin and anti-symmetric wavefunction (I'm aware that it's not clear how this could be and it would probably require a major change in quantum field theory), would we say "we found a fermion whose spin isn't half-integer", or would we say "we found a boson whose state isn't symmetric"? I think the latter, which would imply that the definition of a fermion is a particle with half-integer spin. This is also how Eric Weisstein's World of Physics defines it. (The same applies, obviously, to bosons; I added a link to this discussion on their talk page.) Fpahl 06:15, 8 Oct 2004 (UTC)

I disagree with you. For me, a fermion is an anti-commuting operator (in "second quantization" formalism) and an expression like "spinless fermion" is not self-contradictory. _R_ 14:54, 8 Oct 2004 (UTC)

You're right. A survey of my QFT textbooks yielded overwhelming evidence for this view, with two books explicitly stating the spin-statistics theorem as saying that fermions have half-integer spin, not that they anti-commute. I could be forgiven, though, since Kaku in "Quantum Field Theory: A Modern Introduction", one of the standard texts, writes: "To demonstrate the spin-statistics theorem, let us quantize bosons with anticommutators and arrive at a contradiction. ... Likewise, one can prove that fermions quantized with commutators violates microcausality." This only makes sense if "fermions" is taken to mean "particles with half-integer spin".

Do I understand correctly that in the above admittedly unlikely scenario you would be inclined to say "we found a fermion whose spin isn't half-integer"? Fpahl 15:24, 8 Oct 2004 (UTC)

I would disagree with anticommutation rules - they do not always define fermions. What you say is true if you consider non-interacting systems. If you take bosons on a loop interacting via Dirac-delta potential, then 1) they have ,,Fermion comutation rules (they cannot occupy the same quantum state because of interaction) 2) they have still symmetric wave function.

Interestingly, our own article on the spin-statistics theorem takes the other view. Would you agree it should be changed accordingly? Fpahl 15:28, 8 Oct 2004 (UTC)

A new twist to the story: I emailed mathworld about this since your view suggests that their definitions are wrong, and they sent me this:

The Particle Data Group controls these definitions.  Here are the
definitions from Particle Data Group: 

Fermion: Any particle that has odd-half-integer (1/2, 3/2, ...)
intrinsic angular momentum (spin), measured in units of h-bar. All
particles are either fermions or bosons. Fermions obey a rule called the
Pauli Exclusion Principle, which states that no two fermions can exist
in the same state at the same time. Many of the properties of ordinary
matter arise because of this rule. Electrons, protons, and neutrons are
all fermions, as are all the fundamental matter particles, both quarks
and leptons.

Boson: A particle that has integer intrinsic angular momentum (spin)
measured in units of h-bar (spin =0, 1, 2, ...). All particles are
either fermions or bosons. The particles associated with all the
fundamental interactions (forces) and composite particles with even
numbers of fermion constituents (quarks) are bosons.

Has the Particle Data Group got it wrong?! Fpahl 17:07, 8 Oct 2004 (UTC)

I would argue that the PDG is primarily concerned with particle phenomenology, and thus intends their definition to discriminate solely between existing particles (for which the distinction between the definitions in question is moot). In QFT, it's certainly the case that when we say "fermion" we mean "fields represented by anticommuting Grassman numbers". If we want to talk about a particle that happens to have spin-1/2 but commutes, we'd call it a "pseudofermion". So one might argue that both characteristics are necessary for a particle to be a fermion, and thus that there are really four classes of particle: fermion, boson, pseudofermion and pseudoboson; only two of which actually exist. -- Xerxes 18:53, 2004 Oct 8 (UTC)

(Note: I wrote the following before seeing Xerxes' answer) Of course, it's got it wrong! What else do you expect from particle physicists? Seriously though, the flaw in this definition is the following: how do you count the number of fermion constituents in a phonon?

The more I think of it, the more I believe it depends on whether you focus on individual particles (in which case fermion=half-integer spin) or on their collective behaviour (in which case fermion=anticommuting operator=antisymmetric wavefunction). _R_ 19:05, 8 Oct 2004 (UTC)

[edit] Fermions with odd integer spin

We have all noticed that spin is described as being a multiple of hbar/2. I thought that it would be better to set this value to a constant giving,

hdot = hbar/2 = 5.2728584118222738157569629987­554e-35 J.s

But now the equations for spin did not work with hdot, so I had to correct them.

Here are the corrected equations,

|sv| = sqrt(s(s + 2)) * hdot

and

Sz = ms.hdot

where,

sv is the quantized spin vector,

|sv| is the norm of the spin vector,

s is the spin quantum number, which can be any non negative integer,

Sz is the spin z projection,

ms is the secondary spin quantum number, ranging from -s to +s in steps of two integers

For spin 1 particles this gives:

|sv| = sqrt(3).hdot and Sz = -hdot, +hdot

For spin 2 particles this gives:

|sv| = sqrt(8).hdot and Sz = -hdot, 0, +hdot

Now that the spin equations have been corrected, the definitions for fermions and bosons are incorrect, and must be redefined as follows.

Fermions are particles that that have an odd integer spin.

Bosons are particles that have an even integer spin.

Would these redefinitions have any other effects on the Standard Model? Can these redefinitions explain any currently unexplained phenomena? Are there any experiments that could confirm or refute these claims?

I would like eveyone to have a good think about this, and give me your objections to it, or even data to support it.

The change you're proposing is a trivial renormalization; physicists are perfectly happy with the convention as it stands, tho mathematicians studying lie groups tend to use a normalization with steps of two. -- Xerxes 17:47, 2005 Jun 8 (UTC)

• • Actually, the above explanation is no different than the Standard Model except in notation. In spin-statistics theorem, the only difference is the word "net" where fermions have a "net" energy rotation spin and bosons are particles where the "net" energy rotation cancels in the composite matter structures. The notation of the Standard Model was developed through the evolution of theory. Bosons were discovered first so assigned integer values beginning with 1 as is the common mathematical convention. Had fermions been discovered first, then fermions would have been denoted 1 and no doubt, bosons, would have taken even numbers to show that the net spin effect cancels. Scientific and mathematical notation often simply follows the legacy of the evolving discoveries i.e. s,p,d for azimuthal quantum numbers makes no real sense except it is legacy notation.--Voyajer 19:56, 21 December 2005 (UTC)

[edit] Bosonic field and Fermionic field articles

We need some help defining just what a bosonic field is, and what a fermionic field is. Please see the discussion pages for Bosonic field and Fermionic field. Thanks. RK 19:56, 21 May 2006 (UTC)

[edit] Elementary particles??

I am an ignorant of physics, so I do not understand these two sentences in the article, which look contradictory to me: "[fermions] are sometimes said to be the constituents of matter."

"All observed elementary particles are either fermions or bosons."

The first sentence seems to exclude bosons, and it is stated the same in other articles in Wikipedia ("quark", for example). Will anybody be so kind to explain? Thanks 200.55.118.233 23:42, 31 August 2006 (UTC) Nahuel

[edit] Please Explain

The paragraph begining "Of course..." is not helpfull to the uninitiated (I speak as someone visiting the page without a prior background in the subject matter). If there is a more appropriate topic on why "determining the fermionic or bosonic behavior of a composite particle (or system) is only seen at large (compared to size of the system) distance", perhaps there should be a link to it.

[edit] Confusing paragraph

The article has the following paragraph:

All observed elementary particles are either fermions or bosons. A composite particle (made up of more fundamental particles) may either be a fermion or a boson, depending only on the number of fermions it contains:

   * Composite particles containing an even number of fermions are bosons, such as a meson, or the nucleus of a carbon-12 atom.
   * Composite particles containing odd number of fermions are fermions, such as a baryon, or the nucleus of a carbon-13 atom.

I could not understand how an even number of fermions are bosons, while all elementary particles are either fermions or bosons. Also I could not understand how the fermions have only odd number of fermions. It doesn't make sense.

I guess it could be poor choice of words. Otherwise please explain further about how these particles behave. —The preceding unsigned comment was added by Tenri (talk • contribs) 04:10, 10 March 2007 (UTC).

I think the wording is confusing--fermion simply describes any particle with half integer spin so it can describe both elementary as well as composite particles. Primary fermions are quark and leptons. 3 quarks together is a baryon which is a composite fermion (because it has half spin). 2 quarks together makes a meson (having integer spin). I will edit the text to try to clarify. Can someone please double check and see if it makes sense? ThanksTensegrity 23:45, 2 April 2007 (UTC) Ah, thank you for correcting my brain fart re: the atomic weight/nbr of nucleons. :) Tensegrity 18:10, 4 April 2007 (UTC)