- #1

shounakbhatta

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Fermions have a +1/2 and -1/2 spin. Is there any reason for that or is it just an intrinsic property?

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- Thread starter shounakbhatta
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In summary, spin is intrinsic to particles and it follows directly from the nature of orbital angular momentum.

- #1

shounakbhatta

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Fermions have a +1/2 and -1/2 spin. Is there any reason for that or is it just an intrinsic property?

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- #2

Simon Bridge

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Particles may have whole or half integer spin, the half-integer ones obey the Pauli exclusion principle (the others don't), we call them Fermions, historically, because they obey Fermi-Dirac statistics.

... though there is a niggle in the back of my mind... but I think it's about how 1/2 integer spin means PEP holds. Probably someone else will tell me what I'm almost remembering...

- #3

andrien

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you should say that fermions have half integral spins.The reason why fermions obey fermi-dirac statistics can be answered using qft.If fermions will obey bose statistics then there will not be a minimum energy state.Also if bosons will be treated using fermi statistics then you will find that observables will not commute if they are separated by a space like intervals.Fermions have a +1/2 and -1/2 spin

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Simon Bridge

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Naty1

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Fortunately we have mathematical models which at least give us insights... but not complete understanding.

With regard to spin, the Stern-Gerlach experiment confirmed the intrinsic nature of this 'strange' momentum particle characteristic. At the time of the experiment, it was hypothesized, that is guessed, that this characteristic existed and that's what likely lead to the experiment.

It's a typical science story, and an interesting one: Check just the first few paragraphs in each article linked below for a good 'feel' how such progress occurs.

Wikipedia says this :

The old quantum theory is a collection of results from the years 1900–1925 which predate modern quantum mechanics.The theory was never complete or self-consistent, but was a collection of heuristic prescriptionswhich are now understood to be the first quantum corrections to classical mechanics.

Spin is one of those 'quantum corrections' distinct from our macroscopic world.

The boldface statement is especially interesting as it pretty well describes the Standard Model of particle physics even today: an amalgam of quantum theory, special relativity, and experimental observations. I believe there remain discussions whether even the current model is consistent: One thing we know is that gravity doesn't fit yet.http://en.wikipedia.org/wiki/Stern–Gerlach_experiment

http://en.wikipedia.org/wiki/Old_quantum_theory

- #6

ZealScience

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For the AM, in some particle field rotation of 2π gives non-detectable results, whereas some give rise to negative sign. The former must have integer values of AM since in the transformation there is AM times rotation angle in the argument of a complex exponential. By Maths, it must be integer. And the latter has half integer values. Spin AM follows directly due to the nature of orbital AM.

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Simon Bridge

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That's intreguing... photons have integer spin - do they have orbital angular momentum to be ignored to find the spin? - or for their spin to follow directly due to?Spin angular momentum is the angular momentum left after orbital AM is ignored. ... Spin AM follows directly due to the nature of orbital AM.

- #8

Naty1

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more here for those interested:...supersymmetry exchanges particles of different spin...because their spins are different bosons and fermions transform differently in space...supersymmetry transformations must involved space and time in order to compensate for this distinction...

Fermionic particles have half integer spin, while bosonic particles have integer spin...a supersymmetry transformation turns a fermion into its partner and a boson into its partner fermion...Supersymmetry is a feature of the theoretical description of these particles...{in}a supersymmetry transformation that interchanges bosons and fermions the equations will all end up looking the same...

In a theory with unbroken supersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internal quantum numbers (other than spin), and vice-versa.

You should read that last quote again! I don't think this is yet confirmed experimentally...maybe Wiki says...

Note: WARPED PASSAGES is the only book I own on particles...and most of their characteristics and interactions... fortunately its a great non mathematical discussion of the subject, including string theory and hidden dimensions. Likely still available used, cheap, online.

- #9

Simon Bridge

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The spin of a particle is an intrinsic property that determines how it interacts with other particles. Fermions have a spin of either +1/2 or -1/2 because of the fundamental nature of quantum mechanics. This spin value is a result of the Pauli exclusion principle, which states that no two fermions can occupy the same quantum state simultaneously. This property of fermions is crucial in explaining the structure and behavior of matter.

The spin of a fermion is determined by its quantum numbers, specifically its spin quantum number. This quantum number can take values of either +1/2 or -1/2. The spin value of a fermion is also related to its intrinsic angular momentum, which is a fundamental property of all particles.

No, there are no exceptions to fermions having a spin of +1/2 or -1/2. This is a fundamental property of fermions that is consistently observed in all known particles. In fact, the existence of particles with half-integer spin is one of the defining characteristics of fermions.

The spin of fermions plays a crucial role in determining their behavior and interactions with other particles. For example, fermions with different spin values can have different magnetic properties, which can affect how they interact with magnetic fields. Additionally, the spin value of fermions is also related to their stability and energy levels.

According to current understanding, the spin of fermions is a fundamental property that does not change. However, in certain extreme environments, such as near black holes, the spin of particles can be altered due to strong gravitational forces. Other than these rare occurrences, the spin of fermions is considered a constant property of particles.

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