Need Help on Particle Spin

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Discussion Overview

The discussion centers around the concept of particle spin, particularly the nature of intrinsic spin in elementary particles, its relation to Special Relativity and Quantum Mechanics, and the implications of these ideas in theoretical physics. Participants explore the definitions, interpretations, and underlying principles of spin, including its distinction from classical rotation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that all elementary particles possess intrinsic spin/angular momentum, while others note exceptions, such as the Higgs boson, questioning whether spin zero qualifies as intrinsic spin.
  • There is a claim that the existence of particle spin is a consequence of Special Relativity, but this assertion is challenged with requests for clarification and references.
  • Participants discuss the connection between spin and quantum mechanics, mentioning the spin-statistics theorem and the classification of particles as bosons or fermions based on their spin.
  • One participant suggests that the terminology of "spin" may be misleading, arguing that elementary particles do not literally spin but exhibit non-classical degrees of freedom that behave similarly to angular momentum.
  • Another participant raises the possibility of confusion between spin and Thomas Precession, indicating a need for clarity in definitions.
  • There is a discussion about the implications of group theory in understanding spin, particularly how particles transform under rotations and the significance of the Poincaré group in this context.

Areas of Agreement / Disagreement

Participants express differing views on the nature of particle spin, with some asserting its intrinsic quality while others argue against the classical interpretation of spin. The discussion remains unresolved with multiple competing views present.

Contextual Notes

Some claims rely on specific interpretations of quantum mechanics and relativity, and there are unresolved questions regarding the definitions and implications of spin versus rotation. The discussion also touches on advanced topics such as group theory, which may not be fully explored.

LarryS
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TL;DR
Why do particles spin all the time?
All elementary particles have an intrinsic spin/angular momentum. The fact that particles spin at all is due to Special Relativity. How MUCH they spin, half-integer multiples of reduced Planck's Constant, is due to Quantum Mechanics. Right?

Apparently, the reason particles spin at all is because two non-colinear Lorentz Boosts are the same as one Lorentz Boost followed by a rotation. How do you go from that to all particles spin all the time?

Thanks in advance.
 
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LarryS said:
All elementary particles have an intrinsic spin/angular momentum.
Except those that don't, such as the Higgs. Unless you're counting spin zero as having "intrinsic spin/angular momentum".

LarryS said:
The fact that particles spin at all is due to Special Relativity.
Why do you think this?

LarryS said:
How MUCH they spin, half-integer multiples of reduced Planck's Constant, is due to Quantum Mechanics. Right?
There is a spin-statistics connection which is believed to be due to quantum field theory, that particles with integer spin are bosons and particles with half-integer spin are fermions.

LarryS said:
Apparently, the reason particles spin at all is because two non-colinear Lorentz Boosts are the same as one Lorentz Boost followed by a rotation.
Where are you getting this from? Do you have a reference?
 
LarryS said:
TL;DR Summary: Why do particles spin all the time?

All elementary particles have an intrinsic spin/angular momentum. The fact that particles spin at all is due to Special Relativity.
In addition to #2:
Elementary particles don't spin. Many of them have non-classical degrees of freedom which are named "spin", because their quantum behavior under application of Lorentz transformations (especially spatial rotations) resembles that of angular momentum. The polarizations of light are a well known example of spin. They readily appear in the classical treatment when we regard light as EM waves. You know that a linear polarization of an EM plan-wave in vacuum doesn't "spin", right?

LarryS said:
Apparently, the reason particles spin at all is because two non-colinear Lorentz Boosts are the same as one Lorentz Boost followed by a rotation.
Could it be that you are confused between spin and Thomas Precession?
 
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LarryS said:
How do you go from that to all particles spin all the time?
This question looks like if you were asking for why are particles spinning, do not confuse spin (intrinsic quantum property) with spatial rotation, particles cannot rotate.
 
Spin is angular momentum. Electrons in atoms can change their quantum states by flipping from up to down and at the same time changing their orbital quantum state and emitting or absorbing a photon.

The unit of half-h-bar arises from representations of the symmetry group involved. Specifically the rotational part of the Poincaré group. (Brace yourself. I'm linking a wiki page.)

https://en.wikipedia.org/wiki/Poincaré_group.

Group theory is a lovely but very large subject. But, in extremely shortened form, a particle sits in a representation of the group. It changes into a symmetry related version of itself under rotations. Under rotation, an electron changes into an electron with spin pointing differently. If it didn't sit in a rep then rotations would split up the parts that did sit in reps.

Electrons, being spin half, require 720 degrees of rotation to get back to their original configuration. Photons only require 360 degrees. This is related to the Pauli exclusion principle. But, as I said, this is a very large and very lovely subject. Group theory is one way to deal with a large category of symmetries. And symmetry is one of the more powerful means of understanding things.
 

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