What Is the Significance of Electron Spin in Quantum Mechanics?

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Discussion Overview

The discussion centers on the significance of electron spin in quantum mechanics, exploring its theoretical foundations, implications in experiments such as the Stern-Gerlach experiment, and its relationship with quantum electrodynamics (QED) and the Dirac equation. Participants raise questions about the nature of spin, its prediction, and its role in various quantum theories.

Discussion Character

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

Main Points Raised

  • One participant questions why electrons with non-zero angular momentum in silver atoms do not cause additional beam splitting in Stern-Gerlach experiments, suggesting a need for clarification on the role of intrinsic angular momentum.
  • Another participant inquires whether QED predicts electron spin, indicating a desire to understand the theoretical underpinnings of spin in quantum mechanics.
  • Some participants assert that QED and the Dirac equation are based on the existence of spin, which was experimentally observed prior to the development of these theories, implying that spin is assumed rather than derived.
  • Discussion includes the historical context of Dirac's work, noting that he extended Pauli's formulation of spin into a relativistic framework, which led to the prediction of antimatter.
  • One participant mentions that Majorana derived a Dirac-like equation for photons with spin 1, suggesting ongoing exploration of spin in different contexts.
  • There is a reference to the relationship between Dirac's gamma matrices and Pauli's spin matrices, indicating a technical connection that may warrant further exploration.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the origins and implications of electron spin, with some agreeing on the historical context of spin's development while others raise questions about its theoretical predictions. The discussion remains unresolved with multiple competing views on the nature of spin and its treatment in quantum theories.

Contextual Notes

Participants highlight that the understanding of spin is influenced by empirical observations, and there is a recognition of the limitations in deriving spin from first principles within the existing frameworks. The discussion also reflects on the historical development of theories without reaching a consensus on the foundational aspects of spin.

foobar
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QM texts explain electron spin as an intrinsic spin with intrinsic angular momentum of electron.

And this theory predicts (or postdicts maybe?) splitting of silver or hydrogen atom or electron beams in Stern Gerlach experiments.

a few questions:

1) intrinsic angular momentum with its magnetic moment is invoked to explain the beam splitting in a nonuniform magnetic field. But many electrons in a sivler atom also have non zero angular momentum and associated magnetic moment. Why does this not also cause beam splitting in SG experiment into more than two beams ??

2) Does QED predict electron spin?

3) Does the Dirac equation predict electron spin or handle it, and how or any link to an explanation would be good.

tia
 
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QED and Dirac eq. are based on the existence of spin, spin is found experimentally before one developed theory.
 
malawi_glenn said:
QED and Dirac eq. are based on the existence of spin, spin is found experimentally before one developed theory.

In other words, QED and the Dirac equation do not derive or predict the existence of spin, but assume it as an empirical fact.
 
Dirac wanted an equation for describing the quantum-relativistic motion of electron. He found that for obtaining an hamiltonian linear in mass and momentum he must working in (at least) 4-dimensional space. Those additional dimensions provide the existence of spin and antimatter.

Althought spin was alreay known (and so surely it had to appear in the equation) there is littley logically-derived in the formalism.

Ll.
 
Spin was already formulated by W. Pauli before Dirac did this equation, the thing Dirac did was to extend Paulis eq. into special relativity, and hence he got the negative energy solution, which he postulated were the existence of antimatter.
 
After Dirac italian Maiorana has derive a Dirac like equation for the photon with spin 1.

If you know the experiment results you can slant almost every theory to these results, remember Ptolemeus Universe :)
 
malawi_glenn said:
Spin was already formulated by W. Pauli before Dirac did this equation, the thing Dirac did was to extend Paulis eq. into special relativity, and hence he got the negative energy solution, which he postulated were the existence of antimatter.

i think you are right, thanks for precisation.

Ll.
 
Llewlyn said:
i think you are right, thanks for precisation.

Ll.

also recall the relation between Dirac's gamma matrices and Paulis spin matrices.
 

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