4 component spinor isomorphic to S^7?

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Discussion Overview

The discussion revolves around the relationship between 4-component spinors and the topological structure of spheres, specifically whether the space of all 4-component spinors with norm 1 is isomorphic to S^7. The conversation touches on the nature of Dirac spinors and their classification under various spin groups.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant suggests that since S^3 is isomorphic to the set of all 2-component spinors with norm 1, it might follow that the space of all 4-component spinors with norm 1 is isomorphic to S^7.
  • Another participant argues that "4 component spinor" lacks specificity, noting that Dirac spinors transform under a reducible representation of Spin(3,1) and do not correspond to a compact space like a sphere.
  • This participant also mentions that there are various types of 4-component spinors associated with different spin groups (e.g., Spin(4), Spin(5), Spin(4,1)), but none of these are topologically S^7.
  • Another contribution indicates that the manifold defined by the condition \bar{\Psi}\Psi=1 is actually S^{3}×ℝ^{4}, which suggests a different topological structure than S^7.

Areas of Agreement / Disagreement

Participants express differing views on the isomorphism of 4-component spinors to S^7, with no consensus reached. The discussion remains unresolved regarding the topological classification of these spinors.

Contextual Notes

The discussion highlights the ambiguity in the term "4 component spinor" and the implications of different spin groups on the topological properties of the associated spaces. There are also references to specific mathematical structures that may not align with the assumptions made by participants.

Spinnor
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I was told the space S^3 is isomorphic to the set of all 2 component spinors with norm 1 (see https://www.physicsforums.com/showthread.php?t=603404 ). Can I infer that the space of all 4 component spinors with norm 1 is isomorphic to S^7?

If so is a Dirac spinor isomorphic to S^7?

Thanks for any help!
 
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"4 component spinor" is not specific enough.

2-component spinors transform under Spin(3) which is isomorphic to SU(2), hence S^3. Dirac spinors transform under a reducible rep of Spin(3,1), which is going to be some non-compact space, not a sphere. But there are other 4-component spinors, such as those in Spin(4), Spin(5), or Spin(4,1). None of these are topologically S^7, though.

S^7 is the set of unit octonions, which don't have a group structure (due to the failure of associativity).
 
Spinnor said:
I was told the space S^3 is isomorphic to the set of all 2 component spinors with norm 1 (see https://www.physicsforums.com/showthread.php?t=603404 ). Can I infer that the space of all 4 component spinors with norm 1 is isomorphic to S^7?

If so is a Dirac spinor isomorphic to S^7?

Thanks for any help!

Topologically, the manifold defined by \bar{\Psi}\Psi=1 is S^{3}\times \mathbb{R}^{4}.

Sam
 
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Thanks to both of you, Ben and Sam, for clearing that up!

What a gem Physics Forums is, ask almost any question and get answer.
 
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