Measuring Spin Component of Particle with S=1 & Sz=1

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Discussion Overview

The discussion revolves around measuring the spin component of a particle with spin quantum number S=1 and its projection Sz=1, specifically focusing on the probabilities of obtaining various spin values when measured along a direction at an angle Q to the z-axis. The conversation explores general methods applicable to arbitrary spin values.

Discussion Character

  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the probabilities of measuring different spin components for a particle with S=1 and Sz=1 when measured at an angle Q to the z-axis.
  • Another participant suggests that a system with spin J can be treated as 2J spin 1/2 particles, explaining how to calculate the probabilities of projections along the z-axis based on the orientation of spins.
  • A method for calculating the probability of a specific projection K is presented, involving combinatorial factors and the probabilities of individual spins being "up" or "down".
  • One participant expresses enthusiasm about the method and asks whether it was newly conceived or previously learned.
  • A later reply reveals that the method was derived from a past assignment, indicating its educational context.
  • Another participant mentions that this method is effective for deriving rotation matrices for spin 1 particles by using a tensor product approach with spin 1/2 basis states.

Areas of Agreement / Disagreement

Participants appear to agree on the validity of the method discussed for calculating probabilities, but there is no explicit consensus on the broader implications or applications of the approach. The discussion remains exploratory without definitive conclusions.

Contextual Notes

The discussion does not resolve potential limitations regarding the assumptions made in the calculations or the dependence on specific definitions of spin states. The applicability of the method to arbitrary values of S is also not fully explored.

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For a particle with S = 1 in a state of Sz = 1, what are the probabilities to obtain the various values of the spin component if measured along a direction which makes an angle Q relative to the z-axis?

Is there a general method for an arbitrary value of S?
 
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In general, a system with spin J (and maximum projection along the z-axis of J) can be treated as 2J spin 1/2 particles. A projection of J along z would correspond to all of these pointing up.

If we rotate by an angle &phi;, then each spin 1/2 has a probability P(+)=[cos(&phi;/2)]^2 of having projection "up" along z, and a probability P(-)=[sin(&phi;/2)]^2 of being "down". For that whole system to have projection K (where |K|<=2J), then we will have J+K spins pointing up and J-K spins pointing down. The probability of this projection is expressed in terms of P(+) and P(-), such that

P(K)=(2J choose J+K)*P(+)^(J+K)*P(-)^(J-K)

where "2J choose J+K" is equal to (2J)!/[(J+K)!(J-K)!]
 
Sounds great!

Is this something you thought about now, or you read/learned this trick somewhere in the past?
 
Actually, it was an assignment question a few weeks ago :wink:
 
BTW, this method works very well in other cases. For example, if you're deriving rotation matrices for a spin 1 particle, it is often simpler to express its basis as a combination of the standard spin 1/2 basis (spin up, spin down) using a tensor product and expand things out that way.
 

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