Understanding Angular Momentum States and Clebsh-Gordan Coefficients

The Clebsch-Gordan table shows all the possible combinations of angular momentum for two particles. However, due to symmetry, some combinations may result in a coefficient of zero. This occurs when the sum of the angular momenta is odd and all three m values are zero. This is because of an identity in the Clebsch-Gordan coefficient that involves reversing the sign of the m values. Therefore, not all combinations between |J1-J2| and J1+J2 are possible. In summary, the Clebsch-Gordan table displays all possible combinations of angular momentum for two particles, but some combinations may result in a coefficient of zero due to symmetry.
  • #1
spookyfish
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When we add the angular momenta of two particles, J1 and J2, we get that the resulting total angular momenta is in the range
|J1-J2| < J < J1+J2

but according to the Clebsh-Gordan table some coefficients are zero. Does it mean that not all combinations between |J1-J2| and J1+J2 are possible?
 
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  • #2
No, they are all possible. The Clebsh-Gordan table also takes the components (the m's) into consideration.
 
  • #3
ok, but for a given m - some of them might not be possible
 
  • #4
It's due to symmetry. For example, rotate through 180 degrees, turning z → -z. This reverses the sign of each m value. There's an identity

(j1 -m1 j2 -m2|j3 -m3) = (-1)j1 + j2 + j3 (j1 m1 j2 m2|j3 m3)

So in particular for m1 = m2 = m3 = 0 the Clebsch-Gordan coeffiicent will vanish if j1 + j2 + j3 is odd.
 
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  • #5
spookyfish said:
ok, but for a given m - some of them might not be possible

That's correct
 
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Related to Understanding Angular Momentum States and Clebsh-Gordan Coefficients

What is angular momentum and why is it important in physics?

Angular momentum is a fundamental physical property that describes the rotational motion of an object around an axis. It is important in physics because it helps us understand the behavior of objects in motion, such as planets orbiting around a star or electrons spinning around an atom.

What are angular momentum states and how are they related to Clebsh-Gordan coefficients?

Angular momentum states refer to the different possible states that an object can have based on its angular momentum. Clebsh-Gordan coefficients are mathematical values that describe the probability of an object transitioning from one angular momentum state to another. They are related because the values of the coefficients depend on the specific angular momentum states involved.

How do Clebsh-Gordan coefficients help us understand the behavior of subatomic particles?

Clebsh-Gordan coefficients are used in quantum mechanics to describe the behavior of subatomic particles, such as electrons. They help us understand how these particles transition between different angular momentum states, which is crucial in understanding their behavior and interactions with other particles.

What is the significance of the "selection rule" in determining Clebsh-Gordan coefficients?

The selection rule is a principle in quantum mechanics that states that certain transitions between angular momentum states are allowed while others are forbidden. This rule helps us determine which Clebsh-Gordan coefficients are non-zero and therefore contribute to the overall probability of a transition occurring.

How are Clebsh-Gordan coefficients calculated and what factors can affect their values?

Clebsh-Gordan coefficients are calculated using mathematical formulas that take into account the angular momentum values of the initial and final states. These coefficients can be affected by factors such as the spin of the particles involved, the strength of their interactions, and the presence of external fields.

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