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Is it possible measure simultaneously square of angular momentum and its x projection ( ^L2 and ^Lx )?Why? Proof.

Question

Is it possible measure simultaneously square of angular momentum and its x projection ( ^L2 and ^Lx )?Why? Proof.

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Solution

Yes, it is possible to simultaneously measure the square of the angular momentum and its x projection. This is because the operators corresponding to these two quantities commute, meaning that the order in which they are applied does not affect the result.

In quantum mechanics, the commutation relation between two operators A and B is given by [A, B] = AB - BA. If this commutation relation equals zero, the two operators are said to commute and the physical quantities they represent can be simultaneously measured.

The operators for the square of the angular momentum (^L2) and its x projection (^Lx) are given by:

^L2 = ^Lx^2 + ^Ly^2 + ^Lz^2 ^Lx = ^Lx

The commutation relation between ^L2 and ^Lx is then:

[^L2, ^Lx] = [^Lx^2 + ^Ly^2 + ^Lz^2, ^Lx] = [^Lx^2, ^Lx] + [^Ly^2, ^Lx] + [^Lz^2, ^Lx]

Since ^Lx commutes with itself, [^Lx^2, ^Lx] = 0. The other two terms, [^Ly^2, ^Lx] and [^Lz^2, ^Lx], also equal zero because the components of angular momentum in different directions commute.

Therefore, [^L2, ^Lx] = 0, meaning that the square of the angular momentum and its x projection can be simultaneously measured.

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