- #1

muonion

- 2

- 0

Suppose we measured S

_{z}of particle 1 which happened to be +1/2. Then automatically S

_{z}of particle 2 would be -1/2.

From

*Weinberg's 'Lectures on Quantum Mechanics'*Ch-12 (P. 394)

... the observer could have measured the x-component of the spin of particle 1 instead of its z-component, and by the same reasoning, if a value h/2 or −h/2 were found for the x-component of the spin of particle 1 then also the x-component of the spin of particle 2 must have been −h/2 or h/2 all along. Likewise for the y-components.So according to this reasoning, all three components of the spin of particle 2 have definite values, which is impossible since these spin components do not commute.

I don't get the

**bold**lines above. Whenever we make a S

_{z}measurement of 1 and then the same of 2 and then S

_{x}measurement of 1- this x measurement destroys the previous info of particle 2. Hence particle 2 shouldn't have 2 definite components at a time, let alone 3 definite values.

What did Weinberg imply there?