> On 15 Nov 2018, at 09:04, 'scerir' via Everything List > <[email protected]> wrote: > > Imagine a spin-1/2 particle described by the state psi = sqrt(1/2) [(s+)_z + > (s-)_z] . > > If the x-component of spin is measured by passing the spin-1/2 particle > through a Stern-Gerlach with its field oriented along the x-axis, the > particle will ALWAYS emerge 'up'.. > > In fact (s+)_z = sqrt(1/2) [(s+)_x + (s-)_x] > > and (s-)_z = sqrt(1/2) [(s+)_x - (s-)_x] > > (where _z, _x, are the z-component and the x-component of spin) > > so that psi = sqrt(1/2)[(s+)_z +(s-)_z] = (s+)_x. (pure state, not mixture > state).. > > AGrayson2000 asked "If a system is in a superposition of states, whatever > value measured, will be repeated if the same system is repeatedly measured. > But what happens if the system is in a mixed state?" > > Does Everett's "relative state interpretation" show how to interpret a real > superposition (like the above, in which the particle will always emerge 'up') > and how to interpret a mixture (in which the particle will emerge 50% 'up' or > 50% 'down’)? > With a lot of difficulties, no doubt. But with much less difficulties than with a “physical collapse”, I would say.
What is hard with up’ = up + down (renormalised) is that the physical state up’ *is* the same as two (times infinity) particles in the state up and down, with me being in both universe, without any means to distinguish which one before I do the measurement. It is weird, but that is QM. Similarly a particle with a definite position *is* that particles with an indefinite momentum, and in the many-world, that is an infinity of universes/histories possibles, with each momentum being definite in each universe. Measuring a position is the same as putting myself in *all* those universe where the momentum is unknown. Bruno > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <mailto:[email protected]>. > To post to this group, send email to [email protected] > <mailto:[email protected]>. > Visit this group at https://groups.google.com/group/everything-list > <https://groups.google.com/group/everything-list>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
