> On 15 Nov 2018, at 09:04, 'scerir' via Everything List > <everything-list@googlegroups.com> 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 everything-list+unsubscr...@googlegroups.com > <mailto:everything-list+unsubscr...@googlegroups.com>. > To post to this group, send email to everything-list@googlegroups.com > <mailto:everything-list@googlegroups.com>. > 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 everything-list+unsubscr...@googlegroups.com. To post to this group, send email to everything-list@googlegroups.com. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.