On 7/30/2018 8:02 AM, Bruno Marchal wrote:
*and claims the system being measured is physically in all
eigenstates simultaneously before measurement.*
Nobody claims that this is true. But most of us would I think agree
that this is what happens if you describe the couple “observer
particle” by QM, i.e by the quantum wave. It is a consequence of
elementary quantum mechanics (unless of course you add the
unintelligible collapse of the wave, which for me just means that QM
is false).
This talk of "being in eigenstates" is confused. An eigenstate is
relative to some operator. The system can be in an eigenstate of an
operator. Ideal measurements are projection operators that leave the
system in an eigenstate of that operator. But ideal measurements are
rare in QM. All the measurements you're discussing in Young's slit
examples are destructive measurements. You can consider, as a
mathematical convenience, using a complete set of commuting operators to
define a set of eigenstates that will provide a basis...but remember
that it's just mathematics, a certain choice of basis. The system is
always in just one state and the mathematics says there is some operator
for which that is the eigenstate. But in general we don't know what
that operator is and we have no way of physically implementing it.
Brent
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