On Monday, July 30, 2018 at 7:50:47 PM UTC, Brent wrote: > > > > 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 >
*I can only speak for myself, but when I write that a system in a superposition of states is in all component states simultaneously, I am assuming the existence of an operator with eigenstates that form a complete set and basis, that the wf is written as a sum using this basis, and that this representation corresponds to the state of the system before measurement. I am also assuming that the interpretation of a quantum superposition is that before measurement, the system is in all eigenstates simultaneously, one of which represents the system after measurement. I do allow for situations where we write a superposition as a sum of eigenstates even if we don't know what the operator is, such as the Up + Dn state of a spin particle. In the case of the cat, using the hypothesis of superposition I argue against, we have two eigenstates, which if "occupied" by the system simultaneously, implies the cat is alive and dead simultaneously. AG * -- 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.
