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 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.
