On 7/30/2018 4:40 PM, agrayson2...@gmail.com wrote:
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. *
In general you need a set of operators to have the eigenstates form a
complete basis...but OK.
*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 *
Yes, you can write down the math for that. But to realize that
physically would require that the cat be perfectly isolated and not even
radiate IR photons (c.f. C60 Bucky ball experiment). So it is in fact
impossible to realize (which is why Schroedinger considered if absurd).
Brent
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