On 7/31/2018 10:19 PM, Jason Resch wrote:
On Tue, Jul 31, 2018 at 4:52 PM Brent Meeker <[email protected]
<mailto:[email protected]>> wrote:
On 7/31/2018 2:38 PM, Jason Resch wrote:
On Tuesday, July 31, 2018, Brent Meeker <[email protected]
<mailto:[email protected]>> wrote:
On 7/31/2018 9:46 AM, Jason Resch wrote:
On Tue, Jul 31, 2018 at 1:11 AM Brent Meeker
<[email protected] <mailto:[email protected]>> wrote:
On 7/30/2018 9:21 PM, [email protected]
<mailto:[email protected]> wrote:
On Tuesday, July 31, 2018 at 1:34:58 AM UTC, Brent wrote:
On 7/30/2018 4:40 PM, [email protected] 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).
*
CMIIAW, but as I have argued, in decoherence theory it
is assumed the cat is initially isolated and decoheres
in a fraction of a nano second. So, IMO, the problem
with the interpretation of superposition remains. *
Why is that problematic? You must realize that the cat
dying takes at least several seconds, very long compared
to decoherence times. So the cat is always in a
/*classical*/ state between |alive> and |dead>. These
are never in superposition.
*It doesn't go away because the decoherence time is
exceedingly short. *
Yes is does go away. Even light can't travel the length
of a cat in a nano-second.
What if the cat is on Pluto for this one hour? Would it not
be perfectly isolated from us on Earth, and thus remain in a
superposition until the the several hours it takes for light
to get to Earth from Pluto reaches us?
?? Are you assuming that decoherence only occurs when humans
(or Earthlings) observe the event?
Brent
No, just that superposition is a relative, rather than objective
notion.
OK. Welcome to QBism.
After reading the wiki article on QBism I still can't say I understand
what it is about, as it doesn't seem to offer any core positions.
I am an adherent of bayesianism, and believe it applies generally in
all domains (being an agent having to make decisions/bets), so what
does QBism add if one already accepts a general reliance on Bayes
theorem? It doesn't seem like QBism takes any strong position on any
of the quantum paradoxes, nor offer any insights to addressing or
explaining them. In this it seems like a pretty empty theory, with
hints towards the "instrumentalist" and "shut up and calculate"
mindsets--that only the probability matters. To the extent that is
true, I reject QBism. While QBism might not put forward anything that
is false, the attitude it conveys seems like it would stymie progress
towards advancing our understanding of reality.
QBism says that QM is a theory for predicting personal beliefs. The
"collapse" of the wave function is simply updating one's beliefs based
on an observation.
Brent
That superposition is relative does not require observers or
knowledge, it is a consequence of the postulates of QM. Either system
A has interacted with system B and they are both part of the
superposition together, or they have not interacted yet and system A
will be in a superposition of various possible states to system B, and
system B will be in a super position of various states to system A.
Jason
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