On 29/05/2017 11:21 pm, Telmo Menezes wrote:
On Mon, May 29, 2017 at 3:06 PM, Bruce Kellett
<bhkell...@optusnet.com.au> wrote:
On 29/05/2017 10:42 pm, Telmo Menezes wrote:
On Mon, May 29, 2017 at 2:26 PM, Bruce Kellett
<bhkell...@optusnet.com.au> wrote:
On 29/05/2017 6:26 pm, Telmo Menezes wrote:
On Mon, May 29, 2017 at 3:26 AM, Bruce Kellett
<bhkell...@optusnet.com.au> wrote:
I would say that there is only one history leading to our present
state.
Whether you take an MWI view or a collapse view, the wave function
branches
deterministically at every point, so if you follow your current twig
back
down to the main trunk etc, there will be a unique path.
I don't think we can say we are in a specific twig. Many things about
out present state are unknown/undefined. I can imagine that there are
many well-defined present states that are compatible with my current
subjective state.
Sure, but we are talking about wave functions, not subjective states.
Replace "subjective" with "incomplete knowledge".
Doesn't help. Of course our knowledge is incomplete, the wave function isn't
completely known either -- but the result of specific measurements are what
is at stake here, and they are known quantum states.
In my view, what's at stake here is the possibility of latent
variables having some degree of freedom leading to the same macro
states (provided that there is incomplete information about these
macro states, as is the case for humans).
I am still not clear about what you are trying to say. It is certainly
true that many internal details of your microscopic organization could
be changed and you wouldn't know the difference. For that matter, an
almost infinite number of details about the rest of the universe could
be different and you would still be the same. But that is not what we
were talking about. I understood the issue to be whether there was a
unique past, or whether several (or many) different paths lead to our
current state. The determinism of the Schrödinger equation would suggest
that your unique state has a unique history. Variations in microscopic
details of with your body, or the universe, would correspond to
different decohered worlds with no overlap with our world. So while such
variations are possible, they do not amount to multiple histories
leading to our current state. We don't need to know that state in detail
to be able to argue the consequences of determinism.
In fact you can perform a quantum erasure experiment, and be sure that
your current state goes through at least two different shortest paths
to the root, and it becomes nonsensical to say that one is the
"correct" one. I don't think anyone knows how far this can go into the
macroscopic world, but I don't see any reason to believe that it
doesn't.
I don't understand what you think you are getting in a quantum erasure
experiment. If the "which way" information that was gathered is erased,
normal interference patterns are seen in the double slit situation. The
two
paths (through the separate slits) are in unresolved superposition until
they hit the detector, when decoherence takes over. There are not two
separate worlds, and your state is the result of the superposed paths,
not
of either path separately. There is no ambiguity about which the the
"correct" path -- neither is, both contribute equally.
I would say that the delayed choice version of the experiment makes it
clear that there are two possible pasts that lead to the same present
state -- they differ by one bit of information.
That is not what is implied by delayed choice quantum erasure. Whether an
interference pattern is seen or not is determined by whether the "which way"
information is erased or not. But whether it is or not, the interference is
only seen when coincidence measurements tell one which photons to count. And
the timing information necessary for coincidence determination is available
only *after* all decisions about erasure or not have been made, whether that
decision is made before or after the other photon of the entangled pair has
reached its detector.
"Delayed choice" is perhaps a misleading phrase in this context, and it does
not lead to an ambiguity of path -- it merely tells whether there was an
intact superposition or not.
I know, this is not what I am trying to say. I'll choose something much simpler:
Suppose there is a computer running in an empty room. This computer is
connected to a random number generator. At some point it uses the
random value to decide if it's going to show a screen that is all
green or all red. Nobody witnesses it.
If the random number generator is based on quantum randomness, then in
principle you will get a superposition of red and green screens, but
this is like the question as to whether we see a superposition of live
and dead cats in Schrödinger's thought experiment. Even if there is an
underlying quantum event that would give a superposition, decoherence
steps in and resolves the outcome into separate worlds long before we
reach the macro level of live/dead cats or red/green screens.
Decoherence does not require anyone to witness it....
Bruce
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