Citeren meekerdb <meeke...@verizon.net>:
On 8/14/2013 4:43 PM, smi...@zonnet.nl wrote:
Citeren meekerdb <meeke...@verizon.net>:
On 8/14/2013 7:48 AM, smi...@zonnet.nl wrote:
Citeren Russell Standish <li...@hpcoders.com.au>:
On Tue, Aug 13, 2013 at 05:26:41PM -0700, Pierz wrote:
I need clarification of the significance of quantum theory to
determining
the *past*. I remember having read or heard that the past itself
is subject
to quantum uncertainty. Something like the idea that the past is
determined
only to to the extent that it is forced to be so by the state of the
present, if that makes sense. In other words, there may be more than one
history that could lead to the current state of the world. Let's say it
might have been one way or another and then we make a measurement which
resolves this question, we are 'forcing' the past to be one way
or another.
In MWI, that would be saying my 'track' through the multiverse
is ambiguous
in both directions, both into the future and 'behind me' so to
speak. I'm
unclear on this and what it precisely means. I seem to recall
that it was
critical in calculations Hawking made about the early universe - at a
certain point these uncertainties became critical and it meant
that it was
no longer possible to say that the universe had definitely been
one way or
another. Can someone clarify this for me?
This idea of the past not being determinate until such a time as a
measurement in the present forces the issue is fundamental to my
interpretation of QM. It is also related to the Quantum Eraser. Saibal
Mitra has written some stuff on this too - maybe he'd like to comment?
On the other hand, I don't think this view is particularly
mainstream. Even many worlds people tend to think that the multiverse
has decohered in the past, and that there is a matter of fact which
branch we are in, even if we're ignorant of that fact.
I can't comment on Hawking's work, unfortunately, as I'm not
aware of that.
Cheers
--
Yes, I would agree with the view taken by Russell here. It has
interesting consequences for any future artificial intelligence
who can reset its memory, as I explain here:
http://arxiv.org/abs/0902.3825
So, if you reset your memory at random with some probability p and
you also do that in case of an impending disaster, then if you
find yourself in a state where you know that your memory has been
reset and you need to reload your memory, the reason why the
memory has been reset (routine random memory reset or you were
facing an impending disaster), is no longer determined, you are
identical in the different branches until you find out the reason.
So, while you are firmly in the classical regime and therefore you
won't see any changes in the probabilities of the outcomes of
these sorts of experiments relative to what you would expect
classically, the interpretation of how these probabilities arise
is different; while it is worthwhile to do these memory resettings
in a "single classical world" it wouldn't be worthwhile.
The article I wrote (it was just an essay for FQXI competition
which got the attention from New Scientist), is actually rather
simple, it treats the problem in a non-relativistic way, which is
a bit unnatural (the times at which things happen in the different
different branches seems to matter). You can easily generalize
this, also you can consider thought experiments involving false
memories that may be correct memories in different branches etc.
etc.
Hmm. It seems that "erasing your memory" would encompass a lot
more than what is commonly referred to as memory. Quantum erasure
requires erasing all the information that is diffused into the
environment. So erasing one's memory would imply quantum erasure
of all the information about your past - not just the infinitesimal
bit that you can consciously recall.
Brent
Yes, but then this is not "quantum erasure". If you were to reverse
the act of a measurement then you could experimentally falsify the
Copenhagen interpretation like e.g. in David Deutsch' thought
experiment. However, if you simply erase part of your memory and if
the reason why you did that is not certain (e.g. you do this
randomly and in case of bad news), then after you find that your
memory has been partially erased you are in the same "macro state"
in different branches where the reason of the memory resetting is
different.
I assume that whatever you experience is defined by some suitably
defined macro state that can be isolated from the environment, not
the exact micro state of the system which is always strongly
correlated with the environment; if this is not true, then you'll
have hard time arguing against psychics who claim to be able to feel
what happens somehwere else from basic physics principles alone.
So, of course, the information is present in the environment, but
you are unaware of what is in the environment, and therefore the
outcome of the measurement is uncertain as far as you are concerned.
The relevant physics here is purely classical (except for the many
Worlds aspect, but this enters in the calculations in a trivial way),
I guess I don't understand that. You seem to be considering a
simple case of amnesia - all purely classical - so I don't see how
MWI enters at all. The probabilities are just ignorance uncertainty.
You're still in the same branch of the MWI, you just don't remember
why your memory was erased (although you may read about it in your
diary).
No, you can't say that you are in the same branch. Just because you are
in the clasical regime doesn't mean that the MWI is irrelevant and we
can just pretend that the world is described by classical physics. It
is only that classical physics will give the same answer as QM when
computing probabilities.
If what you are aware of is only described by your memory state which
can be encoded by a finite number of bits, then after a memory
resetting, the state of your memory and the environment (which contains
also the rest of your brain and body), is of the form:
|memory_1>|environment_1> + |memory_2>|environment_2>+...
where |environment_i> is not (necessarily) normalized.
It then follows that a process that can lead to the same memory state
via different paths (routine resetting or resetting in case of bad
news) will lead to a state |psi>, such that projecting out a definite
memory state gives:
|memory><memory|psi> =
|memory> sum over different |environments>
where the different environments contain different information about
the paths that led to the memory.
So, from the point of view of a memory state that has undergone a
resetting, the environment will be in a superposition of different
states, the reason why the reseting has been done is thus undetermined
prior to a measurement.
Saibal
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