Citeren meekerdb <>:

On 8/14/2013 4:43 PM, wrote:
Citeren meekerdb <>:

On 8/14/2013 7:48 AM, wrote:
Citeren Russell Standish <>:

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.


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:

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.


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.


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