Re: Changing the past by forgetting

2009-03-15 Thread Brent Meeker

In "quantum eraser" experiments the erasure is done by making the measured 
value 
ambiguous, e.g. by making a different measurement which does not commute with 
the one to be erased.  In terms of MWI this has the effect or recohering (or 
more accurately, not decohering) the branches rather than cutting one off. 
After the erasure there is no fact of the matter as to what the value was.

Of course in these experiments care must be taken to avoid interaction with the 
environment so that coherence is not lost before the second measurement.

Brent



Stephen Paul King wrote:
> Hi Brent,
> 
> But does not MWI imply that if we could somehow erase all (retrivable!) 
> records of a measurement,  that we would - in effect - be culling that 
> branch from the Tree?
> 
> 
> - Original Message - 
> From: "Brent Meeker" 
> To: 
> Sent: Sunday, March 15, 2009 10:06 PM
> Subject: Re: Changing the past by forgetting
> 
> 
> snip
> 
>> ... Even more telling experiments have already been done in which
>> the "measurement" was just the unrecorded IR radiation from buckyballs.
>> Buckyballs which were sufficiently cold showed the 2-slit interference 
>> pattern
>> in a Young's slit type experiment.  But when they were warm enough to emit 
>> IR
>> radiation that, if detected, could have localized them, the interference
>> disappeared.  So it is not only a matter of the experimenter not looking 
>> at the
>> result, the rest of the universe has to not look too.
>>
>> Brent
> 
> It is sad that Einstein was not made aware of this implication of QM. If 
> he was, his thought about the "moon not being there when I'm not looking at 
> it" would not have formed. I wish we had a better sense of exactly how 
> decoherence worked such that we could quantify how there might be "parts" of 
> a system's wave function that are entagled with its environment and other 
> parts that are not...
> 
> Onward!
> 
> Stephen 
> 
> 
> > 
> 


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Re: Changing the past by forgetting

2009-03-15 Thread russell standish

On Sun, Mar 15, 2009 at 07:06:06PM -0700, Brent Meeker wrote:
> 
> No need to do it.  Even more telling experiments have already been done in 
> which 
> the "measurement" was just the unrecorded IR radiation from buckyballs. 
> Buckyballs which were sufficiently cold showed the 2-slit interference 
> pattern 
> in a Young's slit type experiment.  But when they were warm enough to emit IR 
> radiation that, if detected, could have localized them, the interference 
> disappeared.  So it is not only a matter of the experimenter not looking at 
> the 
> result, the rest of the universe has to not look too.
> 
> Brent
> 

Interesting! Do you have some citations for this, or even let me know
who did the experiment?

Cheers

-- 


Prof Russell Standish  Phone 0425 253119 (mobile)
Mathematics  
UNSW SYDNEY 2052 hpco...@hpcoders.com.au
Australiahttp://www.hpcoders.com.au


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Re: Changing the past by forgetting

2009-03-15 Thread Stephen Paul King

Hi Brent,

But does not MWI imply that if we could somehow erase all (retrivable!) 
records of a measurement,  that we would - in effect - be culling that 
branch from the Tree?


- Original Message - 
From: "Brent Meeker" 
To: 
Sent: Sunday, March 15, 2009 10:06 PM
Subject: Re: Changing the past by forgetting


snip

>... Even more telling experiments have already been done in which
> the "measurement" was just the unrecorded IR radiation from buckyballs.
> Buckyballs which were sufficiently cold showed the 2-slit interference 
> pattern
> in a Young's slit type experiment.  But when they were warm enough to emit 
> IR
> radiation that, if detected, could have localized them, the interference
> disappeared.  So it is not only a matter of the experimenter not looking 
> at the
> result, the rest of the universe has to not look too.
>
> Brent

It is sad that Einstein was not made aware of this implication of QM. If 
he was, his thought about the "moon not being there when I'm not looking at 
it" would not have formed. I wish we had a better sense of exactly how 
decoherence worked such that we could quantify how there might be "parts" of 
a system's wave function that are entagled with its environment and other 
parts that are not...

Onward!

Stephen 


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Re: Changing the past by forgetting

2009-03-15 Thread Brent Meeker

russell standish wrote:
> On Sun, Mar 15, 2009 at 11:06:42AM -0700, Brent Meeker wrote:
>> Saibal Mitra wrote:
>>> If we consider measuring the spin of a particle, you could also say that the
>>> two possible outcomes just exist and thatthere are two possible future
>>> versions of me. There is no meaningful way to associate myself with either
>>> of the two outcomes.
>>>
>>> But then, precisely this implies that after a measurement and forgetting
>>> about the result will yield a version of me who is in a similar position as
>>> that earlier version of me who had yet to make the measurement. If one could
>>> perform measurements in a reversible way, this would be possible to
>>> experimentally confirm, as David Deutsch pointed out. You can start with a
>>> spin polarized in the x direction. Then you measure the z-component. There
>>> then exists a unitary transformation which leads to the observer forgetting
>>> about the outcome of the measurement and to the spin to be restored in the
>>> original state. The observer does remember having measured the z-component
>>> of the spin.
>>>
>>> Then, measuring the x-component again will yield "spin-up" with 100%
>>> probability, confirming that both branches in which the observer measured
>>> spin up and spin down have coherently recombined. This then proves that had
>>> the observer measured the z-component, the outcome would not be a priori
>>> determined, despite the observer having measured it earlier. So, both
>>> branches are real. But then this is true in general, also if the quantum
>>> state is of the form:
>>>
>>> |You>[|spin up>|rest of the world knows the spin is up> + |spin down>|rest
>>> of the world knows spin is down>]
>> You're contemplating reversing three different things:
>>
>> 1) Your knowledge, by forgetting a measurement result.  Something that's 
>> easy to do.
>>
>> 2) The spin state of a particle.
>>
>> 3) The state of what the rest of the world knows.
>>
>> Because of the entanglement, I don't think you can, in general, reverse the 
>> spin 
>> state of the  particle without reversing what is known about it by "the rest 
>> of 
>> the world".
>> If it was a known state (to someone) the particle can easily be put back in 
>> that 
>> state.  But to do so for a general, unknown state, after a measurement would 
>> require invoking time-reversal invariance of the state of whole universe (or 
>> at 
>> least all of it entangled with the particle spin via the measuring 
>> apparatus).
>>
>> Brent Meeker
>>
> 
> By contrast, I think this line of reasoning can be used to create an
> experiment that tests a couple of different versions of MWI.
> 
> Consider a Stern-Gerlach experiment where a particle is prepared in
> the x+ state. Then measure the state of the particle's spin along the
> z-axis, but _do not_ record the result. Finally measure the spin along
> the x-axis.
> 
> According to Saibal's interpretation (which accords with my own
> intuition), the result should be spin up (x+) always. According to the
> interpretation you're suggesting Brent (the decoherence of the
> environment to contain a memory of whether the spin was z+ or z- -
> which I think accords with David Deutch's intuition), the final result
> should be x+ or x- with 50% probability. It may be important to send
> the result of the intervening measurement to a memory store somewhere
> else that the experimenter does not look at.
> 
> This should be a doable experiment, and in fact may already have been
> done. It is similar in some respects to a version of the two-slit
> experiment performed a couple of years ago that generated a spark of
> interest.
> 
> 

No need to do it.  Even more telling experiments have already been done in 
which 
the "measurement" was just the unrecorded IR radiation from buckyballs. 
Buckyballs which were sufficiently cold showed the 2-slit interference pattern 
in a Young's slit type experiment.  But when they were warm enough to emit IR 
radiation that, if detected, could have localized them, the interference 
disappeared.  So it is not only a matter of the experimenter not looking at the 
result, the rest of the universe has to not look too.

Brent

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Re: Changing the past by forgetting

2009-03-15 Thread russell standish

On Sun, Mar 15, 2009 at 11:06:42AM -0700, Brent Meeker wrote:
> 
> Saibal Mitra wrote:
> > If we consider measuring the spin of a particle, you could also say that the
> > two possible outcomes just exist and thatthere are two possible future
> > versions of me. There is no meaningful way to associate myself with either
> > of the two outcomes.
> > 
> > But then, precisely this implies that after a measurement and forgetting
> > about the result will yield a version of me who is in a similar position as
> > that earlier version of me who had yet to make the measurement. If one could
> > perform measurements in a reversible way, this would be possible to
> > experimentally confirm, as David Deutsch pointed out. You can start with a
> > spin polarized in the x direction. Then you measure the z-component. There
> > then exists a unitary transformation which leads to the observer forgetting
> > about the outcome of the measurement and to the spin to be restored in the
> > original state. The observer does remember having measured the z-component
> > of the spin.
> > 
> > Then, measuring the x-component again will yield "spin-up" with 100%
> > probability, confirming that both branches in which the observer measured
> > spin up and spin down have coherently recombined. This then proves that had
> > the observer measured the z-component, the outcome would not be a priori
> > determined, despite the observer having measured it earlier. So, both
> > branches are real. But then this is true in general, also if the quantum
> > state is of the form:
> > 
> > |You>[|spin up>|rest of the world knows the spin is up> + |spin down>|rest
> > of the world knows spin is down>]
> 
> You're contemplating reversing three different things:
> 
> 1) Your knowledge, by forgetting a measurement result.  Something that's easy 
> to do.
> 
> 2) The spin state of a particle.
> 
> 3) The state of what the rest of the world knows.
> 
> Because of the entanglement, I don't think you can, in general, reverse the 
> spin 
> state of the  particle without reversing what is known about it by "the rest 
> of 
> the world".
> If it was a known state (to someone) the particle can easily be put back in 
> that 
> state.  But to do so for a general, unknown state, after a measurement would 
> require invoking time-reversal invariance of the state of whole universe (or 
> at 
> least all of it entangled with the particle spin via the measuring apparatus).
> 
> Brent Meeker
> 

By contrast, I think this line of reasoning can be used to create an
experiment that tests a couple of different versions of MWI.

Consider a Stern-Gerlach experiment where a particle is prepared in
the x+ state. Then measure the state of the particle's spin along the
z-axis, but _do not_ record the result. Finally measure the spin along
the x-axis.

According to Saibal's interpretation (which accords with my own
intuition), the result should be spin up (x+) always. According to the
interpretation you're suggesting Brent (the decoherence of the
environment to contain a memory of whether the spin was z+ or z- -
which I think accords with David Deutch's intuition), the final result
should be x+ or x- with 50% probability. It may be important to send
the result of the intervening measurement to a memory store somewhere
else that the experimenter does not look at.

This should be a doable experiment, and in fact may already have been
done. It is similar in some respects to a version of the two-slit
experiment performed a couple of years ago that generated a spark of
interest.


-- 


Prof Russell Standish  Phone 0425 253119 (mobile)
Mathematics  
UNSW SYDNEY 2052 hpco...@hpcoders.com.au
Australiahttp://www.hpcoders.com.au


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Re: Changing the past by forgetting

2009-03-15 Thread rmiller
At 06:20 PM 3/15/2009, George Levy wrote:
>I agree with Anna. In addition, it all depends on where you define 
>the boundary of the self. Just the brain? Brain + body? Brain + body 
>+ immediate surrounding (prescription glasses being worn, automobile 
>being driven, binoculars or computer being used) ? Brain + body + 
>Whole causally connected universe (CCU)?
>
>There are good arguable reasons for including the CCU as part of the 
>self. Forgetting would then mean resetting the  CCU  to  the last 
>"remembered" state. In this case we have an identity relationship 
>between the self and the universe it inhabits. Resetting the self is 
>the same as resetting the universe. No more problem or paradox 
>associated with forgetting!
>
>George


Helmut Schmidt's "retrocausation" (actually a sophisticated delayed 
choice) experiments suggested that causation might be associated with 
many-worlds---that is, it might be possible to choose a past that 
will lead to a present state of a Causally Connected Universe.   If 
different pasts can be associated with equivalent behavioral sets in 
the present then it would be impossible to differentiate the specific 
world lines that led to the present behavioral set.  If one's present 
behavioral state is a sum of histories over time, then one could 
focus on a particular history (say, one without dinosaurs) to subtly 
alter the present CCU, but if the difference can't be consciously 
detected, then (a la Copenhagen theory) then it would be irrelevent.

As for memory being reconstructive, the fact is we really don't know 
much about it at all.  If time doesn't exist except as a bundle of 
world lines, memory might involve "sampling" a section of the 
preceding bundle.  In that view we create our own particular 
CCU.  Wrong about past events?  Maybe you sampled the wrong world line.

R. Miller



>A. Wolf wrote:
>>>
>>>Thanks!  This is like undoing historical events. If you forget about the
>>>fact that dinosaurs ever lived on Earth and there is an alternative
>>>history
>>>that led to your existence in the multiverse, and you do the memory
>>>erasure
>>>also in sectors were dinosaurs never lived, you have some nonzero
>>>probability of finding yourself on an Earth were the dinosaurs never
>>>lived.
>>>
>>
>>
>>The problem I'm having with this line of reasoning is that "memory" isn't a
>>fixed physical object.  Memory is reconstructive, and depends upon emotional
>>triggers both at the time when the memory was encoded and at the time when
>>it re-examined in the conscious mind.  No memories are particularly
>>accurate.
>>
>>Most of the time, I'm not aware that dinosaurs existed because I'm not
>>thinking about it, or any other part of Earth's history, for that
>>matter...but I don't seem to have the experience that my environment is
>>impoverished of history altogether just because I hadn't been thinking hard
>>enough about it.  As another example, people who have false recovered
>>memories through psychotherapy invariably end up unable to confirm them when
>>they look for facts to back up their new memories, and that happens in my
>>universe even though I personally don't have any information to confirm or
>>deny their memories.
>>
>>In other words, I don't see why forgetting something is any more likely to
>>change events than simply being wrong about having the memory in the first
>>place, the latter of which happens constantly.  If you want to argue about
>>what nonzero probability implies, you'd have a hard time showing that
>>anything non-contradictory at all has a nonzero probability of being true.
>>:)
>>
>>
>>>
>>>Because of the entanglement, I don't think you can, in general, reverse
>>>the spin
>>>state of the  particle without reversing what is known about it by "the
>>>rest of
>>>the world".
>>>
>>
>>
>>The rest of the world?  What's that?
>>
>>Anna
>>
>>
>>
>>
>>
>
>
>
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Re: Changing the past by forgetting

2009-03-15 Thread George Levy
I agree with Anna. In addition, it all depends on where you define the 
boundary of the self. Just the brain? Brain + body? Brain + body + 
immediate surrounding (prescription glasses being worn, automobile being 
driven, binoculars or computer being used) ? Brain + body + Whole 
causally connected universe (CCU)?

There are good arguable reasons for including the CCU as part of the 
self. Forgetting would then mean resetting the  CCU  to  the last 
"remembered" state. In this case we have an identity relationship 
between the self and the universe it inhabits. Resetting the self is the 
same as resetting the universe. No more problem or paradox associated 
with forgetting!

George


A. Wolf wrote:
>> Thanks!  This is like undoing historical events. If you forget about the
>> fact that dinosaurs ever lived on Earth and there is an alternative 
>> history
>> that led to your existence in the multiverse, and you do the memory 
>> erasure
>> also in sectors were dinosaurs never lived, you have some nonzero
>> probability of finding yourself on an Earth were the dinosaurs never 
>> lived.
>> 
>
> The problem I'm having with this line of reasoning is that "memory" isn't a 
> fixed physical object.  Memory is reconstructive, and depends upon emotional 
> triggers both at the time when the memory was encoded and at the time when 
> it re-examined in the conscious mind.  No memories are particularly 
> accurate.
>
> Most of the time, I'm not aware that dinosaurs existed because I'm not 
> thinking about it, or any other part of Earth's history, for that 
> matter...but I don't seem to have the experience that my environment is 
> impoverished of history altogether just because I hadn't been thinking hard 
> enough about it.  As another example, people who have false recovered 
> memories through psychotherapy invariably end up unable to confirm them when 
> they look for facts to back up their new memories, and that happens in my 
> universe even though I personally don't have any information to confirm or 
> deny their memories.
>
> In other words, I don't see why forgetting something is any more likely to 
> change events than simply being wrong about having the memory in the first 
> place, the latter of which happens constantly.  If you want to argue about 
> what nonzero probability implies, you'd have a hard time showing that 
> anything non-contradictory at all has a nonzero probability of being true. 
> :)
>
>   
>> Because of the entanglement, I don't think you can, in general, reverse 
>> the spin
>> state of the  particle without reversing what is known about it by "the 
>> rest of
>> the world".
>> 
>
> The rest of the world?  What's that?
>
> Anna
>
>
> >
>
>   


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Re: Changing the past by forgetting

2009-03-15 Thread George Levy
I agree with Anna. In addition, it all depends on where you define the 
boundary of the self. Just the brain? Brain + body? Brain + body + 
immediate surrounding (prescription glasses being worn, automobile being 
driven, binoculars or computer being used) ? Brain + body + Whole 
causally connected universe (CCU)?

There are good arguable reasons for including the CCU as part of the 
self. Forgetting would then mean resetting the  CCU  to  the last 
"remembered" state. In this case we have an identity relationship 
between the self and the universe it inhabits. Resetting the self is the 
same as resetting the universe. No more problem or paradox associated 
with forgetting!

George


A. Wolf wrote:
>> Thanks!  This is like undoing historical events. If you forget about the
>> fact that dinosaurs ever lived on Earth and there is an alternative 
>> history
>> that led to your existence in the multiverse, and you do the memory 
>> erasure
>> also in sectors were dinosaurs never lived, you have some nonzero
>> probability of finding yourself on an Earth were the dinosaurs never 
>> lived.
>> 
>
> The problem I'm having with this line of reasoning is that "memory" isn't a 
> fixed physical object.  Memory is reconstructive, and depends upon emotional 
> triggers both at the time when the memory was encoded and at the time when 
> it re-examined in the conscious mind.  No memories are particularly 
> accurate.
>
> Most of the time, I'm not aware that dinosaurs existed because I'm not 
> thinking about it, or any other part of Earth's history, for that 
> matter...but I don't seem to have the experience that my environment is 
> impoverished of history altogether just because I hadn't been thinking hard 
> enough about it.  As another example, people who have false recovered 
> memories through psychotherapy invariably end up unable to confirm them when 
> they look for facts to back up their new memories, and that happens in my 
> universe even though I personally don't have any information to confirm or 
> deny their memories.
>
> In other words, I don't see why forgetting something is any more likely to 
> change events than simply being wrong about having the memory in the first 
> place, the latter of which happens constantly.  If you want to argue about 
> what nonzero probability implies, you'd have a hard time showing that 
> anything non-contradictory at all has a nonzero probability of being true. 
> :)
>
>   
>> Because of the entanglement, I don't think you can, in general, reverse 
>> the spin
>> state of the  particle without reversing what is known about it by "the 
>> rest of
>> the world".
>> 
>
> The rest of the world?  What's that?
>
> Anna
>
>
> >
>
>   


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Re: Changing the past by forgetting

2009-03-15 Thread A. Wolf

> what nonzero probability implies, you'd have a hard time showing that 
> anything non-contradictory at all has a nonzero probability of being true.

Er, I typed too quickly.  I mean you'd have a hard time of showing that 
anything non-contradictory has zero probability.  Anything that isn't 
mathematically contradictory has a nonzero probability, as far as I'm 
concerned.

Anna


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Re: Changing the past by forgetting

2009-03-15 Thread A. Wolf

> Thanks!  This is like undoing historical events. If you forget about the
> fact that dinosaurs ever lived on Earth and there is an alternative 
> history
> that led to your existence in the multiverse, and you do the memory 
> erasure
> also in sectors were dinosaurs never lived, you have some nonzero
> probability of finding yourself on an Earth were the dinosaurs never 
> lived.

The problem I'm having with this line of reasoning is that "memory" isn't a 
fixed physical object.  Memory is reconstructive, and depends upon emotional 
triggers both at the time when the memory was encoded and at the time when 
it re-examined in the conscious mind.  No memories are particularly 
accurate.

Most of the time, I'm not aware that dinosaurs existed because I'm not 
thinking about it, or any other part of Earth's history, for that 
matter...but I don't seem to have the experience that my environment is 
impoverished of history altogether just because I hadn't been thinking hard 
enough about it.  As another example, people who have false recovered 
memories through psychotherapy invariably end up unable to confirm them when 
they look for facts to back up their new memories, and that happens in my 
universe even though I personally don't have any information to confirm or 
deny their memories.

In other words, I don't see why forgetting something is any more likely to 
change events than simply being wrong about having the memory in the first 
place, the latter of which happens constantly.  If you want to argue about 
what nonzero probability implies, you'd have a hard time showing that 
anything non-contradictory at all has a nonzero probability of being true. 
:)

> Because of the entanglement, I don't think you can, in general, reverse 
> the spin
> state of the  particle without reversing what is known about it by "the 
> rest of
> the world".

The rest of the world?  What's that?

Anna


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Re: [Fwd: NDPR David Shoemaker, Personal Identity and Ethics: A Brief Introduction]

2009-03-15 Thread Bruno Marchal


Hi Günther,

>
> Hi Bruno,
>
> thanks for your interesting answer, I have some questions though.
>
>> course, as I said, this will depend of what you mean by "you". In  
>> case
>> you accept the idea of surviving with amnesia, you can even get to a
>> state where you "know" you are immortal, because your immortality  
>> is a
>> "past event".
>
>
> I would equate total amnesia with death (we've been through this  
> before,
> Stathis has written about this, if I remember correctly).


I remember Quentin identifying himself with his memory, and very  
logically, identifying total amnesia with death.
It is a complex matter. Total amnesia concern only some form of  
declarative knowledge, you cannot loss your "procedural memory",  
because it is part of ... arithmetic, and common to all elementary  
knowers.



>
>
> I agree with you that you can't have a universal machine stopping
> relatively to all others from it's POV; but I don't see why we can't
> think of it having total amnesia. So, for the time being, let us take
> surviving as meaning to keep (at least large parts) of one's memory.


All right. Comp leads also to that form of immortality, at least for  
awhile (if I can say).
The problem of death is intrinsically difficult, because you will  
survive with amnesia or not according of the level of substitution,  
which we cannot know, but only bet on (or perhaps even choose in some  
circumstances?). Total amnesia seems to lead to the remembering of you  
being the universal person, plausibly Plotinus "Soul-Universe", or the  
arithmetical S4Grz (the third hypostase).
Total amnesia is complete fusing. We "remember" we are all the same  
person. We can lost that memory only by differentiating oneself again.

>
>
>> facts, like the continuum of many-worlds. If Loop Gravity is 100%
>> correct, and if the big bang has a finitely describable origin then
>> comp is false!
>
> Could you elaborate? I don't see why LG should be bad news for comp?
> You mean because LG proposes a fundamental spacetime quantization?
> I don't see how it would falsify comp?

It is obviously an open problem. But taking literally the UDA, and  
making abstraction of some still possible (logically) conspiration of  
the numbers, it seems clear that comp predict that an electron, for  
going from A to B, with respect to you, will be supported by a  
continuum of computational histories. I have no clue how to select a  
finite or countable infinity of subcomputations. But if that is the  
case, the UDA shows that such a conclusion (like LG) has to be derived  
from arithmetic. Otherwise, it would be treachery.



>
>
> And why the finitely describable Big Bang? It seems you have a problem
> when there are some finite limits (outside of the effective  
> computation
> of mind). Is this because you need the continuum in the AUDA to get
> Quantum logic or something like that?


Not at all. That would be treachery too. If the continuum was not  
possibly emerging from AUDA, I would take this as an evidence that  
comp is false. The reason comes really from computer science and the  
(enumerable) redundancy of the computational states, and the non  
enumerable redundancy of the "1-pov" infinite stories.



>
>
>> Our bodies can be considered programmed to stop (by sex and death),
>> our soul just cannot, there is always a consistent continuation (even
>> without amnesia
>
> Why do you believe that latter?


Consider someone who dies, relatively to you. Well, La Palisse was  
found of "tautology": after he died, someone said" 5 minutes avant sa  
mort, Monsieur de La Paice vivait encore". "Fives minutes before he  
died, Sir de La Palice was still alive". Now, from the point of view  
of the dying person, the UD generates 2^aleph_zero histories going  
through that state "where he is is still alive", and which are below  
its computational relevant substitution level. Even with just QM, you  
can see stories which will "repair anything wrong with whatever needed  
to generate a short consistent computation. At his substitution level  
he is finite, and finite machine can always be fixed, and that is all  
the soul need to survive an instant. But this is true for all  
instants, by exactly the repetition of the argument. Perhaps some Gods  
can be mortal, but no souls, as supported by finite entities. That  
would be like my mechanics: "no, you car is definitely broken". It is  
a lie, or a simplification, what it means is "it would be more  
expensive to fix it than to buy a new one". In the ud, the first  
person has unbound-able resources.

I think, and Quentin disagrees, that, would I be dying, I would feel  
myself surviving more probably in the "amnesic stories", probably  
because I tend to believe (those days) that, if comp is true, my  
substitution level could probably be rather high. So I would survive  
in those "normal world" with a "lesser brain". But if Quentin is right  
and all my memory are necessary for being me, I will

Re: Changing the past by forgetting

2009-03-15 Thread Brent Meeker

Saibal Mitra wrote:
> If we consider measuring the spin of a particle, you could also say that the
> two possible outcomes just exist and thatthere are two possible future
> versions of me. There is no meaningful way to associate myself with either
> of the two outcomes.
> 
> But then, precisely this implies that after a measurement and forgetting
> about the result will yield a version of me who is in a similar position as
> that earlier version of me who had yet to make the measurement. If one could
> perform measurements in a reversible way, this would be possible to
> experimentally confirm, as David Deutsch pointed out. You can start with a
> spin polarized in the x direction. Then you measure the z-component. There
> then exists a unitary transformation which leads to the observer forgetting
> about the outcome of the measurement and to the spin to be restored in the
> original state. The observer does remember having measured the z-component
> of the spin.
> 
> Then, measuring the x-component again will yield "spin-up" with 100%
> probability, confirming that both branches in which the observer measured
> spin up and spin down have coherently recombined. This then proves that had
> the observer measured the z-component, the outcome would not be a priori
> determined, despite the observer having measured it earlier. So, both
> branches are real. But then this is true in general, also if the quantum
> state is of the form:
> 
> |You>[|spin up>|rest of the world knows the spin is up> + |spin down>|rest
> of the world knows spin is down>]

You're contemplating reversing three different things:

1) Your knowledge, by forgetting a measurement result.  Something that's easy 
to do.

2) The spin state of a particle.

3) The state of what the rest of the world knows.

Because of the entanglement, I don't think you can, in general, reverse the 
spin 
state of the  particle without reversing what is known about it by "the rest of 
the world".
If it was a known state (to someone) the particle can easily be put back in 
that 
state.  But to do so for a general, unknown state, after a measurement would 
require invoking time-reversal invariance of the state of whole universe (or at 
least all of it entangled with the particle spin via the measuring apparatus).

Brent Meeker

> 
> although you cannot directly verify it here. But that means that you cannot
> rule out an alternative theory in which only one of the branches is real
> when performing a measurement in this case. But if the reality of both
> branches is accepted, then each time you make a measurement and you don't
> know the outcome, the outcome is not fixed (proovided, of course, there is
> indeed more than one branch).
> 
> 
> - Original Message - 
> From: "Jack Mallah" 
> To: 
> Sent: Thursday, March 12, 2009 03:47 AM
> Subject: Re: Changing the past by forgetting
> 
> 
> 
> 
> --- On Tue, 3/10/09, Saibal Mitra  wrote:
>> http://arxiv.org/abs/0902.3825
>>
>> I've written up a small article about the idea that you could end up in a
> different sector of the multiverse by selective memory erasure. I had
> written about that possibility a long time ago on this list, but now I've
> made the argument more rigorous.
> 
> Saibal, I have to say that I disagree.  As you acknowledge, erasing memory
> doesn't recohere the branches.  There is no meaningful sense in which you
> could end up in a different branch due to memory erasure.
> 
> You admit the 'effect' has no observable consequences.  But it has no
> unobservable meaning either.
> 
> In fact, other than what I call 'causal differentiation', which clearly will
> track the already-decohered branches (so you don't get to reshuffle the
> deck), there is no meaningful sense in which "you" will end up in one
> particular future branch at all.  Other than causal differentiation
> tracking, either 'you' are all of your future branches, or 'you' are just
> here for the moment and are none of them.

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Re: Changing the past by forgetting

2009-03-15 Thread Saibal Mitra

If we consider measuring the spin of a particle, you could also say that the
two possible outcomes just exist and thatthere are two possible future
versions of me. There is no meaningful way to associate myself with either
of the two outcomes.

But then, precisely this implies that after a measurement and forgetting
about the result will yield a version of me who is in a similar position as
that earlier version of me who had yet to make the measurement. If one could
perform measurements in a reversible way, this would be possible to
experimentally confirm, as David Deutsch pointed out. You can start with a
spin polarized in the x direction. Then you measure the z-component. There
then exists a unitary transformation which leads to the observer forgetting
about the outcome of the measurement and to the spin to be restored in the
original state. The observer does remember having measured the z-component
of the spin.

Then, measuring the x-component again will yield "spin-up" with 100%
probability, confirming that both branches in which the observer measured
spin up and spin down have coherently recombined. This then proves that had
the observer measured the z-component, the outcome would not be a priori
determined, despite the observer having measured it earlier. So, both
branches are real. But then this is true in general, also if the quantum
state is of the form:

|You>[|spin up>|rest of the world knows the spin is up> + |spin down>|rest
of the world knows spin is down>]

although you cannot directly verify it here. But that means that you cannot
rule out an alternative theory in which only one of the branches is real
when performing a measurement in this case. But if the reality of both
branches is accepted, then each time you make a measurement and you don't
know the outcome, the outcome is not fixed (proovided, of course, there is
indeed more than one branch).


- Original Message - 
From: "Jack Mallah" 
To: 
Sent: Thursday, March 12, 2009 03:47 AM
Subject: Re: Changing the past by forgetting




--- On Tue, 3/10/09, Saibal Mitra  wrote:
> http://arxiv.org/abs/0902.3825
>
> I've written up a small article about the idea that you could end up in a
different sector of the multiverse by selective memory erasure. I had
written about that possibility a long time ago on this list, but now I've
made the argument more rigorous.

Saibal, I have to say that I disagree.  As you acknowledge, erasing memory
doesn't recohere the branches.  There is no meaningful sense in which you
could end up in a different branch due to memory erasure.

You admit the 'effect' has no observable consequences.  But it has no
unobservable meaning either.

In fact, other than what I call 'causal differentiation', which clearly will
track the already-decohered branches (so you don't get to reshuffle the
deck), there is no meaningful sense in which "you" will end up in one
particular future branch at all.  Other than causal differentiation
tracking, either 'you' are all of your future branches, or 'you' are just
here for the moment and are none of them.









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Re: Changing the past by forgetting

2009-03-15 Thread Saibal Mitra

Thanks!  This is like undoing historical events. If you forget about the
fact that dinosaurs ever lived on Earth and there is an alternative history
that led to your existence in the multiverse, and you do the memory erasure
also in sectors were dinosaurs never lived, you have some nonzero
probability of finding yourself on an Earth were the dinosaurs never lived.

- Original Message - 
From: "Bruno Marchal" 
To: 
Sent: Wednesday, March 11, 2009 06:54 PM
Subject: Re: Changing the past by forgetting


>
> Nice! I did refer often to the Saibal Mitra backtracking procedure (in
> immortality discussions). I will take a further look on your paper.
> If valid, it should work in the comp frame. Amnesia could lead you to
> the "original singularity", which could be a kind of blind spot of
> "universal consciousness", except that with comp such a singularity
> should looks like a "little Mandelbrot" set, at first sight, I mean
> something like a compact view of a universal dovetailing.
>
> Bruno
>
> On 10 Mar 2009, at 19:55, Saibal Mitra wrote:
>
> >
> > http://arxiv.org/abs/0902.3825
> >
> > I've written up a small article about the idea that you could end up
> > in a
> > different sector of the multiverse by selective memory erasure. I had
> > written about that possibility a long time ago on this list, but now
> > I've
> > made the argument more rigorous.
> >
> >
> > >
>
> http://iridia.ulb.ac.be/~marchal/
>
>
>
>
> >


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