In "quantum eraser" experiments the erasure is done by making the measured 
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.


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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