On 5/9/2013 7:49 AM, Telmo Menezes wrote:
I was thinking to quantum erasure experiments. We can make a wave
>>"collapse", by some measurement, and still make it cohere again, by erasing
>>the memory of the experience/the result of the experiment. If observation
>>did collapse or select irreversibly, that could not make sense.
>But it isn't a "measurement" if you can make it cohere again.  A measurement
>is irreversbile, "erasing" means reversing the process that, if it were not
>erased could have become a measurement.
But this implies that a human observer is needed for something to be a
measurement. The fact that the outcome of the measurement is
physically available in the universe is not enough.

No, it doesn't imply that. Being available in enough, c.f. buckyball interference experiments.

Would you agree that, knowing the outcome of the quantum eraser
experiment, the Copenhagen interpretation implies that observation by
a conscious entity is necessary to define a quantum state?

No. But I'm not a fan of the Copenhagen interpretation because it implies the Heisenberg cut is physical, yet is determined by the experimenter.

The Copenhagen interpretation seems to require two magic steps
(consciousness causing wave collapse and "pure randomness"). It seems
to be a more extraordinary claim than the MWI.

Also, CI + the quantum eraser experiment implies that stuff doesn't
exist when no one is looking. Am I wrong (honestly asking)?

I think you are assuming that there are only two possible QM interpretations: Copenhagen and MWI. There is also the epistemological interpretation; the wave function is just our description of the system and so it changes when we get new information (i.e. a measurment). There is also decoherence theory, which attempts to described the physical process of MWI. And there are several variants of MWI. And Mermin's "relations without relata". As well Bohm's guide waves and some actual collapse theories.


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