# Pointer states

## Main Question or Discussion Point

How do you understand pointer states? I was trying to comprehend Zurek papers that without Einselection, Decoherence can even affect classical states and every tourist who looks at the Statue of Liberty would see different thing because its state was affected by the previous tourist. So he proposed something called Quantum Darwism where the photons reflecting off the Statue of Liberty can make all tourists see the same thing. But assuming the photons not there.. how can decoherence still affect classical states? In other words, can you give an example where classical states can also decohere because the pointer states were not resistant to decoherence (remember in this formalism the wave function never collapse, superposition still occurs at all levels),

## Answers and Replies

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bhobba
Mentor
In other words, can you give an example where classical states can also decohere because the pointer states were not resistant to decoherence (remember in this formalism the wave function never collapse, superposition still occurs at all levels),
Some of those into decoherence as an explanation for the measurement problem (and I am one of them) like to think no collapse occurs. That's false - its simply replaced by the assumption an improper mixture is somehow a proper one which is really collapse in disguise - but of a more refined form. I like to call it the problem of why we get any outcomes at all - decoherence doesn't and cant explain that - other assumptions are required.

My view, called the ignorance ensemble, is to just after decoherence say we have a proper mixed state so, classically, you don't get decoherence - it occurs well before then.

Personally I have never been really been able to understand Quantum Darwinism nor the validity of their proof of the Born rule - it looks circular to me - or at least simply assuming, without stating it explicitly, the non-contextuality of Gleason which is the real basis of Born. But even beyond that decoherence requires the Born rule anyway so it looks pretty suspect from the start.

That said I am no expert on it (as evidenced from my not really getting it anyway) so someone else may be able to help - and I would be very interested in what they say.

Thanks
Bill

atyy
Science Advisor
Personally I have never been really been able to understand Quantum Darwinism nor the validity of their proof of the Born rule - it looks circular to me - or at least simply assuming, without stating it explicitly, the non-contextuality of Gleason which is the real basis of Born. But even beyond that decoherence requires the Born rule anyway so it looks pretty suspect from the start.
I too don't know if Zurek's derivation is correct. However, Zurek is well aware that the Born rule is used in decoherence when one uses reduced density matrices, and explicitly claims not to use reduced density matrices until after he's gotten the Born rule.

bhobba
Mentor
I too don't know if Zurek's derivation is correct. However, Zurek is well aware that the Born rule is used in decoherence when one uses reduced density matrices, and explicitly claims not to use reduced density matrices until after he's gotten the Born rule.
He indeed does know that, and specifically states his derivation relies on symmetry considerations.

I haven't gone through it carefully, but my suspicion is its really non-contextuality is disguise.

Thanks
Bill

naima
Gold Member
every tourist who looks at the Statue of Liberty would see different thing
Read Ensemble interpretation
The interpretation states that the wave function does not apply to an individual system – or for example, a single particle – but is an abstract mathematical, statistical quantity that only applies to an ensemble of similarly prepared systems or particles
You can understand now that when you have a hudge number of The Statue of Liberty, there is no more problem!