I Mixed states vs pure states - physical POV

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Pure states are superpositions of different quantum states that can interfere, while mixed states represent statistical mixtures where the system is definitely in one of the states but the observer lacks complete knowledge. A pure superposition of states like |0> and |1> can manifest in various ways, but measurement will yield a definitive outcome of either state. In contrast, mixed states cannot be represented as pure states in any basis and arise from incomplete knowledge or entanglement. The distinction lies in that pure states can yield a single outcome with certainty upon measurement, while mixed states result in probabilistic outcomes. Understanding these concepts is crucial for grasping the nature of quantum systems and their measurements.
  • #61
vanhees71 said:
Well, can you prove that the measurement devices used to measure the polarization of photons or the spin of particles etc. etc. are not described by quantum statistics of macroscopic "very-many-body systems"

Absolutely, I can't prove that, and I don't believe it. I believe that there should be a non-schizophrenic formulation of quantum theory, but the current formulation is schizophrenic.

If so, then you'd be right in saying that there is a division of the world in microscopic and macroscopic behavior.

I'm saying that the formalism has this division. I'm not saying that the division is inherent in nature. I believe it's not.
 
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  • #62
stevendaryl said:
I'm calling your approach schizophrenic. If the only meaning of the quantum state is to give probabilities for measurement outcomes (or other macroscopic quantities such as mean values) in terms of preparation procedures, then that's necessarily schizophrenic. A non-schizophrenic theory would not mention macroscopic quantities such as measurements or preparation procedures in the fundamental laws, but would be able to derive whatever is being claimed about those things.
Physics is defined as describing preparation procedures, measurements, quantitative observations of Nature. All theories of physics are right about this way to investigate what's objectively going on. If you want something else, it's not physics!
 
  • #63
stevendaryl said:
Absolutely, I can't prove that, and I don't believe it. I believe that there should be a non-schizophrenic formulation of quantum theory, but the current formulation is schizophrenic.
This is your very individual idea about what the natural sciences should provide. Nature doesn't care about our feelings and expectations how she should behaves. She is just is she is.
 
  • #64
vanhees71 said:
This is your very individual idea about what the natural sciences should provide. Nature doesn't care about our feelings and expectations how she should behaves. She is just is she is.

My complaint is not in how nature behaves, but the schizophrenic way that we describe it. I don't believe that nature is schizophrenic.
 
  • #65
vanhees71 said:
Physics is defined as describing preparation procedures, measurements, quantitative observations of Nature. All theories of physics are right about this way to investigate what's objectively going on. If you want something else, it's not physics!

You're confusing your own philosophy of physics with physics itself.
 
  • #66
It's not my philosophy of physics. It's what physicists do in all kinds of physics labs and theory institutes around the world.
 

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