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It seems like the Copenhagen interpretation is just self contradictory. These two axioms are contradictory:

  1. Quantum Mechanics describes all the particles in the universe

  2. Measurement devices evolve superpositions into eigenstates.

Suppose an electron is in a state $|\psi \rangle $ and all the particles of a measurement device are in a state $|m\rangle$.

If we apply axiom #1 on the state $|\psi \rangle \otimes |m\rangle$, we can evolve it using the Schrodinger equation. The decoherence theorem, which is an application of the Schrodinger equation, says that the electron will evolve into a mixed state. The decoherence theorem applies because the measurement device has 10^23 particles.

If we apply axiom #2 on the state $|\psi\rangle \otimes |m\rangle$, it says that the electron will evolve into an eigenstate.

A mixed state contains all eigenvectors, and not just one. Since a mixed state $\neq$ an eigenstate, we have a contradiction.

What is the way out of this contradiction?

Leopard Mamba
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1 Answers1

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Equations of physics are all time-reversal symmetric. But we know the Universe is in fact not (second law of Thermodynamics). How can modern Physics be redeemed of this contradiction?

The truth is that all physical theories have limitations. What you wrote is true, but it only means that the Quantum Theory the way we know it now has a limited sphere of applicability. We have to live with the theories we have, be mindful of their shortcomings and be careful to not to use them beyond the intended range of validity (here belong the Schodinger cats and the like) until something more general (=wider range) comes about. In the meantime we try to come up with something more universal. This is what they call research.

John
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