Koch vs Penrose

[thefunkychook]

In the 30 March issue of Nature, Christof Koch and Klaus Hepp brought out an article ("Quantum Mechanics in the Brain") which puts a fairly strong case for why quantum mechanics is unlikely to play a significant role in mediating conscious experience

Comments

[pappubahry]

Doesn't decoherence explain how a superposition becomes a regular macroscopic state? The system will have long decohered before the brain gets around to processing stuff.

[thefunkychook]

Ok... I'm going to have to admit that I don't know what the current status of the decoherence approach to superposition breakdown is

[pappubahry]

I don't see why the measurement problem remains.

You let the quantum system and the environment evolve for a bit, and they become entangled. You take the full density operator for this system and trace over the environment, since you don't measure that. This gives a reduced density operator for the system as

rho = a|0><0| + b|0><0|.

This describes a classical mixture of two states, not a quantum superposition.

[thefunkychook]

Ok... I guess that would depend on your philosophical position on what quantum mechanics should be able to explain.

After the quantum system is entangled with the environment, we describe the state of the universe by |0>|A>+|1>|B>, and the local description of the subsystem is now rho = 1/2|0><0| + 1/2|1><1|. I'm happy with that. That will also allow us to predict (statistically) the outcome of any measurement we choose to make.

But if we want to describe everything in existence with one formalism (including ourselves and our perceptions), we still need to acknowledge that the state of the universe is |0>|A>+|1>|B>; when we actually receive a specific result, say 0, from a measurement of the quantum subsystem, should we then describe the state of the universe by |0>|A>+|1>|B>, or |0>|A>? What does it actually *mean* to receive the outcome of an individual measurement (as opposed to describing the statistical outcome of a large number of measurements)?