Psi-Collapse in Nature: Definition and Implications

[The thinker]

Hello,

I'm studying decoherence at the moment, as a result I've also been looking a wavefunction collapse.

I was wondering what the definition of a full measurement is and whether they occur outside of the lab? - I've been looking at 'measurement like' interactions, which cause decoherence, but not actual collapse.

I suppose my question boils down to: what's the difference between a measurement and a measurement like interaction? ... at the moment it seems to me that simply consciousness is involved in one but not the other!

Thanks a lot!

 

[Fredrik]

Many interactions will create correlations between two subsystems (e.g. a silver atom interacting with the field of a Stern-Gerlach magnet will develop correlations between position states and spin component states), but it's only a measurement when the states of one of the subsystems can be considered "macroscopically distinguishable". So what subsystems have macroscopically distinguishable eigenstates? The answer is "the ones that interact strongly with their environments". (See e.g the discussion about air molecules bouncing off a book in Schlosshauer's book).

Consciousness involves a brain that goes through a series of well-defined memory states, and those memory states are macroscopically distinguishable because those degrees of freedom of your brain can't be isolated from their environment. So if we count the light-sensitive cells of your retina as part of your brain, then your brain is collapsing some superpositions, i.e. the superpositions of states of the photons that they interact with. But those states aren't correlated with the measurement results (say the spin-z states of a silver atom), because the light you're observing is just light scattered by some part of your measuring device, which behaves classically because of its strong interactions with the environment. So your consciousness certainly can't collapse the superposition of spin component states that a silver atom is in when it enters your Stern-Gerlach apparatus. The device has already done that, by interacting with the environment so that information about its state has been carried off into the environment.