Copenhagen interpretation claims this, but have we actually verified this? There exist two options:
- Objective collapse: Consider a large isolated box inside which there is a macroscopic classical system and a microscopic quantum system. At some time $t$, the microscopic system becomes entangled with the macroscopic system. After some more time, there will be decoherence and the density matrix will diagonalise. The decoherence itself doesn't lead to the "picking" of any one eigenvalue. Copenhagen interpretation claims that, after this decoherence, one of the eigenvalues will get "picked" according to some unknown collapse mechanism.
The problem is that this not just an interpretation. This is a different theory! Its predictions deviate from standard quantum mechanics. Standard quantum mechanics says that the entire box will evolve according to the Schrodinger equation. But this Copenhagen theory is saying that the system collapses mid-way before we even open the box.
Since this Copenhagen theory is verifiably true or false, has it been verified by experiments? I believe experimental evidence is so far against this. People have shown that decoherence is reversible in many complicated entanglement experiments. Decoherence is unitary.
If objective collapse is experimentally false, then the other option is:
- Subjective collapse: This says that one system can collapse another quantum system from its own subjective point of view, but the combined system still evolves unitarily from an external observer's point of view. According to this, that system in the box will evolve completely according to the Schrodinger equation.
The problem with this is that it's making completely unverifiable claims. Whether a quantum system can subjectively collapse another quantum system or not, it will have no observable effect on the outcome of our experiments, as our experiments only evolve according to the Schrodinger equation. Subjective phenomena are, by definition, unverifiable, in the same way consciousness in unverifiable from an external point of view. You can only verify that you yourself can collapse the wavefunction. You can't verify if other people can do it, you can't verify if inanimate objects can do it.
Many worlds interpretation has identical problems. You can only verify that you yourself end up in one of the worlds. The rest of the world must always be in a superposition relative to you, for agreement with experimental results.
SUMMARY : Based on all of this, I am saying that consciousness collapsing the wavefunction is the only thing that has been experimentally verified. If we insist that all macroscopic objects can do it, we'll have to verify objective collapse, which deviates from the Schrodinger equation. If we insist that all macroscopic objects can do it subjectively, then that's unverifiable and it's the same as saying that inanmiate objects have a subjective experience.
But, the consciousness collapse theory has long been discarded from mainstream science. Then, what is wrong with my reasoning? Are physicists hoping that there will be a non-linear "objective collapse" correction term to the Schrodinger evolution?