Did he really say that? https://physicstoday.scitation.org/doi/10.1063/1.1768652Feynman said the Copenhagen interpretation of quantum mechanics was simple, "Shut up and calculate."
If you insist on a collapse then the only way it could make perhaps sense is to take it as subjective, i.e., you just call to adapt your probability description by gaining knowledge through obtaining new information, but I still don's see, why you ever need a collapse. As I've shown in the quoted posting the objective state of the cat as seen by Bob is objectively defined by the preparation and time evolution of the system, no matter whether he knows that Alice is observing the cat and that this would collapse something or not. All he knows is that with 50% probability the cat is dead and with 50% it's alive.So collapse is subjective?
Yeah! An Feynman new that in fact he knew everything relevant about quantum theory, namely how to calculate and make predictions that can be tested in the lab. If there's anyone who in fact understood QT very well then it was Feynman ;-).Mermin didn't establish if Feynman did or didn't say it. Feynman's approach was clear though. He said he didn't understand quantum mechanics, but he did know how to calculate,
Good! That brings us closer to the OP question. Does subjectivity require consciousness?If you insist on a collapse then the only way it could make perhaps sense is to take it as subjective, i.e., you just call to adapt your probability description by gaining knowledge through obtaining new information, but I still don's see, why you ever need a collapse. As I've shown in the quoted posting the objective state of the cat as seen by Bob is objectively defined by the preparation and time evolution of the system, no matter whether he knows that Alice is observing the cat and that this would collapse something or not. All he knows is that with 50% probability the cat is dead and with 50% it's alive.
The old argument by Schrödinger et al (btw if there's anyone of the founding fathers really describing their view in an understandable way then it's Schrödinger) is that you need to ensure that after a measurement a repeated measurement with certainty reproduces the just measured result. That's of course only possible if you make a preparation (based on a measurement) a la a von Neumann filter measurement, but then indeed you don't need a collapse, because everything is well explained by the local (sic!) interaction of the system with the measurement device and filter. You don't need an instantaneous action at a distance.
Certainly in the sense that some popularizers of science present it.So are you denying quantum theory applies to macroscopic objects, or ...?
I am not familiar with Quantum Decoherence, but I find it difficult to treat macroscopic states like; alive and deal on the same footing as quantum states that describes position, time, momentum and energy.I've no clue what you want to tell me. If you believe in a collapse (which in my opinion is pretty absurd) then the presence of the 2nd observer for the external observer is (let's call them Alice and Bob for convenience) that if Bob knows that the cat is continuously watched by Alice "collapsing" the poor creature to a definite state "alive" or "dead", he'll use the stat op
$$\hat{\rho}=p_{\text{alive}} |\text{alive} \rangle \langle{\text{alive}} + p_{\text{dead}} |\text{dead} \rangle \langle \text{dead}|.$$
That's because then he knows that the cat is no longer entangled with the state of the unstable particle ("still there" vs. "decayed").
In the other case, i.e., if Alice is not present he'd use the pure state
$$\hat{\rho}_{\text{particle}+\text{cat}}=|\Psi \rangle \langle \Psi | \quad \text{with} \quad |\Psi \rangle=\frac{1}{\sqrt{2}} |\text{particle there},\text{cat alive} \rangle + |\text{particle decayed},\text{cat dead} \rangle.$$
Now the funny thing is, ignoring the particle, Bob will also assign the state
$$\hat{\rho}_{\text{cat}}'=\mathrm{Tr}_{\text{particle}} \hat{\rho}_{\text{particle}+\text{cat}} = \hat{\rho}.$$
So definitely nothing has changed for Bob. It's just different ways to describe the cat's state given the knowledge Bob has due to the knowledge about the setup of Schrödingers devilish experiment.