Would it be possible to Loschmidt reverse Wigner's friend using future technology? Why or why not? Can a human be isolated in a decoherence shielded box? Can we have fine control over every single atom and photon in a room?
Background: Wigner's friend and intersubjectivity in quantum mechanics
Please don't say the second law of thermodynamics, because it doesn't apply to microscopic descriptions of isolated reversible systems.
Don't say the act of reversal itself will have to force the room to be permanently entangled with the reversal machine. We can imagine the reversal machine is quantum and shielded from decoherence, and controlled by a quantum computer. Call the internal states of the reversal machine and quantum computer the ancilla. Initialize them to the "default pure state". In the act of reversing the room, the ancilla will become entangled with the room. However, as Bennett had shown, as both the machine and computer are also reversible, it is possible to uncompute the ancilla back to a different pure state which is unentangled with the room while keeping the room itself reversed.
But we must also ask ourselves this metaphysical dilemma. Take a human A. Show him an apple and let it register in his memory. The following day, using future technology, edit his brain memories changing the apple in his memory to that of a pear. Was it true that A actually saw a pear instead of an apple the day before? He vehemently claims he really really saw a pear. The pear existed in front of him the day before, and the apple didn't exist. Call A of the previous day $A_p$, and A of the following day $A_f$. Is the consciousness of $A_f$ a continuation of the consciousness of $A_p$? Was $A_p$ even conscious? What about $A_f$? Is $A_f$ even the same person as $A_p$? Did $A_p$ even exist? Did $A_p$ ever once existed? After all, the memories of $A_p$ are not saved forever.
Reversing Wigner's friend is like that.
You claim the brain tamperer's memory and the brainwashing machine records are witnesses to the fakery? Well then, reverse the brain tamperer and the brainwashing machine as well.
These issues are all very convincingly explained in this chapter.
but there's still the objection if in actuality, every single record and memory trace of the apple and brain tampering had been truly erased, how come the narrator of the story and the readers know about them?
