# Settling Copenhagen Interpretation / Measurement in QM & Existence of Quantum Gravity

It is my understanding that there are many interpretations of observation/measurement in Quantum Mechanics (I am only familiar with the Copenhagen one).

The Schrodinger's Cat experiment forms a paradigm in terms of understanding the state of a quantum system before its observation. Sometimes the analogy is taken too far and interpreted in terms like "If you don't observe the moon isn't there / it could be or not be there" i.e. "If you don't observe it then the moon isn't necessarily there". Such an argument of course ignores the decoherence due to the size of moon. But consider the following where one could argue the existence of a quantum system in a definite state at the moment in time when it hasn't been observed.

TLDR - Modifying the cat experiment one could have two observers such that one is blind and the other is deaf. If the blind observer hears a meow then can't the deaf observer (who can only consult the blind observer) say that the cat is alive (also assume that this is a weird universe with only the two observers as supernatural beings floating in space and a mystery box)?

Detailed version - Suppose that I assume the existence of a quantum theory of gravity (just that it exists and not its particular form). Now, consider two sets of events A=$$\{A1=(x_{A1},t_{A1}) ; A2=(x_{A2},t_{A2})\}$$ and B=$$\{B1=(x_{B1},t_{B1}) ; B2=(x_{B2},t_{B2})\}$$ where

$$x_{A1} - x_{B1} = 0 \\ x_{A2} - x_{B2} = 0 \\ t_{A1} - t_{A2} = 0 \\ t_{B1} - t_{B2} = 0 \\ x_{A1} - x_{A2} = 10,000 \mathrm{\ light\ years} \\ t_{A1} - t_{B1} = 10,000 \mathrm{\ years}$$

Now, assume that there is a black hole merger at A2. Of course, A1 will not be able to detect it immediately and cannot make a statement whether the black hole (or the associated Hawking Radiation) exists or not and like the Cat experiment the existence of Black Hole at A2 is not guaranteed. But an observer at B1 can detect the merger thereby saying that even at the time of events A1 and A2 Black Holes existed.

Doesn't this settle the argument? I am sure there is some fallacy here otherwise people would have pointed this out ages ago.

Note : One could argue that I am making a logical fallacy here and I will reply that: Assume that you detect a merger at B1 and then the argument can be run backwards without any fallacy.

• Mixing a non-existent accepted theory of quantum gravity into the unproveable (one way or another) choice of interpretation of quantum theory does not sound like a good plan to me. The (awful) "Cat experiment" which has done so much to hopelessly confuse decades of students just isn't going to help here, IMO. My advice is to Run Away, as Monty Python would put it. :-) – StephenG Sep 16 '20 at 18:31
• @StephenG I agree but the motivation to ask this is to determine whether, just as a thought experiment, this would be enough to settle the issue? – self.grassmanian Sep 16 '20 at 19:44

I believe this is a badly drawn space-time diagram of the four events: $$\begin{array} & t\uparrow & B1(x_1,t_2) & B2(x_2,t_2) \\ & A1(x_1,t_1) & A2(x_2,t_1) \\ & & x\rightarrow \end{array}$$ Events A1 and A2 are at the same height, so they are at the same time. Likewise, events B1 and B2 are at the same height, so they are at the same time. Events A1 and B1 are at the same horizontal position, so they are at the same spatial location. Likewise for A2 and B2.
• So the Cat experiment analogy is just meant to serve as TLDR. As for the future evolution it is already provided that a merger has been detected from gravitational waves. Now, the question is not whether they merged but whether they existed at the time $t_{A1}$ or $t_{A2}$. The thought experiment is meant to address a question like : If you don't "look" is a quantum system really there? The proposed resolution here is that it is possible to detect (in certain cases) future indications which can resolve for us whether the system existed in past or not & then universality takes care of the rest. – self.grassmanian Sep 17 '20 at 0:53