Why doesn't gravity act as a measurement?

Question

I think this must be a very basic question but I couldn't find the answers anywhere. I was starting reading about Quantum Mechanics and these questions came in mind:

As I understand the quantum universe is considered stochastic, and the act of measurement is what makes the wavefunctions collapse into a single state, what is called wavefunction collapse, making the system randomly assume only one of its possible states.

Copenhagen says that the system transforms from a superposition of states to a single unique state.

The problem is shouldn't gravity make all particles keep measuring themselves and act as observers all the time (and thus making the wavefunctions keep collapsing and denying an uncollapsed state)? In the sense that all particles are in constant interaction with each other's gravitational fields.

E.g. The schrodinger cat for instance, say if he is dead he is emitting gravity from a lay down position and if he is alive from a straight up position. The problem I'm seeing is that if gravity carries information and it's a wave or force, then how a superposition of states is possible to exist?

Is gravity being a particle a requirement for superposition being true?

I hope my question was clear enough now, thank you!

Answer 1

No, gravity doesn't "cause" a measurement in any greater way then the electric force or another force. In fact, a difference is that between two particles, gravity is about $10^{40}$ times weaker than the electric force, so its effects – and your hypothetical "auto-measurement" effects – are negligible relatively to the electric force. None of them exists.

The idea that gravity makes the wave functions "collapse" was vaguely proposed by Roger Penrose and one may easily show that it is completely wrong. If such an effect existed, it would lead to the loss of coherence and interference in experiments we know to remain coherent and in the violations of the equivalence principle (all forms of energy must accelerate in the same way in the gravitational field).

The fact that the results of a measurement are "sharp" boils down to the fact that the wave function isn't a classical "objectively existing" wave but just a probability amplitude wave and the actual phenomena that exist, the results of measurements, are the sharp outcomes we know; the wave function is just a tool to predict the probabilities. The "sharp" results of the measurements have nothing to do with gravity or any other particular "mechanism" one could think of. There is no mechanism; the probabilistic character of the wave function is a fundamental postulate of quantum mechanics.

When you throw a die, the probability of each outcome is 1/6: $$(1/6,1/6,1/6,1/6,1/6,1/6)$$ When a particular outcome, like 4, is obtained, the probability distribution "shrinks" to $$(0,0,0,1,0,0)$$ But the shrinking isn't due to any "force". It's due to our learning about the result. In classical physics, one may imagine that the number 4 was already "guaranteed" before the die landed, due to determinism. But in quantum mechanics, it's not possible: the results are really random. But the "shrinking" of the wave function is still just about the change of the knowledge, not about the physical shrinking of any classical wave.

Another, related way to falsify your thinking is to point out that the wave function, because it is not a classical wave or field, doesn't self-attract. In fact, any such self-attraction would mean that the dynamics is nonlinear in the wave function. But linearity of the evolution operators as a function of wave functions is a principle of quantum mechanics, linearity, and it holds completely generally, including cases with gravity.