system
Given the following post, respond as if you're commenting on Reddit:
human
If measurement causes the wavefunction to collapse, and everything is constantly interacting with everything else in the universe via gravity, why haven't all wavefunctions collapsed all the time?

I don't get why bouncing a photon off something is enough to get its wavefunction for position to collapse, but interacting with it via gravity isn't.
gpt
Crudely, wavefunction collapse means the following. You allow your quantum system S to interact with a set of degrees of freedom M. The interaction entangles the state of S with the state of M, so that the probability distribution of some observable OS of S are correlated with observable OM of M. But perhaps you don't care about OM and only want the marginal distribution for OS. Averaging over OM produces an effective collapse: the probabilities governing future behavior of S now behave as though, at the time of interaction with M, OS took various values with probability set by the Born rule. This is what you would expect from classical probability theory, but differs from quantum mechanics because it eliminates the effects of interference between quantum mechanical amplitudes.

So, you might think that everything is interacting with gravity, and we don't care about the state of the gravitational field, so why doesn't averaging over it classicalize all probabilities? The reason is that gravity is weak: interactions with gravitons are rare. The quantum systems we look at are affected by gravity in the sense that their motion is influenced by the background gravitational field, but they are very unlikely to change the state of that field by entangling with a graviton.