Pardon my ignorance, I'm pretty sure this has an obvious answer, but it's something I've been wondering about. In QM it's said that particles don't exist in a definite state between measurements, but rather exist as a wave function, with a probability of being found in a certain place when they're measured. Neutrinos are very weakly interacting, and travel through the partial vaccuum of space. Since they're not being "measured" when they're not interacting with other particles, they should exist in their wave-function state during those times. How come, then, don't they zig-zag as the fly accross the universe? My intuition tells me that between interactions (measurements), their position and momentum should be determined by a probability function, and there should be a possibility of them changing direction during those times. Obviously, this doesn't seem to happen (from what I've read), so I'm guessing my intuition is wrong. Could anyone shed some light on this for me? Thanks.