Does laser beam expand along distance or not?

Question

Why does laser dot become bigger at bigger distance? e.g: from millimeter to inches so that it distracts pilots in aircrafts. I knew that laser beam should't expand over distance, but remain stable. Is that true?

Answer 1

This problem can be answered if we treat Light as a Particle. Due to Heisenberg's Uncertainty Principle, $\Delta x\Delta p\geq \dfrac{\hbar}{2}$, saying that we cannot exactly know the particle's Position and Momentum at the same time. In the case of a laser, a photon cannot have 0 momentum in any arbitrary direction so the photons cannot just go in one direction together. So what little momentum in other directions will cause the photons to spread out over time.