Two quantum particles can interact with each other without either's wavefunction collapsing. That's pretty much what makes the double slit experiment work. It s

Two quantum particles can interact with each other without either's wavefunction collapsing. That's pretty much what makes the double slit experiment work. It seems that generally, interacting with a "macroscopic" object tends to collapse the wave function. You can find an interesting discussion of one way scientists try to explain this phenomenon here: http://en.wikipedia.org/wiki/Quantum_decoherence http://en.wikipedia.org/wiki/Quantum_decoherence