Actually, to be more precise about the collapse of the wave-function and the 'observer' here's a slightly more technical explanation that deals away with the misunderstandings that crop up with these terms:Koloman Varady said:That interpretation has trouble (I think that's the one they call the Copenhagen interpretation) because the whole "observing" thing is pretty weird. Not only does it raise weird questions about what is an observer (if you observe something, is it still in a superposition until I observe it? does it have to be human? does it need a PhD in physics?) but the whole process of the "collapse of the wavefunction" isn't even a process that can be described by Shrodinger's equation (which describes quantum mechanics).

In macroscopic world, when two objects (even if they are identical) collide you can tell which is object A and which is object B both before and after the collision if you only do not take your eyes off of them. Their speed and positions are always known.

Now take for example two electrons. They are represented by a wave-function which describes their state in terms of probabilities. When two electrons collide, you can't actually tell which is electron A and which is electron B. To an outside observer, they are interchangeable. And thus, to describe this pair of electrons, you need a combined wave-function, made up from the two individual wave-functions.

During this time of interchangeability, the electrons are in a state of superposition: out of all possible trajectories for the electrons following the impact, both the electron A and B are at both, neither, just one and just the other of these trajectories.

However, when a new event occurs, a second collision with Electron C for example, the combined wave-function of A and B are resolved: We know A collided with C over

*here*and B continued towards there. The combined wave-function of A and B collapsed, as the interchangeability of the particles ceased.

This was caused by the particle C, who in this case acted as the observer. So when in QM we talk of an observer, we actually mean any event that solves the wave-function-indeterminancy. The term 'Observer' is used due to the origins of this realization in the double-slit experiment: It is impossible to construct a device that can observe the slit the electron goes trough, without the device at the same time acting as a sufficient outside influence to collapse the wave-function. The observing device, just by taking a measurement, disturbs the observed particle enough to solve the indeterminancy.