Wave function collapse! What is wave function collapse!?

Question

can someone explain, wikipedia aint making sense!

"A particle is described by a wave function which gives you a probability function of where it might be."

When you say particle you mean an "elementary particle" yeah?


"You don't actually know until you take a measurement."

With what? some sort of gizmo on a particle accelator?

"Once you take a measurement, the space wave function collapses to a point (causing the momentum wave function to spread to infinity). "

like dipping your fingers into a liquid and a ripple effect? or are you saying its like a policeman asking a ganster what happened and the ganster drops dead? or explodes?

Answer 1

In quantum physics, a particle is described by a wave function which gives you a probability function of where it might be.

You don't actually know until you take a measurement.

Once you take a measurement, the space wave function collapses to a point (causing the momentum wave function to spread to infinity).

edit:
question 1: I mean ANY particle, elementary or otherwise. Macroscopic particles tend to have very compact wave functions though (short deBroglie wavelengths) so they are already practically collapsed.

edit
question 2: For a simple example of a measurement, consider a two-slit experiment (like Young's experiment). If you don't check which slit the particle goes through, you get wave functions which interfere with each other. If you check, however, the wave function collapses and you know where the particle has to end up (give or take slight diffraction effects after the measurement).

edit: question 3. This is simplest to see in terms of a particle with a plane wave function. If you fix a wave in one place, it has fourier components of many, many momenta. Conversely, if you let the wave be perfectly monochromatic (one momentum), it spreads over all space. You can't know momentum and position exactly at the same time. (Heisenberg uncertainty principle). It's hard to explain any better without resorting to advanced math (Fourier analysis of waves). Fortunately, for macroscopic objects like gangsters, Heisenberg restrictions don't keep us from knowing reasonably exact positions and momenta for the same reason I cited in question 1.

Answer 2

When quantum theory was originally developed, Bohr proposed the "complimentary principle", which says that, for example, photons are "either waves or particles". It was originally an "either-or" presumption. To describe the wave properties of matter, quantum physicists use the wave function to describe position in quantum probabilistic terms. But once a detector has been triggered, then it is said to be a particle with a definite location and therefore a much more compact wave function of position. This sudden change from an extended wave function of position to a much smaller one is called a "wave function collapse", and for decades, nobody really understood how the process occurred. It was as if there was a black box of time in which a big wave function turned into a small one, and some magician waved a wand to make this happen.

Today, we do have a better idea of what's going on, and while this "collapse" is pretty sudden, it's not truly instantaneous, and the cause of this change in wave function is due to quantum decoherence. Check the wiki article on quantum decoherence. It's a new field and few laymen know of this. There has been much work in the past few decades exploring the "bridge" between classical and quantum behavior. As an example which might help illustrate this bridging is the case of super-excited "Rydberg atoms", where the outer electron has an orbit so far away from the nucleus that it actually begins to take on the characteristics of a tiny satellite whirling around it!

Answer 3

drl wave function collapse occurs when the energy of the spread out wave is condensed to a point or points where the intensity of the wave (amplitude) is the greatest. In the case of light this results in photon or photons formation. The amount of photons formed is directly proportional to the wave intensity and the energy carried by the photon is directly proportional to the waves frequency. E=Fh