*Originally posted by forkedknight*

**The "waveform collapses when observed" thing comes form the fact that even if only ONE electron is sent though the double slits, it produces a wave pattern.
**

How does a single electron produce a wave patten? That's why they try to observe which slit it passes through, but once they've done that, there is no longer a wave pattern.

The wavefunction gives the probability of where the particle is (via the modulas squared). Imaging there is a screen behind the slits and wherever and electron hits the screen a dot appears (say 1mm in diameter so it is easily seen with the eye), the dots are permenant so do not fade.

If electrons are fired at the slits one and a time (ie: with only one electron "in flight" at any given time, so a single electron is fired then the next electron is only fired after the first one has hit the screen) then over time a pattern will built up on the screen.

You are correct that indervidual electrons behave as waves, it is not an effect which needs multiple electrons interfering with each other. Each indervidual electron has a probability that it will hit the screen at any given point, this probability is found by considering the modulas squared of the wavefunction at that given point.

If lots of electron as fired then the resulting spot pattern is a way of visualising the modulas squared of the wavefunction, as the density of spots is proportional to the wavefunction magnitude.

This is why electrons are said to behave both as a wave and a particle. It is an example of wave-particle duality - where something has the properties of both a wave and a particle. Another common example is light; light behaves as a wave, but it is also made up of quanta called photons, and the quantised nature of light has been varified by many experiments.

Coming back to the topic of this thread. The electron has a wavefunction, but upon measuring the properties of the electron its values are determined, and so the wavefunction collapses to give particle-like behavour. In otherwords, if you dont know where the electron is you can say the probability of where it could be, but if you watch where it is you know for certain so there are no other possibilities.

This phenomenom is better understood by considering more quantum properties as well as position. There are lots of interdependant quantum numbers,( like total angular momentum and the 3 orthoganal angular momentum components). In many cases if you observe on property, it limits the values that other properties can take, as you know the values of one property and you know how the others behave on it. In this way, once you know a property by observing it the wavefunction is collapsed, as there are now less possible states the particle is in. However, there are also properties which you cannot know at the same time, (like position and momentum, time and energy etc...) due to quantum uncertainty.

There is a rival theory to the genrally accepted QM theory. It involves "quantum fields" which quide particles. So an electron would be a particle which floats in a quantum field, the flow lines of the field would be related to the wavefunction of the electron in normal QM, so the behavour of the electron is the same as in normal QM. This is perhaps as way to "think round" the problem of wave-particle duality, - particles are still particles but they are quided by waves.

I do not know the details of this theory tho, so it could have some massive flaws in it. The small amount I have read about it suggests it gives the same behavour as standard QM, but again, I do not know enougth about it to say for sure how it compares.