# Why does the wave function have to collapse upon measurement?

1. Nov 27, 2013

### TriKri

How does the Copenhagen interpretation motivate its claim that the wave function of a quantum system collapses when a property of the system is being measured? I mean, was there some problem they were trying to solve by introducing the notion of wave function collapse? What would happen if the wave function would not collapse? Wouldn't it work to just continue to describe it with the Schrödinger equation like normal?

2. Nov 27, 2013

### atyy

The idea is that in some cases, repeated measurements are possible, and these indicate that immediate repetition of a measurement gives the same result.

3. Nov 27, 2013

### Staff: Mentor

Descriptions without collapses are possible, but then they are not called Copenhagen interpretation:
a) describe the wave-function as purely mathematical tool without physical reality (many different variants)
b) describe the measurement process itself and everything else with quantum mechanics (-> many-worlds, de-Broglie Bohm)

You cannot do "nothing", as repeated measurements (can be the same observable, but does not have to) will show that your measurement did influence the particle in those setups where Copenhagen gives a collapse (this is true for all interpretations).

4. Nov 27, 2013

### ModusPwnd

Is it ok to think of it the other way around? If the wavefunction collapsed then a measurement was made. Measurement is defined as that which collapses the wavefunction. Is it ok to think of it this way?

5. Nov 27, 2013

### bhobba

In that interpretation the wavefunction is like probabilities - it encodes the outcomes of observations. Its very purpose is to change when you observe it. Its like throwing a dice. Prior to throwing it each side has a 1/6th probability of coming up. Throw it and one side is a dead cert - its 'wavefunction' has collapsed.

Some other interpretations explain the 'how' - Copenhagen simply accepts it.

Thanks
Bill

6. Nov 27, 2013

### bhobba

Measurement is whatever leaves a 'mark' here in he macro world.

Nowadays its usually associated with decoherence - a measurement has occurred once decoherence has happened. This means it can happen even without reference to an actual measurement apparatus. As an example, a dust particle is decohered to have a definite position by a few stray photons. One can think it has been measured.

Thanks
Bill

7. Nov 27, 2013

### audioloop

imo there are collapses without measurements, due just to a non linear character of the schrodinger equation, i.e. non linear quantum mechanics

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