# Double slit interpretation - what is the mainstream view?

1. Dec 3, 2009

### peteratcam

Can someone summarise what is the currently accepted interpretation of how measurement affects the interference pattern in a double slit experiment? (Answers which say things like 'the electron knows if it is being observed' are not acceptable - :-) ) To quote from the PF rules, I'm looking for "the current status of physics as practiced by the scientific community", with regards the double slit measurement thing.

The general internet is full of so many crank opinions on this and the literature is far too big and also full of speculative nonsense. There must be someone here who has the right description and is able to summarise succinctly?

I have my own explanation, which I'm hoping is the majority view among people of good sense.

Thanks!

2. Dec 3, 2009

### Matterwave

The current interpretation (copenhagen interpretation) is that observation collapses the wavefunction into either moving through one or the other slit, and thus no interference occurs. It's not that the electron "knows" it's being observed, it's that the act of observation is forcing a collapse of the wavefunction.

It's not a very intuitive, or satisfying result, I know. :P

There are a myriad other interpretations (e.g. many worlds, decoherence, ensemble, piolet wave, etc)

3. Dec 3, 2009

### Demystifier

Decoherence is not an interpretation. Decoherence is an experimental fact (predicted by the Schrodinger equation) that plays an important role in all interpretations, especially in many world and pilot wave.

4. Dec 3, 2009

### peteratcam

To clarify, I suppose I should have said, what is the mainstream view on the mechanism by which measurement destroys interference, rather than the interpretation.

I firmly believe that (almost) everything is describably by unitary time evolution according to the TDSE, so I don't like collapsing wavefunctions.

5. Dec 3, 2009

### Demystifier

It is generally accepted that measurement affects interference pattern through decoherence. Decoherence explains why quantum statistics may be replaced by classical statistics. However, it solves only a part of the problem. Decoherence does not explain how and why the particle ends up in one particular state among many states allowed by classical statistics. There are many interpretations that suggest an answer to this question, but NOBODY knows with certainty which answer is the correct one.

6. Dec 3, 2009

### Matterwave

Sorry about my mixup with decoherence, it's been a while...

I believe there are models that include the observer within the TDSE and observations are therefore wavefunctions interacting with wavefunctions or some such. I'm not very familiar with this.

In the mainstream interpretation, the mechanism by which wavefunction collapses is unknown. It's just saying for example if the atom could either be in room A or room B. Before you look, there is a chance it is in A or room B (both rooms contain a finite probability), but if you look in room A and find the atom, it's in room A, it's not in room B, and vice versa...(the probability distribution collapses)...

7. Dec 3, 2009

### Demystifier

For more details see

8. Dec 3, 2009

### peteratcam

So I looked at a number of those links. Only the Rev Mod Phys article mentioned entanglement much.

My view is that the electron and the which-path measuring apparatus become entangled and hence interference pattern disappears. I don't see the need to include a macroscopic number of degrees of freedom which seems to be the point with decoherence.

Eg:
$$\phi_1(x)+\phi_2(x)\rightarrow|\phi_1|^2+|\phi_2|^2+Re(\phi_1\phi_2)$$
The last term is the interference.
whereas if we entangle the different slit states with orthogonal states of the measuring apparatus (ie, make a good measurement):
$$\phi_1(x)|1\rangle+\phi_2(x)|2\rangle\rightarrow|\phi_1|^2+|\phi_2|^2$$
No intereference this time.

It seems obvious to me that this is what's going on - measurement doesn't collapse a wavefunction, it just entangles the measuring device and the system of interest, and by orthogonality of the states of the measuring device all interference terms are killed and classical behaviour emerges.

Is this the same idea as decoherence?
Thanks.

9. Dec 3, 2009

### Demystifier

Yes it is. But you need a macroscopic number of the degrees of freedom, because it provides the stability of orthogonality and decoherence. Otherwise, the original coherence could be restored.

Besides, the orthogonality between the states of the measuring apparatus in not exact. They are only approximately orthogonal, and the approximation is better for a larger number of the degrees of freedom. When their number is macroscopic, then the approximation is almost perfect.

To summarize, decoherence as you described it may work even without a macroscopic number of the degrees of freedom, but when this number is macroscopic then decoherence is almost inevitable.

Last edited: Dec 3, 2009