# Wave function collapse with Young's double slits.

1. Feb 15, 2012

### inquiring

I have a question regarding Young's double slits experiment.

To my understanding, wave function of a photon somehow collapses according to the probability function (which has a interference pattern).
But at the very moment the wave hits the screen, it seems to me that there should be no interference. (just two peaks, also at the last moment)
It would take a fraction of time (that the photon travels a few wave length distance) before the interference pattern emerges.

So I wonder, the wave function might collapse even before the interference pattern shows up? at the very moment when the wave touches the screen? (oppose to collapsing only when sufficiently interfered)
I think experiments can tell the difference; the double-slit pattern would enclose a small portion of two single-slit patterns, and that portion would vary according to the distance between the slits and the screen.

My question is:
1. My understanding of wave function collapse is correct?
Does one need a new interpretation than the wave function collapse?
Or I got it all wrong?

2. How's the experimental result?
Double slits only show purely interferential pattern?
Did they look at the chance that it may contain single-slit patterns?

Could somebody help me on this topic?
Thank you.

2. Feb 15, 2012

### Karl Coryat

Generally (at least with the Copenhagen interpretation), we have to resist the desire to describe what the wave is "doing," "on its way" to the screen. The question of whether there is a real wave function that collapses is a matter of interpretation. Followers of Bohmian mechanics will give you one answer, and many-worlds or relational people will give another.

The best way to think of it is that the apparatus setup (distance between the slits, wavelength of light etc.) corresponds to a specific configuration of probability values at the screen, which can be predicted using a basic formula. The distribution of photons on the screen ends up "obeying" this formula. The question of why is harder to answer, but visualizing each photon as a wave that passes through both slits certainly works.

But this only goes so far. You'll invariably fail if you try to make an intuitive picture, like imagining the photon as a literal wave of water that undergoes a literal physical change at some point in space and time, a change that we could see if we just looked at it in a certain way. It just doesn't happen like that. This is part of the challenge of understanding quantum mechanics.

3. Feb 15, 2012

### lugita15

According to the standard interpretation of quantum mechanics, known as Copenhagen, the collapse of the wave function occurs instantaneously throughout space, i.e. the collapse propagates faster than the speed of light. So you don't need to even consider how long it would take light to go from one point on the screen to another.

It should be noted, however, that the same theory that suggests that there might be something going on that's faster than light also tells us that this faster-than-light mechanism (if it exists as the Copenhagen interpretation states) is totally useless to us, in the sense that we cannot transmit information or travel using it.