# Wave Function Collapse

1. Mar 23, 2015

### Corran Paterson

While I was daydreaming in my boring math class today I came across a weird problem. What ACTUALLY causes a wave function to collapse. I had a discussion with my teacher the week prior and she was telling me that it has something to do with your brain. I didnt really believe her, I just thought it was something that everyone had to accept, however, during my daydream I had the idea that the interactions between photons and objects forces wave function collapse. If that makes any sense.

Photons have really small mass, and if they act as a particle and smack into objects at c wouldnt that in some way change the object?! [In this case force it to collapse?]

This may be a really silly question but I am only basing this off of my limited knowledge of QM.

2. Mar 23, 2015

### Staff: Mentor

Your teacher is mistaken. The idea that wave function collapse is caused by conscious observation was abandoned three-quarters of a century ago. You are definitely on the right track with the idea that it's the interaction that matters.

You should also be aware that wave function collapse is not an integral part of quantum mechanics, and hasn't been since the the theory was formulated in its modern form, also more than 75 years ago. It's just one of several ways of thinking about what the math is telling us.

A good equation-free layman-friendly book that will explain the modern understanding of this stuff is "Where does the weirdness go" by David Lindley. You can also google for "quantum decoherence" to get the real story, math and all.

Last edited: Mar 23, 2015
3. Mar 23, 2015

### Staff: Mentor

Its usually not explained in beginning texts, but collapse is not really part of QM - only some interpretations. In some it's 'magical' and no explanation offered (eg Baysian type interpretations - but since its all subjective in those interpretations its debatable if any is required), in others its explicit (so called collapse theories like GRW).

And Nugatory is correct - its got nothing to do with the brain, conciousness or anything like that - that's an old idea, that without going into the details, is now well and truly outmoded with the reason for its introduction long gone.

Thanks
Bill

Last edited: Mar 23, 2015
4. Mar 23, 2015

### atyy

But if collapse is subjective as in Quantum Bayesian interpretations, then isn't it in the brain?

5. Mar 23, 2015

### atyy

In the orthodox Copenhagen interpretation, collapse is how we update our information about the physical situation when the classical measuring apparatus interacts with the quantum system - so it potentially is in the brain and also the result of an interaction - but we don't know how much of each is happening, because the wave function is not necessarily real in the Copenhagen interpretation, and just a tool to calculate the probabilities of measurement outcomes.

To try to answer what is really happening in collapse, one needs another approach. For example, in Bohmian Mechanics, there is no wave function collapse, but additional variables are introduced, so that wave function collapse is derived. Another approach that tries to derive the collapse of the wave function without having it as a postulate is the Many-Worlds interpretation.

6. Mar 23, 2015

### Staff: Mentor

Its like the Baysian view of probabilities - you update a subjective belief based on new knowledge. Its a subjective belief - but one any rational agent will agree on.

Its a philosophical minefield and one reason I eschew Baysianism. Large ensembles suit me a lot better.

Thanks
Bill

7. Mar 23, 2015

### Corran Paterson

Thanks Guys you da best! :')

8. Mar 23, 2015

### stevendaryl

Staff Emeritus
We've had this interchange before, but just for the record, I have to say that I don't see how any of the conceptual difficulties of subjective probability are helped by having a large ensemble. With a large ensemble, you can turn the question of how to interpret what "probability 1/2 of heads" means into the question of how to interpret what "probability 99.99% of getting 5000 $\pm$ 5 heads out of 10,000 tosses". But you still don't have any better idea of what that $99.99%$ means.

9. Feb 5, 2016

### drl

Try this The transfer of the work function energy from a wave to a bound particle as an atom to a molecule results in the formation of a free atom particle which rapidly changes to its wave form.i.e. particle +work function energy=wave and wave -work function energy =particle. Using this concept De Broglies macro particles do not show wave properties because the work function necessary to do this would be enormous. I know this is contentious but hope some one can improve on it so as to lead somewhere.