Question about the double slit experiment

There is your problem from the start - it's neither wave or particle:
https://www.physicsforums.com/showthread.php?t=511178

What happens in the double slit experiment is probably best explained by Feynmans approach. Particles take all paths. With both holes open they can take both paths so you get interference. Close one hole and since it can only go via one path you don't.

Now put a detector in one path and its like closing one hole - if it goes through that path it's detected so cant interfere - if it doesn't its not detected - but either way we know by which path it went so cant go via both paths.

It's subtle, and its best to have it explained in detail from the start. I suggest having a look at Lenny Susskinds lectures on it:
http://theoreticalminimum.com/courses/quantum-entanglement/2006/fall

Thanks
Bill

 

If you put a detector it cant go through both holes simultaneously - it goes through one or the other. Either its detected at the detector or not. If it is then it went through that hole - if it wasn't it went through the other hole.

To understand Feynman's view and why you get dark and light bands check out his lectures:


But just as an overview each path has a little arrow attached to it that twirls around. They sum at the screen - if they are opposite they cancel and you never get a particle there - if they are the same then they reinforce and you are more likely to get a particle there.

I also STRONGLY suggest you view the Susskind lectures I linked to.

Thanks
Bill

 

Try the images here: http://mcat-review.org/light-geometrical-optics.php

The analysis is also worthwhile.

 

Note that removing some of the electron won't necessarily make a band dimmer. It might make it narrower instead

 

The answer is yes, the intensity changes as you describe. :)

 

Great questions. The answer is that the detector is not responsible for creating the interference pattern. This is a function of whether or not it is possible to know that the particle went through a specific slit.

In one version of the experiment, polarizers are placed in front of each slit. This blocks half the light. When the polarizers are aligned parallel, there is an interference pattern. But none when aligned perpendicular. That is because it is possible - even if you don't attempt to find out - which slit the particle went through when the polarizers are perpendicular. The only variable here is the orientation of the polarizers!

 

the interaction is physical.

the detector, I think, interacts physically with the photon.

Maybe someone can clarify this.

good point.

To add: The perpendicular polarizers create a difference in the direction of the oscillation of the two beams. The beams are now oscillating perpendicular/orthogonal to each other. Because of this the beams can no longer interfere.

Before encountering the perpendicular polarizers, the two beams were oscillating in the same directions (axes).

 

It does - via the process of decoherence:
http://en.wikipedia.org/wiki/Quantum_decoherence

Technically its changed from a superposition of both paths (loosely speaking) to what's called a mixed state that can be interpreted as having a probability of going via one path or the other - but not both.

I wont hide from you looking at it this way is controversial and discussions on it can get a bit heated. I wont be drawn into that but simply give a link with the detail so you can make up your own mind - if you want to pursue it that is:
http://philsci-archive.pitt.edu/5439/1/Decoherence_Essay_arXiv_version.pdf

BTW its not decoherence that's controversial - its implied by the formalism of QM and no one doubts it - its the interpretation that's at issue.

Thanks
Bill

 

I would advise you to read first four chapters of Feynman Lectures vol III. If you don't have one then buy one. It is really worth reading.
You will find answers to all the questions you have asked.You don't need to be familiar with anything but high-school mathematics to understand those chapters.

 

Yes they have significance but probably not in the way you are thinking. In Quantum Mechanics you do physics which is very much different from Newtonian Mechanics.
If you take postulates of Quantum Mechanics (translated into a simple english by Feynman) for granted then you will find that the interference (so called wave nature) appears in absence of detector and it disappears in presence of the detector. You can actually see the interference disappearing through the equations, without considering any "extra forces" but the rules stated by Feynman.

While I am not very much comfortable with collapse and how/why it happens, I can tell you that if you apply Feynman rules you can get the result without having to answer these questions.

Of course it doesn't mean that Quantum Mechanics can't answer these questions. But then you will have to consider stuff like "de-coherence" which is not something you want to learn in the beginning.

 

Dear m_robertson,

Keep in mind that the detector and polariser are interacting in ery different ways with the photon.

The detector is collapsing the wavefunction (decoherence),
the polarizer is not, the polarizer is simpling changing the axis of oscillations.

 

M_Robertson,

One thing is that:

Particle interactions are assumed (and it might actually be that way) to be frictionless.

For example: There is not energy loss during, for the photon during, say, decoherence or when the direction of polarization of a photon is changed etc.

For QM - The brain has to be trained to not think classically and perhaphs think beyond time-space.

 

This sort of stuff is very much tied up with interpretation.

If collapse is even a physical process is far from certain.

In some interpretations, such as the ensemble interpretation I hold to, and even Copenhagen, its like probability theory. Before you throw a dice there is a probability of 1/6 that any face can come up. This is represented by a vector with 6 entries and in a sense is the state of the dice. Now throw it and a face comes up - it is now a dead cert that face is up. The state changed from a vector with 1/6th in each entry to one with a 1 in one entry. The state has collapsed. But nothing in a physical sense changed - simply our knowledge about the dice. Some interpretations of QM view state in a similar way.

If so questions like where does it get the energy to collapse are pretty meaningless.

One view of QM is its simply an example of a generalized probability theory:
http://www.mathematik.uni-wuerzburg.de/sommerschule2011/download/ln-janotta.pdf [Broken]

Thanks
Bill