Question about the double-slit experiment.

BruceW

I am not sure what they use in actual experiments, but the example I have seen is using a loop of wire around one of the slits. So this will let the particle pass through one of the slits, and you can tell which slit it went through because a current will be induced in the wire loop surrounding the particular slit that the particle went through.

Physics4

Thanks Bruce. Allow me to elucidate. Home experiments reveal the principles of the effect, but don't actually use single photons or single electrons because the equipment isn't capable of doing that. I've also seen several computer animations of the experiment, but nothing that describes the method and principle that allows the detector to both detect a single particle and allow that same particle ( wave form or whatever ) to pass through the detector undisturbed to the target.

Typically we simply see a simplistic portrayal where an artist inserts an eye or camera into the animation that is supposed to represent a detector. The problem with such interpretations is that for our eyes or a camera to detect a single photon, that photon has to strike the surface of our retina or the camera sensor where it's converted into energy that is sent down a signal path and registered. The photon has been absorbed. Therefore if you were to actually use such a detector in the double slit experiment, the detector will catch the particle and the particle will never never reach the target. Logically this should have the same effect as covering one slit, and consequently the interference pattern should disappear.

The missing information is therefore, how does the detector actually work to detect a single particle? Logically, if it cannot interact with the particle in some way it cannot detect it, and if it does interact with it in some way it must have some interactive effect, and that interactive effect may well be the cause of the change in the pattern seen on the target. Am I making any sense here?

pilotwave

The double slit experiment can be understood in a completely "classical" way by considering a definite particle passing through one of the slits while an accompanying guiding wave passing through both. A macroscopic pilot-wave system has reproduced this (and there is no trouble with observing the particle):

http://www.youtube.com/watch?v=W9yWv5dqSKk

If one of the slits were closed, the wave would be disrupted and the pattern is not found. Additionally you can easily imagine a measurement technique so heavy-handed (i.e. a floating buoy measuring the surface deflection) that would also destroy the wave-pattern. No mystery here, just a particle AND a wave.

Another recent experiment shows how a complex underlying pilot-wave dynamics can underlay a simple statistical description via deterministic chaos:

http://www.youtube.com/watch?v=nmC0ygr08tE

Physics4

Thanks pilotwave, it’s an interesting theory

Nugatory

Good thing you used scare-quotes around the word "classical"

BruceW

Yeah, of course we have to interact with the particle to be able to tell which slit it went through. And this is why the interference pattern changes when we choose to place detectors over the slits.

And yes, I have also seen videos explaining quantum mechanics, where someone just watches the particle, and then the particle chooses one slit to go through. Of course this is only telling part of the story, because the person must interact with the particle. You shouldn't take such videos too literally.

Physics4

I would have chosen “the professor of ignorance” if allowed as a user name, as I’m a member from a completely different forum, asking a question for someone else. However, allow me to ask you this. If only having the option of choosing one video explaining what the term “observer effect” really meant, which video would you suggest?

BruceW

I would suggest a lecture series. maybe MIT have something online. Or even type QM lecture into youtube. Some of the introductory lectures might have a good explanation of some of the concepts of QM, before actually teaching it.

The term 'observer effect' is a bit misleading itself. The way I see it, is that if we have some particle, then generally it will not be in an eigenstate of some measurable quantity. So when we do measure that quantity, we are forcing the particle into one of the eigenstates of the quantity we are measuring. So we are not merely 'observing', we are forcing it into a different state. It is only when the particle is already in an eigenstate of the quantity we want to measure, that we can take the measurement without affecting the particle.

Out of curiosity, what is the forum you usually go on?

Physics4

Thank’s, I can see why your recognized as a curious homework helper. You never know.., perhaps the professor of ignorance may show up again