Grilling the Double Slit Experiment

[richerrich]

A detector screen far from the slits will show wave patterns whereas a detector near the screen will show particles. My question is what if we maintain the detector screen far from the slits but put some say block of wood near the slits, will we detect wave or paticles?

 

[Matterwave]

why do you think that waves/particles will appear depending on how far the detector is...?

 

[richerrich]

I was referring to an article in New Scientist May 8 2010. To quote from its diagram: A. "Place a detector far behind the slits and a single electron will produce a characteristic interference pattern - a wave has seemingly passed through both slits at once". B. "Place separate detectors close enough behind the slits and only one registers a click as if the electron were a single particle."

 

[xts]

Could you post a link to the article?
It seems to be totally crap (single electron always produce single click)

 

[richerrich]

Could you post a link to the article?
It seems to be totally crap (single electron always produce single click)

Please click on attachment. Recently though I've come across an article which gave me more answers than what i expect from my question here. But I am interested on what's the take of people in this forum.

 

[DrChinese]

Here goes.

This is a misleading picture. One particle leaves one impression, always, as xts says. The patterns only emerge after many particles are detected.

 

[richerrich]

A detector screen far from the slits will show wave patterns whereas a detector near the screen will show particles. My question is what if we maintain the detector screen far from the slits but put some say block of wood near the slits, will we detect wave or paticles?

Sorry for my mistake, above should have been:

A detector far from the slits will show wave INTERFERENCE whereas a detector near the SLITS will show CLUMP OF particles. My question is what if we maintain the detector far from the slits but put some say block of wood near the slits, will we detect interference or CLUMP OF particles?

 

[Ken G]

The point is that waves do both of the things you see in that picture, and particles do both of the things you see in that picture, because waves and particles are the same thing. For example, everything you see in that picture also happens with water waves, but the water wave is many many particles of "water wave excitation"-- it won't make a single blip because there's no way to "turn down the intensity" of a water wave like you can a particle beam. But if you could turn down the intensity of a water wave, that's just what it would do. I don't know why so many people frame "wave/particle duality" like behaving like a particle sometimes and like a wave sometimes-- the real message of quantum mechanics is that we were mistaken to ever think that waves and particles were any different from each other. They do all the same things.

 

[richerrich]

The point is that waves do both of the things you see in that picture, and particles do both of the things you see in that picture, because waves and particles are the same thing. For example, everything you see in that picture also happens with water waves, but the water wave is many many particles of "water wave excitation"-- it won't make a single blip because there's no way to "turn down the intensity" of a water wave like you can a particle beam. But if you could turn down the intensity of a water wave, that's just what it would do. I don't know why so many people frame "wave/particle duality" like behaving like a particle sometimes and like a wave sometimes-- the real message of quantum mechanics is that we were mistaken to ever think that waves and particles were any different from each other. They do all the same things.

Ken, There's a reason while this duality STILL can't be explained. Classic experiment demonstrates that electrons or photons behave as either wave or particle dependent on the observer (and like you at the back of my mind i was thinking waves are a bunch of particles anyway). I refused to accept this explanation from the beginning since it doesn't make sense. Most physicists don't mind this at all since they'd rather calculate with QM than explain it.

But note that in the double slit experiment there was no "turning down the intensity" since using the same setup the ONLY difference when the screen showed either particles or waves was the result of the detector being counted (say by a computer while the experimenter was outside the room for coffee break).

I've been looking for articles or documents detailing the double slit experiment further. I've come across one and it says that they behave as particles once the observer starts counting how many particles have passed through the slit, but not because somehow the detectors themselves have influenced how electrons or photons behave.

Recently though I'm glad that my sentiment is justified by a latest New Scientist October article that there are Physicists making the effort of explaining the duality independent of the observer. And it has something to do with entanglement.

 

[xts]

Classic experiment demonstrates that electrons or photons behave as either wave or particle dependent on the observer

That seems to be one of the most common misunderstandings of QM.
Try to think about photons/electrons as:
- waves - always when they propagate (also if only one propagation path is available);
- particles - only when they are emitted/detected/interact with others.

The picture is misleadingly captioned. Single electron always produce single click. Wave mechanics explains both interference fringes on a distant screen and lack of them on a screen very close to the slits: the amplitude coming from other slit is negligibly small, so the pattern seen in such case will be just a two (not one, like they painted!) bulky spots in front of each slit.

As you move the screen further, the spots in front of slits will grow wider, and as they grow large enough to overlap, the fringes starts to be visible in an overlap area.

 

[Ken G]

Ken, There's a reason while this duality STILL can't be explained.

No one is ever going to "explain" the duality, any more than anyone ever explained why we thought particles followed trajectories (is that a "natural" thing for a particle to do? Why?). We just witnessed trajetory-like and wave-like behaviors, and we thought they were different things, and now we know they are the same thing. That's all we're likely to get, I don't see much point in hoping for more than that. Maybe there'll be additional layers added there, but at the lowest layer we will always wonder why.

Classic experiment demonstrates that electrons or photons behave as either wave or particle dependent on the observer (and like you at the back of my mind i was thinking waves are a bunch of particles anyway).

Yes, we need to move away from "particle or wave" and see that the lesson is "particle and wave." There never was any difference between particles and waves, they were always two sides of the same coin, but we only now realize it (if we stop getting told that things are turned from one thing into another by an observer). If you spend your whole life thinking there are two kinds of coins, ones with heads on both sides and ones with tails on both sides, and someone flips a coin over and shows you the actual state of affairs, you don't say "that's amazing, sometimes coins act like they have heads on both sides, and sometimes like tails", you say "aha, heads and tails come on the same coin, not two different coins." That's understanding, that's the explanation.

But note that in the double slit experiment there was no "turning down the intensity" since using the same setup the ONLY difference when the screen showed either particles or waves was the result of the detector being counted (say by a computer while the experimenter was outside the room for coffee break).

The difference was where the detector was placed. My point was, that is entirely explained by wave mechanics-- water waves will do those exact same things if you put a seawall behind two holes in a jetty. The only thing that makes it seem strange is if you turn down the intensity until you notice you have individual particles there, but then you don't say "wait, sometimes these things are particles", you say "oh, these things are both particles and waves, I guess waves and particles are the same thing." Which they are.

I've been looking for articles or documents detailing the double slit experiment further. I've come across one and it says that they behave as particles once the observer starts counting how many particles have passed through the slit, but not because somehow the detectors themselves have influenced how electrons or photons behave.

That's just not the right lesson. Waves do all those same things if you turn up the intensity, there is not a single thing that a high-intensity source of particles does that waves don't do, because there's no difference between what we used to think waves were and a high-intensity particle source. It's all a question of what the wavelength is (that length compared to length scales of the obstacles encountered), and what the intensity is (whether or not we can distinguish individual detection events). That's it, that's where all the differences between what we thought were waves, and what we thought were particles, comes in. There's your "duality"-- there are actually two dualities there, the duality of high and low intensity, and the duality of long and short wavelength. That's it, that's the whole enchilada, the deepest level of our theory. That's what you want to understand, but you won't, because we never understand the deepest level of any of our physical theories. That's not a bad thing, it's just how science works. The point of science is to try to unify everything you don't understand into the simplest possible set of principles that you don't understand.

Recently though I'm glad that my sentiment is justified by a latest New Scientist October article that there are Physicists making the effort of explaining the duality independent of the observer. And it has something to do with entanglement.

They will never explain the duality independent of the observer, that's obvious. Because they are always going to have an observer in there, I guarantee it-- that's how science works. What they mean is really something quite different-- they will be trying to include the observer in as unimportant a way as possible, they want the "least violent" observation to happen-- and so they can learn a lot about what observers are not doing by eliminating various kinds of observer influences. But what they can never learn is what observers are doing, because they can never completely remove the observer and make a comparison-- they will always be doing science, and science is, quite demonstrably, about what observers do. Again that's not a bad thing, it's what science is.

 

[richerrich]

That seems to be one of the most common misunderstandings of QM.
Try to think about photons/electrons as:
- waves - always when they propagate (also if only one propagation path is available);
- particles - only when they are emitted/detected/interact with others.

xts: "only when they are emitted/detected/interact with others." can also be misleading since if one places glass in the path of photons/electrons they interact with the glass but still behave as waves.

i am more inclined to understand that photons/electrons are quanta of the same thing whose wave function collapses once anything (observer, measuring tool, etc) causes them to entangle.

i am still waiting for someone to drop by here and provide another perspective. But if one has come across recent research by Daniel Bedingham then that'll be more like it.

 

[richerrich]

...oh, these things are both particles and waves, I guess waves and particles are the same thing." Which they are.

Ken, i think most physicists would not use water molecules and water waves as analogy to explain the double slit experiment. Just to ease on someone's imagination let's suppose these molecules are big as marbles. Using waves of marbles will always give the impression that there can be interference from the resulting waves. Thus experimenters need to use one marble at a time. And surprise! sometimes a single marble is acting like it's on its own and sometimes it's acting like it's riding on some invisible waves that can interfere with itself and that single marble just follows the resulting path. (As to what are these waves is another thing to argue). Thus in the double slit experiment particle and wave are not the same thing.

 

[Ken G]

Ken, i think most physicists would not use water molecules and water waves as analogy to explain the double slit experiment.

That's because the wave/particle duality in a water wave has nothing to do with water molecules, so it would not be a good idea to bring them in. The physics of a water wave is normally treated as a disturbance of a continuous medium with a gravitational restoring force. The excitation has an action associated with it, so quantum mechanics says that action is quantized. The quantum of action is the "particle" that is dual to the water "wave", and it is that particle, and that wave, that are the same thing-- there are just so many particles of wave excitation involved (again, nothing to do with water molecules here) that we would normally ignore the particle aspect of the wave. That is typical of classical waves, and is why we used to think waves and particles were different things. The waves that are dual to the water molecules have nothing to do with water waves, they are the wave functions of the water molecules.

Using waves of marbles will always give the impression that there can be interference from the resulting waves.

No. One can realize that waves and particles are the same thing without it following necessarily that waves will always interfere or that particles will always have trajectories. Wave interference, for example, is always an issue of wavelength and coherence length, and many types of wave do not show interference. It is time to move past these wrong concepts, and get to the right one: waves and particles are the same thing. This is one of the main lessons of quantum mechanics.

Thus in the double slit experiment particle and wave are not the same thing.

Then you have missed the main lesson of the double slit experiment, which is that waves and particles are the same thing, but they of course have various limiting behaviors. When you see what we used to call "waves" and "particles" as just limiting behaviors of the same thing, then you will see why waves and particles are the same animal, it's just an animal with a head and a tail (if you will). This is the essential recognition-- you, and anyone else reading the thread, have not really heard the message of the double slit experiment until you see this.

 

[Lambert]

Forgive the cartoon, unfortunately I am not a physicist but it worked for me ;-/ :

http://www.youtube.com/watch?v=zKdoE1vX7k4&feature=related

 

[Ken G]

Beware of a few inaccuracies in that video though. For one thing, if you look which slit the particles go through, you still get a wave pattern, it is just the wave pattern of a single slit repeated twice, rather than the wave pattern of a double slit. Do not think for a moment that the particles "turn into particles" when we watch them, they are always particles whether we look or not, and they are always ruled by wave mechanics-- in every situation. It is time to get away from all this incorrect "particle vs. wave" language, the real lesson is, whether you know which slit the particle went through or not, particles and waves are still just aspects of the same thing. There is never any situation where they are not aspects of the same thing.

 

[aspades]

Isn't the entire principle of how a detector can change the state of something from wave to particle or Visa Versa frankly I think the double slit experiment is a great way to demonstrate this Quantum weirdness on a detectable level. As for the detectors close to the slits obviously they will change the state because they are measuring what slit the electron goes through whereas the moving detector further away could only measure an electron that could have passed through either slit. Every particle has a wave function so all you are doing by observing it is settling it into one of these functions which is basically the same principle as schrodingers cat. By observing the electron going through the slit we collapse it into a particle which settles the path into a definite state resulting in the pattern we would expect.

 

[Ken G]

Isn't the entire principle of how a detector can change the state of something from wave to particle or Visa Versa frankly I think the double slit experiment is a great way to demonstrate this Quantum weirdness on a detectable level.

You do often see language to the effect that the detector "changes a particle into a wave" or some such thing. I am saying that I feel that is very unfortunate language, because it conceals the whole message of quantum mechanics and the double slit: particles and waves are the same thing. It's like if your whole life, you noticed that some U.S. quarters have a picture of George Washington on them, and some have an eagle on them. Then someone showed you that it is possible to "turn the eagle into George Washington or vice versa", would that really be the right lesson? Or would it be that a quarter had two sides all along and you just hadn't noticed until now? Waves and particles are two sides of the same thing, and nothing that waves do particles can't do, and nothing that particles do waves can't do. We just don't always have the ability to observe it, as with the quanta of excitation of a water wave or the diffraction of a baseball.