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Double-slit Experiment Questions


RobbieG
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the computer you are using right now to read this is proof of quantum mechanics. lots of quantum mechanics is involved in the operation of a semiconductor. and the LED's on your screen(or the phosphors if you use a CRT) then there's the way the data is sent around the globe.

 

if it was bunk then pretty much none of it would work.

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  • 9 months later...

What i have never understood is how do you detect a photon without intereacting with it, either by capturing it or in some other way modifying it and thus compromising the entire experiment?

 

Generally speaking, you don't. There are some measurements that can be done which don't perturb the system, called quantum nondemolition measurements.

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Is there anywhere that could describe this in a terrestrial language, say, English? Does it fulfil the criteria that the journey of the photon from source, A, to target screen, C, via the slit wall, B, is not modified by observation to consist of two journeys, A-B and B-C? what is the criterion for non-perturbation of the system? Because, since the outcome appears to depend upon whether or not there is an observation, this is a crucial point. The fact of a changed outcome would strongly indicate that the observation did have a non-transient, permanent interaction with the photon.

If memory serves, this experiment and its adherent theory was carried out before quantum nondemolition measurements were available.

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Is there anywhere that could describe this in a terrestrial language, say, English? Does it fulfil the criteria that the journey of the photon from source, A, to target screen, C, via the slit wall, B, is not modified by observation to consist of two journeys, A-B and B-C? what is the criterion for non-perturbation of the system? Because, since the outcome appears to depend upon whether or not there is an observation, this is a crucial point. The fact of a changed outcome would strongly indicate that the observation did have a non-transient, permanent interaction with the photon.

If memory serves, this experiment and its adherent theory was carried out before quantum nondemolition measurements were available.

 

The 2slit experiment is about the wave nature of photon, not about perturbation due to measurement or observation. Correct me if I am wrong.

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As I understand it, the experiment concerns the wave/particle duality of the photon.

 

In the latter phases of the experiment, single photons are fired at the dual slit on the assumption that the photon particles can only pass through one slit or the other, but not both. It has been suggested that observation of the photons affects the outcome.

 

To me, this seems obvious as I do not see how a photon can be observed without interaction. At the very minimum, information has to be extracted from the system in order to give an output observation. As I see it, this means that the observation station is no longer a "waypoint" on the photon's journey Ato C, but a "staging point" between two journeys A-B and B-C.

 

That changes the entire nature of the experiment.

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Knowing which path the photon takes, i.e. forcing the photon to pass through one slit or the other, destroys the interference. Having multiple paths allows for interference to occur.

 

Where is the photon being observed without interacting with it?

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There's something wrong here. I need to dig out my books with proper details of the experiment. there is an itch on the inside of my skull that tells me it ain't right.

 

With you.

I don't have the proper details. look at how they manage to avoid relection from plate C. As I understand the experiment photons arriving at plate C are 100% absorbed: how do they do that?

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anyone know where I can find a complete account of the experiment, with dimensions, materials, readings, etc.

 

all I ever see are reports summarising - well, what exactly?

 

And conclusions drawn from heresay, which is a euphamism for conjecture

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And conclusions drawn from heresay, which is a euphamism for conjecture

 

is this a dastardly combination of heresy and hearsay?

 

Michel and RMW - if you have access to JSTOR then you can find thomas youngs original write up here http://www.jstor.org/pss/107135 . the later experiments showing destruction of the interference when pathway is known are easier to find. And whilst we must be careful to avoid anecdotalism and defending the paradigm merely for its own sake; we must also acknowledge that science is enabled by "standing on the shoulders of giants". QM is remarkably well evidenced, and rigorous.

 

 

 

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is this a dastardly combination of heresy and hearsay?

 

Michel and RMW - if you have access to JSTOR then you can find thomas youngs original write up here http://www.jstor.org/pss/107135 . the later experiments showing destruction of the interference when pathway is known are easier to find. And whilst we must be careful to avoid anecdotalism and defending the paradigm merely for its own sake; we must also acknowledge that science is enabled by "standing on the shoulders of giants". QM is remarkably well evidenced, and rigorous.

 

Found this

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So I thought "better give it a go".

apparatus:

DIY laser level

Vernier caliper held in machine vise to genrate single slit

Kitchen wall

bottle of wine and chocolate doughnut as back-up.

good,, innit?

 

Distance laser housing to slit 240mm

Distance laser housing to wall 3048mm

slit width 0.1mm

post-32901-041320400 1287524016_thumb.jpgpost-32901-095162700 1287524039_thumb.jpgpost-32901-091560600 1287523967_thumb.jpgpost-32901-091749400 1287523992_thumb.jpg

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So I thought "better give it a go".

apparatus:

DIY laser level

Vernier caliper held in machine vise to genrate single slit

Kitchen wall

bottle of wine and chocolate doughnut as back-up.

good,, innit?

 

Distance laser housing to slit 240mm

Distance laser housing to wall 3048mm

slit width 0.1mm

post-32901-041320400 1287524016_thumb.jpgpost-32901-095162700 1287524039_thumb.jpgpost-32901-091560600 1287523967_thumb.jpgpost-32901-091749400 1287523992_thumb.jpg

 

I suppose you shooted perpendicularly to the target. The disposition showed in the picture don't show it that way.

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This was just "qualitative", i.e. just checking out apparatus and "how to"s.

In the absence of an optical bench, workshop drill press vise makes a very stable component holder. Vernier caliper gives easily adjustable slit width, but a craft knife blade gives a much thinner slit. by this I mean the actual depth of the slit along the beam path. Vernier caliper jaws are about 1-2mm thick which is rather like the Grand Canyon compared to wavelength of light.

 

First impressions are that imperfections in the internal surfaces of the laser and the laser housing itself are responsible for a lot of the side scatter i.e not in the plane of the expected interference pattern. dismantled the housing and cleaned the optics, but this did not give much improvement.

 

Before I take any serious measurements, I want to understand why the laser spot is not pin sharp.

 

I also need to buld a fully adjustable slit device and work out a method of photographing the interferenfce patterns so that I can use my PC's graphics package to make measurements.

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This was just "qualitative", i.e. just checking out apparatus and "how to"s.

In the absence of an optical bench, workshop drill press vise makes a very stable component holder. Vernier caliper gives easily adjustable slit width, but a craft knife blade gives a much thinner slit. by this I mean the actual depth of the slit along the beam path. Vernier caliper jaws are about 1-2mm thick which is rather like the Grand Canyon compared to wavelength of light.

 

First impressions are that imperfections in the internal surfaces of the laser and the laser housing itself are responsible for a lot of the side scatter i.e not in the plane of the expected interference pattern. dismantled the housing and cleaned the optics, but this did not give much improvement.

 

Before I take any serious measurements, I want to understand why the laser spot is not pin sharp.

 

I also need to buld a fully adjustable slit device and work out a method of photographing the interferenfce patterns so that I can use my PC's graphics package to make measurements.

 

Maybe due to the surface of the target. Of course, if you use a better surface, like a metallic one, you will get a sharper result, but more reflection (my objection upon the experiment, how do you avoid reflection?).

_also the back side of the Vernier is higly reflective*.

_And as you said, the thickness of the Vernier is also problem.

 

IIRC in other experiments they use a smoked glass with a ray traced with a pin. Higly reflective on the back side, as the Vernier is.

 

*when you take a photograph of the target in the experiment, you are taking an image due to reflection.

 

And, after all, you made a single slit experiment, with interference, isn't it?

Edited by michel123456
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Thanks

 

working through the problems.

 

The scatter that I refer to is not from the target, you can shield it out. So, it comes from other components. Using a craft knife edge instead of the vernier gives a different slit depth, but the scatter remains. having cleaned the optics, that leaves either the materials of the optical elements, or the internals of the sealed laser unit. Took the laser out of the housing, eliminating the optics. So that leaves the internals of the sealed laser housing. If it were the air column between the source and target, would expect thes catter to move about, but it is fixed and therefore possibly due to machining imperfections in the housing or the laser itself.

 

Simply don't have the equipment to make measurements directly on the beam, so a photographic method is the way to go. Using the resolution of a graphics package on a digital photograph should give good results. Need to work on getting good photographs at low light levels with a cheap camera.

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Thanks

 

working through the problems.

 

The scatter that I refer to is not from the target, you can shield it out. So, it comes from other components. Using a craft knife edge instead of the vernier gives a different slit depth, but the scatter remains. having cleaned the optics, that leaves either the materials of the optical elements, or the internals of the sealed laser unit. Took the laser out of the housing, eliminating the optics. So that leaves the internals of the sealed laser housing. If it were the air column between the source and target, would expect thes catter to move about, but it is fixed and therefore possibly due to machining imperfections in the housing or the laser itself.

 

Simply don't have the equipment to make measurements directly on the beam, so a photographic method is the way to go. Using the resolution of a graphics package on a digital photograph should give good results. Need to work on getting good photographs at low light levels with a cheap camera.

 

Try put the knife edges at some angle in a disposition /\ or \ / and look if the result changes.

 

And again, with a single slit, you shouldn't theoretically observe no interference at all.

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Don't rush me, don't rush me! When I have the "shortest"( i.e. length along the beam) slit, will then vary blade angles to see at what point the "canyon" effect begins to happen.

 

will also pass beam down polished brass tube to see what total internal relfection does. Because that will give beams of differing lengths.

 

All these things need to be accounted for before can even begin any experiments.

 

but, have to finish a couple of other projects first.

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Yes, actualy it is still behaving somewhat as a paraticle, because it hits one exact spot on the test screen, but when we fire more and more electrons, the dots on the screen begin to resemble an interference pattern. It's like the electrons intefere with eachother... at different time. The really curiuous part, however, comes when we put some device on one of the slits to watch if the electrons pass through slit #1 or slit #2.

The result is, that if we try to measure exactly trough which slit did they pass, the electrons do not form interference pattern anymore, but just usual scattered pattern (like golfballs). As if the act of observation* turns off the wave-like properties of electrons. This is called, i believe, wave function collapse.

 

One way to explain this is the wave nature of an electron is associated with its emitted EM waves. The particle natures stems from an electron being a tiny zone in space, which is the center from which the EM waves are emitted. If we try to measure an electron at one of the slits, we will need some type of device tuned to an electron which means we will use EM. This will interfere with the electron's EM waves so the interference pattern will be altered.

 

As an analogy, we have a small boat moving on the water making a wake. Since EM waves can travel at C which is faster than an electron, in the analogy the wake precedes the boat. If there are two openings under the bridge for the boat to travel, the boat can only use one at a time. But since the wake is a wave, preceding the boat and expanding, it can enter both at the same time. If we stand on the other side of one of the two tunnels and are making a wake (use the wave nature of electrons to detect the electron), we can cancel the waves.

 

If we separate the openings of the double slit enough, until the interference stops, one might be able to calculate the electron velocity since we know the speed of the EM wake is C. If our boat was making a wake but the openings are to far apart, the wave may not spread out enough to enter the second tunnel. Knowing the speed of the wake and the critical distance, we can sort of figure out the speed of the boat needed to prevent the wave from spreading out enough.

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After reading up on particle-wave duality and the double-slit experiment, I came upon this in the article.

 

The most baffling part of this experiment comes when only one photon at a time is fired at the barrier with both slits open. The pattern of interference remains the same, as can be seen if many photons are emitted one at a time and recorded on the same sheet of photographic film. The clear implication is that something with a wavelike nature passes simultaneously through both slits and interferes with itself — even though there is only one photon present. (The experiment works with electrons, atoms, and even some molecules too.)

http://en.wikipedia....slit_experiment

 

 

Would someone care to try and explain this? Or do we just not know enough as of yet to understand why this happens?

 

See if this helps. As I understand it, per quantum field theory, a particle (photon, electron,etc.) spreads out as it travels from place to place like a wave; but is detected only in a single location like a particle. What does the traveling? The so-called wave function; which represents the probability of finding the particle at a certain place and time.

 

So in the double-slit experiment, a single particle's wave function goes through both slits! This causes interference. But the electron shows up (is detected) at only one location on the photographic film. (This is called the collapse of the wave function). Repeating with many particles, we see the interference pattern on the film.

 

Take a look at the following animation:

 

http://phet.colorado...ve-interference

Edited by I ME
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sst. I hope RMW is working on it.

 

---------------

Hi RMW.

I suspect something.

1. you observed interference with a single slit.

2. your slit is close to the laser.

3. move the slit closer to the screen wall till interference disappear.

4. at this distance, make a second slit

5. observe interference

 

Right?

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