Do I Misunderstand The Double Slit Experiment

[ZeeJustin]

Metric, could you clarify what you mean by "being entangled with the state of the photon."

Are you saying that we only get the "weird" results when observing to such a degree that we can determine what slit(s) was entered by the photon?

That would imply that watching the experiment, or pointing a normal camera at it would have no effect on it, but using some kind of super camera would instead produce the "weird" results.

Is that understanding correct?

[Metric]

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Metric, could you clarify what you mean by "being entangled with the state of the photon."

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The photon P is in an entangled state with some system A if the state of the combined system PA cannot be factored into a state of the form |Psi_P> (x) |Psi_A>. Most states of composite systems are entangled.

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Are you saying that we only get the "weird" results when observing to such a degree that we can determine what slit(s) was entered by the photon?

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You don't necessarily have to perform the measurement yourself -- if the photon becomes entangled with anything that potentially holds information about "which slit," the interference pattern will vanish.

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That would imply that watching the experiment, or pointing a normal camera at it would have no effect on it, but using some kind of super camera would instead produce the "weird" results.

Is that understanding correct?

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See above.

[Metric]

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Metric, is the eraser experiment similar to a double split experiment with a moving detector? Let's say you have a moving detector between the slits and the screen. When the detector is very close to the slits, close enough to determine which slit a photon went thru, the interference pattern disappears. If you move the detector further back toward the screen, to a point where it becomes impossible to answer a which slit question, the interference pattern is displayed.

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I'm not sure I understand the question -- it's true that in a "close up" measurement, it will be hard to see the interference fringes, and that the position of detection of the individual photons will hold a good deal of "which path" information.

[Max Raker]

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3. If the movie camera has no film the pattern will be waves.

4. IF THE CAMERA HAS FILM AND WE DON'T LOOK AT THE SCREEN UNTIL AFTER WE LOOK AT THE PICTURES, AND ON THE WAY TO THE DRUGSTORE WE FALL AND RUIN THE FILM, WE WILL SEE WAVES ON THE SCREEN. In other words the photons "know" that we will not be able to see them go through the slits, even though our inability to do that is because of an event in the future!

Am I wrong?

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I think you don't fully get what constitutes a measurement. Like when you say "we don't look at the screen", the measurement takes place not when a person looks at it but when the screen interacts with the photon. And if 4 was true, you could use it to send information faster than the speed of light. DO you see why?

[Max Raker]

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I'll end with something spooky. Lets say you set up the experiment and dont measure which slit the photons go through. You'll see a wave pattern on the screen. Now, if you replace the screen with a very sensitive potodetector that can in principle show where individual photons are arriving... then you'll see individual photons arriving... BUT with many more photons arriving in the bright fringes, and very few arriving in the dark fringes. In other words... you see individual photons, but arriving in the intensity pattern expected from a wave going through the slits and interfering with itself!

Spooky, isnt it?

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This is really the only spooky part of this. It is even weirder to think that you can slow down the rate at which you emit the photons to something like 1 per hour and you will still get an interference pattern. So it has nothing to do with 2 different photons interacting with each other.

So though an individual photon can only go through one slit, it "knows" that the other one is there. The standard explanation is it doesn't make sense to talk about photons until a measurement forces localization. Others claim that the many worlds interpretation of QM is needed to explain this.

[NotReady]

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Hey Not Ready come back! Did you know about this stuff?


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Very flattering but you have more than one expert explaining it in this thread. My only take on it is that we change the quantum world by observing it. If you try to grab something very light that's floating in water the object will move because of the action of your hand causing waves which move the object. I think something like that is what happens when we try to "grab" the quantum world by observation. Just a total layman's view.

[Max Raker]

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This is another example of a paradox that I have been wondering about for awhile.

Apparent Paradox:

I run the double-slit experiment but do not observe the slit the electrons travel through. Someone a light-minute away with a powerful telescope flips a coin to decide whether to observe them. Will I see an interference pattern or not?

If I see an interference pattern then the observer should be able to see which slit each electron went through and find a contradiction.

If I do not see interference then I know that the observer will not observe the experiment one minute from now and in effect know the future result of his coin toss. I have observed an effect before the cause.

I am about to earn a B.S. in Physics and yet I don't really understand what will happen in this situation.

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After the guy flips the coin, it will take him 1 minute to be able to determine which slits the photons are going through. So you will know the result of his flip but only after a time of distance/c from the flip. Does that seem right to you?

[teampursuit]

Do you mean the Feynman Lectures? There's a really good discussion of this there. Where you're gettng bogged down is in the definition of 'observed'.

[PairTheBoard]

We already understand light to be a self propagating electromagnetic wave. That's what a photon of light is. Electric and Magnetic Forces fluxing back and forth against each other and propagating forward as a result. This Electromagnetic Fluxing Field of forces constituting a photon in travel is spread out in space. I don't think it's well understood what happens when that spread out travelling fluxing field of EM forces gets its energy absorbed when it makes contact with something like a Film. The Field of Forces is spread out in space, but the energy doesn't get absorbed in the same smeared out way. It somehow all gets pulled down into a single "point". That's what we see on the Film when we interpret it as a "particle" hitting the screen. But it's not really a particle hitting the screen. What we see is the result of a spread out self propagating wave of Fluxing EM forces getting the spread out energy pulled into a single "point" of absorption.

With such a view it's not so hard to see why a single self propagating photon "wave" might pass through both slits and thereby have it's structure of electromagnetic flux lines altered so that the way the altered wave has it's energy pulled together into a point of absorption is also altered in a way that can be measured by looking at the statistical pattern of absorption points on the Film for a number of such photons.

So it's our mistaken notion that because a photon looks like a particle at the Film it must have gone through one slit or the other. It's only when we take a measurement to observe it going through one of the slits that this is really the case. What happens then is that we alter the spread out EM Flux lines with a kind of Partial Absorption, forcing the the spread out energy to get pulled into the slit local, but without complete absorption into a point. The structure of the EM flux lines is then altered in a way that the new structure is absorbed at the Film with the same statistical earmarks as if the slit were the origin of the photon.

I don't know how well this explanation holds up against all the experiments they've done. But if it does, then the spooky thing is not so much that a photon of light - which we already model as a self propagating EM wave - behaves this way. But that all so called "particles" do as well. Not all "particles" are as easily understood as smeared out distributions of self interacting Forces of some kind. If that's what they are then I don't think they've identified the "Forces".

PairTheBoard

[hexag1]

the explanation given by Feynaman in "The Character of Physical Law" sums it up best:

first we must keep in mind the following:
When we look at anything under a microscope, our smallest resolution is given by the frequency of the light we use too look. In other words, if we have two dots, we can only distinguish them from each other if the light we use to look has a shorter wavelength than the distance between the two dots. Anything shorter and they look like one dot.

Now, with that in mind:

If we let the electron pass through the two slits we will get a pattern that indicates that that electron behaves like a wave.

If the intensity of the electron beam is turned down low enough, then we can see that they come in lumps; individual units. The probability of us catching a lump at any point after the two slits is given by a sinusoidal wave function. When all the probilities are mapped, the pattern of electron arrival looks like a ripple.

If we try too 'look' at the electrons by shooting light at the electrons before they hit, it will seem like the electrons definitely pass through one slit or another. The pattern of arrival for the electrons on the other side of the slits will look like electrons obey ballistic mechanics.

We can try to look at the electrons before they pass through the slits. We do this by shooting light at them. But the light affects the path of the electrons. When we shine a light, the path of the electrons is drastically affected. They no longer behave like waves, but instead like ballistic particles.

We can try to turn down the energy of the light we use to look at them. If we turn it down low enough, the light no longer affects the electrons enough, and they behave like waves again. However, a new problem arises. Now that we have decreased the energy of the light (and therefore the wavelength of the light) we shine on the electrons, we can no longer distinguish between the two slits the electrons pass through. The wavelength that equals the distance between the two slits, equals the wavelength that no longer affects the path of the electrons.

Therefore it is IN PRINCIPLE impossible to tell through which slit the electron will pass. Nature is rigged in just such a way that we can never tell what happens. Some say that nature herself doesnt know through which slit the electron will pass.

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