Do I Misunderstand The Double Slit Experiment
 

What I think I know about it comes from a couple of forty year old books. And what I remember reading is so spooky that it gives one pause about whether some supernatural entity could have something to do with it. But every time I mention it the scientists pooh pooh me. Which makes me think that I am thinking things about the experiment, and related exoeriments that aren't true.

So without going into detail, are the following things true?

1. If we watch individual photons as they are going through the slit(s) they make patterns on the screen like they are particles. If we don't watch them, the patterns are those of a wave.

2. If we aren't watching, but we have a movie camera pointed at the slits, the pattern is of particles.

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?

Re: Do I Misunderstand The Double Slit Experiment
 

Well, if individual photons are coming through the slits, if they make it to the camera, that means they scattered off something and didn't get to the screen. So to be pedantic, they shouldn't tell you anything about the interference pattern, meaning you'll see interference behaviors.

There are ways in which you can do experiments similar to your case 4. They tend to go by the name "delayed choice quantum eraser," and it's been a while since I've read about them, so I can't say offhand if your impressions are exactly correct. It's a pretty subtle issue, obviously.

EDIT: From the Wikipedia on delayed choice quantum eraser,

[ QUOTE ]
In the double slit experiment, a photon passes through one of two slits in a double slit apparatus, and then registers on a detector. The detector shows where the photon hit it, like an image projected on a screen. If many photons individually pass through the double slit apparatus, and nothing observes which slit a given photon travels through, an interference pattern emerges on the detector. The interference pattern indicates that the light beam is in fact made up of waves. However, if someone observes which of the two slits each photon passes through, a different result will be obtained. In this case, each photon hits the detector after going through only one slit and a single concentration of hits in the middle of the detection field. This result is consistent with light behaving as individual particles, like tiny bullets. It is counterintuitive that a different outcome results based on whether or not the photon is observed after it goes through the slit but before it hits the detector.

In a quantum eraser experiment, one arranges to detect which one of the slits the photon passes through, but also construct the experiment in such a way that this information can be "erased" after the fact. It turns out that if one observes which slit the photon passes through, the "no interference" or particle behavior will result, which is what quantum mechanics predicts, but if the quantum information is "erased" regarding which slit the photon passed through, a wavelike interference pattern can be observed.

However, Kim, et al. have shown that it is possible to delay the choice to erase the quantum information until after the photon has actually hit the target. But, again, if the information is "erased," an interference pattern can be recovered in a certain subset of the photons which reach the detector, even if the information is erased after the photons have hit the detector

[/ QUOTE ]

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
What I think I know about it comes from a couple of forty year old books. And what I remember reading is so spooky that it gives one pause about whether some supernatural entity could have something to do with it. But every time I mention it the scientists pooh pooh me. Which makes me think that I am thinking things about the experiment, and related exoeriments that aren't true.

So without going into detail, are the following things true?

1. If we watch individual photons as they are going through the slit(s) they make patterns on the screen like they are particles. If we don't watch them, the patterns are those of a wave.

2. If we aren't watching, but we have a movie camera pointed at the slits, the pattern is of particles.

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?

[/ QUOTE ]

This is essentially correct, barring the fact that you can't just use a "movie camera". After all, you can see the interference pattern while *looking* at the beam hitting the slits. You have to actually be able to *detect* which slit an individual photon passes through, which requires interacting with it, and localizing it, which is what destroys the interference pattern. But my understanding is that if you do as you say, that is, not observe the screen and wipe out the data of which slit each photon went through, and then observe the screen, you will observe an interference pattern.

Edit: Er, what gump said.

Re: Do I Misunderstand The Double Slit Experiment
 

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.

Re: Do I Misunderstand The Double Slit Experiment
 

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

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
This is essentially correct, barring the fact that you can't just use a "movie camera". After all, you can see the interference pattern while *looking* at the beam hitting the slits. You have to actually be able to *detect* which slit an individual photon passes through, which requires interacting with it, and localizing it, which is what destroys the interference pattern. But my understanding is that if you do as you say, that is, not observe the screen and wipe out the data of which slit each photon went through, and then observe the screen, you will observe an interference pattern.

Edit: Er, what gump said.


[/ QUOTE ]

So what you are saying is if light hits the particles they become localized and no interfence pattern is observed whether or not the light is observed?
I was under the impression that if light interacts with the particles, but the light is not observed, that an interfence pattern would be observed.

Re: Do I Misunderstand The Double Slit Experiment
 

so ... what happens if we set up the cameras to watch the slits, and we bring two people into the room.

Person A does not know the cameras are set up and can see the screen. What does he see?

Person B knows the cameras are set up, but can't see the screen. Afterwards, he takes the camera, falls down and the film is destroyed. He returns to the screen, what does he see?

At this point in time, Person A returns, does what he see differ than what he saw earlier?

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
so spooky that it gives one pause about whether some supernatural entity could have something to do with it.

[/ QUOTE ]

What would the entity do?

Re: Do I Misunderstand The Double Slit Experiment
 

There's an excellent chance that you do.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
[ QUOTE ]
This is essentially correct, barring the fact that you can't just use a "movie camera". After all, you can see the interference pattern while *looking* at the beam hitting the slits. You have to actually be able to *detect* which slit an individual photon passes through, which requires interacting with it, and localizing it, which is what destroys the interference pattern. But my understanding is that if you do as you say, that is, not observe the screen and wipe out the data of which slit each photon went through, and then observe the screen, you will observe an interference pattern.

Edit: Er, what gump said.


[/ QUOTE ]

So what you are saying is if light hits the particles they become localized and no interfence pattern is observed whether or not the light is observed?
I was under the impression that if light interacts with the particles, but the light is not observed, that an interfence pattern would be observed.

[/ QUOTE ]

Both are correct.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
so ... what happens if we set up the cameras to watch the slits, and we bring two people into the room.

Person A does not know the cameras are set up and can see the screen. What does he see?

Person B knows the cameras are set up, but can't see the screen. Afterwards, he takes the camera, falls down and the film is destroyed. He returns to the screen, what does he see?

[/ QUOTE ]

Do A and B ever communicate with each other?

[ QUOTE ]
At this point in time, Person A returns, does what he see differ than what he saw earlier?

[/ QUOTE ]

I didn't know A ever left? What difference would that make? The screen cannot magically change from what he has previously seen.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
[ QUOTE ]
so ... what happens if we set up the cameras to watch the slits, and we bring two people into the room.

Person A does not know the cameras are set up and can see the screen. What does he see?

Person B knows the cameras are set up, but can't see the screen. Afterwards, he takes the camera, falls down and the film is destroyed. He returns to the screen, what does he see?

[/ QUOTE ]

Do A and B ever communicate with each other?


[/ QUOTE ]

no they never communicate with each other (nor do they even know someone else is in the room) until after they have both seen the screen for the first time.

[ QUOTE ]

[ QUOTE ]
At this point in time, Person A returns, does what he see differ than what he saw earlier?

[/ QUOTE ]

I didn't know A ever left? What difference would that make? The screen cannot magically change from what he has previously seen.

[/ QUOTE ]

Ok well this question isn't that important, but the question is if A will observe the particle pattern even if he does not know the slit is being observed AND the film is destroyed in the future (but before the film is actually destroyed).

Re: Do I Misunderstand The Double Slit Experiment
 

From wiki as well "the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics." So why is this spooky again?

Re: Do I Misunderstand The Double Slit Experiment
 

I don't know much about Quantum physics nor photons, but it seems if a person or a light recording decive is viewing the photon experiment as it occurs that it will interfere with it. Photons would inevitably be refracted off the eye or camera lens and 'ruin' the experiment compared to an unobserved experiment.

Just curious, how the hell do they only send one photon at a time toward a plate?

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
so ... what happens if we set up the cameras to watch the slits, and we bring two people into the room.

Person A does not know the cameras are set up and can see the screen. What does he see?

Person B knows the cameras are set up, but can't see the screen. Afterwards, he takes the camera, falls down and the film is destroyed. He returns to the screen, what does he see?

[/ QUOTE ]

Do A and B ever communicate with each other?


[/ QUOTE ]

no they never communicate with each other (nor do they even know someone else is in the room) until after they have both seen the screen for the first time.

[/ QUOTE ]

Well then they communicate. I said "ever".

Still thinking about my response though.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
From wiki as well "the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics." So why is this spooky again?

[/ QUOTE ]

Hmm. This is news to me. Also, it doesn't quite make sense. How could you look at a subset of photons that were seen at a particular detector if you have destroyed the information from the detectors? [img]/images/graemlins/confused.gif[/img]

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
so ... what happens if we set up the cameras to watch the slits, and we bring two people into the room.

Person A does not know the cameras are set up and can see the screen. What does he see?

Person B knows the cameras are set up, but can't see the screen. Afterwards, he takes the camera, falls down and the film is destroyed. He returns to the screen, what does he see?

[/ QUOTE ]

Do A and B ever communicate with each other?


[/ QUOTE ]

no they never communicate with each other (nor do they even know someone else is in the room) until after they have both seen the screen for the first time.

[/ QUOTE ]

Well then they communicate. I said "ever".

Still thinking about my response though.

[/ QUOTE ]

does the answer differ if A and B never communicated (ever), or if they only communicate after they have both seen the screen?

does the answer differ if A tells B what he saw before B sees the screen (but after the film is destroyed)?

but i guess this also depends on what A actually sees and if its even possible for A and B to see different things.

Re: Do I Misunderstand The Double Slit Experiment
 

what about ebr, action at distnance thingee, einstein-bohr-(rosenberg?) pair.

Re: Do I Misunderstand The Double Slit Experiment
 

If A & B communicate, then obviously they will have observed the same thing on the screen (whatever that might be).

IF A & B never communicate, then I believe they can have seen different things.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
[ QUOTE ]
so ... what happens if we set up the cameras to watch the slits, and we bring two people into the room.

Person A does not know the cameras are set up and can see the screen. What does he see?

Person B knows the cameras are set up, but can't see the screen. Afterwards, he takes the camera, falls down and the film is destroyed. He returns to the screen, what does he see?

[/ QUOTE ]

Do A and B ever communicate with each other?


[/ QUOTE ]

no they never communicate with each other (nor do they even know someone else is in the room) until after they have both seen the screen for the first time.

[/ QUOTE ]

Well then they communicate. I said "ever".

Still thinking about my response though.

[/ QUOTE ]If the camera measures which slot each photon went through, it's the same pattern on the screen for all.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
[ QUOTE ]
From wiki as well "the total pattern of signal photons never shows interference, and it is only when one looks at a subset of signal photons whose idlers were seen at a particular detector that an interference pattern can be recovered. So, the experiment would certainly not allow one to send a message back in time, and whether the experiment requires any sort of backwards causality to understand it would depend on one's interpretation of quantum mechanics." So why is this spooky again?

[/ QUOTE ]

Hmm. This is news to me. Also, it doesn't quite make sense. How could you look at a subset of photons that were seen at a particular detector if you have destroyed the information from the detectors? [img]/images/graemlins/confused.gif[/img]

[/ QUOTE ]It's all there in the wiki article they use a beem spliter on entagled particles. And test only a subset. I'm not an expert on this stuff, so grain of salt.

Re: Do I Misunderstand The Double Slit Experiment
 

In my experience the biggest problem people have when trying to understand these kind of experiments is not really understanding what it means to observe something.

People feel like, based on everyday interactions with the macroscopic world, that they can passively observe things without disturbing or interacting with them. In reality this isn't the case in either the macro or quantum world. When you see a person in a telescope you are detecting many photons that are being emitted or bouncing off of them. You couldn’t see the person if these photons were not interacting with the person.

This has already been said in this thread, but, you can not observe a photon without interacting with it. That interaction fundamentally changes the quantum state of the photon.

To further complicate matters you can have a quantum system of entangled particles where the state of one particle depends on the state of the other particle. By observing one of the entangled particles you know something about the other particle in the system, thus collapsing the wave function of both particles. Without explaining in great detail this is the fundamental concept of observing a photon but not developing the film... that is never observing the particle that was entangled with the photon.

It turns out that even though at first glance people think you can use this to send information back in time or faster than light, you actually can't.

Re: Do I Misunderstand The Double Slit Experiment
 

In the delayed choice quantum eraser experiments which are suppose to do something like what you describe in #4 the effect is not quite as dramatic as you describe it. At least according to my understanding when I last read about it. I believe a system of photon splitters is set up which allow photons to continue to the screen normally and produce duplicate photons which can then be directed elsewhere for study. I'm pretty fuzzy on the details, but when the erasure is done you don't actually see a change in the pattern on the screen. You use the duplicate photons with slit information erased to produce or deduce some kind of wavy subpattern of what you see on the screen. If you're really interested you should read up on it yourself. I think Wiki describes it in detail or at least links to something that does.

It's still spooky, just not as dramatic as in your OP #4.

PairTheBoard

Re: Do I Misunderstand The Double Slit Experiment
 

I know it can be very complicated but it seems that if this is a true experiment the interference pattern speaks for itself.Simply put, the interaction of light with light breeds darkness. Following the light after the darkness reveals that light leeches through until another moment of interference. This is what the experiment reveals and no more.

Now, to assume that light is particles(photons) is consequent to the Newtonian look at light and of course the Wave theory is around the corner. To deal with light as both particle and wave is a preconceived notion because of our dependence on particularity(matter broken into parts and pieces)and the consequence or retort is the wave theory. The idea of particles(photons) in no way is brought to life by this experiment.

As a follow up on this conceive that light and darkness are realities(i.e. darkness is not the absence of light but a reality of its own.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
1. If we watch individual photons as they are going through the slit(s) they make patterns on the screen like they are particles. If we don't watch them, the patterns are those of a wave.

[/ QUOTE ]
If by "watch" you mean "entangle the photon with some system in such a way that information on the path is stored in that system", then yes.

[ QUOTE ]
2. If we aren't watching, but we have a movie camera pointed at the slits, the pattern is of particles.

[/ QUOTE ]
Again, this is true as long as the state of the movie camera is being entangled with the state of the photon.

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

[/ QUOTE ]
I.E. if the state of the movie camera (and everything else in the room) does not become entangled with the photon, then we will see waves.

[ QUOTE ]
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!

[/ QUOTE ]
If by "destroy the film" you mean that you disentangle the photon/screen state from everything else (including the camera film), then this is right.

This seems shocking at first, but these are all subtly the same phenomenon at work. If the photon/screen state is entangled with anything else, interference will not be observed. It doesn't matter if it was or will be entangled -- the only thing that matters as to what you will see right now is whether path-information is held by some other system via entanglemet right now.

Re: Do I Misunderstand The Double Slit Experiment
 

I defer to Metric on all this voodoo [censored].

Re: Do I Misunderstand The Double Slit Experiment
 

There's a disclaimer in Wikipedia.

[ QUOTE ]
This article may require cleanup to meet Wikipedia's quality standards.

[/ QUOTE ]

Re: Do I Misunderstand The Double Slit Experiment
 

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.

Re: Do I Misunderstand The Double Slit Experiment
 

Metric gave a good answer. Here is a different take on your questions:

[ QUOTE ]

1. If we watch individual photons as they are going through the slit(s) they make patterns on the screen like they are particles. If we don't watch them, the patterns are those of a wave.

[/ QUOTE ]

Correct, sort of. A better phrasing would be:

If you determine which slit each individual photon goes through, then you will see two bright spots on the screen in front of each slit, as if you shot a stream of particles straight at each slit. If you do not determine which slit each photon goes through, then you'll see an interference pattern on the screen, with a bight blob in the middle, between the slits, accompanied by 'fringes' as if you'd fired one big wave at the slits.

[ QUOTE ]

2. If we aren't watching, but we have a movie camera pointed at the slits, the pattern is of particles.

[/ QUOTE ]

Correct, as long as the movie camera is measuring which slit each individual photon goes through.

[ QUOTE ]

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

[/ QUOTE ]

I would say this is incorrect. This would be taking the measurement, but not writing the result down [img]/images/graemlins/wink.gif[/img]

[ QUOTE ]

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!

[/ QUOTE ]

This is wrong. You will see particles. If you measure which slit the photons go through then you wont see an interference pattern on the screen. It doesn't matter if you record the result on film or not. Its the action of measurement that is important.

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?

Re: Do I Misunderstand The Double Slit Experiment
 

And to add something - if you set up apparatus to measure which slit each photon goes through, and then look at the screen, but not the film, you are in effect looking at the film. By looking at the screen you become 'entangled' with the screen/film/slit/photon system.

Re: Do I Misunderstand The Double Slit Experiment
 

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?

Re: Do I Misunderstand The Double Slit Experiment
 

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

[/ QUOTE ]
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.

[ QUOTE ]
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?

[/ QUOTE ]
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.

[ QUOTE ]
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?

[/ QUOTE ]
See above.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
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.

[/ QUOTE ]
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.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
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?

[/ QUOTE ]

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?

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
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?

[/ QUOTE ]

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.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]

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


[/ QUOTE ]

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.

Re: Do I Misunderstand The Double Slit Experiment
 

[ QUOTE ]
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.

[/ QUOTE ]

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?

Re: Do I Misunderstand The Double Slit Experiment
 

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'.

Re: Do I Misunderstand The Double Slit Experiment
 

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

Re: Do I Misunderstand The Double Slit Experiment
 

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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