Do I Misunderstand The Double Slit Experiment

[DougShrapnel]

[ 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.

[UprightCreature]

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.

[PairTheBoard]

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

[carlo]

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.

[Metric]

[ 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.

[Borodog]

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

[jogsxyz]

There's a disclaimer in Wikipedia.

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

[/ QUOTE ]

[DougShrapnel]

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.

[Arp220]

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?

[Arp220]

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.