Double slit and measurement

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Main Question or Discussion Point

Let's say we do a double slit experiment with the standard laser/two slit setup with a screen to view the pattern.

1. In the first run, we just turn on the laser and observe the screen with our unaided eyes. We see an interference pattern consistent with the wave-like nature of light, correct?

2. In the second run, we place a video camera on the screen and start recording while we observe the laser light between the source and the two slits with our unaided eyes. When we view the recording from the camera we still see an interference pattern on the screen, correct?

3. In the final run, we place a video camera on the screen and start recording while we observe the laser light between the two slits and the screen with our unaided eyes. When we view the recording from the camera we still see an interference pattern on the screen, correct?

Would not this experiment show that the unaided human eye and brain (or consciousness if you want to use that term) have no effect on the laser light displaying wave-like behavior as opposed to particle-like behavior?
 

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  • #2
PeterDonis
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The answer to all three of your questions is "yes, correct".

Would not this experiment show that the unaided human eye and brain (or consciousness if you want to use that term) have no effect on the laser light displaying wave-like behavior as opposed to particle-like behavior?
No, because your human eye and brain still has to look at the recording from the video camera and observe that it shows an interference pattern. So a person who wanted to claim that only human consciousness collapses the wave function (or affects the wave-like vs. particle-like behavior of the light, or whatever) could still claim that, until the human looked at it, the video camera recording was actually in a superposition and had not yet collapsed.

A better response to this kind of claim (i.e., a claim that only human consciousness can collapse the wavefunction, etc.) is that, since it is inherently untestable (I've just explained how it can explain away any possible test result you can imagine), it's not a scientific claim.
 
  • #3
Khashishi
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How do you intend to observe the laser light before it hits the screen? In practice, there's some dust in the air which will cause the laser to scatter and be visible. Is that what you are talking about?
 
  • #4
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How do you intend to observe the laser light before it hits the screen? In practice, there's some dust in the air which will cause the laser to scatter and be visible. Is that what you are talking about?
I hadn't really thought about that. Can't you see some laser beams with the unaided eye?
 
  • #5
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The answer to all three of your questions is "yes, correct".

No, because your human eye and brain still has to look at the recording from the video camera and observe that it shows an interference pattern. So a person who wanted to claim that only human consciousness collapses the wave function (or affects the wave-like vs. particle-like behavior of the light, or whatever) could still claim that, until the human looked at it, the video camera recording was actually in a superposition and had not yet collapsed.

A better response to this kind of claim (i.e., a claim that only human consciousness can collapse the wavefunction, etc.) is that, since it is inherently untestable (I've just explained how it can explain away any possible test result you can imagine), it's not a scientific claim.
But would not the recording on the video camera then show two marks on the screen consistent with particle-like behavior, even if the result was delayed until I watched the video.
 
  • #6
Khashishi
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Of course you can see the beam if you stick your eye in the beam. But you can't see the beam just looking at it from the side (except the parts that scatter on dust).
 
  • #7
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Of course you can see the beam if you stick your eye in the beam. But you can't see the beam just looking at it from the side.
Then if we had some fine particles in the air, like dust, in which to scatter the laser to view it from other angles, would this compromise our experiment?
 
  • #8
Khashishi
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The dust helps you see the laser but it doesn't give you any which way information for individual photons hitting the screen, since the photons that hit the dust don't make it to the screen. You will see a double slit pattern.
 
  • #9
DrChinese
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I hadn't really thought about that. Can't you see some laser beams with the unaided eye?
You can certainly see laser light that reflects in your direction from dust or gas or other particles the light encounters. But otherwise you would not see that light unless it was going straight into your eyes.
 
  • #10
PeterDonis
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would not the recording on the video camera then show two marks on the screen consistent with particle-like behavior
Why? Particle-like behavior only happens if you have something at each slit that records which slit the light went through. Your description of the setup didn't include that, so I assumed it wasn't there. Putting it there changes the experiment.
 
  • #11
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I am just trying to sort out what seems to be the claim that "observing" or "looking" at the laser light is enough to change the behavior from wave-like to particle-like. Some of the more reputable QM physicists that I have watched on different programs seem to be saying that experimental results change if we "look" or "observe". But if the "photons or waves" or whatever you want to call them in this superposition state never make it to your eyes, you have not "observed" them. So how do you "observe" a "photon or whatever" without interacting with it, and changing it's momentum or position?
 
  • #12
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Why? Particle-like behavior only happens if you have something at each slit that records which slit the light went through. Your description of the setup didn't include that, so I assumed it wasn't there. Putting it there changes the experiment.
No, you are correct. I did not have such a device in this experiment.
 
  • #13
PeterDonis
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I am just trying to sort out what seems to be the claim that "observing" or "looking" at the laser light is enough to change the behavior from wave-like to particle-like.
Then you need to point us at a reputable source (textbook or peer-reviewed paper) that makes the claim, so we can look at what it says. Pop science videos or articles or TV shows don't count. Physicists will say all kinds of things in such venues that they know they can't get away with in a textbook or peer-reviewed paper.
 
  • #14
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Then you need to point us at a reputable source (textbook or peer-reviewed paper) that makes the claim, so we can look at what it says. Pop science videos or articles or TV shows don't count. Physicists will say all kinds of things in such venues that they know they can't get away with in a textbook or peer-reviewed paper.
Busted. I was hoping that Filipenko, Carroll, Greene, Susskind, and Weinberg from PBS Nova couldn't steer me wrong. :frown: Stupid pop-sci misinformation.
 
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  • #15
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if you have something at each slit that records which slit the light went through
How does such a device interact with the "light wave"?
 
  • #16
PeterDonis
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How does such a device interact with the "light wave"?
It depends on the device. The key point is that, with such a device present, the outcome of the experiment is now not "what is observed on the screen", but "what is observed on the screen, plus which slit's device registers the passage of a photon".
 
  • #17
DrChinese
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How does such a device interact with the "light wave"?
To add to PeterDonis' answer: there are a number of approaches. I like one in which the slits have polarizers in front.

a. Orient the polarizers PERPENDICULAR, and it is possible to identify which slit the light goes through by its polarization. Regardless of whether you check the light's polarization or not, there will be NO interference pattern on the screen.

b. Orient the polarizers PARALLEL, and it is impossible to identify which slit the light goes through by its polarization. There WILL be an interference pattern on the screen.

Obviously, it is a bit difficult to assert that the individual polarizer itself modified the momentum of each light particle so as to collapse the interference. Because there are polarizers present in both a. and b. It is only the relative orientation that is a factor.
 
  • #18
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It depends on the device. The key point is that, with such a device present, the outcome of the experiment is now not "what is observed on the screen", but "what is observed on the screen, plus which slit's device registers the passage of a photon".
Thanks, I appreciate the responses. Is there a free paper online that you could recommend that discusses the different mechanisms for detection of said photons and what implications the mechanism may have on the nature of the light ray?
 
  • #19
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To add to PeterDonis' answer: there are a number of approaches. I like one in which the slits have polarizers in front.

a. Orient the polarizers PERPENDICULAR, and it is possible to identify which slit the light goes through by its polarization. Regardless of whether you check the light's polarization or not, there will be NO interference pattern on the screen.

b. Orient the polarizers PARALLEL, and it is impossible to identify which slit the light goes through by its polarization. There WILL be an interference pattern on the screen.

Obviously, it is a bit difficult to assert that the individual polarizer itself modified the momentum of each light particle so as to collapse the interference. Because there are polarizers present in both a. and b. It is only the relative orientation that is a factor.
Fascinating.
 
  • #21
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Do electrons have polarization also?
 
  • #22
Nugatory
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Let's say we do a double slit experiment with the standard laser/two slit setup with a screen to view the pattern.
That's a standard experiment, but not one that has anything to do with quantum mechanics. You shine light through a barrier with two slits, you see an interference pattern on the screen behind the barrier because according to ordinary non-quantum electrodynamics light is a wave and waves interfere behind a barrier with two slits. This is all stuff that was observed early in the 19th century (Young's double-slit experiment) and completely explained by 1861 (Maxwell's equations of classical electrodynamics), more than a half-century before any quantum phenomena had been observed.

To demonstrate quantum effects, you need a source that sends a single photon at a time towards the barrier. Because you're sending a single photon, it's just going to make a dot where it lands on the screen - there's no pattern to be seen. To get a pattern, you use something like a piece of photographic film that records each individual dot as it forms; and then when you're done with the experiment and develop the film you'll see a whole bunch of dots that form some pattern (A google image search for "quantum interference pattern builds up" will find many good examples).

Now we see something that is uniquely quantum mechanical and that has no classical explanation: We send the particles one at a time, and each one makes a single dot on the screen; but when two slits are open the dots form an interference pattern even though classically we'd expect each particle to go through one slit or the other and make a dot behind that slit. Then if we close one slit, or place a detector (such as drChinese's polarizers if we're using photons) we find that the dots don't form an interference pattern. We get either a clump behind each slit (we have a detector in one of the slits) or we get one clump behind one open slit (we closed the other slit).

Either way, conscious observation is pretty much irrelevant. The pattern is in the photographic film whether we develop it and look at it or not.
Do electrons have polarization also?
No, but they have other properties that we can use (electric charge, magnetic moment) to build a detector that will tell us which slit an electron went through. However, this entire question of observing which slit the particle went through is a bit of a red herring - the uniquely quantum mechanical phenomenon is that when both slits are open and there is no detector, the one-particle-at-a-time experiments produce an interference pattern. Not getting an interference pattern isn't surprising, it's what you'd expect out of a single particle moving from source to screen.
 
  • #23
PeterDonis
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Do electrons have polarization also?
They have spin, which has similarities to photon polarization, but is not quite the same (because electrons are spin-1/2 fermions and photons are spin-1 bosons).
 
  • #24
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Busted. I was hoping that Filipenko, Carroll, Greene, Susskind, and Weinberg from PBS Nova couldn't steer me wrong. :frown: Stupid pop-sci misinformation.
Susskind? I know his QM book and he doesn't say that.

But overall books, shows, etc about QM written for a lay audience, to try and get some feel for QM across, take liberties with the 'truth'. Come here if you want the correct answer.

Thanks
Bill
 
  • #25
Lord Jestocost
Either way, conscious observation is pretty much irrelevant. The pattern is in the photographic film whether we develop it and look at it or not.
Can you prove this scientifically?
 

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