6 ms·

I'm still confused: where is the interaction in the double-slit experiment? Here's a quote: In the famous double-slit experiment, single particles, such as ph

I'm still confused: where is the interaction in the double-slit experiment? Here's a quote: In the famous double-slit experiment, single particles, such as photons, pass one at a time through a screen containing two slits. If either path is monitored, a photon seemingly passes through one slit or the other, and no interference will be seen. Conversely, if neither is checked, a photon will appear to have passed through both slits simultaneously before interfering with itself, acting like a wave. What does "monitored" mean here? The problem is that everywhere I look, I find a synonym of "monitored". My question is, "How does that break down?"

Strilanc 9y ago
Well, the simplest way to "monitor" it is to just block one of the slits. Then you definitely know which slit photons that hit the screen went through! Alternatives include: - Marking the photon itself (e.g. by rotating its polarization with a waveplate). - Placing a crystal that does spontaneous parametric down-conversion after one of the slits. This splits the photon into two photons. Aim the secondary photon at a detector. Detector clicks -> went through that slit. No click -> went through other slit. But this will also mark the photon itself with a change in frequency, so really should do it to both slits. - Having a magical device that stores a bit and toggles it anytime a photon passes through a piece of glass. Place the magical toggle-glass over one of the slits. Anything that can distinguish the case where a photon goes through the left slit from the case where a photon went through right slit will work fine.
- david927 9y ago
  I understand. Thanks.
monktastic1 9y ago
There's an important distinction here which is often misunderstood. The loss of interference is due to the photon (really, light wave) interacting with the detector. That results in an entanglement, and it's easy to show (mathematically) that interference is lost. That's not the difficult part. Any physical interaction that causes a correlation between the particle's state and some other state (effectively making that thing a detector) will cause this. This includes air molecules, which is why we have to isolate the systems very well. After interacting with the detector, the particle+detector system is still in a superposition of two states (corresponding to the two slits). When the particle hits the screen, the screen gets in on the entanglement. At all times there are (effectively) two paths, including when your body and brain interact with the experimental setup. So why do you see only one result? One answer (the Many-Worlds Interpretation) is that you don't see one result. One "copy" of you sees this one and "another" sees the other. Another possibility is that there really is some particular event that "collapses" the wave function, but AFAICT people mostly use this interpretation for practical reasons, bypassing the philosophical conundrum. Or, if you insist that you you sees only one result, but you still like MWI, then the question becomes: why did I end up on this branch rather than another one? But any explanation would have to come from outside the system (the multiverse) more or less, so I wouldn't hold my breath waiting for an answer.
- david927 9y ago
  Fascinating. And the fact that it's entanglement makes it information and thus not subject to space/time constraints.
  - monktastic1 9y ago
    Depends on what precisely you mean by that. It can allow you to exhibit correlations that are hard to explain within a classical framework, yes. But that requires very precise control over the system, so has no practical import most of the time. Things are being entangled pretty much nonstop around you all the time. BTW, I added a paragraph to my answer above.
  - EGreg 9y ago
    I think that basically the observer is considered "entangled" with a phenomenon after they have interacted with it. That's all. The observer can be anything. You are you, so by the time information comes to you, you've become entangled. I find this interpretation to be mostly wordplay. After all how can something be "real" if you can never observe it or detect it? It's sort of saying the theoretical construct "those other worlds" are real, but in what sense? Bohmian mechanics makes way more sense to me.
    - goatlover 9y ago
      Do we ever observe or detect the larger universe outside our light cone? Yet astronomers say it exists. What about other minds? Do you ever observer/detect someone else's experiences, thoughts, feelings? Or just their behavior from which you infer a mind?
- jodrellblank 9y ago
  but you still like MWI, then the question becomes: why did I end up on this branch rather than another one? If you like MWI, then the answer is you end up on every branch. It's like another invocation of the hateful anthropic principle - in every branch you are asking about the result you saw in that branch. You are asking why you ended up in this branch because this is the branch in which you exist to ask that specific question. The real question of MWI for me is, where the heck are all these squillions of universe copies every second and where does the energy come from and go to making squillions of universes worth of matter and creating universe sized places to put them and how does you interacting with a photon on Earth cause a copy of the Andromeda galaxy anyway?

Strilanc 9y ago

Well, the simplest way to "monitor" it is to just block one of the slits. Then you definitely know which slit photons that hit the screen went through! Alternatives include: - Marking the photon itself (e.g. by rotating its polarization with a waveplate). - Placing a crystal that does spontaneous parametric down-conversion after one of the slits. This splits the photon into two photons. Aim the secondary photon at a detector. Detector clicks -> went through that slit. No click -> went through other slit. But this will also mark the photon itself with a change in frequency, so really should do it to both slits. - Having a magical device that stores a bit and toggles it anytime a photon passes through a piece of glass. Place the magical toggle-glass over one of the slits. Anything that can distinguish the case where a photon goes through the left slit from the case where a photon went through right slit will work fine.

david927 9y ago
I understand. Thanks.

monktastic1 9y ago

There's an important distinction here which is often misunderstood. The loss of interference is due to the photon (really, light wave) interacting with the detector. That results in an entanglement, and it's easy to show (mathematically) that interference is lost. That's not the difficult part. Any physical interaction that causes a correlation between the particle's state and some other state (effectively making that thing a detector) will cause this. This includes air molecules, which is why we have to isolate the systems very well. After interacting with the detector, the particle+detector system is still in a superposition of two states (corresponding to the two slits). When the particle hits the screen, the screen gets in on the entanglement. At all times there are (effectively) two paths, including when your body and brain interact with the experimental setup. So why do you see only one result? One answer (the Many-Worlds Interpretation) is that you don't see one result. One "copy" of you sees this one and "another" sees the other. Another possibility is that there really is some particular event that "collapses" the wave function, but AFAICT people mostly use this interpretation for practical reasons, bypassing the philosophical conundrum. Or, if you insist that you you sees only one result, but you still like MWI, then the question becomes: why did I end up on this branch rather than another one? But any explanation would have to come from outside the system (the multiverse) more or less, so I wouldn't hold my breath waiting for an answer.

david927 9y ago
Fascinating. And the fact that it's entanglement makes it information and thus not subject to space/time constraints.
- monktastic1 9y ago
  Depends on what precisely you mean by that. It can allow you to exhibit correlations that are hard to explain within a classical framework, yes. But that requires very precise control over the system, so has no practical import most of the time. Things are being entangled pretty much nonstop around you all the time. BTW, I added a paragraph to my answer above.
- EGreg 9y ago
  I think that basically the observer is considered "entangled" with a phenomenon after they have interacted with it. That's all. The observer can be anything. You are you, so by the time information comes to you, you've become entangled. I find this interpretation to be mostly wordplay. After all how can something be "real" if you can never observe it or detect it? It's sort of saying the theoretical construct "those other worlds" are real, but in what sense? Bohmian mechanics makes way more sense to me.
  - goatlover 9y ago
    Do we ever observe or detect the larger universe outside our light cone? Yet astronomers say it exists. What about other minds? Do you ever observer/detect someone else's experiences, thoughts, feelings? Or just their behavior from which you infer a mind?
jodrellblank 9y ago
but you still like MWI, then the question becomes: why did I end up on this branch rather than another one? If you like MWI, then the answer is you end up on every branch. It's like another invocation of the hateful anthropic principle - in every branch you are asking about the result you saw in that branch. You are asking why you ended up in this branch because this is the branch in which you exist to ask that specific question. The real question of MWI for me is, where the heck are all these squillions of universe copies every second and where does the energy come from and go to making squillions of universes worth of matter and creating universe sized places to put them and how does you interacting with a photon on Earth cause a copy of the Andromeda galaxy anyway?