Double slit experiment observation on and off

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Discussion Overview

The discussion revolves around the double slit experiment, particularly focusing on the concept of observation and whether it is possible to turn observation on and off during the experiment. Participants explore the implications of observation on particle behavior and the conditions under which interference patterns are formed.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions if scientists have considered the possibility of turning observation on and off during the double slit experiment, suggesting that particles react differently when observed versus when not observed.
  • Another participant clarifies that quantum mechanics does not provide information about what happens when particles are not observed, implying that switching observation on and off would not yield new insights.
  • A different participant explains that "observation" may be misleading, emphasizing that it is the interaction with other particles or detectors that determines the behavior of the particles, rather than observation itself.
  • It is noted that the experiment has been conducted in various forms, consistently aligning with quantum mechanical predictions regarding interference patterns based on which-path information.
  • Participants discuss the historical context of the term "observation" in quantum mechanics, indicating that its usage has persisted despite being potentially misleading.
  • One participant describes an experimental setup using polarizers to control which-path information, demonstrating that the presence or absence of interference patterns can be manipulated by adjusting the polarizers.
  • A question is raised about the relationship between particle localization and the completeness of path information, suggesting a potential mathematical function to describe this relationship.

Areas of Agreement / Disagreement

Participants express differing views on the implications of observation in the double slit experiment, with some agreeing on the complexity of the interaction and others emphasizing the limitations of the term "observation." The discussion remains unresolved regarding the broader implications of turning observation on and off.

Contextual Notes

Limitations include the dependence on definitions of observation and interaction, as well as the unresolved nature of how to quantify path information in relation to particle localization.

wes hunter
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Might I be so bold as to ask a question about the "double slit experiment". Was wondering...have scientist ever considered...turning the observation on and off...? Is it even possible...? Experiment says...when observed...particles react one way...and when not observed...they react a different way. Have they ever tried turning the camera 'on and off'...throughout...?
 
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wes hunter said:
Experiment says...when observed...particles react one way...and when not observed...they react a different way.

That's not quite what it says.

QM is a theory about observations. When not observed the theory is silent. Switching it on and off will tell us nothing new because what's going on when not observed it doesn't say.

Thanks
Bill.
 
The word "observation" may be misleading you here. What matters is whether there is any interaction of any kind (for example, the particle collides with a random air molecule floating around somewhere between source and screen, or light shines on it, or there is a detector in one ofthe slits, or a polarizing filter behind one of the slits, or... ) whose result requires that the particle had to go through a particular slit. If there is, then the particle will behave as if only that one path was available to it. If there is not, then the particle will behave as if both paths are available to it.

The experiment in which sometimes both paths are possible and other times only one is possible has been done many times, and always matches the quantum mechanical prediction: An interference pattern is formed by those particles that could have gone through either slit and is not formed by those that could not. It's worth mentioning that you can't see this with a single particle, as each particle just makes a dot on the screen either way. To see the interference you have to run the experiment with a large number of particles and watch the individual dots build up the pattern.

If you google for "Kim delayed choice quantum eraser" you'll find some good descriptions of one of the more interesting experiments along these lines.

And if you are wondering why we keep on hearing about "observations"... It's historical. Physicists were using that word early in the 20th century when these concepts were first being thrashed out. By the time is became clear that "observation" was the wrong word it was too late - that usage had caught on in the popular press.
 
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Nugatory said:
... the particle will behave as if only that one path was available ... the particle will behave as if both paths are available ...
That's a good choice of word(s) IMO. I don't think I've ever heard it said that way before. But maybe I just haven't read enough.
 
dlgoff said:
That's a good choice of word(s) IMO. I don't think I've ever heard it said that way before.
I picked it up from @DrChinese who explains the double-slit experiment in terms of the "availability" of which-path information.
 
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Nugatory said:
I picked it up from @DrChinese who explains the double-slit experiment in terms of the "availability" of which-path information.

Ha! I probably learned the idea from you first...
 
wes hunter said:
Might I be so bold as to ask a question about the "double slit experiment". Was wondering...have scientist ever considered...turning the observation on and off...? Is it even possible...?

Not surprisingly, this has been done (assuming I understand your question correctly). You place perpendicular polarizers in front of each of the 2 slits. The polarizer acts to tell you which slit a photon goes through - but only because of the polarizers' relative positioning (as perpendicular). So there is NO interference pattern. If you rotate a polarizer so they are now parallel: there IS an interference pattern.

In both cases, there are polarizers in the path. If observation was as simple as having a polarizer present, neither case would yield an interference pattern. But that's not what happens. So it is more complex than that, as Nugatory, bhobba and dlgoff allude.

You can therefore alter the setup so you have 100% knowledge of which slit the photon went through, 0% knowledge, 50%, or any value between 0 and 1. This is done by rotating the polarizers to a suitable relative positioning. You can turn it on and off as you like without otherwise disturbing the setup.
 
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A question about path information. If you know that the particle is near the slit, this information is complete, if it is localized on the screen it is very small or null. Is there a function of the position giving the value of this information?
 

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