Q - Double slit experiment, delayed slit selection observation

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Discussion Overview

The discussion revolves around the double slit experiment, specifically addressing whether an interference pattern can be observed if a detector is used to determine which slit a photon passes through, even if the observer does not see the measurement result. The scope includes conceptual exploration of quantum measurement and wave-particle duality.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants assert that no interference pattern will be observed if there is a detector that provides which-slit information, regardless of whether the observer sees the result.
  • One participant argues that the interaction with the measuring device constitutes an observation that collapses the wave function, thus eliminating the possibility of an interference pattern.
  • Another participant explains that even without a measuring device in the slits, having the observation screen close enough to the slits provides which-way information, preventing interference due to the lack of overlap of spherical waves.
  • A later reply discusses the concept of complementarity, suggesting that one can either observe particle aspects or wave aspects depending on the placement of the observational screen, but not both simultaneously.
  • There is mention of the possibility of placing the observational screen at an intermediate distance to achieve uncertain which-way information and a blurred diffraction pattern.

Areas of Agreement / Disagreement

Participants generally disagree on the implications of having a detector and its effect on the observation of interference patterns. Multiple competing views remain regarding the nature of observation and its impact on quantum behavior.

Contextual Notes

The discussion highlights limitations in understanding related to the definitions of observation and measurement in quantum mechanics, as well as the dependence on experimental setup and conditions.

Matt711
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TL;DR
The result of slit selection is shown on a PC monitor, observer does not see the result, is there an interference or particle pattern on a interference screen?
Do I see an interference pattern on a interference screen in Double slit experiment if there is a detector watching which slit photon went trough but the observer does not see the result of detector measurement?
Thanks :]
 
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No
 
BvU said:
No
why not?
 
Why would you expect a diffraction pattern?

:welcome: ##\qquad##!​

##\ ##
 
Matt711 said:
why not?
You have to have some sort of measuring device to determine which slit the particle went through if the PC is going to display that information on its screen. The interaction with that measuring device is an “observation” sufficient to collapse the wave function - it doesn’t matter whether a human being gets involved.

Many older threads here, such as https://www.physicsforums.com/threa...e-of-quantum-measurement.919279/#post-5796193
 
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You don't even need to put something in the slits to have which-way information. It's sufficient to put the screen, where the particles are observed close enough to the double slits. It's easy to understand within the wave picture, why you don't see interference fringes: Due to Huygens's principle the wave behind the slits is given by the superposition of the spherical waves coming from each point in the slit openings. If you are close enough to the slits these spherical waves don't overlap there, and thus they do not interfere at the observation screen and you have which-way information for each particle. If you put the screen far away from the slits you get the usual Fraunhofer diffraction pattern, because the partial waves from the slits overlap at the screen and thus you can't know from which slit each of the individual particle came. In this sense you have a typical case of what Bohr called "complementarity": You can either observe "particle aspects" or "wave aspects" of the "quanta", depending on the choice of observation (in this case putting the observational screen close to or far from the slits. You can't have both aspects in one experiment, but of course you can put the observational screen somewhere in between. Then you have some "uncertain" which-way information and a somewhat "blurred" diffraction pattern.
 
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