Detecting a photon passing through a slit without destroying it?

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Discussion Overview

The discussion revolves around the detection of photons passing through slits, particularly focusing on whether a photon can be detected without altering its properties and how this affects the formation of interference patterns. The scope includes theoretical considerations and conceptual clarifications related to quantum mechanics.

Discussion Character

  • Debate/contested, Conceptual clarification, Exploratory

Main Points Raised

  • Some participants express skepticism about the possibility of detecting a photon without it being altered, questioning the implications for interference patterns.
  • One participant suggests that detecting a photon's effect on the slit could provide information without directly measuring the photon itself.
  • Another participant argues that nondestructive detection methods exist, but questions remain about whether these methods change the energy of the photon.
  • There is a discussion about the relationship between the detection process and the interference pattern, with some suggesting that knowing which slit the photon passed through inherently disrupts the pattern.
  • One participant introduces the idea of using polarizers to control interference, indicating that their orientation can determine whether interference occurs, regardless of direct detection.

Areas of Agreement / Disagreement

Participants generally disagree on the nature of photon detection and its implications for interference patterns. Multiple competing views remain regarding whether detection can occur without altering the photon and how this relates to the formation of interference patterns.

Contextual Notes

There are unresolved questions about the definitions of detection and alteration of photons, as well as the assumptions underlying the discussion of nondestructive detection methods.

Cato
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TL;DR
I occasionally read descriptions of the double slit experiment in which the author says something like, "If a detector is place at each slit so that we know which slit the photon passsed through, the interefence patten does not form." Is this even possible? Is it possible to detect a passing photon and have it remain a photon? An electron, yes, but a photon?
I cannot see how a photon can be detected and yet remain the same photon. I am thinking that the description "If a detector is place at each slit so that we know which slit the photon passsed through, the interefence patten does not form" is sloppy and in error.
 
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Cato said:
I cannot see how a photon can be detected and yet remain the same photon.

First, you don't have to detect the photon itself to know which slit it passed through. You could detect the photon's effect on the slit.

Second, there are nondestructive ways of detecting photons, as a simple Google search will show you.
 
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Isn't detecting the photon's effect on the slit the same as detecting the photon? And wouldn't that change the energy of the photon? Same for the second -- wouldn't detecting a photon nondestructively change the photon? Is it possible to detect a photon without altering its energy? So that an interference pattern would not be created not because we "knew" which slit the photon passed through, but because the energy of the photon was changed in the process of being nondestructively detected?
 
Cato said:
Isn't detecting the photon's effect on the slit the same as detecting the photon?

No. For example, you could measure the recoil of the slit as the photon passes through it and interacts with it. That does not involve any measurement of the photon.

Cato said:
wouldn't that change the energy of the photon?

The photon might not even have a definite energy to begin with. But it will certainly change the behavior of the photon, since detecting which slit the photon went through destroys the interference pattern.

Cato said:
wouldn't detecting a photon nondestructively change the photon?

See above.

Cato said:
So that an interference pattern would not be created not because we "knew" which slit the photon passed through, but because the energy of the photon was changed in the process of being nondestructively detected?

These aren't two different alternatives. They're two different descriptions of the same process.
 
Cato said:
I cannot see how a photon can be detected and yet remain the same photon. I am thinking that the description "If a detector is place at each slit so that we know which slit the photon passsed through, the interefence patten does not form" is sloppy and in error.

PeterDonis is correct. If you place polarizers in front of each slit, their relative orientation controls whether there is interference or not. If they are parallel, there IS interference.

But if they are perpendicular, there is NO interference. It is possible in this case to determine whether the photons went through one slit or the other (even if you do not attempt to determine that). So of course there is no interference, and the light gets through.
 
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