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If we send a particle with mass through the double slits we observe a interference pattern. That is of course if we don't observe which slit the particle goes through. If we do then we get two peaks as if a particle had passed through the slits. But what if we detect what slit the photons pass through when using light. Is this even possible? and if so would we get two peaks (opposite each slit) or an interference pattern?

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  • $\begingroup$ If one paticle goes through slits we observe this particle as a particle on the observers screen. Only if you repeat this experiment many times you get fringes on the screen. $\endgroup$ – HolgerFiedler May 24 '15 at 8:06
  • $\begingroup$ How many peaks one get depends from the width of the slit, the distance betwenn the slits and all this is related with the energy of these particles, in the case if photons it is related to wavelength only. $\endgroup$ – HolgerFiedler May 24 '15 at 8:08
  • $\begingroup$ Particles don't exist (not even in classical physics), and you can't send something that doesn't exist trough a slit. You can send a quantum field trough a slit and observe it with a measurement that will return a bunch of discrete quanta, though. $\endgroup$ – CuriousOne May 24 '15 at 8:09
  • $\begingroup$ Since a single photon gives his moment partially to the electron which we have to detect, the electron will be deflected and the fringes on the screen will be blurry or will not more exist at all. $\endgroup$ – HolgerFiedler May 24 '15 at 8:10
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It's not about disturbing the photon. If you observe it, your detector gets entangled with it. It's possible for the photon going through the left slit and going through the right slit to end up in the same state and interfere, but if you get a detector involved then one of those will set off the detector and the other won't, so they won't be in the same state and they won't interfere.

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