Feynman's double slit quantum eraser with "redder" photons

Discussion in 'Physics & Math' started by eram, May 5, 2015.

  1. eram Sciengineer Valued Senior Member

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    In one of his famous lectures, Richard Feynman explains that when we try to use photons to "see" which slit an electron went through, there is quantum decoherence and the electrons end up behaving like particles, not waves.

    What if we keep increasing the wavelength (hence decreasing the energy)of each photon? As the momentum of the photons becomes decreasingly significant, how will this affect the electrons?
     
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  3. Billy T Use Sugar Cane Alcohol car Fuel Valued Senior Member

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    If the wave length of the observing "light" becomes as large as the slit separation (or greater) the interference pattern will be restored as the spatial resolution with EM wave is not better than the wave length. I. e. you will not have determined which slit the photon went thru. In some sense hard to understand it goes thru both. Another way to see this is true is when wave length is large compared to silt separation, the both slits are always in essentially the same electric field. You would not expect same E field to destroy the interference pattern would you?
     
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  5. Layman Totally Internally Reflected Valued Senior Member

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    I had watched a lecture where Feynman also explains that this same experiment even showed to happen the same way detecting particles indirectly with particles that don't even react or affect one another. In this way, they proved that it was not the interaction between the particles itself that was creating the affect, and that would be part of why it is just said that it is the act of observation itself that causes it to happen this way.
     
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  7. exchemist Valued Senior Member

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    Yes this must be right.
     
  8. Layman Totally Internally Reflected Valued Senior Member

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    I can't be sure it would be exactly right, though. I think it would in a way be saying that there is extra knowledge about what is going on there that is beyond the standard model. No one would have ever observed the particle going through both slits of the experiment to prove it. That is why there are so many interpretations of quantum mechanics, like the many worlds interpretation and the Copenhagen interpretation, among many others. That would just be one interpretation of what is happening.

    Then all particles are identical to the same types of particles. If you observed what happened with the same particle with the same electric field, it would still destroy the interference pattern. The only thing we know with absolute certainty is that there is no known experiment that could show that the particle was seen to actually go through both slits at the same time. The reason why it looks that way could be from something else completely different. It could still be possible that one day someone comes up with a theory that is perfectly accurate that describes this particle behavior as happening because of something completely different and proves the Copenhagen and many worlds interpretation to be completely wrong.
     
  9. exchemist Valued Senior Member

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    No it is simply saying that you cannot resolve optically an object whose size is comparable with the wavelength of the light. You get diffraction of the light and cannot form an image. This is just A-level physics, no mystery about it at all.
     
  10. Layman Totally Internally Reflected Valued Senior Member

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    I was more or less referring to the statement that the photon goes through both slits. That is still a mystery, but you guys act like that has already been resolved. It hasn't. I don't know how you could even say that, when it is one of the biggest mysteries in physics. Then if using larger wavelengths prevented accurate information about the particle location to be obtained, it would seem only natural that it would not collapse the wave function. As far as I know, the experiment would work just as well with the lights on than it would with them off. That alone wouldn't give information about the exact location of the electron.

    If you took the mathematics of it completely literally, the electron would have gone through every single possible section of both slits all at the same time, missed the slits completely in every square inch of the board, and it also just went through every section of the board completely altogether. Then it would have only made one dot at the end of the experiment. That in itself would have determined the exact location of the particle...

    For all intents and purposes, it is just as though God doesn't want physicist to know or have exact information about particles. That would be a bit mysterious to me...
     
  11. exchemist Valued Senior Member

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    I was referring to Billy T's answer.

    You are right about the mystery associated with wave-particle duality, of course. The electron appears to explore all possible paths as a wave, but can only be detected as a discrete particle at one specific location out of all those where the amplitude of the wave is not zero.
     
  12. Layman Totally Internally Reflected Valued Senior Member

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    Ya, I didn't mean to start nit picking, but I always interpret or think about it as the particle always actually taking all the possible paths. I think it would be important for more people to think about it that way too, but a lot of people don't like that idea and make up different interpretations of it. Then I think that is just a way to start setting physics down the wrong path. People would have to start thinking about it that way if all of relativity theory was to be combined with quantum mechanics. For example, a particle traveling the speed of light could observe all other frames contracting to zero. If everything was contracted to a single point, then it would literally take every single path. It would be at every point along it's world line at once. Then Feynman's theories could just be a nice mathematical trick to be able to describe something like that occurring. They would consider the particle being at every possible location. Then the hidden variables that would link these two theories together could just be a lot of zero's and infinities... As a consequence of my own interpretations of it, I would think the equations could be more literal than most people think. It could really just be an ingenious way to describe something that couldn't be mathematically describe in another way, by just considering all possible trajectories. It really makes me wonder if dealing with hidden variables could really tell us much more, if anything, about quantum mechanics at all.

    As far as the final conclusion, I don't see anything wrong with that. Sometimes it is just getting there the right way that matters. I don't think an electric field would give you much of any information about the exact speed and position of a particle.
     
  13. brucep Valued Senior Member

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    It doesn't matter whether the detection device uses photons or not as soon as we attempt to detect which slit the electron is going through the interference pattern is extinguished. It seems like you're asking if changing the frequency of photons in a detection device will alter the outcome of the experiment? Such as detecting the electron [particle] at a slit while maintaining it's wavelike interference pattern? There's only two outcomes. With detector particle behavior without detector wave behavior.
     
  14. wellwisher Banned Banned

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    The analogy is the wake of a boat, with the boat the particle and the wake the wave duality. The boat can only go through one tunnel at a time, but the wake can go through two or more tunnels, if the wave is spread out like shown below.

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  15. exchemist Valued Senior Member

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    You've made this analogy before, and it is ballocks, as my old physical chemistry tutor would have said.

    The wave does not spread out behind the particle. The wave explores space and its amplitude at any point is the square root of the probability of finding the particle at that point. With your analogy, the duck might be detected at any point in its own wake (think about that) and most likely where the wave crests and troughs are highest - absurdly self-contradictory.

    Do not repeat this nonsense again, please.
     
  16. Fednis48 Registered Senior Member

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    This is only true if you're talking about strong measurements; that is, measurements that tell you unambiguously which slit the electron went through. There are also weak measurements, which give some information about which way the electron went but don't establish 100% certainty one way or the other. If we shine visible light on the electrons, the wavefront of the scattered light will be very different depending on the electron's location, so we are effectively performing a strong measurement and the interference pattern goes away. If we shine radio-frequency light on the electrons, the wavelength of the light will be large compared to the spacing between the slits, so the scattered wavefront will change only slightly depending on where the electron is. This is just a weak measurement, and as a result the interference will decrease somewhat (i.e. the bright spots won't be quite as bright, and the dark spots won't be completely dark) but will still be visible. As Billy T pointed out, the measurement effect gets weaker and weaker as we continue to decrease the photons' energies, up to the limit where a static EM field won't disrupt interference at all.
     
  17. brucep Valued Senior Member

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    So in essence that's the answer to eram's query. Thanks for the interesting information.
     

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