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Question about the double-slit experiment.

  1. May 16, 2012 #1
    If a particle has the probability of being in two places at once, then is it possible to use a particle accelerator to accelerate the particle to a high energy and into the two slits and then place two detectors, with one at each slit, to convert the energy of the particle into electricity? Since the particle has become two particles, will this allow the total power output of the two detectors to be higher than the input power from the accelerator? This is assuming that the accelerator is 100% efficient.
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  3. May 16, 2012 #2


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    The particle will never appear in two places at once. If it appears at the first detector then you can be sure that the other detector did not go off. If there are no detectors at the slits, then one cannot say that the particle must have gone through slit one or that it went through slit two.
  4. May 16, 2012 #3


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    If there are no detectors at the slits, and all you observe is the final arrival of the particle(s) at a detector/screen further "downstream", then there is no general agreement on what "really happens" to the particles in the vicinity of the slits, beyond the probabilities that the wave function gives us. This sort of question is basically what the arguments about interpretations of QM are about.
  5. May 17, 2012 #4
    So there is only probability if the particles are not being observed by detectors, but once the particles are observed then there is a definite result.
  6. May 17, 2012 #5
    What if the detectors merged signals, so no information on which slit the particle went through, but had the energy of both particles/
  7. May 17, 2012 #6
    There aren't multiple particles that go through the detectors at a given moment in the dual-slit experiment. There is only a single particle that travels through one of the slits, and then another is thrown, and another, etc. There aren't multiple particles being thrown at one time, and the single particle doesn't become multiple particles, it has the probability to be one of many different particles, based on it's location in space, which once observed is known.

    This is my naive understanding on the topic, as i'm only 14, but still I can pretty confidently say that you wouldn't be creating energy by capturing the energy of one accelerated particle, as it's still one particle.
  8. May 18, 2012 #7
    My post was just a possible way around the problem with the opening post, but chances are the info given by what percent of the slits (0 50% or 100%) the particle went through would cause it to act as a particle.
  9. May 18, 2012 #8
    Brilliant idea but, only 50% of a particle goes through each slit, nice try.

    I really don't like thinking about the double slit experiment. Has anyone actually PREFORMED the double slit experiment? I have about as much faith in the double slit experiment as I do alien autopsies. In fact I have EXACTLY as much faith in the double slit experiment as I do alien autopsies. But what if you.. but.. um... so.... the fudge is BUILT IN to the particle?? But you can unfudge it if you touch it wrong?? I mean first of all you wouldn't say all balls are made of fudge just because your sister threw a ball of fudge at you. You would say that fudge is made of eggs and milk and maybe some other stuff and is delicious and was made by your sister! But what would you say if the ball hit the cat first and then wasn't made of fudge anymore.. Curiouser and curiouser...
  10. May 18, 2012 #9
    See here. The double slit experiment has been performed with photons, electrons, neutrons, and entire atoms.

    Let me explain it as simply as possible:

    There is only one particle. Except, it no longer has a defined position. So, we treat it as a probability wave, a wavefunction. The wavefunction assigns probability amplitudes to different possible results. Once you make an observation on the wavefunction, probability amplitudes determine the probability of finding the particle in a particular place - you only find it in one place. These probabilities can interfere with each other, which is why interference patterns build up over time in DSEs.

    That's all there really is to it. The act of interfering with the wavefunction will give you a particle, so it is pointless to speculate what happens before this. You cannot 'extract energy' from in it.

    This is totally incorrect. You can only speak of the probabilities of finding the particle in a particular state. It's not like the particle becomes some kind of cloud that passes through both slits.
  11. May 18, 2012 #10
    Please don't try to tie in Quantum Mechanics with everyday experiences, you get stuff in direct contradiction with observational evidence. Just because something isn't intuitively explained doesn't mean it's wrong.
  12. May 18, 2012 #11


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    I don't think we could really call them 'detectors' anymore, but I see what you're getting at. I'll try to explain as best as I can how I see the situation:

    When the particle is given a certain amount of energy at the start of the experiment, this means it is put into an energy eigenstate. And as it travels to the detector, the wave gets spread out a bit, but it is still an energy eigenstate. So at the 'detectors', we have a superposition of energy eigenstates. So a measurement of the energy will still equal the energy which was given to the particle at the start.

    To explain this in terms of the states 'upper path' α and 'lower path' β, we know that α and β are energy eigenstates, so any linear combination ψ = cα + dβ (where c and d are constants), will mean that ψ is also an energy eigenstate of the same eigenvalue.

    Edit: Also, to finish my explanation: When a system is not in an energy eigenstate, the outcome can have several possibilities for a measurement of the energy of the system, but in this particular case, the system is in an energy eigenstate, so a measurement of the energy will always yield the same value.
  13. May 23, 2012 #12
    Thanks for clearing that up
  14. May 24, 2012 #13


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    Millions of students everywhere in elementary physics classes.
  15. May 24, 2012 #14
    Look up "de broglie bohm pilot-wave interpretation of quantum mechanics" for an intuitive/understandable explanation of quantum theory (no funny business there).
  16. May 24, 2012 #15
    On topic but a different question. As I understood is that during the slit experiment it can both react as waves or marbles depending if there is an observer or not.
    What i'm wondering is, if this is because of the exlusion principal.

    By observering would you be changing the energy levels of the "marble" enough, so that its charastics in energy levels would have an increasing difference in levels from its surroundings so it's drag over distance becomes greater then when not observed.

    basically like the bow of the ship creating a wave because it's in the water then having it hit the wall. Or just lifting it just above the water and hitting the wall.

    and the height of the boat is determined by the difference of energy levels between the boat and its surrounding. If there is little, its in the air. And if the difference is bigger, the ship drops in the sea thus more drag leaving a wake wave. And the wave is representing the other elektrons adjusting to the changes energy levels of the marble since none elektron and be in the same energy level.
    Last edited: May 24, 2012
  17. May 25, 2012 #16


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    No. The way it's stated that light or electrons behaves either like a wave or a particle depending on the observer gets the whole idea wrong.

    It depends on what the observer is actually observing. The double slits experiment. I think the most impressive version is the one done with single electrons. What you observe in that experiment is the electrons hit the screen, as if they are particles - but they build up a pattern on the screen, this indicates there has been interference - that the particles had to be waves before they hit the screen. And not only that - since the experiment is done, sending one electron at a time - for the interference pattern to build up as it does, the electron must interfere with itself - so it must pass through both slits. So in the double slits experiment, you, the observer, are actually observing it both being a particle and a wave simultaneously.

    Where it gets all screwy. And I don't know the fancy maths - and I have a feeling the fancy maths does not answer the question - so what if the electron has a probability of hitting the screen where it does. The really confusing thing, in the world way above the quantum level, we expect waves when they hit something to evenly distribute the energy they're carrying against whatever they hit. That's not what happens with electrons and photons. When they hit something, all the infinite points collapse down to one single point.

    A thought experiment I've thought up myself that's been really bothering me. Say if we do the double slits experiment in deep space. We make the distance between the slits and the screen 10 light years, and we make the screen 20 light years long. Unless I'm completely wrong, the wave function will be light years wide before it hits the screen. Yet it will be able to instantaneously collapse across all those light years to a single microscopic dot, somewhere on the screen.

    I've had similar worrying thoughts about light travelling through the cosmos. A photon can travel for billions of years. Its wave must be billions of light years wide, and that wave can collapse when its observed on earth by an astronomer - or just anyone looking at the stars.
  18. May 25, 2012 #17
    A proper explanation of it all can be found here
    Last edited by a moderator: Sep 25, 2014
  19. May 26, 2012 #18


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    That video does not explain the Two slits experiment. He describes the observation - which is the quantum particles simultaneously behaving as waves and particles.

    What is strange, and unexplained, is why the wave decides to collapse at a single point, and not distribute its energy evenly over the screen.

    The basic Young slits experiment - done with light, does not show photons to be particles, The observer would only be able to determine that light was waves.

    When the experiment is done with single electrons, then it becomes apparent that the electron is both simultaneously acting as a wave an particle. Or at least, behaves as a wave - even splitting into two wave fronts, but both wave fronts collapse as a particle - or discrete quantity. That's the real mind boggling bit.
    Last edited: May 26, 2012
  20. May 26, 2012 #19
    Welcome to Quantum Mechanics. Or at least the one interpretation of it you're using.
  21. May 26, 2012 #20


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    Believe it or not, I actually studied this - not to a huge depth. I studied applied physics - on my course we didn't get too deeply into the quantum stuff. I haven't done a differential equation in years. I haven't had the skills to investigate the maths heavy end of the theory.

    I'm not sure if I personally have an interpretation of it. I'm clear on what is observed in the Youngs slit experiment. But I couldn't give you a mathematical explanation of why the wave collapses to a point. Is there an explanation for that? An explanation that says there is a probability of the wave collapsing where it does, doesn't really explain it. And I wonder about some of the conventional theory - that the elements may be mathematical fictions that agree with experimental results. (what I mean by mathematical fictions is like astronomical calculations that agree with an earth centred universe - but they're actually contortions - the results are correct but the theory is incorrect)

    Is there an answer to my little thought experiment - making the slits apparatus billions of light years wide and long? Making the waves billions of light years wide - and then having them collapse to a point. When the wave reaches the screen, and the photon finds an atom it's going to collapse on, how long does it take the wave to collapse.
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