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Question about double slit experiment

  1. Apr 6, 2009 #1
    i've always been curious...WHY did somebody come up with the experiment? why did they think anything special would happen? hope this doesn't sound stupid :)
  2. jcsd
  3. Apr 6, 2009 #2
    I assume you mean the double slit experiment with electrons? The double slit experiment with photons seems kind of obvious, especially given how long the wave-particle nature of light debate had been going on. Extending it to electrons seems sort of natural in that light, I think (no pun intended).
  4. Apr 6, 2009 #3
    yes, electrons.
  5. Apr 6, 2009 #4
    Double slit with electrons wasn't actually done until the 1960s... so there was certainly enough quantum mechanics on the books already to suggest something very weird would happen.
  6. Apr 6, 2009 #5
    this may sound stupid as well, but what would be the difference between a double-slit experiment involving electrons as opposed to photons? would the wave form be any different? do electrons and photons have different tendencies as to where they would appear during a quantum leap?
  7. Apr 6, 2009 #6
    There are many big difference between photons and electrons... classically, photons are expected to product an interference pattern in the double slit experiment as classically they are waves. Electrons, on the other hand, are classically particles... with mass and other properties... making them very different from photons.

    Classically, the two would have incredibly different results, since an electron shouldn't be able to interfere with itself if it's just a particle.

    Actually though, the experiments look very similar... I don't know what you mean by "tendencies" though... but if you allow the experiments to run for a long time/with lots of photons or electrons, they'll basically look the same (interference patterns on the screen, that is)
  8. Apr 6, 2009 #7
    so it was first with photons then electrons? but still, how come they thought of exactly a double slit experiment?! i'm not any good at physics or quantum emchanics so what i'm asking might have an obvious answer. it just sounds weird that a person could think to himself - if i send electrons throught 2 slits, so and so might happen- then one at a time etc... again, i have a very small understanding in this field so don't laugh :) why did he think that something odd would occur with electrons? from what i gather it was quite astonishing even for people that work in the field so it wasn't somethink they thought would happen.
  9. Apr 6, 2009 #8

    yeah sorry, maybe i should have been a bit more clear there. what i meant was basically whether or not the electrons and photons could interfere with themselves, thus changing their probabilities to pass through one slit or the other.
  10. Apr 6, 2009 #9
    Historically, there was a debate as to the nature of light, whether it was a particle... like a rock... or a wave... more like water.

    When a flat water wave goes between one slit, it comes out again as a circular wave - this was easily observed. When water would pass through a double slit, it would come out as two waves and interfere- as interference is a thing waves do. They knew this about waves for a long time, so when asking if light was a wave, testing to see if it interfered with itself in a double slit type set up was a fairly obvious choice. Double slits probably just occurred naturally and were observed with water, so it was just a natural choice for light.

    The double slit experiment was a long time thought experiment in quantum mechanics even before they were actually able to preform it in the 1960s. People expected the results when they happened.

    de Broglie did theoretical work in the early 1900s on "matter-waves" - Light was already accepted/thought of as a wave and a partcile. De Broglie suggested that was true for electrons (and everything else really) too, but being able to describe an associated wave length with material objects (similar the the fact that light has a wave length). If electrons have a wavelength, then they must also have some wave properties, like self interference.

    Yes, wave-particle duality of matter was really weird at the time. Not by the 60s though.
  11. Apr 6, 2009 #10
    Photons and electrons both interfere with themselves, part of their "wave nature".
  12. Apr 6, 2009 #11
    thanks for taking the time to make it clear for me. really appreciate it.
  13. Apr 6, 2009 #12


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  14. Apr 7, 2009 #13
    yeah, thanks. that helped me out a lot too.
  15. Apr 7, 2009 #14
    - Indeed, the wave nature of the light was very much expressed by Young’s double slit experiment.
    - Einstein explained the photoelectric effect by suggesting that light is made of particles (the photons). Before that, Plank realized that energy is made of discrete packets. Bohr proposed a quantum model of the atom, where photons (having discrete energies) are emitted by electrons jumping from an “orbit” to another. The corpuscular nature of the photon required reinterpretation of Young’s double slit experiment. The photons are particles which interfere like waves. This looked weird at that time, and still does.
    - de Broglie explained Bohr’s quantization conditions for the electrons in the atoms by the condition that the electrons are also waves, and that they oscillate an integral number of times while going on the orbit around the nucleus (reminding of resonance of musical instruments).
    - there were made experiments with electrons, by Davisson and Germer in 1927, showing diffraction exactly as predicted by their (theoretical, until then) wavelike nature.
    - then, of course, the two slit experiment with electrons.

    The reason is that physicists wanted to see how deep can push the wave nature of matter. The same reason led them to test interference with molecules consisting on 60 atoms of Carbon (buckyballs).

    And they would do this with cats too, if they could :smile:
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