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Science Forum

  1. 20 Dec '08 19:45
    Can anyone explain to me how a particle can be in 2 places at the same time please. I've read about how particles can be waves also, and that in a certain experiment a photon (i think it was) managed to pass through two separate slits in a diffraction grating.

    In lay persons terms if anyone can help - or can be bothered to?

    Thanks
  2. 20 Dec '08 20:37 / 1 edit
    A particle is not in 2 places at the same time, rather, the concept of "place" becomes blurred in the same sense as you can't assign a "place" to a wave in a rope.

    Also, it's not that a particle can be a wave. A particle is always a wave.
  3. Standard member sonhouse
    Fast and Curious
    21 Dec '08 12:22
    Originally posted by KazetNagorra
    A particle is not in 2 places at the same time, rather, the concept of "place" becomes blurred in the same sense as you can't assign a "place" to a wave in a rope.

    Also, it's not that a particle can be a wave. A particle is always a wave.
    What about the energy content of a wave that seems to be in two places at once? If you have a certain wavelength, you have a certain amount of energy, but if it is in two places in quantum superposition experiments, which particle/wave has the full amount of energy inherent in that wave?
    Suppose you did a double slit experiment where the waves split and then goes to a flourescent molecule set, one for each photon, where it takes X amount of energy to flouresce. Will both molecules be activated?
  4. 21 Dec '08 13:01
    Originally posted by sonhouse
    What about the energy content of a wave that seems to be in two places at once? If you have a certain wavelength, you have a certain amount of energy, but if it is in two places in quantum superposition experiments, which particle/wave has the full amount of energy inherent in that wave?
    Suppose you did a double slit experiment where the waves split and th ...[text shortened]... r each photon, where it takes X amount of energy to flouresce. Will both molecules be activated?
    A photon does not split into two photons when going through a double slit. One photon goes through both slits. So in your hypothetical experiment, you will not see any fluorescence, unless there also happens to be a transition of about 2 times X of energy.
  5. Standard member sonhouse
    Fast and Curious
    21 Dec '08 22:50
    Originally posted by KazetNagorra
    A photon does not split into two photons when going through a double slit. One photon goes through both slits. So in your hypothetical experiment, you will not see any fluorescence, unless there also happens to be a transition of about 2 times X of energy.
    But if you tried to measure the energy in the photon that went through the one slit, would you have to measure the energy of both slits?
  6. Standard member forkedknight
    Defend the Universe
    22 Dec '08 05:55
    If you tried to measure the energy as it went through one of the slits, it would only to through one slit.
  7. 22 Dec '08 08:00
    Originally posted by sonhouse
    But if you tried to measure the energy in the photon that went through the one slit, would you have to measure the energy of both slits?
    A photon cannot go through just one slit in the double-slit experiment. It will always go through both. If you measure its energy, you will get the energy of the photon. If you measure the location of the photon as it goes through the double slit, it will have a certain probability of being in one slit and a certain probability to be in the other depending on the exact geometry of the experiment. This does not mean the photon "actually" went through one slit and not the other - it went through both, but assumed a certain position after measurement.
  8. 22 Dec '08 09:40
    Originally posted by KazetNagorra
    A photon cannot go through just one slit in the double-slit experiment. It will always go through both. If you measure its energy, you will get the energy of the photon. If you measure the location of the photon as it goes through the double slit, it will have a certain probability of being in one slit and a certain probability to be in the other depend ...[text shortened]... it and not the other - it went through both, but assumed a certain position after measurement.
    How is it possible for something to travel through two separate spaces at the same time as you describe. Is the photon moving through space/time in some way, or is it a more parsimonious explanation where the photon simply doesn't move in a single plane and direction?

    If it can only be explained through complicated physics then I guess I'll pass as it's not a strength! Thanks anyway.
  9. 22 Dec '08 10:54 / 1 edit
    Originally posted by divegeester
    How is it possible for something to travel through two separate spaces at the same time as you describe. Is the photon moving through space/time in some way, or is it a more parsimonious explanation where the photon simply doesn't move in a single plane and direction?

    If it can only be explained through complicated physics then I guess I'll pass as it's not a strength! Thanks anyway.
    It's not complicated, the photon simply behaves as a wave, so something analogous to a wave passing through an obstruction with 2 holes happens. It's just that the nature of the wave is changed after measurement (note that "measurement" in a quantum mechanical sense does not require a conscious "measurer", but it's used metaphorically to describe events in which the wavefunction collapses - you could see it as "an interaction with a macroscopic object" ).
  10. Standard member sonhouse
    Fast and Curious
    22 Dec '08 23:27
    Originally posted by KazetNagorra
    It's not complicated, the photon simply behaves as a wave, so something analogous to a wave passing through an obstruction with 2 holes happens. It's just that the nature of the wave is changed after measurement (note that "measurement" in a quantum mechanical sense does not require a conscious "measurer", but it's used metaphorically to describe events ...[text shortened]... efunction collapses - you could see it as "an interaction with a macroscopic object" ).
    But the slits are spaced way wider than the wavelength of the individual photons, right? And they still end up going through both slits?
  11. 23 Dec '08 09:38
    Originally posted by sonhouse
    But the slits are spaced way wider than the wavelength of the individual photons, right? And they still end up going through both slits?
    If I remember correctly the spacing should be on the order of the wavelength of the particle to properly see the interference, but I'm not sure on that one. But yes, they will go through both slits even if spaced far apart.
  12. 28 Dec '08 11:18 / 1 edit
    We can send the light one photon at a time and the interference pattern is still shown. You will not see the light in two spots concentrated diametrically opposite each of the holes. clearly the photons are interfering with themselves, a given photon goes through each hole simultaneously. The interesting thing is if you set up a mechanism that tries to see exactly which hole the photon is going through, then there is no interference pattern. If you try to measure the holes you end up with photons acting like particles, the wave pattern disappears. But if you choose not to look the photon retains the wave patterns. It is extraordinary that the observer can influence the outcome of the experiment. In any given experiment you can only measure one property at a time. This property of light turns out to be a property of normal matter as well, of protons, of electrons, of people. It is just harder to see.
  13. 28 Dec '08 12:56 / 1 edit
    Originally posted by omg1337
    We can send the light one photon at a time and the interference pattern is still shown. You will not see the light in two spots concentrated diametrically opposite each of the holes. clearly the photons are interfering with themselves, a given photon goes through each hole simultaneously. The interesting thing is if you set up a mechanism that tries to see ...[text shortened]... roperty of normal matter as well, of protons, of electrons, of people. It is just harder to see.
    So the observer effect is impossible to control for, increasingly so the smaller the object? I'm no physicist, just interested, but would this suggest that light is not subject to the laws of time and space? (as currently understood).
  14. 28 Dec '08 16:34
    Originally posted by divegeester
    So the observer effect is impossible to control for, increasingly so the smaller the object? I'm no physicist, just interested, but would this suggest that light is not subject to the laws of time and space? (as currently understood).
    Light is subject to the laws of time and space. The laws of time and space as we currently know them imply that you cannot simultaneously measure position and momentum (which in the case of photons, is proportional to energy) of any particle.
  15. 28 Dec '08 17:13 / 15 edits
    Originally posted by omg1337
    We can send the light one photon at a time and the interference pattern is still shown. You will not see the light in two spots concentrated diametrically opposite each of the holes. clearly the photons are interfering with themselves, a given photon goes through each hole simultaneously. The interesting thing is if you set up a mechanism that tries to see ...[text shortened]... roperty of normal matter as well, of protons, of electrons, of people. It is just harder to see.
    …It is extraordinary that the observer can influence the outcome of the experiment...…

    I know that it is common scientific wisdom in quantum physics that the outcome of the slit experiment is influenced by the observer but I think that is bases on flawed logic for you cannot safely conclude that from the premises you just pointed out.
    The only thing that can safely be concluded from the premises you just pointed out is that the outcome is influenced by what property the physical experiment itself is set up to respond to -that is all! I think you cannot logically safely conclude that the observer or whether or not something is being “observed” is what is directly relevant here although I am sure there are many physicists that would very strongly disagree with me here.