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Photons do not travel in straight lines

  1. Aug 11, 2009 #1
    I've been thinking about this for some time now and I wanted to get some feedback on it. If photons are supposed to travel along straight lines (for the sake of simplicity lets neglect the curvature of space-time), then how could one photon possibly be used in a double slit experiment? More specifically, how could one photon traversing a straight line possibly create an interference pattern with itself? I have never heard of even pseudo-logic that would allow for something this absurd.

    I understand that upon absorption (or some other event) the possible paths of the photon will "reduce" to a single path that takes it, in essence, in a straight line from emission to absorption; I'm not concerned with the reduction part of it, I'm concerned with the "all possible paths" that we assume the photon to be traversing simultaneously. What possible logic could then say that the photon travels along a straight line, and why do we still teach that this is the case? If we're saying that the photon travels along an infinite number of straight lines, simultaneously, during the time between absorption and emission, that would make sense, but every single day I have to struggle with the fact that different sources explain the propagation of light in different ways. If I'm reading about optics, I'm going to get the "photons travel in straight lines" interpretation. If I'm reading about pure quantum physics, I'm going to get a completely different story, where photons are evolving over multiple paths simultaneously, until they reduce to having traveled a single path.

    If one is true, the other is not. Which one is it?
     
  2. jcsd
  3. Aug 11, 2009 #2

    negitron

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    Both are (or at least appear to be) true. Light sometimes seems to act as a pointlike particle that travels in straight lines but at others it seems to act as a wavelike entity. Chances are it is actually neither of these things, but some other construct altogether haivng some properties of both.
     
  4. Aug 11, 2009 #3

    Demystifier

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    Even classical light does not travel in straight lines. (Otherwise, mirrors and lens couldn't exist.) Hence, there is no reason to believe that photons travel in straight lines.
     
  5. Aug 11, 2009 #4
    photons travel by all possible paths. read QED by Feynman - excellent presentation of this topic by a true master.
     
  6. Aug 11, 2009 #5
    They only instantaneously travel in straight lines.

    :biggrin:
     
  7. Aug 11, 2009 #6

    negitron

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    This makes no sense.
     
  8. Aug 11, 2009 #7

    Dale

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    Yes, in general photons do not travel in straight lines, they travel as waves. However, it is often a reasonable simplification to treat a wave as a ray or otherwise to say that waves travel in a straight line. They are not mutually contradictory explanations, one is an approximation of the other and both are correct in situations where the simplifying assumptions are valid.
     
  9. Aug 11, 2009 #8
    Photons are EMR waves which travel in straight lines, so in essence it travels both in a straight line and in a wave motion. One photon cannot create an inference pattern with itself, it needs another photon which may arrive in phase or out of phase to create an interference pattern.
     
  10. Aug 11, 2009 #9
    It's not helpful to think of photons having permanent existence as either particles or waves. The underlying "real" structure is unknown and may be unknowable, but in certain interactions the underlying structure will manifest itself as a wave, and in certain interactions it will manifest the properties of a particle. Neither "waves" nor "particles" are persistent objects, they are just useful theoretical tools for visualizing something, depending on the situation, that we don't have complete knowledge of.

    Take the property of being red. We say that being red is NOT an intrinsic property of an apple. It is a property that manifests itself in specific situations. When a person is looking at an apple, to that person, the apple manifests the property of being red.

    The particle-property of momentum works the same way. Momentum is a property of the underlying photon structure that manifests itself during collisions. When the photon is not actively colliding, it has no intrinsic property of momentum. When it is colliding, then, to whatever it is colliding with, it manifests the property of having a certain momentum. The same can be said of the other particle-properties of location, mass, etc.

    Similarly, a photon does not have a persistent wave property of polarization. A photon will manifest the property of polarization while passing through polarized glasses.

    The property of being red, the property of being 1 gram, and the property of being linearly polarized at 0 degrees, are all fleeting, extrinsic properties that manifest themselves in certain (experimental / observational) situations.

    Realize that any framework you use to visualize basic objective reality will be inconsistent and illogical. We do not have a consistent framework for ascribing persistent properties to real objects. It is very likely that as humans we are necessarily limited to visualizing reality in the falsified, macro, classical mechanical sense of billiard-ball style particle determinism.

    So which is real, particles or waves? Well... neither is objectively basic, as the properties of each can only exist in certain situations, but particles and waves are both just as real as the color red and the meaning of the words I'm typing. And that may be as "real" as we can get.

    FYI that (above) is Bohr in a nutshell, with a little Kant for flavor.
     
    Last edited: Aug 11, 2009
  11. Aug 11, 2009 #10

    negitron

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  12. Aug 11, 2009 #11
    Science gets things a little backwards. We believe that photons can interfere with each other because they DO. We know from experiment. After we can accept that, we come up with a theory about it retroactively.

    Science has many moving parts, all stitched together. That light travels in straight lines is a consequence of classical electromagnetism and general relativity. That light is discrete and can interfere with itself comes from quantum mechanics. Classical theories don't always agree with quantum theories because massive objects (beams of light) act very differently than small objects (individual photons).
     
  13. Aug 11, 2009 #12
    In general I agree with your post, but I have to disagree on this. Individual photons can act exactly the same way beams of light do, wave properties and all. The differences aren't a question of scale. We are also seeing entanglement experiments of larger and larger systems, bringing small object phenomena to a larger scale.
     
    Last edited: Aug 11, 2009
  14. Aug 11, 2009 #13
    Just curious, How do you know that only one photon is hitting the slit or how to make only one photon to hit the slit? It can't be done just by pinhole.
     
  15. Aug 11, 2009 #14
    You only record one photon at a time on the other side of the slit. Notice the experiment shows individual dots showing up on the screen on the other side. They show up one at a time. I've seen this done with electrons as well in radioactive situations where you can only be emitting one electron at a time from each atom.

    The nature of particles as discrete objects necessarily means that, at arbitrary precision, they must be passing through the slit one at a time, in a discrete order. Only when considered as continuous waves is it even possible to talk about photons simultaneously passing through a slit... although "passing through" isn't exactly the right terminology for talking about waves either.
     
    Last edited: Aug 11, 2009
  16. Aug 11, 2009 #15

    ZapperZ

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    Look at the SIZE of the light source!

    For example, if you have a beam spot that is 1 mm in diameter, and your slit is more than 1 mm apart, do you think a photon will then bend its path to pass through both slit? Try it!

    The spot size of 1 mm requires that the slits be separated at LESS than 1 mm. This means that AT THE SOURCE, a photon can be emitted anywhere within that 1 mm cross section! Given that scenario and the ability for each path to interfere with each other, only then will you get the interference pattern.

    It is why, when many of these experiments are done using thin laser beam, one often use a beam expander - to increase the spot size!

    Zz.
     
  17. Aug 11, 2009 #16
    I understand. But the source is flashlight bulb which emits many photons at a time, then how one can isolate one photon to hit the slit? Is it possible by simple pinhole arrangement?
     
  18. Aug 11, 2009 #17

    jtbell

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    Photons do not interfere with each other except via a rare higher-order process, Delbrück scattering.
     
  19. Aug 11, 2009 #18
    then what's creating the interference patterns on Young's Double Slits?
     
  20. Aug 11, 2009 #19
  21. Aug 11, 2009 #20

    ZapperZ

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    It is the "single photon interference".

    2-photon interference almost never happen - it is a higher order interaction.

    Zz.
     
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