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The Double Slit Experiment from a Photon Point of View

  1. Aug 17, 2010 #1

    When reading about the Double Slit Experiment, often the author will rave about something like 'the photon must pass through both slits at the same time'.

    I'm struggling a bit with the simple phrasing of this statement, and can't come to terms with two questions I have:

    • Isn't the fact that a photon is traveling at the speed of light means that time doesn't pass within its inertial frame? That is, all of its motion is on the space axis of spacetime? As such wouldn't it be very possible for the photon to travel through both slits? By way of analogy, if I would be in a room where there are 2 open doors on one wall, and for me, within the room, time wouldn't pass, I can quite easily go through 1 door, then back, then through the other. For someone outside the room, where time does pass, it will appear as if I 'passed through both doors at the same time' - but I could only have done so because time didn't pass within my inertial frame?
    • Also, what does the double slit screen looks like - in terms of size in space - to the photon? Isn't the fact that it travels at the speed of light means that space outside its inertial frame shrinks to infinitely small dimension? And so the photon does not see the double slits as two spaced apart slits but rather as two slits in the same space? Saying 'it passed through both slits' needs the addition 'because they were, from the photon point of view, at the same point in space'?

    As probably obvious, there are child-like questions that don't show deep understanding of concepts like spacetime, so please forgive me for what might be a complete mess of misunderstanding.

  2. jcsd
  3. Aug 17, 2010 #2
    One immediate problem with this logic, is that it implies that the entire universe is shrunk to a single point (and the entire history of the universe is shrunk to a single instant) from the photon's point of view, so how does it have any spatial awareness so that it can appear at the correct location on the final screen and produce those precise interference patterns?
  4. Aug 17, 2010 #3
    Excellent question. So what space looks like for something that travels at the speed of light?

    In a way, maybe saying that space 'shrinks' is not right, but how about saying that as you only travel through the space axis of space time, you are actually traveling (obviously at the same time) through all the space through which you are allowed to travel?
  5. Aug 17, 2010 #4
    A) The distance dilation only occurs in the direction of travel. Thus the photon would still only be able to be aimed at single slit.

    B) For the photon to go through one slit, back up, and go through the other would require some cause for it to be backing up and changing directions. Just getting a photon to stop and backup without reflecting off of something would be a bit of an impossible occurrence.
  6. Aug 17, 2010 #5


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    A photon doesn't have an inertial rest frame, all inertial frames move slower than light. This has been discussed on many previous threads, see this one for example.
  7. Aug 17, 2010 #6
    So based on this is my question meaningless as well? Asking what space and time are like for something traveling at the speed of light is meaningless?
  8. Aug 17, 2010 #7


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    I suppose you could arrgue it's not a meaningless question, but relatvity doesn't offer much in terms of procedures for constructing reference frame sof particles (like photons) whose worldlines are null.
  9. Aug 17, 2010 #8


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    I think it's a mistake to conceive of inertial frames as "what space and time are like" for even slower-than-light inertial observers. Inertial frames are simply one way of defining distances and time-intervals, in terms of a system of rulers and clocks at rest relative to the inertial observer, but the inertial observer is free to use a different non-inertial definition if desired (though inertial frames have some 'special' properties that make them especially useful, like the fact that the laws of physics obey the same equations in different inertial frames)
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