Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Superposition-airy disc-snell's law-qbits.

  1. Jun 24, 2010 #1
    The airy pattern is generated by a photon interfering with itself at the aperture. How is this possible? How can such a thing be visualized?
    Also, since we can now predict with around 80% certainty the position of the qbit (superposed between a 1 and a 0), doesn't it stand to reason that we can also predict with equal certainty where the photon will land on the screen as it generates its airy pattern? After all, both particles are considered to be 'superposed' on themselves.
  2. jcsd
  3. Jun 24, 2010 #2
    I think you're a bit off course here, but there is a thread, albeit a very old one, which seems to address this issue: https://www.physicsforums.com/showthread.php?t=11095

    I think you're confusing the long-term statistical effects, with single particles, which do show up as a "dot" on the detector.
  4. Jun 25, 2010 #3
    I went to the thread you posted, and it was the usual: some people proposing crackpottery; some people mastering the obvious and some people rehashing stuff from their 6th-grade science texts, with noone really answering the questions set fourth.

    Every first-year phys. Student at caltech ggets to see the diffraction demonstration whereby a photon gets sent thru the slits one at a time, showing up as a 'dot on the detector'; eventually culminating in a usual diffraction pattern.

    There is no difference between airy disc diffraction effects, single-slit diffraction effects, and multiple-slit diffraction effects-superposition of the photon upon itself (continued...)
  5. Jun 25, 2010 #4
    (continued from above...)
    is the phonomena that's supposed to be implicit in all diffraction effects. but how is superposition even possible? we can't describe a photon (or particle) as a 'wavepacket', since wavepackets can't really exist in nature (neither can strings, branes or other forms of physics crackpottery).
    Nor can we say that a 'photon is a wave until we look at it', since Dirac's pilot wave hypothesis was routinely dismissed shortly after it was proposed.

    What happens, then, to the single photon as it exits its aperture? Assume the photon has a 600nm wavelength, and the aperture is 600nm round.
  6. Jun 25, 2010 #5


    User Avatar
    Science Advisor

    The airy disc is a diffraction phenomenon, and well-described by classical wave mechanics. So this best thought of as a 'wave-like' property of light, rather than a 'particle-like' property. Speaking of a photon (a 'particle' picture) 'interfering with itself', is of course non-intuitive.

    Where did 80% certainty and qbits come from here?
    A "qbit" is something which has discrete states (e.g. spin up/down of an electron), just like a digital bit. Unlike a digital bit it can be in a superposition of both states at once. But the position of a photon isn't a qbit, because position is not a discrete state.

    If you are dealing with single photon, you can predict the probabilities of where it will it will be detected.

    Both particles?

    The same way it is for classical waves, and anything else that obeys a linear differential equation.

    That's just silly. All mathematical concepts are abstract things which "don't exist in nature", yet this doesn't mean you can't describe things mathematically. Whether or not you take the philosophical position, for instance that the wave function is a 'real thing', has no bearing at all whether or not it works as a physical description.

    It wasn't Dirac's theory.
    Last edited: Jun 25, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook