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    No, one of your twenty bastards
    • CommentTimeDec 30th 2016
    Love children, be a good politically correct socialist.
    • CommentAuthorloreman
    • CommentTimeFeb 14th 2017
    A girl I went to school with worked on the LIGO mirror
    • CommentTimeMar 29th 2017
    Piketty said it better. And Piketty was pretty obscure.
    Some lovely old stories from Freeman Dyson
    LHC is back up again
    Thought experiment for all you big brains out there (and Angus):

    First, a quick refresher about Young's Slits and the delayed quantum eraser (one link inside the other)

    Now, what happens if we start to mess with the inter-slit separation while the particle is in flight between the slits and the screen? Imagine some stretchy material and an actuator for it.
    • CommentTimeMay 24th 2017
    Is this some sort of sex game?
    • CommentTimeMay 24th 2017 edited
    You don’t know exactly when the photons will be generated and therefore when one will arrive at the slits, so these must be in motion from the start of the experiment. At the time the diffraction occurs they will have some unknown separation. You will be able to determine the separation when you measure the fringe interval after the experiment. Except that you can’t because you have to use multiple photons to get fringes.

    Well, never mind, let’s imagine doing it over and over until there is some average result. The fringe separation gives you the wavelength of the photon if you know the slit separation, which you don’t unless you know the time the photon passed through them (as well as the separation velocity of the slits). But the wavelength (i.e. the photon energy) and the time form a famous pair of properties subject to the uncertainty relation, so you can’t know both at once beyond an accuracy limit.

    I’m guessing that if the slits move fast enough to create an effect* the uncertainty principle will smear out the fringes and cancel it.

    *effect: that is, other than the classical result that the fringe spacing decreases as the slit separation increases.

    Can I play with the big boys now, please?
    This is a one-photon-at-a-time experiment - or could be. The effect is still observed for the regular experiment, so we can do this slit separation modulation on this too.

    The simplest way to begin approaching what I mean is to close one slit while the photon is (photons are) in transit between slits and screen.

    The key observation for the single photon double slit experiment is that the interference pattern which builds up depends on 3 parameters: the laser wavelength, and the geometry of the system: the inter-slit separation (d) and the slit-screen distance (D).
    • CommentTimeMay 24th 2017
    You said the slits were changing their separation during the passage from slit to screen. I addressed that. What is wrong with what I said?
    Because you muddy the waters in an unnecessary way I think (could be wrong!). We know the emission time post hoc accurately enough to determine when we're free to mess with the slits. We know this accurately enough to know when to change something about the slits. So we can be pretty sure that the photon passed through the slits before we do anything to the slits. The time of flight is repeatable to within Uncertainty as you say (wavelength-time complementarity).

    Don't we know the (invariant) wavelength rather accurately anyway?
    • CommentTimeMay 24th 2017
    Posted By: Andrew PalfreymanWe know the emission time post hoc accurately enough to determine when we're free to mess with the slits.

    How? You have to detect the emission time of the photon with sufficient accuracy to determine when to move the slits. I don't know how you plan to do that, but if you do manage, it interferes with how well you know its energy, and therefore with how well you know what the interference pattern should look like for any particular slit separation.
    If I measure when a spot appears on the screen, and I have independently measured the emitter-screen distance, then I know the emission time of that particular spot post hoc. Independently I know the laser wavelength and its spread delta-lambda between the 3dB points of its distribution.

    I should be able to tell when the photon is well past the slits.
    • CommentTimeMay 24th 2017
    It doesn't matter where you detect the spot. It is the resolution of the time measurement that matters. Contrary to what you seem to think, lasers do not emit photons of identical wavelength, but a band of wavelengths set by the Q of the cavity, so you don't know the wavelength with accuracy a priori. If you did, there would have to be some slop in your ability to detect the arrival time.
    Not where, but when, yes. Which is what I said. As opposed to contrary to it.

    I also addressed the Q. Did you not read?
    You do this regularly - nit-pick at some irrelevant detail and ignore the big picture. You're claiming that it's impossible to know when the photon has passed the slits.

    Look. Single photon sources are available. They are characterised such that you know, within a defined resolution given by inter alia the uncertainty principle and the Q, the delay between hitting the tit and a photon squirting out.

    All that's necessary is make the apparatus of sufficiently large dimensions such that this time uncertainty is ignorable, and such that you can always say whether the photon has passed the slits or not with sufficient certainty.

    The simplest embodiment of my slit modulation concept is to cover one of the slits while the photon is in transit between slit and screen.