Inspired by another quantum thread, I am going to put some messy thoughts down on paper to try and make sense of them. IANA physicist as you can tell.

If one takes a multiple slit photon experiment to get a diffraction pattern, the precise pattern one gets is determined by the total arrangement of slits. But some of those slits could be so far away that the photon “potential field” would have to travel faster than the speed of light to have been influenced by them (this is also true for photons hitting the middle of the screen as the precise pattern can be influenced by further away slits). This does not seem to be true of other potential fields which propagate at the speed of light. (I know I am mixing up field concepts here but there does seem to be some analogy)

Now imagine a photon in flight. If one changes just one of the slits and it affects the pattern, whats the latest that one can change it before having an effect on the photon passing though the collective? Is the idea of non-locality in the EPR experiment also relevant here? Does the fact that the photon appears to interact with the entire slit device at once imply some sort of universal time (now) for the device or are quantum interactions governed by the speed of light too?

I have probably mangled many fine concepts and I suspect Heisenberg has something to say on it too.

A couple of nitpicks here: First, there’s only one other field that we know of that we think travels at the speed of light, namely the gravitational field. And second, you can get interference patterns with electrons and other massive particles just as well as you can with photons; in fact, this is one of the ways that scientists can investigate molecular structure.

Photons, in this regard, behave just like waves. If you drop a pebble into a pond, the pattern made by the waves as they interact with obstacles in the water is determined by what the obstacles look like when the waves actually get there. So if I were to fire off a photon towards a double slit, and then close one of the slits before the photon could get there, then the photon would just “see” a single slit and wouldn’t interfere with itself; there’s nothing terribly out of the ordinary here, as long as you’re familiar with how waves act.

Now, where things get weird is in the fact that you can “erase” quantum information even after it’s been collected and the waveform should have “collapsed”, thereby restoring the quantum nature of the system. See the quantum eraser experiment and its even freakier cousin the deayed choice quantum eraser experiment, for details.

The color field (responsible for the strong force) is also presumed to travel at the speed of light, though it’s probably hard to measure at those short distances.

And other fields (or particles, or whatever) which propagate slower than c don’t have any particular speed (it depends on the energy compared to the mass of the particle). One way of phrasing the principle of Lorentz invarience is that c is the only inherent speed in fundamental physics.

Wow, this has really been the week for questions about quantum mechanics. I need to figure out a way to turn this into a best-selling airplane novel before the trend dies out and I’ll be set for life.

This is not a trivial or nescient question, and in fact the answer is at the heart of interpreting what is going on here. According to Heisenberg interpretations, there is no “realism”, nothing exists until you look at it, and everything that seems to require particles doing strange things over fast differences is just an artifact of the probabilistic nature of QM. They try really really hard not to think about this kind of thing and just crank through the equations, which give the right answers. Boosters of the Bohm interpretations would say that yes, the photon is nonlocally connected (i.e. spread out), very much real whether you’re looking at it or not, and communication from one point on it to another is instantaneous, or at least not dependant upon the spacetime distance between the points. The relative state/Many Worlds advocates will say there is an infinite number of photons and you happen to have ended up in the universe where this photon took that path. The consistant histories romantics believe that it was all statistically classical behavior all along, but doctored up by nature to look probabilistic and conforming to the Schrödinger equation. And nobody can prove anything; it’s not just a bit like reading Murder On The Orient Express, in that all interpretations appear to satisfy the theory, but none more than the others.

Another slight nitpick is that we expect that gravity moves at c, but we don’t know that it does for certain. In general relativity, gravity is shown to be a property of spacetime (that is, its curvature) and propagation occurs at a single speed for all fields. As I understand it, this speed not necessarily equal to c; however, it if goes slower then you have a potential for non-conservation of momentum, and if you go faster than light, it violates causality. The former is a stock principle of physics which gives every evidence of applying on all scales, from the cosmological to the quantum. The latter (causality) is something we’ve always assumed to be true, and violating it would have serious problems not only for the grandfather you’ve always wanted to go back and assassinate, but also for thermodynamics, and more general, being able to make any kind of consistant rules about the mechanics of nature.

Similar arguments exist for the color field, but because the interactions are mediated by virtual gluons that, owing to color confinement, can’t come out and play for hardly any time at all, certainly not enough to get a lock on them and measure anything.

This is an interesting question – one that the complementarity of the quantum world may make impossible to answer. In the double-slit experiment, the space between the interference fringes on the detector is approximately wL/d, with w the wavelength of the light, d the distance between the slits, and L the distance from the slits to the detector.

As you make the slits farther and farther apart, increasing d, the fringes will get closer and closer together, and the pattern on the detector begins to look more and more classical. By the time d gets large enough for you to have any hope of “fooling” the photon by making a change to one of the slits on the time scale of d/c (with c=3x10[sup]8[/sup]m/s, a microsecond corresponds to 30 kilometers), or to have any reasonable expectation that the field might not interact with itself, your system may be too large to make any precision measurement (too many “observer” atoms; I like the thermodynamic analogue you suggested in the other thread).

A really nice, accessible piece on interference and complementarity is here. If you search google video, for “phase space interferences of neutrons”, you’ll find a talk by a charming fellow describing how the interference patterns of neutrons vanishes as you move the clusters farther apart (caveat: I think it’s that one, but I haven’t watched it in some time).

thanks for the replies. Glad it wasnt a totally stupid question - I suspected that as illoe pointed out the scales involved means that you may never be able to test it (or that nature will find a way to prevent you)

It certainly doesn’t seem testable given what we know now. It’s certainly possible that someone will come up with a novel method of testing the premises of various interpretations, and if they do they’ll no doubt win one of those fancy awards from Sweden, but with what we have at our disposal we’re at a loss to say more than “Urrrow?” It’s also quite possible (and in my estimation, likely) that all interpretations are wrong and that something fundamentally stranger than we can possibly imagine is going on.

And to think that Keats whined of the destruction of beauty and mystery by science “unweaving the rainbow.”

Or Wordsworth (even less mellifluously) that “we murder to dissect.” Those consarned quantum particles just can’t be properly interrogated, but if they could, they’d be quivering in their socks right about now.