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Old 23rd February 2015, 01:17 PM   #47
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Originally Posted by wogoga View Post
Let us imagine a big slit and two radio antennas next to each other. Having the same distance from the slit, both antennas send a signal of the same frequency and intensity to the slit. Let us assume a phase shift (resp. difference) of Pi/2 (i.e. wavelength/4) between the two radio signals: S1 with phase 0 and S2 with phase Pi/2. The superposed signal S then gets phase Pi/4 at the slit.

Now let us divide the united signal S behind the slit into Sa and Sb, and reunite these two signals again at a given point P after a path length difference of half a wave length. If "the only relevant phase" is the superposed phase Pi/4 of S, then we get 100% destructive interference, as a phase-shift of Pi of the two otherwise identical signals Sa and Sb reduces their joint intensity to zero.
OK, so you took what's effectively now a single source, split it into two paths of different length, and then combined the two paths after to destructively interfere.

However, if we assume that the original signals S1 and S2 of the combined signal S do not interact in any way when passing the slit, then we get four different sub-signals behind the slit:
S1 becoming Sa1 with phase 0
S2 becoming Sa2 with phase Pi/2

S1 becoming Sb1 with phase 0 + Pi
S2 becoming Sb2 with phase Pi/2 + Pi
This obviously cannot result in 100% destructive interference between Sa and Sb as in the already abovementioned case, where "the phase difference at the slit" is "the only relevant phase difference":
Actually, this situation DOES result in 100% destructive interference. Sa1 cancels Sb1, Sa2 cancels Sb2. It's right there in your numbers. Treating the components separately doesn't change the end result.

Therefore: [b]Fully coherent light (photon groups) is only possible if photons passing the slit (within coherence length) adjust their phase-shifts, or if they already have been coherent before passing the slit.
The mistake that you are making is in thinking that this "adjustment" consists of anything other than BLOCKING the component of the waves which are not already in phase at the slit. The component of the waves which is out of phase doesn't get changed to be in phases, it just interacts with the screen with the slit rather than what's beyond the screen.

But there is no slit in our astronomical case. The components which are not in phase don't get blocked, they aren't absorbed anywhere, they aren't reflected away. There is nothing which makes that light become coherent the way you are envisioning it.
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