I have seen some contributors (e.g. annaV) state that em-wawes are built up by coherent photons. Now imagine 2 identical thin laser beams (same intensity, same source using a beam splitter and prisms) that meet head-on. The Mie-scattering from a few microscopic particles in their common path make the beams visible. Now adjust the phase of one beam so that it is exactly opposite the other. This will make the beams more or less invisible. So what happened to the (stream of) coherent photons and their energy/momentum. Did they collide, annihilate or change into something else ? And when reajusting the phase, so that the beams are in sync again, everything is back to normal (the photons reappear)! I also wonder if the coherent photons are inseperable from the wawe contrary to what others postulate, namely that photons and em-wawes are two different worlds that are mutually exclusive (Accidental Fourier Transform).
1 Answers
This will make the beams more or less invisible.
Here is an MIT course video showing destructive interference using beam splitters and mirrors from one laser source.
Here is the set up, at the point where it is arranged that the beam on the left is the one returning to the source.
From a larger distance interference fringes appear
Here are the two beams , the one returning to the source(left) and the one coming from the source (right). The interferences are complementary.
bringing the mirror closer concentrates on the central spot
The instructor ( Shaoul Ezekiel) presses on the table to change the wavelengths, and force total destructive on the right,
and then total destructive on the left
Please note that I am making screen shots of the video. I advise you to watch it.
So what happened to the (stream of) coherent photons and their energy/momentum.
The photons are back in the source. It is not only the crystal (or lasing matter) of the laser that is in a quantum mechanical state of emitting and absorbing photons . Once a setup with lasers is constructed the whole is in a quantum mechanical state with the source (or sources, if there is more than one laser). The photons that go back to the crystal reduce the energy needed to excite the laser for that delta(t).
Did they collide, annihilate or change into something else ?
Optical range photons cannot collide measurably since the two photon interaction is very very small. They are in a superposition in making up the classical electromagnetic wave.
One should stress that the destructive and constructive interference patterns are local , when there is destructive interference in the outgoing beam there is constructive in the return to the source, but also there are always rings of destructive and constructive, the beam does not disappear as a whole.
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