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I've read several QA here: Is coherent light required for interference in Young's double slit experiment?

Why can't we duplicate the double slit experiment with two parrallel sources of light.. why must the light enter the two slits externally?

Why don't two identical monochromatic lamps form interference patterns?

Why can't we duplicate the double slit experiment with two parrallel sources of light.. why must the light enter the two slits externally?

From answers and comments I could not see definite answer if photon "interfere" only with itself, somewhere it was claimed but mostly discussion was around coherence requirement, I do not recall experiments where slits were separated by a wall. Were such experiments done with a wall and synchronized lasers something? TIA

Web search for "double slits experiment slits isolated by a wall" found no relevant top results, "separated slits" finds separation by distance variations. Making a "wall" at home is not complex, making two lasers coherent is not trivial to test for myself.

I expect photons to be out-of-phase with each other from a laser and, in one replaces them with classical waves there will be no clear ridges pattern, but I do not know exactly how lasers work, maybe lasers can create in-phase radiation. Or maybe there are more factors in play.

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Forget photons: interference is a wave phenomenon. It predicts where you will detect photons, but it does not involve photons.

You can do this experiment by tuning in to weak stations at night (to get better range) using an AM radio. The sound will generally "flutter". That's two or more stations on the channel producing a moving interference pattern. The "carrier waves" are coherent, but they are not locked in phase, and have slightly different frequencies.

The same thing happens (in principle) with two lasers, but they must either be extremely coherent and tuned almost to the same frequency, or they must be extremely bright so you can do the experiment very quickly. The problem is that in a practical experiment, while the interference is present, the fringes move so rapidly that they are difficult to detect.

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  • $\begingroup$ AM is interesting idea, as for the doubt-slit, e.g. personal.math.ubc.ca/~cass/courses/m309-03a/m309-projects/fun/… shows where ridge is, but if not same photon, then 1st slit is out-of-phase (even same length results in arbitrary phase difference) with the other, is it not? And so ridge position will shift each time. $\endgroup$
    – Martian2020
    Commented Nov 10, 2022 at 2:44
  • $\begingroup$ To understand AM radio, I think I need to understand deeply how receiving electronics work. Fact that sound loudness varies is not by itself proves individual photons (long wave ones here) interfere with each other. $\endgroup$
    – Martian2020
    Commented Nov 10, 2022 at 2:53
  • $\begingroup$ @Martian2020 AM radio is well modeled by classical waves, no photons are apparent. At shorter wavelengths, photons are detectable, but it is a mistake to think of waves as composed of photons in flight: it will lead you to confusion and incorrect reasoning. My friends who design x-ray diffraction gratings use classical electrodynamics. "Looks like antenna theory to me." But photons show up at my detectors. The intensity of the waves predicts where the photons will appear. $\endgroup$
    – John Doty
    Commented Nov 10, 2022 at 13:21
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Let's say we have a red laser and a green laser, pointed at the same area on a color film.

Now a red dot on the film is produced by a red photon, which is produced partly (99.9999%) by the red laser, and partly (0.0001%) by the green laser.

This photon's probability of making a dot on a film varies in space, because at some points the photon is coherent (the two parts have the same phase) and at some points the photon is anti-coherent (the two parts have the opposite phase)

Now we say that red and green lasers do not produce an interference pattern, because it is very hard to detect the 0.00001% variation on density of dots. And also because we consider it an "error" when a green laser produces a red photon.

Now maybe a green laser produces a red photon only once a day, and a red laser produces million red photons a day. So then that is the reason why each red photon produced by these two lasers is partly (0.00001%) produced by the green laser, and partly (0.99999%) produced by the red laser.

The above story is an answer to the question, if you think about it carefully enough. I just left out all unnecessary stuff, like the wall and the slits.

Of yes, interference pattern of two lasers pointed at a screen has been observed. Those lasers were made as identical as possible.

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  • $\begingroup$ started to think, as for experiment "interference pattern of two lasers pointed at a screen has been observed" - each was pointed at its only one slit of the slit pair and slits were isolated to make sure light from one laser could not get to the other slit? do you know how to find it? $\endgroup$
    – Martian2020
    Commented Nov 10, 2022 at 12:29
  • $\begingroup$ "because at some points the photon is coherent (the two parts have the same phase)"..." I just left out all unnecessary stuff, like the wall and the slits." if there are no slits, what two parts are you talking about? $\endgroup$
    – Martian2020
    Commented Nov 10, 2022 at 12:33
  • $\begingroup$ @Martian2020 see my answer please. There is no pattern for separate lasers on separate slits. $\endgroup$
    – PhysicsDave
    Commented Nov 11, 2022 at 21:22
  • $\begingroup$ @Martian2020 u.arizona.edu/~jiefeiw/final.htm $\endgroup$
    – stuffu
    Commented Dec 26, 2022 at 17:57
  • $\begingroup$ @PhysicsDave u.arizona.edu/~jiefeiw/final.htm $\endgroup$
    – stuffu
    Commented Dec 26, 2022 at 17:58
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The saying “interferes with itself” is historical and misleading … the modern way is to say “each photon determines its own path”.

The photon is actually coherent with the apparatus and that it why it can show up on the screen. Similar photons of similar coherence are also present on the screen. For a non laser source many photons are rejected and are actually reflected at the slit.

It can even be said that the apparatus setup/ geometry influences the production of the photon … i.e. photons will generate from a close source that fit the geometry, similar to a laser cavity.

The best theory is the Feynman path integral. By summing phases of each possible path we get it’s probability. It turns out that paths with lengths of integer wavelengths are most possible.

The excited electron in the atom is already working with the em field even before the photon is emitted …. This a possible reason why the photons path is determined.

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  • $\begingroup$ Thanks for "No" answer in the comment. "The photon is actually coherent with the apparatus" - did you mean "entangled"? Anyway from you answer I do not see why photons do not influence each other paths (if that is the reason for no ridges). $\endgroup$
    – Martian2020
    Commented Nov 13, 2022 at 8:46
  • $\begingroup$ @Martian2020 you could say entangled, yes. …. Although the most common use is when 2 photons are created simultaneously and have opposite polarizations, process called parametric downconversion. $\endgroup$
    – PhysicsDave
    Commented Nov 14, 2022 at 17:27
  • $\begingroup$ The excited electron is generating forces, the only physics explanation for ‘‘em forces” is called virtual photons … there is NO energy exchange, only real photons transfer energy. We can say the em field has many parts or forces just for one excited electron …. That is why one of the answers refers to “2 parts” but it could be many parts per Feynman ( my preference), all parts virtual …. no real photons until virtual forces are happy! $\endgroup$
    – PhysicsDave
    Commented Nov 14, 2022 at 17:35

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