As I am reading ** ,the individual photon (ideally I guess) in the double slit experiment is created from any light source (but ideally a laser) that is pointed at barely translucent material - like darkened glass- so that there emerges at the end a very faint amount of light so that it can be described as (probably?) being a quantum of light or a single photon

If the foregoing is more or less correct is it true that the photon in question has emerged from an interaction with the material at the end of the "darkened glass" so that it can be viewed as an expanding sphere of probable locations where it is likely to be detected?

In other words ,whilst the initial beam of light at the source was/is pointing more or less in the direction of one or the other slit,the actual photon that emerges from the apparatus is not pointing anywhere (it has no momentum,I think) and instead shoots off in all directions until it is detected either at the wall or somewhere else (even back into the apparatus or maybe missing the screen)

So,is it impossible to actually aim a photon at the screen even though the apparatus itself does point in that direction?

**I am actually looking at the Feynman lecture (there are 2, I think in Volume 3) on the subject and I don't think he goes into this detail in it.

Edited 21 hours ago21 hr by geordief

11 minutes ago, geordief said:

As I am reading ** ,the individual photon (ideally I guess) in the double slit experiment is created from any light source (but ideally a laser) that is pointed at barely transparent material like darkened glass so that there emerges at the end a very faint amount of light such that it can be described as (probably?) being a quantum of light or a single photon

If the foregoing is more or less correct is it true that the photon in question has emerged from an interaction with the material at the end of the "darkened glass" so that it can be viewed as an expanding sphere of probable locations where it is likely to be detected?

In other words ,whilst the initial beam of light at the source was/is pointing more or less in the direction of one or the other slit,the actual photon that emerges from the apparatus is not pointing anywhere (it has no momentum,I think) and instead shoots off in all directions until it is detected either at the wall or somewhere else (even back into the apparatus or maybe missing the screen)

So,is it impossible to actually aim a photon at the screen even though the apparatus itself does point in that direction?

**I am actually looking at the Feynman lecture (2 I think in Volume 3) on the subject and I don't think he goes into this detail in it.

"A very faint amount of light" is described as relatively few photons, rather than one photon. Intensity is related to the number of photons as well as energy of each photon.

Photons in the double-slit experiment are not "pointed at" one slit or the other; they are just let go off in every direction or a range of directions. Some of them hit the screen, others go through. For those that go through, it's really not possible to say "they go through this or that slit". When photons interact with matter, I'm not sure it makes much sense to speak of this or that individual photon anymore.

It is actually an essential part of the Young experiment that photons not be very high-energy, otherwise difraction would be harder to notice. Lower-energy photons diffract better, and therefore are not "being pointed" at all. High-photons point better than low-energy ones. But this has nothing to do with the faintness of the beam.

A spherical wave does have momentum, only it is not precisely determined. It has a lot of dispersion. You could say the photon is an observable of that spherical wave. These "waves" do not have a little thing inside that you can picture as the individual photon going certain way. Much of it is making peace with the fact that you cannot say what you want to say, as, eg, it doesn't make sense to say that the electron has a smile.

1 hour ago, joigus said:

High-photons point better than low-energy ones. But this has nothing to do with the faintness of the beam

Thanks for your reply.

I am reassured that the photons do not have a direction based on the orientation of the apparatus and that they are "free agents" once they emerge from it in front ot the 2 slits and the barrier.

(I was,at first scratching my head as to why we can't just point them at one or other of the two slits with a fine accuracy but that was apparently based on an initial misconception)

I am still a little unclear as to why high -energy photons would "point better" than low energy ones.

Why would one not see them as expanding spheres which end up traveling in the direction of their first interaction?

(exactly as low-density photons)

Consider two rooms and an open doorway between them. You are in one room, and a friend is in the other. There is no direct line of sight between the two of you. However, although you can't see each other, you can clearly hear each other. That is because sound can propagate around obstacles. This is called diffraction and is a property of waves in general. Because light is also a wave, light also exhibits diffraction. But you can't see your friend, so evidently the light isn't diffracting the way the sound is. That is because diffraction depends on the wavelength of the wave, and the wavelength of visible light is very small compared to the wavelength of audible sound.

I will have to be careful here in answering your last point and great explanation @joigus . The reason being is a large part of the answer will involve the probaility nature described by Joigus who also mentioned the particle number density. The higher the energy as well as the known properties one can define a state used for the particle identity factors such as mass spin energy momentum etc. This is the initial state, the more energy in the beam (intensity as mentioned above) gives us step or ladder operator based on the a classical formula used (likely in your Feymann lectures )

\[E=\hbar \omega\]

for photons the number of quanta derived by that formula is the number of photons.

given the above a higher intensity beam will generate more photons giving a higher probability of photons that will make it through the slit without interference.

The topic of location of a wave often comes up. QM has a rather handy probability wavefunction for this called the Dirac Delta function it acts much like a switch when a hit occurs and in essence gives us relations to the billiard ball like aspects a particle. The wavefunction under graph is a sharp spike to infinity easily (localized) . Other wavefunctions are not easily localized such as the sinusoidal wavefunction. Keep in mind much of the graph of any function will include those probabilities into the graph itself.

once an interaction or measurement is performed the wavefunction changes as you now have more details to better localize a particle. The Dirac Delta function is often used for localization of a particle another closely related is the Heaviside step function.

I only mention these as one can readily pull up images of each and they give a better sense of a localized wavefunction.

cross posted with KJW good answer

8 hours ago, Mordred said:

given the above a higher intensity beam will generate more photons giving a higher probability of photons that will make it through the slit without interference

Is that with the 2 slits open?

By the way I seem to be being disabused of the idea that it is possible to send one photon at a time.Or is it possible but not relevant at this point or based on the question I asked?(considering one photon at a time might seem simpler if that was possible)

4 minutes ago, geordief said:

Is that with the 2 slits open?

By the way I seem to be being disabused of the idea that it is possible to send one photon at a time.Or is it possible but not relevant at this point or based on the question I asked?(considering one photon at a time might seem simpler if that was possible)

It does not matter how many photons are sent out at a time: by moving your source farther away from the screen you can get the intensity at the screen as low as you wish.

Edited 9 hours ago9 hr by Genady

12 minutes ago, Genady said:

It does not matter how many photons are sent out at a time: by moving your source farther away from the screen you can get the intensity at the screen as low as you wish.

Down to one photon?

And you can't achieve that while maintaining a greater proximity to the slit(s)?

I mean ,for simplicity it would be nice to be able to "drop off" a single photon directly in front of slit A so that there would be only the very faintest possibility of it being able to reach slit B (the angle being so tight)

Edited 9 hours ago9 hr by geordief

7 minutes ago, geordief said:

Down to one photon?

And you can't achieve that while maintaining a greater proximity to the slit(s)?

I mean ,for simplicity it would be nice to be able to "drop off" a single photon directly in front of slit A so that there would be only the very faintest possibility of it being able to reach slit B (the angle being so tight)

You know that only one photon passed through because you get only one mark at a time on the second screen, say one mark a minute on average.

If your setup makes it certain in any way which slit a photon went through, there is no interference.

If it makes it almost certain, there is almost no interference.

11 minutes ago, Genady said:

You know that only one photon passed through because you get only one mark at a time on the second screen, say one mark a minute on average.

Yes ,I think I had thought of that.

Is there any point (I expect this has been done) in having detectors on the barrier with the slits in as well?

That way ,if you had one detection at the main detection screen you might also have some multiple at the first barrier alongside the slits?

I am not sure what that would add to the scenario but it might show a little more detail possibly.

Just now, geordief said:

Yes ,I think I had thought of that.

Is there any point (I expect this has been done) in having detectors on the barrier with the slits in as well?

That way ,if you had one detection at the main detection screen you might also have some multiple at the first barrier alongside the slits?

I am not sure what that would add to the scenario but it might show a little more detail possibly.

See my later addition to the previous post.

26 minutes ago, Genady said:

If it makes it almost certain, there is almost no interference

Ah yes,I see it now (your addendum)

It is interesting (and something that I think I did pick up on in the Feynman lecture) that you can apparently get a blend of bullet like detections and an overlay of interference pattern on the detection screen.

I hope I have that right. It seems like an important clarification (to my previous understanding - which was it was 100% one or the other)

Thats an accurate description

2 minutes ago, Mordred said:

Thats an accurate description

Thanks. One less misunderstanding is always good (as if a little bit of "decoherence" occurred in my brain,ironically -leading no doubt to further bouts of more informed incoherence* down the road 😉

*for clarity ,I meant "incoherence" in a personal,psychological sense in this particular case 😉

Edited 7 hours ago7 hr by geordief

12 hours ago, geordief said:

Thanks for your reply.

I am reassured that the photons do not have a direction based on the orientation of the apparatus and that they are "free agents" once they emerge from it in front ot the 2 slits and the barrier.

(I was,at first scratching my head as to why we can't just point them at one or other of the two slits with a fine accuracy but that was apparently based on an initial misconception)

I am still a little unclear as to why high -energy photons would "point better" than low energy ones.

Why would one not see them as expanding spheres which end up traveling in the direction of their first interaction?

(exactly as low-density photons)

The photons do have a direction; you point your laser at the double slit and the photons go in that direction. What you lack is precision in knowing exactly where they go. i.e. you can’t aim them at one slit or the other, because they aren’t localized to that level. But they hit the screen on the wall and not e.g. the ceiling

The energy is associated with how well you can localize the photon. You can shine visible light down a metal tube while microwaves won’t propagate

2 hours ago, geordief said:

Is that with the 2 slits open?

By the way I seem to be being disabused of the idea that it is possible to send one photon at a time.Or is it possible but not relevant at this point or based on the question I asked?(considering one photon at a time might seem simpler if that was possible)

Generating a few photons and waiting for one to go in a particular direction is possible but generating multiple photons and then attenuating the signal is easier.