B Double Slit Experiment: Dumb question that needs to be asked

[kered rettop]

I understand the physics just fine, thanks. If you guys think "any possible measurement" is clear enough then so be it.

 

[sillyputty]

Perhaps a more interpretation free approach: Instead of saying "You can only have the double-slit interference patterns (with full contrast), if it is impossible to gain which-way information by any possible measurement on the photons." it might be better to say "Measurements which yield which-way information will not contribute to a full-contrast interference pattern."

Seems cleaner to get rid of the measurement term and especially the which-way info term. This seems to work:
The interference pattern always occurs but we see it only when it is enhanced enough to notice it.

 

[PeterDonis]

The interference pattern always occurs

But it doesn't. If you have polarizers in orthogonal directions at the two slits, there is no interference pattern at all.

 

[sillyputty]

That's not true.
Not only do you get IP, but you get 2 of them.

 

[PeterDonis]

That's not true.
Not only do you get IP, but you get 2 of them.

What specific experiment do you have in mind? Can you give a reference?

 

[sillyputty]

I'm not sure if it was you or someone else here, who provided the experiment .You were talking about a simple double slit, with 1 slit blocked and you stated that there is still 1 slit diffraction that occurs (the OP was thinking that blocking a slit causes the pattern to be completely gone and change into as if little stones were shot through that one slit, no fringes at all). I know that polarizers over slits (oriented perpendicular to each other), means certain photons are blocked at one slit, and are let to pass at the other. So (unless there is something that makes this special and different) this polarizer experiment would also produce fringes for each slit, albeit perhaps less noticeable (than the prior example) because not all that 'head' to that slit make it through.

 

[Nugatory]

The interference pattern always occurs but we see it only when it is enhanced enough to notice it.

But it doesn't occur at all unless paths through slits are available to the photon, which is to say that there is no which-way information. If that which-way information is available we get only the single-slit diffraction pattern at each open slit.

 

[PeterDonis]

You were talking about a simple double slit, with 1 slit blocked and you stated that there is still 1 slit diffraction that occurs

There was a recent thread about that (and I was not the only one posting clarifications there along the lines you describe), but one of the points made in that thread is that a diffraction pattern (what you get with one slit open, or with both slits open but orthogonal polarizers at each slit, as @Nugatory describes) is different from an interference pattern (what you get with both slits open and no polarizers anywhere).

 

[sillyputty]

But it doesn't occur at all unless paths through slits are available to the photon, which is to say that there is no which-way information. If that which-way information is available we get only the single-slit diffraction pattern at each open slit.

Yes that's what I am saying (single slit x2)

 

[sillyputty]

There was a recent thread about that (and I was not the only one posting clarifications there along the lines you describe), but one of the points made in that thread is that a diffraction pattern (what you get with one slit open, or with both slits open but orthogonal polarizers at each slit, as @Nugatory describes) is different from an interference pattern (what you get with both slits open and no polarizers anywhere).

I see what the snag is. You are thinking of a diffraction pattern as a completely independent separate phenomenon to an IP. I don't think of it as separate.

 

[PeterDonis]

I don't think of it as separate.

In other words, you're ignoring the actual physics. The whole point is that the pattern you get with orthogonal polarizers at each slit (i.e., "which-way information") is different from the pattern you get with no polarizers. That's why we call the first a "diffraction pattern" (or two of them, one for each slit) and the second an "interference pattern". Thinking of them as "the same" is a misrepresentation of the physics.