# B Interference pattern, one particle at a time

1. Feb 6, 2017

### MindWalk

[Mentor's note: Split off from another thread because it's a different question]
I have a question about the single-photon double-slit experiment's results that isn't about the role of consciousness. Should I go ahead and ask it here or ask it elsewhere? (Briefly: If the results are lots of dots accumulating into what looks for all the world like an interference pattern, how is that equivalent to our seeing a lot of tiny individual interference patterns, which is what I'd expect were individual photons interfering with themselves?)

2. Feb 6, 2017

### phinds

? What does that mean? How would these "tiny individual ... " be arranged?

3. Feb 7, 2017

### m k

It isn't.
You can't see individual photons interfering only calculate.

And you do it by adding waves, single also, sort of.
That's the only way to get the answer as you see it, lots of dots as they appear.

4. Feb 8, 2017

### MindWalk

OK, here's my question. I frequently hear or read physicists say that the individual photons in the double-slit experiment interfere with themselves. But the individual photons strike the screen as individual dots, not as tiny interference patterns, don't they? And that means that what we see is an accumulation of dots that builds up a pattern looking for all the world like an interference pattern. That doesn't look the same as what I'd expect to see were the individual photons' interfering with themselves, which would be not dots but tiny interference patterns (one for each photon). Now, it may well be that a mathematical description of those dots' building up into what looks for all the world like an interference pattern and a mathematical description of photons' interfering with themselves would look the same--possibly because a probabilistic description of an ensemble and a probabilistic description applied to individuals looks the same?--but when it comes to interpreting what is happening, the "interfering with themselves" view sure looks unjustified, from my admittedly limited knowledge. Why is this said so often, then?

5. Feb 8, 2017

### Staff: Mentor

Yes and yes... But a photon has no position at all (that's "no position"! Not "it has a position but we don't know what it is", but "has no position" the same way that I have no lap when I'm not sitting down) until it interacts with the grain of light-sensitive material at a particular spot on the screen; and when it does that grain and only that grain will be affected. That's why you get individual dots on the screen even though the entire screen is being illuminated. The photon always delivers all of its energy to a single point, namely the point where it was detected.

To reconcile this picture with the notion that the photon "interferes with itself", you will want to read Richard Feynman's (math-free, layman-friendly) book "QED: The strange theory of light and matter".

6. Feb 8, 2017

### MindWalk

As to a photon's "having no position" (I'll get to the rest after this is cleared up).... It is unclear to me what you mean. Here's my guess as to what you mean: the photon is a *something*--wavicle, vibrating string, something not well described as a particle but which, when observed, sometimes yields results suggesting waveness and sometimes yields results suggesting particleness--but when whatever it is interacts with the grain of light-sensitive material at a particular spot on the screen, then you get a dot.

7. Feb 8, 2017

### Mentz114

In QFT one speaks of a beam of photons with occupation $1..n$ photons. What makes it a beam is that the photon(s) have momentum $\vec{{k}}$.
In no sense is anything moving, but in an interaction energy $\hbar \omega$ and momentum $\vec{{k}}$ is available. This is an oversimplification, it could be more subtle ...

8. Feb 9, 2017

### m k

No, there is no "sometimes."
It's always a dot.

Problem was the combination of those dots.
You can't handle that without waves.

Other thing is that between start and end you can't get anything out of those dots but at the same time you know it is more than nothing.
Some(many) call that no position, others(same) superposition and so on.
But for what ever you still need those waves.
And get dots.

9. Feb 10, 2017

### bhobba

QM is a theory about observations (with observation having a very general meaning that can be made precise in QM - it does not, repeat does not, require a conscious observer) that occur in an assumed common sense classical world.

What's going on when not observed the theory is silent about. And that silence includes if the particle has position or not.

If you want a much better explanation of the double slit see:
http://cds.cern.ch/record/1024152/files/0703126.pdf

Thanks
Bill

10. Feb 10, 2017

### bhobba

Wrong.

See the link I gave previously.

What the double slit demonstrates is two basic principles of QM:

1. The Heisenberg uncertainty principle.

2. The practical application of the principle of superposition.

The wave-particle duality usually invoked to explain it is seen as not correct as you get more advanced. Its OK to start with, but strictly speaking its wrong.

Even the explanation in the paper I gave above is wrong:
https://arxiv.org/pdf/1009.2408.pdf

But slowly grasshopper, slowly - full understanding of QM is a slow step by step process with many things at the start changed as you progress. QM is bad like that, but QFT is even worse - so bad we have professors that teach QFT, post here with the correct answers, and they are not believed because some pop-sci or beginner text says otherwise - even professionals can get a bit confused about it (but they have the excuse they write for other professionals who know what they mean) - amazing but true.

Thanks
Bill

11. Feb 11, 2017

### m k

My bad.
Never thought OP as a student.

12. Feb 11, 2017

### MindWalk

Of course "sometimes." You run one kind of experiment, you get results that look like the results you'd expect were photons particles. You run another kind of experiment, you get results that look like the results you'd expect were photons waves. That's where the particle-wave duality originates.

13. Feb 11, 2017

### phinds

As bhobba has already pointed out to you, "wave particle duality" is the wrong way to look at things, really. It's still used in pop-sci presentations but not in actual physics texts. It was dropped about 90 years ago.

14. Feb 11, 2017

### MindWalk

<Sigh> Are you saying that the results of experiments involving photons never appear as one would expect were photons particles (like, say, dots on a screen) and never appear as one would expect were photons waves (like, say, diffraction patterns)?
By "wave/particle duality," I do not have in mind that the photon *is* a particle and *is* a wave. I only have in mind the dual sorts of experimental results, so that the photon must be something that can result in such observations.

15. Feb 11, 2017

### MindWalk

Anyway, I'm just trying to understand Nugatory's meaning when he says the photon has no position. Clearly, it has position when it interacts with the grain of light-sensitive material at a particular location on the screen. I do not mean to say that it has a definite position before then. It is possible that what Nugatory intends is that the photon, *whatever kind of thing it is*, is capable of somehow delivering all of its energy to a specific location, just as though it were a particle interacting with another particle at that location, even though before that, its energy cannot be assigned a specific location (as a particle's could). Is that what you mean, Nugatory?
Then my question remains that of how this counts as "interfering with itself." If it is necessary to look up the Feynman book, I will. Can this be explained in a forum like this one?

16. Feb 11, 2017

### rootone

It is, it's a quantum object.

17. Feb 11, 2017

### phinds

Unfortunately for you then, "wave particle duality" has a very specific meaning in physics and it's not your self-made definition so using it to communicate with other is not a good idea.

18. Feb 11, 2017

### phinds

Then you are not in conflict. He's just saying that absent any interaction, it has no specified location.

19. Feb 11, 2017

### Staff: Mentor

Not really, because this interaction destroys the photon. What has position is the grain of light-sensitive material.

20. Feb 11, 2017

### Staff: Mentor

I think you have to be careful here. As I noted in my previous post just now, this interaction destroys the photon (it is absorbed by the light-sensitive material), so I don't think it's correct to attribute the position of the light flash to the photon; it's the position of the grain that absorbs the photon.

This is a better way to say it, because it doesn't require you to say that this point is "the position of the photon". It's just the position at which the energy is delivered (by the photon being absorbed and therefore destroyed).

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