# Imaging of Light as a Particle and a Wave

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## Main Question or Discussion Point

In this article, researchers have used electrons to image light as both particle and wave at the same time.

https://phys.org/news/2015-03-particle.html

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Orodruin
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tech99
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If we use an electron as a sensor, it will respond to reactive as well as radiated fields. It appears to react to the evanescent fields around thr wire. So is it acting as a true sensor for photons?

vanhees71
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It's soooooooo sad! Planck once said that old-fashioned theories in physics die out by people believing in them passing away. That seems not the case with the wrong pictures given in the introductory lectures and in popular-science writing.

What's indeed shown by this experiment is that plasmon modes of the em. field are quantized. That's all. There's no particle picture for photons proven ever. Photons cannot even by localized, because they do not admit the definition of a proper position operator to begin with. This is true even for free photons, let alone for cavity photons or plasmon modes, as investigated here. The Nature article is pretty clear on what's measured (and it's open access!).

Orodruin
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Planck once said that old-fashioned theories in physics die out by people believing in them passing away.
What is the $R_0$ of belief in wave-particle duality? Can it be properly treated or vaccinated against?

vanhees71
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It can be properly treated by just teaching modern quantum theory right away. In the next semester I've to teach my teachers students quantum mechanics, and I'll start my manuscript with a longer qualitative chapter on this didactical problem. I think, it's high time to make particularly future teachers aware of all the bad didactics in the introductory chapters of textbooks.

Of course, I surveyed the existing (German) literature on "Quantum Mechanics for teachers students", and I was shocked that the books provided utterly wrong pictures about quantum theory. Particularly all start with photons, which is the most complicated real-world quantum you can discuss. Allready free photons cannot be adequately discussed without a quite thorough analysis of the Poincare group, and unfortunately there's no time to teach this in this three-semester theory course. Nevertheless, on a qualitative level, one can describe all the modern single-photon experiments correctly without "drawing" the wrong picture of photons as "pointlike particles". One of the textbooks (there's even an English translation: it's Pade, Quantum Mechanics for Pedestrians, Springer 2014, 2 vols) which in principle is a very nice book, discussing modern applications of quantum theory on a level, which is well suited for this target group of students, using discrete-observable examples like spin and polarization states particles or photons etc., discussing Bell inequalities, entanglement, and all that. Hoever, I cannot recommend this book without a bad consciousness, because it's all wrong in its qualitative explanations. Rather than being careful to explain photons in a way of what's really observed and what comes out from the proper analysis of the Poincare group (which of course you cannot give to this target group; it's usually not even fully treated in BSc/MS lectures on QED, because you'd need an entire semester for the mathematics of the Poincare group rather than discussing real physics problems). On this qualitative level you can however still explain it correctly by sticking to what's observed and then just telling the students what comes out of the proper treatment within modern QED.

PeroK
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Maybe it's just the difference between science and journalism. It seems strange to me, having learned QM without the wave-particle duality, that it's still strongly promoted, apparently.

Perhaps it's part of "selling" the subject. Like the Scottish Tourist Board promoting the Loch Ness Monster!

vanhees71
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It's not strongly promoted anymore. It has been shown over 90 years ago that there's no wave-particle duality but a consistent mathematical description of quantum phenomena that so far withstood the hardest tests by experiment ever. That's why I find it so sad that people seem to think they still have to use outdated theories to introduce the subject to students. I'm not so much talking about science journalists, who are excused a bit, because they have to write in a way to generate buyers of their newspaper and clicks on their online sites. One must not expect too reliable information from science journalists. Often I don't understand at all what they want to say, and I have to go to the scientific publications they write about to understand it.

What I'm really angry about is that new textbooks are written, particularly with an emphasis in didacticts in the sense to simplify the theory such that physics-minor students can easily understand the essence, which are plain wrong in the picture they convey about QT. It's the more sad, because that's not even necessary to make the subject simple enough!

E.g. you can explain the Mach-Zehnder experiment with single photons without ever relying on a naive and plain wrong particle picture of photons. Even emphasizing the localizability of photons which is impossible from first principles mathematically in such books is a sin!

@vanhees71, how would you classify Weinberg's books? They are good in qualitative explanations?

vanhees71