In Wikipedia's description of the Observer Effect wrt particle physics, we have this:-

For an electron to become detectable, a photon must first interact with it, and this interaction will inevitably change the path of that electron.

Surely a photon is not the only way to detect an electron. Photon emission means that we might actually see the electron's presence (with a magnifying glass or something). But electrons have charge and interact with other charged thingies, so they could be detected indirectly through other means couldn't they? And that's not even getting into detection via the electron's mass or momentum.

Is this just an example of sloppy language, or are photons always involved even if electrons interact with other +/- charges?

  • 4
    $\begingroup$ "But electrons have charge and interact with other charged thingies" - Quick question: how do thingies with (electric) charge interact with other charged thingies? $\endgroup$ Dec 19 '17 at 3:48
  • $\begingroup$ @Alfred Centauri - Is the electrostatic interaction between an electron and another charged particle also mediated by "photons"? $\endgroup$
    – freecharly
    Dec 19 '17 at 4:15
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    $\begingroup$ @freecharly - yes. $\endgroup$
    – Prahar
    Dec 19 '17 at 6:14
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    $\begingroup$ @Prahar Thingies aren't just particles. What about electrical fields? A charged plate can deflect an electron beam as in a TV. Where are the bending photons (other than coming off the phosphor)? $\endgroup$
    – Paul Uszak
    Dec 19 '17 at 12:39
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    $\begingroup$ Paul, QED is the quantum description of the electromagnetic field and its interaction with electrically charged fundamental particles. The electromagnetic field quanta are photons and the interaction of charged particles with the electromagnetic field is in terms of , e.g., electron-photon interaction. Yes, even a static electric (and/or magnetic) field has a photon description. See, for example, Virtual photon description of B and E fields $\endgroup$ Dec 19 '17 at 16:30

The language is sloppy, but the sentence right after the one you cited actually does mention: "It is also possible for other, less direct means of measurement to affect the electron." The idea the article is trying to get at is that detection requires interaction, and interaction means changing some properties of the object you are trying to detect.


It is correct for the mathematical models used in particle physics studies. Because the electron interacts electromagnetically, and there are not only the real photons but also the virtual photons


which transfer the momentum and the quantum numbers in the interaction described by integrals over the variables. So conceptually there is always a photon intermediary in any everyday interaction of an electron. ( it also interacts with the weak and the gravitational but these are very weak effects, outside our measuring capabilities). An electron interacting with an electron , (when we touch something for example it is all electron interactions,) will exchange a virtual photon as in the diagram above.

Now the "change in path" is misleading . In detectors high energy electrons are identified by their energy loss in calorimeters. Low energy electrons are seen in bubble chambers:


The curly line was produced by an electron that was struck by one of twelve passing beam particles in a liquid hydrogen bubble chamber. It curves in an applied magnetic field and loses energy rapidly, spiraling inwards.

A virtual photon is involved in the "strike" , and a multitude of virtual photons kick off the electron from hydrogen atoms, little energy is lost in each hit because the ionisation energy of hydrogen is ~13eV and the electron has some hundreds of keV, these create the small bubbles of the path.

"it is all photons , all the way down"

spoof from " it is all turtles , all the way down"


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