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In the double slit experiment, it is important whether a particle is observed or not.

Does observation mean I illuminate my particle actively, making photons interact with it and look what comes back, or can I see it because the particle emits photons?

To describe a macroscopic equivalent:

Is it like seeing a white ball fly by, because I have ambient light, and I would not see it if I switched the light off,

or like seeing a glowing ball fly by, and still see it if I switch the light off?

Volker Siegel
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  • Active illumination, which requires interaction with the particle. (I'm not an expert.) – electronpusher Dec 02 '21 at 18:08
  • "In the double slit experiment, it is important whether I observe a particle or not." It is not important what you or anyone else observes. – my2cts Dec 02 '21 at 18:38
  • Echoing something that anna v. said at the very end of her answer: If it's the kind of quantum-behaved particles that can form an interference pattern on the screen, then there is no way to "detect" them before they reach the screen without destroying the coherence upon which the pattern depends. – Solomon Slow Dec 02 '21 at 20:13
  • @my2cts Right, I did not even think of this meaning of "I observe" as a person - and did not notice that it looks like I mean just that - thanks! – Volker Siegel Dec 02 '21 at 23:09
  • @VolkerSiegel I could have been nicer. What I should have said is that no observation is needed. – my2cts Dec 02 '21 at 23:20
  • @my2cts It was perfectly right, an had a valid reason. There is enough confusion about this in the world, I do not want to add more. – Volker Siegel Dec 02 '21 at 23:23

2 Answers2

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Usually, elementary particles are not detected using light, nor photons in general. They are detected by putting a "detector" on their path. The particle transfers some of its energy to the detector, in some form. In some cases, the particle is able to cross the detector without loosing too much energy and without changing too much its speed; in other cases, it can be completely stopped.

Maybe, the most simple example is the photographic film. Particles with enough energy can generate a dark spot on the film, exactly as photons. In this sense, you "directly see" the particles, without the use of light. Modern digital cameras are in principle able to detect particles passing through their sensors, although I do not suggest to make the test: it could permanently damage the sensors.

Finally: is it possible to "see" the particle using light? Interaction of a single particle with a light beam is possible, in principle, but, practically, it is never used in real detectors.

Doriano Brogioli
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  • Installing a detector and not using its result - is that observing, or not? I would think it is observing. – Volker Siegel Dec 02 '21 at 18:34
  • Yes, it is! The word "observing" comes from a misconception, according to which the wavefunction collapse takes place when a person looks at it. It's a misconception: whenever the particle interacts with something that keeps memory of some of its features, that feature it is "measured", unless we can erase that memory. There is an even more general concept of "implicit measurement", requiring even less. – Doriano Brogioli Dec 02 '21 at 20:32
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The double slit experiment for light or a beam of an accumulation of single particles footprints on a screen

dblslit

Photons or particles of matter (like an electron) produce a wave pattern when two slits are used

This is the double slit experiment for electrons, one at a time, a former picture from this link.

dblslitsinglele

The accumulation of electron footprints from a to e on the "screen" shows the wave nature in the interference pattern.

Their traversal through space to hit the "screen" is not seen, because to see a particle one would have to interact with it and change its direction and coherence with the beam.

anna v
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