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Since an electron is smaller than visible light, then what what color would a group of electrons (trillions of electrons) be if there were enough of them to be seen by the eye? What color would a group of trillions of protons be? Color of trillions of neutrons? I don't mean a group of electrons, protons, and neutrons mixed together into atoms, I mean a group of each of them separately. Would they be an actual color (red, black, green, etc), clear but visible (the color of water, glasses's lenses), or invisible?

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    $\begingroup$ see this for true trillions of electrons physics.wisc.edu/museum/Exhibits-2/EM/ElectBeam/… $\endgroup$
    – anna v
    Jun 11, 2016 at 6:08
  • $\begingroup$ @annav it's not color "of electrons" — it's fluorescence of the phosphor placed in the tube. $\endgroup$
    – Ruslan
    Jun 11, 2016 at 10:39
  • $\begingroup$ Well, sovlated electrons are purple. $\endgroup$
    – JDługosz
    Jun 11, 2016 at 10:40
  • $\begingroup$ @Ruslan sure, but that is why one can see them $\endgroup$
    – anna v
    Jun 11, 2016 at 11:02
  • $\begingroup$ In general I feel like a group of electrons would act somewhat like a perfect electrical conductor. Maybe the most perfect one ever. So it would have complete reflection, possibly looking like a metal as @JohnRennie suggested. $\endgroup$
    – M Barbosa
    Jun 11, 2016 at 12:08

3 Answers 3

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There isn't a simple answer to that.

Colour arises when the light absorption or emission of a system is dependent on the wavelength. For example chlorophyll (i.e. plants) is green because it absorbs red and blue light so only the green light is reflected and reaches our eyes. So the question would be how does the light absorption and emission of trillions of electrons etc depend on wavelength?

The problem is that the light absorption and emission of electrons is dependent on their environment. For example an electron in a hydrogen atom absorbs and emits light differently to free electrons. This dependence on environment will also apply to protons and neutrons.

However, as a general rule a gas of charged particles is going to interact with light much as a metal does, so if you can make it dense enough a gas of electrons or protons will look silvery in reflected light. Neutrons aren't charged and their interaction with light is a lot weaker than electrons and protons so at similar densities a neutron gas would be transparent.

As for emission, electrons and protons will emit light due to black body radiation, so the colour of the emitted light will depend on temperature. As you heat the particle gas it will glow first red then yellow then white just as a heated metal does. Again neutrons are the odd one out since they have no charge. To a first approximation a neutron gas will not emit light.

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The 'color' would be an ultra-bright burst of gamma rays as the trillions of electrons rush apart, frying both you and your eyes to a crisp.

More seriously, if you confined the cloud of electrons, it wouldn't emit any particular color on its own -- for instance, there could be no optical transitions since there are no nuclei.

If you shined light on it, it might behave like the electrons in the 'free electron model' of metals. Since the electron response determines the color, I guess it'd be a shiny grey, like typical metals. For more detail, see section 5 here.

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  • $\begingroup$ What would be the color of the group of protons and the group of neutrons? $\endgroup$
    – Schneider
    Jun 11, 2016 at 5:58
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    $\begingroup$ @Lollipop Visible light is made by oscillating charges. Protons are much heavier than electrons and oscillate slower, so I would guess they'd have less effect, making the group mostly transparent. Neutrons are not charged, so they'd definitely be totally transparent. $\endgroup$
    – knzhou
    Jun 11, 2016 at 6:01
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    $\begingroup$ Confined electron gas wouldn't emit anything only if you freeze it to its ground state (zero temperature). Otherwise it'll emit thermal radiation. The confinement itself creates discrete states, so there are certain optical transitions possible. $\endgroup$
    – Ruslan
    Jun 11, 2016 at 10:42
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I cant comment on large groups of electrons, but we do actually have two real world examples of groups of neutrons, and one of groups of protons.

Neutrons

  1. In labs, we have Bose-Einstein condensates. But since they evaporate with the slightest motion or exposure to light, I dont know how we could ever see them.

  2. Possibly more helpful could be neutron stars. I think they have a ~4 inch thick atmosphere of some gas and a crust of iron in the way though, so seeing bare neutrons might still be impossible.

Protons

  1. There is an isotope of hydrogen called protium that has no neutrons. Cationic protium (positively charged protium, ie stripped of the electrons) is nothing but protons. To find out the color of pure protons, just find out what color cationic protium matter is. This is just a conjecture, but, it may be possible that isotope and/or charge is irrelevant to the color of hydrogen, so maybe you just have to look at liquid hydrogen to find out what color protons are.
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