Bunch of electrons - is it visible?

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Discussion Overview

The discussion revolves around the visibility of a bunch of electrons, particularly in the context of whether they could be seen with the naked eye if confined by an electromagnetic field. Participants explore the mechanisms of light emission from electrons, the conditions required for visibility, and comparisons to fictional representations in media.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions if a bunch of electrons could be visible, noting that atoms emit light when electrons transition between energy levels, but wonders if free electrons exhibit similar behavior.
  • Another participant argues that electrons would not be visible unless they were sufficiently dense and confined, suggesting that high energy fields would be necessary for visibility.
  • A different viewpoint suggests that if electrons are confined in orbits by a static electromagnetic field, they could emit photons in the visible range, provided energy is continuously supplied to the system.
  • One participant introduces the concept of synchrotron radiation, stating that accelerating charged particles emit light, which could imply that confined electrons would glow.
  • Concerns are raised about the feasibility of grouping electrons without them accelerating, and the implications of using electrically neutral particles like neutrons in magnetic confinement.
  • Another participant emphasizes the limitations of human vision as a detector, questioning the relevance of visibility in the context of electromagnetic detection methods used in particle accelerators.
  • One participant discusses the Compton scattering of light by electrons as a potential method for detecting them, providing a calculation for the number of photons that could be reflected from a bunch of electrons.
  • Another participant acknowledges the limitations of human vision but reiterates curiosity about what the electrons would look like if visible, including the nature of emitted light.
  • A later reply references reports suggesting the possibility of visualizing single electrons, adding complexity to the discussion.

Areas of Agreement / Disagreement

Participants express a range of views on the visibility of electrons, with no consensus reached. Some argue that visibility is unlikely without specific conditions, while others propose mechanisms that could allow for visibility under certain circumstances.

Contextual Notes

Participants note various assumptions, such as the need for continuous energy input for visible light emission and the challenges of confining electrons without acceleration. The discussion also highlights the limitations of human perception in detecting electromagnetic phenomena.

Nick89
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Suppose we have a bunch of electrons, held together by an electromagnetic field (magnetic bottle kind of thing). Suppose this bunch is large enough to view with the naked eye (say, like a water droplet).

Would it be visible?

If so, what would it look like, and what mechanism makes this happen? Atoms can send out light when electrons jump to a lower energy level. I don't suppose a bunch of sole electrons has this behavior..?

Of course, they can be protons, positrons for that matter.

The reason I'm asking is because of the Angels & Demons movie... They supposedly have a bunch of positrons in a magnetic bottle in the movie, large enough for the naked eye to see. And it looks like a blue fire ball... That doesn't seem very realistic to me. But when a friend asked me what it WOULD look like in real life, I found I had no answer...

Thanks!
 
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I don't think that the electrons would be visible. In a cathode ray tube you have a large number of electrodes flying through vacuum and you cannot see them from the sides. Proof: look at the gap between the glowing cathode and the phosphor screen. For your electrons to be visible you would need them to be fairly dense and some kind of energy levels, I assume the necessary fields would be very high... I don't think anyone has made something like that, and maybe there was even some QED conservation principle that suppresses the absorption of photons by single free electrons.
 
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I imagine the answer would depend on how exactly the electrons were being confined, but we can assume for argument's sake that an appropriately configured static electromagnetic field is capable of confining the electrons. That means that they would be likely spinning around in little orbits - perpendicular to the lines of a magnetic field, for example. Well, as soon as you say "orbits", you have to say "energy levels", so it seems reasonable to me to say that there would be different energy states that the electrons could jump between.

If the confining field were robust enough to contain the electrons even as they absorbed and re-emitted photons, then it seems reasonable that some of these photons might be in the visible range. You'd have to imagine some way for energy to be continually fed into the system, however, just as a mercury vapor lamp need to have a source of power.

Interesting question, though - I'm looking forward to seeing what any real experts (i.e. not me) have to say about it.
 
Oh, wait a minute - what am I thinking?? The electrons would have to be continually accelerated in their circular paths (to be confined) and charged particles always emit light when they are accelerated. So now I'm thinking yes, they would definitely glow.

This is what's called synchrotron radiation in a particle accelerator.

BTW, this is also why the cathode ray example is different - there the electrons are not being accelerated in the tube. There were accelerated as they passed from the anode, but the tubes typically glow there.
 
I like the answers so far! I didn't even think about the electrons not being stationary. If they were accelerating (orbits) then yeah they would radiate. Whether or not this is in the visual range I don't know, but it doesn't really matter for the sake of the question.

Is there no conceivable way (practical or not) to group this many electrons together without them accelerating?

Furthermore, what if we use electrically neutral particles (eg neutrons)? I have read about the possibility of 'storing' neutral particles in a magnetic field, by using their magnetic moment or something. The magnetic field has to be non-linear or something... I can't remember really well, and I'm also not sure if this applies to neutrons. I'm not that into particle physics (yet).
 
Nick89 said:
I like the answers so far! I didn't even think about the electrons not being stationary. If they were accelerating (orbits) then yeah they would radiate. Whether or not this is in the visual range I don't know, but it doesn't really matter for the sake of the question.

Is there no conceivable way (practical or not) to group this many electrons together without them accelerating?
"Stationary" doesn't really work, once you're into the world of Quantum mechanics! The more tightly you confine the electrons, the faster they move around, so yeah, they've got to be accelerating.
 
I'm not sure why being visible to the naked eye is that important, considering how narrow the visible range is. In other words, our eyes are very BAD detectors as far as range of electromagnetic spectrum that it can detect. So using a bad detector as the de facto standard to observe anything is rather dubious.

I work at a particle accelerator that accelerates bunches of electrons, ranging from 0.5 nC per bunch, all the way to 110 nC per bunch (the latter is the current world's record, as far as we know, for single bunch of electrons being accelerated in an L-band structure). I can tell you that even at 1 nC per bunch confined to a bunch length of barely a couple of mm and bunch diameter of about 0.5 cm, there is a huge amount of space charge involved that causes the bunches to want to diverge. So having such a thing being "stationary" is out of the question.

How do we detected them, i.e. how do I know I have 1 nC, 10 nC, 110 nC, etc? We detect them when they pass through a ceramic break in the accelerator beamline that is equipped with something called an ICT - integrated charge transformer. It is a fancy coil that operates via Faraday's law of induction, except this one integrates the total charge in each bunch. This means that I don't have to see the charge directly with my bad eye detector to detect it and to measure it.

Zz.
 
Outside of synchrotron radiation, which occurs at many MeV, the best way to see say 1 nC (1 nanocoulomb) (6 x 109 electrons) of low energy electrons in 1 cubic cm is to shine a bright flashlight at them, and look at the reflected "Compton scattered" light. Suppose we had a flashlight that could shine 1000 watts of 2 eV (red) photons (3 x 1021 photons/sec) into 1 cm2. The Compton cross section is about 8 pi/3 r02 (where r0 is the classical electron radius) = 0.66 x 10-24 cm2. So the electrons will Compton scatter
N = (6 x 109 electrons)(3 x 1021 photons/cm2 sec) (0.66 x 10-24 cm2) = 1 x 107 photons/sec into 4 pi steradians. This might be enough "reflected" photons to "see" 1 nC of electrons.
 
I realize the human eye is a very bad detector when it comes to the large EM spectrum. I was merely wondering if we (as humans) would be able to see the electrons floating around, and what they would look like. Of course they can be detected easily using other methods, but that wasn't really what the question was about.

So, so far we have emitted EM radiation due to their acceleration, and due to Compton scattering. How much of this can we expect in the visible range? And what would we see? Merely a sort of hazy light? Multiple colors, or only a small band of wavelengths?
 

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