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Bunch of electrons - is it visible?

  1. May 31, 2009 #1
    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...

  2. jcsd
  3. Jun 2, 2009 #2
    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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  4. Jun 2, 2009 #3
    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.
  5. Jun 2, 2009 #4
    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.
  6. Jun 2, 2009 #5
    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).
  7. Jun 2, 2009 #6
    "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.
  8. Jun 2, 2009 #7


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    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.

  9. Jun 2, 2009 #8
    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.
  10. Jun 3, 2009 #9
    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?
  11. Jun 3, 2009 #10


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