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Color of sub-atomic particles

  1. Aug 6, 2010 #1

    I am not quite sure if this is a valid question, but I'll give it a shot. What my education managed to teach me about how materials emit or reflect colors is, that it's (usually) related to electrons changing their energy levels. Though there are other causes like bremsstrahlung.
    Thus I was wondering, assumed it would be possible to do in any way, if there is a way of telling what a nugget of some elementary particle would look like.
    Say, if you could overcome the repulsive forces, and create a macroscopic ball of electrons. Or protons, some quark, or any other particle that is not an atom (or nucleus). I know that this is probably highly unlikely to be actually accomplished, but it would be nice if there was a way to tell this from some theory.
    Would such a material emit light? Reflect? What frequency? Or would such a thing just be black and dark on any level, even if bombarded with EM radiation? Maybe fully transparent?

    Thanks in advance for any insight!
  2. jcsd
  3. Aug 9, 2010 #2
    Okay, from the silence I conclude, that this was in fact not a valid question :)
  4. Aug 9, 2010 #3
    Un-confined electrons or other charged particles are called "plasma" and they are opaque and usually glow since they are hot. See the sun or a candle flame or a tube-style light bulb, for example.

    Elementary particles that do not have any electric charge would be invisible; light would just pass through it unaffected. Note that neutrons are made of more elementary units which are charged. But it would be interesting to imagine what neutron-star matter would look like!
  5. Aug 9, 2010 #4


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    Shiny or black, depending on the interaction of any incident light. Shiny if the charges could move without appreciable energy loss like electrons in a metal or black if the losses were high. I suggest that black is the more likely option, if we're just talking classical Physics.
  6. Aug 9, 2010 #5
    JDługosz, is there a way to tell what such a plasma might look like if cooled down enough to not radiate (at least in the visible spectrum)?

    Neutrons are a nice question too! And I guess then, that if someone managed to press a bunch of quarks so tightly together that atoms won't pass through, and make it stable, we'd have THE perfect window glass :eek:)
    But seriously, this was the kind of information I was looking for. If there is any mechanic present other than moving charges that makes interaction with or emits light. It's kind of sad though that we'll never know for real what quarks look like, since by that definition they don't have a look at all.
  7. Aug 10, 2010 #6
    If you cooled it down, it would re-combine into normal gas.

    However, in vast outer space, such clouds exist and may be cold. Although very thin (so it doesn't recombine right away) if the cloud is big enough it would be noticed to be opaque. An astronomy forum might be able to answer with an example.
  8. Aug 10, 2010 #7
    Quarks are charged. A quark-gluon plasma is very very hot indeed! Although brightly glowing, you can still ask what happens to other light that is shined in through it. It would be scattered mixed up and absorbed.
  9. Aug 10, 2010 #8
    I was more thinking of an impossible scenario, in which, for example, cooling plasma wouldn't recombine. More like as if you put Legos together, seen from a distance where you couldn't make out a single brick, but once enough of the same color are put together, you could make out it's red, without the need to put it on fire or move at all.

    I am aware of that this is an entirely hypothetical question, and has no relevance in our reality at all. It's more a gedankenexperiment of mine, with which I got stuck, and deliberately defies a good hand full of physical laws. It would be quite cool if we could say that up-quarks are, say, deeply blue :) (Which we probably couldn't, but anyway.)
  10. Aug 10, 2010 #9


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    lol :wink: the thing is, once you start asking about hypothetical scenarios, it's kind of a meaningless question. The way science works is that you develop a model, check it against experimental evidence, and once you're satisfied that it agrees, then you can use it to answer questions. But if you start ignoring any of the experimental evidence, then you might have a whole bunch of different models that all make different predictions, and you can't tell which one might be right. So for example, one model might say that a large ball of up quarks is blue, and another might say that it's red, and another might say that it's tye-died and has a tiny little picture of a cow on it. There's no way to tell which one of these is "most correct" because all of them are excluded by the experimental observation that you can't have a large ball of up quarks.

    I will say this, though: the main force that governs the motion of quarks is the strong force, which (roughly speaking) acts at much higher energies than the energies of visible light. So the visible color of a material has essentially nothing to do with what kind of quarks make it up. The quarks group themselves into hadrons, the hadrons group themselves into atomic nuclei, the nuclei group themselves along with electrons into atoms, and the atoms group themselves into molecules. It's the molecular structure that mostly determines what something looks like visually. If you had gamma-ray vision, then you might actually see something that relates to the quarks themselves (but then again the universe would look pretty dark).
  11. Aug 10, 2010 #10
    Hehe, I suspected that asking such kind of a question on these boards might fall trapped into the "not possible, not important, not relevant" pot. I was merely wondering, if I would be able to see a very tightly packed 10ccm cube of up-quarks if it stood right in front of me on my desk, or if I might feel it, but is entirely transparent although very dense. (Would it have a refraction index? ...or... I better take that back ;) )
    All assuming, that said cube is not in interaction with any other particles, doesn't combine into larger particles, isn't hot enough to emit anything, etc., but just stays as it is, deviously waiting for me to look at it.

    But you're implying that extremely high frequency radiation could interact with bare particles on this small scale, this is quite interesting! Would a quark be actually able to "reflect" this in the conventional sense, if it were only at a high enough frequency?
  12. Aug 10, 2010 #11


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    I understand your curiosity :wink: but really, you can't get a meaningful scientific answer. What I was trying to say before is that in order to answer your question, you'd have to have some explanation for why it doesn't combine into larger particles or emit thermal radiation or, more likely, explode. The visual appearance of a cube of up quarks could depend on all that stuff.
    Well sure, it happens all the time. Or at least, it's a reasonably common process, although there isn't really that much high-energy radiation floating around.
    I'm not sure about a single quark, but a group of quarks, a.k.a. a hadron, certainly could. Just like electrons occupy energy levels in an atom, quarks occupy energy levels in a hadron - with much larger energies, of course. So high-energy gamma rays interact with hadrons in much the same way that visible light rays interact with atoms and molecules.
  13. Aug 11, 2010 #12
    Then you can make up whatever you like for the color, too. I suggest a sublime shade of yellowish purple.
  14. Aug 11, 2010 #13
    I get it. I really do. My way of thinking was: It should be quite possible to build some amount of nice red balls, that repel each other magnetically, have a sensor in them that makes them blow up in close proximity to another, and have some more ways of ensuring that it won't be possible to put more of them together in close space. But in the end, these balls would still be red, even if not visible from too far away with just a single one looked at. It it weren't for all the mechanisms that prevented them from coming close, it would be quite possible to build a wall of balls, that then are well visibly still red. That I simply tried to think in small, and wondered. Not possible, okay with me.

    But the notion of interaction with appropriately high energies comes quite close to what I had in mind. Not the visible spectrum, but still interaction with electromagnetic forces, which I can perfectly accept. My deduction from that would then be, that IF particles could be put together in my unscientific way, they'd simply be transparent to us, no matter how thick. A (heavily loose) comparison then might be materials that are opaque to infrared, but translucent in the visible spectrum.

    I guess my question is answered as far as it is possible without getting really crazy, and I'd like to leave it at that. Thanks to all of you!
  15. Aug 12, 2010 #14


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    OK, well, I wouldn't agree with your deduction, but if you'd like to leave the topic alone, that's fine. Glad you got some useful information out of this.
  16. Aug 12, 2010 #15
    could i get a neutron to emit photons , i mean a neutron has a magnetic field so could i cause a disturbance in its field and get photon emission , It seems like you could i mean a neutron is made of quarks just like a proton , and protons can emit light .
    And if i shoot an electron over a piece of steel with ridge cut in it , it will start to oscillate
    the electron and get it to emit light .
  17. Aug 12, 2010 #16
    I'll throw in my 2-cents.

    The reason plasma glows is because the excited electrons are emitting photons. There may or may not be any visible light being reflected in that mess, but the amplitude of the light being emitted makes it nearly impossible to see any natural visible light being reflected. The color of the glow is mainly dictated by the element in the plasma state emitting variable wavelengths of light.

    As for the color of other particles, a neutron or proton, for example, it may be possible to discover the answer to this if there were a way to strip the electrons completely off of an atom. If there's no way to do that, then there's no point in even asking the question because the electron(s) would dictate the color of the reflected light.

    Keep in mind, however, that electrons are a negative charge. IF we were able to get through to the protons, which have a positive charge, one might imagine a simple reverse of the visible spectrum. So whatever color you would see on the object normally would be inverted at the proton level.

    Neutrons, however, are a different story. Since neutrons hold no net charge, it could make sense they would reflect back the exact light that was shot in, kind of like a mirror. I'm not sure though.

    Take all this into consideration, if you were to remove the electrons, I think you would have an inverted color reflected back at you. The neutrons would play no role in the changing of the color, so the only thing left to act on would be the protons.
  18. Aug 12, 2010 #17
    you can have protons by themselves in space and they can emit photons ,
    and why wouldn't a neutron play a role in changing the color, all tho a neutron has no charge it has a B field . And a photon is an excitation of a E or B field .
  19. Aug 12, 2010 #18
    True, neutrons would definitely play some role, but I think that role would be minimal and probably not noticeable to the naked eye.
  20. Aug 12, 2010 #19

    How does the direction of the charge "reverse" the color, and what do you mean by reversing a color?

    Anti-matter, i.e. positive electrons, would look just the same. Maybe polarization effects would be reversed, so you could determine whether a chunk of Iceland Spar was anti-matter by looking at the polarization, but in general the color, or the energy level between electron states, would be the same for positrons in the same conditions.

    Now if you looked at a stripped nucleus, and understanding that it's very small so you'd need a large cloud of them or very careful aim, the positive charge won't matter so much as the available energy levels and interaction with the E and B fields. Even with a macroscopic amount of nuclei, I expect two things:

    The individual particles are too small to be affected by visible light. It would be transparent.

    The individual charged particles would be very heavy, so in bulk the loose charges would be disturbed very little, so it would be only very slightly metallic in nature; e.g. shiny and reflective.
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