1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Would a massive collection of neutrons be invisible?

  1. Feb 26, 2012 #1
    Can accelerating neutrons emit electromagnetic waves? Will neutrons absorb or reflect EM waves? If not, what would a very large collection of neutrons look like? (I am assuming there are no protons/electrons present and the neutrons are not decaying)
     
  2. jcsd
  3. Feb 26, 2012 #2
    This is a great question. I think they would be clear!
     
  4. Feb 27, 2012 #3
    When you say clear, do you mean invisible? Would EM waves pass through the neutron? Or would it somehow deflect the EM waves?
     
  5. Feb 27, 2012 #4

    Drakkith

    User Avatar

    Staff: Mentor

    Neutrons are composed of up and down quarks which are electrically charged, so I would expect them to interact with light similar to how a neutral atom can.
     
  6. Feb 28, 2012 #5
    I'm not sure what you mean - can you clarify? In a neutral atom, it is the electron (lepton) that absorbs and reflects light, not the neutron.
     
  7. Feb 28, 2012 #6

    Drakkith

    User Avatar

    Staff: Mentor

    Not always. High energy gamma rays can interact with nucleons. One result is photodisintigration. http://en.wikipedia.org/wiki/Photodisintegration

    I am unsure how a mass of neutrons would interact with less energetic light though. Although such a mass is probably not possible. Even neutron stars have multiple layers, with the surface thought to be composed of degenerate proton and electrons.
     
  8. Feb 29, 2012 #7

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Boy, that's a huge assumption. The reason that a neutral atom interacts with light is because of the energy levels involved and not just the presence of charge (remember all that stuff that elementary QM tells us?). What sort of energy would you suggest that an EM wave would need in order to interact with quarks?
     
  9. Feb 29, 2012 #8

    cmb

    User Avatar

    My guess would be that if the EM radiation had a wavelength less than 656nm then it would appear much as hydrogen does in a fluid state (of equivalent mass density) as there would be no electron interaction - the energy would be insufficient to cause any electron shell activity.

    At EM energies with wavelength <656nm but >91nm I guess the hydrogen would look a little different as there would be some absorption/re-emission going on.

    Neutrons do have electric dipole and magnetic moments, and not so dissimilar to protons, so I'd guess their 'optical properties' in the X-ray spectrum would be similar to protons?

    All guesses I'm afraid - who's hearded neutrons together before??
     
  10. Feb 29, 2012 #9

    Drakkith

    User Avatar

    Staff: Mentor

    I think it's gamma-ray energies. By "similar" I mean that it can interact with EM waves if they have sufficient energy, just like electron energy levels do. Can't you excite a nucleon or quark with a high energy photon? I thought I remember reading that it happens.
     
  11. Feb 29, 2012 #10
    Well, since neutron consists of oppositely charged quarks (one up quark and two down quarks), plus they are not agglomerated into a blob, there is a dipole moment. Therefore, I would expect interactions in EM fields would generate radiation somehow.

    And I think that neutron stars could be an example, where protons were transformed to neutrons via capturing electrons under immense pressure. Thus they comprise of purely neutrons. And there is pulses due to the rotation up in the sky.
     
  12. Feb 29, 2012 #11

    Drakkith

    User Avatar

    Staff: Mentor

    I don't believe the current thinking is that they are composed purely of neutrons. See here:
    http://en.wikipedia.org/wiki/Neutron_star#Structure
     
  13. Feb 29, 2012 #12
    This assumption changes everything. If the neutrons don't decay, that means they don't interact. The force that forms a neutron star is the weak interaction, among others. You are not considering "real" neutrons then.

    With "real" massive neutron collection it would be similar to neutron star. First of all, they would have temperature, which means they would be glowing. Also the neutrons would form some kind of orbits just like electrons in atoms and they would be jumping over those orbits emitting photons or something (maybe mesons). Also, the neutrons would be decaying into fast electricaly-interacting particles: photons, electrons, protons, which would yield visible Cherenkov radiation if you want to keep your little neutron star at home. This neutron star would be also very dense, so you would see many relativistic fireworks like curved light beams or Doppler light shift. I also suspect that this thing would suck the air from the room and generate accretion disk which is a show on its own.

    You may ask: what happens when everything finally comes to its end, the star will have emitted all its energy and reached the ground state in zero Kelvin. I suspect this one would really be invisible, but I bet some kind of superconductivity would appear in theese conditions.

    So the answer for your question is: no one really knows.
     
  14. Feb 29, 2012 #13
    Sorry, I didn't learn that much, but at least I think there is certain contribution from neutrons to the characteristics of neutron stars, isn't there?
     
  15. Feb 29, 2012 #14

    Drakkith

    User Avatar

    Staff: Mentor

    Uhh, did you look at the article? It has a diagram of the inside of a neutron star right there when you click on the link.
     
  16. Feb 29, 2012 #15
    I mean the pulses that the stars are generating. Although there are great many other particles according to the diagram, but there are enormous amount of neutrons inside as well. Is it possible that those neutrons are also contributing to the EM pulses?
     
  17. Feb 29, 2012 #16

    Drakkith

    User Avatar

    Staff: Mentor

    What pulses are you referring to?

    Edit: The article has this to say:

    Is that what you meant?
     
  18. Feb 29, 2012 #17
    Some rotating neutron stars are emitting EM pulses at poles, right?
     
  19. Feb 29, 2012 #18

    Drakkith

    User Avatar

    Staff: Mentor

    Yes, pulsars do this. Sorry, I edited my post right after posting but I guess I wasn't fast enough.

    What effect the neutrons themselves have on this phenomenon other than interacting gravitationally, I do not know.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Would a massive collection of neutrons be invisible?
  1. Neutron ? (Replies: 6)

Loading...