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Why doesn't light interact with dark matter?

  1. Apr 2, 2012 #1
    Namaste,

    Do we understand why light doesn't interact with dark matter? If a photon were to collide with a dark particle, would they be unaffected?
     
  2. jcsd
  3. Apr 2, 2012 #2

    Cthugha

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    In first approximation anything interacting with electromagnetic fields needs some dipole moment. That means it should not be electrically neutral. When having a closer look even particles which are neutral only on average (think about a simple hydrogen atom for example) may interact with light due to fluctuations which induce temporary dipole moments. However, particles which do not carry any electric charge at all will not interact with light and are thus dark. Neutrinos are a good example for that.
     
  4. Apr 2, 2012 #3
    but ive read some where that neutrinos do .
    even if it has to travel great lengths.
    but probability tells me it could also happen twice in a short distance maybe
    i read that if a neutrino were to go through lead it could go many light years in distance before actually doing so.if this is true then is this not still frequent in the grand size of things?
     
  5. Apr 2, 2012 #4

    phinds

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    Neutrinos have nothing to do with dark matter.
     
  6. Apr 2, 2012 #5

    mfb

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    Neutrinos do interact - but not with photons. They interact via the weak interaction (and gravity), which is the exchange of W- and Z-bosons.
    As the weak interaction is weak, these reactions are very unlikely. A neutrino has a very small probability to interact with an object in its way.

    "If a photon were to collide with a dark particle"
    There is no collision. They just continue to fly in their direction, without any interaction.

    Dark matter does not interact with photons because it is uncharged. Charged particles can be seen, therefore they are not called "dark matter" (which is quite trivial, once you think about it ;) ).
     
  7. Apr 2, 2012 #6

    Drakkith

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    Let's look at another example. One could ask why electrons aren't affected by the strong nuclear force. While I'm sure there's some complicated answer involving all kinds of quantum mechanics, the simple answer is that they simply don't interact via that force. Similarly, dark matter doesn't interact via the electromagnetic force.
     
  8. Apr 4, 2012 #7

    Bobbywhy

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    Namaste. Welcome to Physics Forums!

    Of course light DOES interact with dark matter! Gravitational lensing is the most convincing evidence.

    "That's what Dan Coe and his colleagues at JPL have done in their study of Abell 1689. The cluster's mass bent the surrounding space time and thus the light from even more distant galaxies behind it produced warped and greatly magnified images of those galaxies. Coe and company were able to analyze those distorted images and estimate how much dark matter should be present within the cluster. And the image definitely provides further evidence that dark matter does, indeed, exist; if the gravitational effects were only due to the visible matter, the lensing effect would be much weaker, with less distortion."

    http://news.discovery.com/space/mapping-dark-matter-with-a-cosmic-lens.html
     
    Last edited: Apr 4, 2012
  9. Apr 4, 2012 #8

    Nabeshin

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    Well, this is unfair, since this is a gravitational rather than electromagnetic interaction. Surely the OP is aware that dark matter interacts gravitationally, so the fact that lensing takes place is obvious.
     
  10. Apr 5, 2012 #9

    Bobbywhy

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    I am not presumptuous enough to know “Surely the OP is aware” of anything.

    Two questions were posted in the OP. The first, “Do we understand why light doesn't interact with dark matter?” is not ambiguous. The answer to that question, as in my post #7, is that light does interact with dark matter during the gravitational lensing process. If that was an incorrect answer then I await correction from more educated members here.

    The second question in the OP, “If a photon were to collide with a dark particle, would they be unaffected?” apparently requires a different answer. I suspect that since no one knows the nature of that “dark particle” it is impossible to answer this question definitively.
     
  11. Apr 5, 2012 #10

    Drakkith

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    Except that current theories say that light simply doesn't interact with dark matter at all. So we can say that pretty definitively.
     
  12. Apr 5, 2012 #11

    Bobbywhy

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    Definition: Interact: to act one upon another

    Statement: The curvature of space-time near any gravitating mass (including dark matter) deflects passing rays of light - observably shifting, distorting and magnifying the images of background galaxies.

    Therefore: Light interacts with dark matter.

    Dark matter appears not to interact via the electromagnetic force, and therefore neither emits nor reflects light.
     
  13. Apr 5, 2012 #12

    Drakkith

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    Sorry, I meant "Doesn't interact with dark matter, other than gravity, at all".
     
  14. Apr 6, 2012 #13

    Bobbywhy

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    No problemo! What is important is that OP gets answered...correctly, if at all possible.

    Cheers,
    Bobbywhy
     
  15. Apr 6, 2012 #14
    Question: gluons are electrically neutral. So, a glueball should not scatter light, should it? Or, are there radiative corrections of quark-antiquark virtual pairs, which carry electric charge, that would lead to some sattering?

    Come to think of it, similar corrections for the neutrino due to charged W vector bosons should make the neutrino scatter light, shouldn't they?
     
  16. Apr 8, 2012 #15

    Nabeshin

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    Eh, this is fine. I just don't want the OP to be confused and think dark matter couples to the electromagnetic force, that's all.
     
  17. Apr 9, 2012 #16

    mfb

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    Maybe you can get some dipole moment out of the calculation. Similar for neutrinos. Maybe.
    But I think you would have to do the actual calculations for this. Maybe positive and negative contributions just cancel each other, and the result is 0.
     
  18. Apr 9, 2012 #17

    Chronos

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    The reason it is called dark matter is because it does not appear to interact with photons, ordinary matter or even other dark matter particles.
     
  19. Apr 10, 2012 #18

    Bobbywhy

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    Dark matter DOES interact with photons. In post # 7 of this thread I wrote “Of course light DOES interact with dark matter! Gravitational lensing is the most convincing evidence.”

    In post # 8 Nabeshin remarked that “dark matter interacts gravitationally”

    In my post #9 I reaffirmed the fact that Dark Matter does indeed interact with photons (light rays).

    In post #10 Drakkith took issue with that statement.

    In post #11 I elaborated, defined terms, made a statement of scientific fact, and concluded: “Therefore: Light interacts with dark matter.”

    In post #12 Drakkith corrected his statement and wrote “Sorry, I meant "Doesn't interact with dark matter, other than gravity, at all.”

    In post #15 Nabeshin, referencing my conclusion that dark matter interacts with light in post #11, agreed with my conclusion when he wrote “Eh, this is fine. I just don't want the OP to be confused and think dark matter couples to the electromagnetic force, that's all.”

    Now, this:

    I repeat: Dark Matter interacts with light rays (photons) during gravitational lensing.
    If this is mistaken, will you please correct it with evidence to the contrary?
    If it is accurate, will you please revise your post so as to clearly describe this natural process?

    Thank you, Bobbywhy
     
  20. Apr 10, 2012 #19
    Photons do not interact with anything during gravitational lensing. They follow the geodesics in curved space-time caused by the presence of matter/energy, and we interpret it as their deflection from the original path. But no event of absorption, and re-emission, whether its elastic or inelastic, of a photon took place during such a process. The gravitational interaction is not incorporated in the current scheme of interactions (the Standard Model), simply because it is not treated as a true force, but as a classical theory whose effect looks as a pseudoforce.
     
  21. Apr 10, 2012 #20

    Drakkith

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    Bobby, everyone understands what is meant by "doesn't interact with". There is no reason to get bent out of shape.
     
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