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Before the discovery of dark matter, the prevailing popular understanding of Space content is that of celestial bodies (planets, stars..etc) floating in 'void' and emitting particles and waves (light-photons, electro-magnetic..etc)

Now, if we still assume Space is void, logically why wouldn't particles travel endlessly? That is, a 'solar' flair from Betelgeuse would eventually reach Earth, since there would be nothing that would stop/slow it.

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Bear in mind that the angular diameter of the Earth from Betelgeuse is absolutely tiny (1.2*10^-10 degrees!). As such, unless an event is very tightly collimated, the inverse square law will attenuate the intensity of the radiation dramatically. Which perhaps isn't saying much as supernovae (which I assume you are interested in by reference to Betelgeuse) are very very very very bright. –  Richard Terrett Apr 10 '12 at 9:29
    
Makes me think of another question - would the movement of dark matter in space be highly quantum mechanical? In in order to decohere, there has to be interaction, and DM is notorious for interacting infrequently. Why would this matter? I don't know. This is as far as I've thought it out. But yes, if there's nothing to interact with, nothing interacts. –  AlanSE Apr 10 '12 at 13:19
    
Betelgeuse being an example in the question, yet the information provided about it is very interesting! Position of celestial bodies is very important... –  k.honsali Apr 11 '12 at 20:00
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Quite correct, a solar flare from Betelgeuse will (probably) eventually reach Earth.

Photons of light from Betelgeuse obviously reach Earth otherwise we wouldn't be able to see it. Solar flares contain charged particles, and there is no reason why these won't eventually reach Earth as well. Interstellar space isn't empty, it does contain a very low density of matter, and this can interact with charged particles. See http://en.wikipedia.org/wiki/Interstellar_medium for details. In principle dark matter would interact with the particles generated by solar flares, but in practice the interaction is so weak as to be negligable.

Note that in some places the interstellar medium can get quite concentrated and in fact can be thick enough to block light. These are generally called nebulae, and these will absorb particles from solar flares. However in general a charged particle from a solar flare would travel a very very long way before it hit anything.

Although they probably don't come from solar flares, the Earth is hit by cosmic rays, and some of these are thought to come from distant galaxies. This shows that charged particles can travel for very great distances.

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If we're talking about weak interactions, do you know to what extent the galactic magnetic field would deflect charged particles traversing interstellar space? –  Richard Terrett Apr 10 '12 at 10:32
    
Good question, and the answer is that I don't know. I did the obligatory quick Google and found arxiv.org/abs/1012.2932, which looks interesting though I haven't had a chance to sit down and read it so I don't know if it comments on charged particle motion. –  John Rennie Apr 10 '12 at 10:39
    
I had a quick look: the review cites a field of 6 picotesla near the solar system (cf. earth's field of 25-65 microtesla). I imagine that over large distances this will probably lead to profound deflections of charged particles. The review also notes that charged particles moving through the field will give off synchrotron radiation. –  Richard Terrett Apr 10 '12 at 13:45
    
@RichardTerrett The effects of galactic and intergalactic magnetic fields on cosmic rays are something that people have tried to understand. At medium energies the flux is essentially uniform, but at the very highest energies there is a strong possibility that they point back to their extra-galactic origins. The science, however, is not settled. –  dmckee Apr 10 '12 at 17:19
    
Very interesting explanation & comments. I am studying the indicated references. It is understood that charged particles can indeed 'travel very great distances', but thanks to interstellar medium and local magnetic fields, our environment is not crushed by the giants far away :) –  k.honsali Apr 11 '12 at 20:20
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