[jackdawjack]

The usual energy loss mechanism for cosmic-rays (fast protons) is reverse-compton scattering off the cosmic-microwave background photons, which is quite amazing really. Some of the most energetic particles scattering off the ubiquitous but very low energy background.

Anyway, if you propose that this is the mechanism for energy loss and you know how dense the CMB photons are (which we do very well) then you can predict interesting things like the maximum distance a cosmic ray can travel before it runs out of steam entirely, see

http://en.wikipedia.org/wiki/Greisen–Zatsepin–Kuzmin_limit

Although i'm not sure how the OMG particle fits in with this scheme yet

[Groxx]

I was looking at that too... the trick that I see is that that's a statistical limit, not a "travel X miles, lose X% energy, no matter what" limit. It's still entirely possible for a particle to be over the limit, it's just highly unlikely.

We detect super-energetic particles frequently, but only a couple "over the limit". Seems to fit the statistical model to me.