The hydrogen-ion speed limit

Courtesy HBDChick, Ethan Siegel posts at Forbes a fine and informative article about the speedlimits for matter in intergalactic space.

Light in a pure vacuum has a speed: C, which is the limit for everything. Between galaxies, matter is so sparse that a particle traveling through it might not encounter another particle. Such might evade the particles in our own galaxy long enough to hit us down here - such is a Cosmic Ray. We've been talking about dodging those Cosmic Rays in the fortnight-long night of our Moon, where metals breed Bremsstrahlung. Since their speed is boring, some ridiculous fraction of lightspeed C, and since we've been assuming a rest mass, which is that of a proton, these 'rays are measured in energy, by the electron-volt.

Siegel explains that a cosmic proton should own a speedlimit meaning an energylimit. When such go through intergalactic space, not hitting more protons, they swim against a tide, a tide of photons-with-an-h. Between galaxies you can see other galaxies, by their photons. And the Cosmic Microwave Background bathes us all. A proton-with-a-r will hit photons, even low-frequency photons, and, if the proton is going fast enough, this interaction bleeds energy. At 5 × 10¹⁹ electron-volts, the proton hitting light should be spawning off some pions which are a major drain for it. Pions then decay into other energetic particles and of course much more light, but the proton doesn't care; it cares it's not exceeding the limit.

As the universe expands we can expect the photon count to lower and those photons left over to lower their frequencies, so this speed limit will increase. By extension, as it were, the speed limit is higher near us than it is - was - out by the most distant and visible-youngest galaxies.

Anyway we've been seeing some 'rays already in the 10²⁰ eV range, down here. Since they are coming from all over, like comets, that rules out protons from, say, the Sagittarius black-hole or the nearest galaxies which haven't had the time to lose energy to pions.

Siegel says the strongest 'rays must not be protons, by themselves; but whole atomic nuclei of protons and neutrons. Iron would do it. UPDATE 7/8: more on acceleration of nuclei.