I’m not sure if it is even possible explain it to simple folk like myself, but in my free time, I enjoy learning about Earth and space sciences. Recently, I read about the Oh-My-God Particle and how it exceeds the GZK Limit. Unfortunately, I was the dumb kid and class clown in my Honors Physical Science class, so physics has never really registered to my ‘arts and humanities’ brain.
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True eli5: space has particles of light in it. If a cosmic ray is moving too fast, it will crash into these particles and slow down. The faster it moves, the more it crashes, the more it slows down. So there is effectively a max speed where it crashes too much to go any faster.
Eli15: It has to do with three things. First, if you hit particles with enough energy, they break apart. What they break into depends on, in part, how much energy you hit them with. Makes sense.
Second, there is thing called the cosmic microwave background (CMB), which is basically the light left over from the Big Bang (not quite, but that’s another eli5). This light exists everywhere in empty space, and in our frame of reference is at microwave wavelengths.
(If you don’t know, light is a type of wave, but it interacts with things like a particle, which we call a photon. The energy of a photon is uniquely determined by its wavelength–shorter wavelength, or equivalently, higher frequency, means higher energy).
Third, you may have heard of something called a Doppler shift. This is what happens when an ambulance siren sounds higher pitched while it is approaching you and lower pitched after it passes. Sound is a wave, and while the source of the wave (the ambulance) is moving towards you, the wavelength of the sound is effectively shorter relative to you, so it sounds higher pitched. The same thing happens with light (not quite, but eli5)
Ok, so what does this have to do with cosmic rays? Cosmic rays are high-energy (aka, fast) particles moving through space. The photons from the CMB are everywhere in space. But the faster a cosmic ray is moving, the shorter the wavelength of the CMB photons is in its reference frame. And if the cosmic ray is moving fast enough, these photons have enough energy to break them apart. But remember how I said what they break into depends on how hard they get hit? Well, above the GZK limit, the CMB photons have enough energy to break them apart in a way that lowers their energy quite a bit. So you can’t (theoretically) have many cosmic rays above this limit, because the CMB is Doppler shifted enough to them to rapidly slow them down.
The idea is that cosmic rays(various energy levels of protons, typically) are theoretically limited by, basically, ‘drag’ from the cosmic background microwave field pervading the universe; they can only have so much energy due to losing it to the CBM field.
There’s a lot of hypothesis out there about it, ranging from ‘instrument error’, ‘closer source than we thought’ and ‘was actually a heavier nucleus instead of just a proton’ all the way to ‘weird exotic neutrino decayed and spat that thing out closer to home than the source of the neutrino’.