The Universe Has A Speed Limit, And It Isn’t The Speed Of Light
“We believe that every charged particle in the cosmos — every cosmic ray, every proton, every atomic nucleus — should limited by this speed. Not just the speed of light, but a little bit lower, thanks to the leftover glow from the Big Bang and the particles in the intergalactic medium. If we see anything that’s at a higher energy, then it either means:
1. particles at high energies might be playing by different rules than the ones we presently think they do,
2. they are being produced much closer than we think they are: within our own Local Group or Milky Way, rather than these distant, extragalactic black holes,
3. or they’re not protons at all, but composite nuclei.”
If you were to try and travel as close to the speed of light as possible, you’d never get there because of Einstein’s relativity and the fact that you have mass. But even if you pumped an arbitrary amount of energy into you, you still wouldn’t get arbitrarily close to the speed of light. Instead, you’d find that there was a barrier or cutoff just a little bit below the speed of light: about 80 femtometers-per-second below the ultimate cosmic speed limit. That’s because the leftover glow from the Big Bang, the cosmic microwave background, exists no matter where you go, and prevents you from going any faster. Even if you beat that speed, it will knock you back down below it in short order.