Illustration of cosmic rays striking Earth’s atmosphere, where they produce particle showers. By building large ground-based arrays of detectors, the original energy and charge of the incoming cosmic ray can frequently be reconstructed, with observatories like Pierre Auger leading the way. (Credit: Asimmetrie/INFN)

Why the cosmic speed limit is below the speed of light

As particles travel through the Universe, there’s a speed limit to how fast they’re allowed to go. No, not the speed of light: below it.

If you want to travel as fast as you can through the Universe, your best bet is to pump as much energy into as small a mass as you can. As you add progressively more kinetic energy and momentum to your particle, it will travel through space more quickly, approaching the ultimate cosmic speed limit: the speed of light. No matter how much energy you manage to add into the particle in question, you can only get it to approach the speed of light, but it will never reach it. Since the total amount of energy in the Universe is finite, but the energy required for a massive particle to reach the speed of light is infinite, it can never get there.

But in our real-life Universe, not the idealized “toy” version we play with in our heads, we don’t simply have arbitrary amounts of energy to give to particles, and we also have to accept that they’re traveling through the space that actually exists, rather than what we imagine as a complete, perfect vacuum. While the Universe is capable of imparting far more energy to particles through natural accelerators — like neutron stars and black holes — than we can ever give…