It’s common knowledge that the universal speed limit is equal to the speed of light in a vacuum, but it’s not accurate to say that it’s impossible to travel faster than light. For example, if a beta particle, a high-energy, high-speed electron emitted by certain types of radioactive nuclei, passes through a dielectric medium, such as water, it’s possible for that particle to travel faster than the light it emits. This is because the speed of light through a medium is slower than it is through a vacuum. In the case of water, about 25% slower, slow enough that these particles can outpace it. The result is something akin to a sonic boom of light called Cherenkov radiation. This is visualized as an eerie blue glow, characteristic of nuclear reactors.