Four things that break the speed limit of the universe

Einstein predicted that nothing with mass can move faster than light but there are objects disobeying the laws of physics

1. Superluminal motion Distant black holes and active galaxies with supermassive black holes at their hearts can spit out high-speed jets of particles that appear to be travelling faster than light. This form of so-called ‘superluminal’ motion is in fact an optical illusion that occurs when jets moving at close to light speed are pointing roughly in our direction. Because the radiation emitted at different points along the jet’s path is not moving much faster than the jet itself, the train of emitted light is compressed – light emitted at different points over the course of a century reaches Earth in just a few years, creating the impression of faster-than-light motion.

3. A tachyonic universe Even if no particles with mass can travel at the speed of light itself, some physicists have pondered whether there could be a whole universe of particles that travel faster than light. Such particles, known as tachyons, would exhibit a variety of strange properties; for example, their energy would decrease as their speed increased. However, attempts to detect evidence for this strange mirror universe have so far proved unsuccessful.

2. Entanglement In quantum physics, particles with entangled properties remain connected by a strange link however widely they are separated. Measuring and ‘resolving’ the properties of one causes the other to instantaneously resolve itself in the other direction, with no time needed for a message to travel between them. Einstein called this strange effect “spooky action at a distance” – the laws of physics seem to permit it because it cannot be used to send information at FTL speeds.

4. Cherenkov radiation An often overlooked point is that the universe’s ultimate speed limit is the speed of light in a vacuum. When light passes through transparent materials it can slow down significantly, and sometimes particles moving through the same medium can break this artificially lowered light-speed barrier. The result is a burst of bluish-coloured Cherenkov radiation – the light equivalent of a ‘sonic boom’, which can be used to detect high-speed particles such as elusive neutrinos.