Why can nothing travel faster than light?

According to Einstein's theory of special relativity, published in 1905, nothing can exceed the speed of light. Light's speed is a fundamental constant that appears the same to all observers. Objects with mass cannot even reach it, because doing so would take an infinite amount of energy, and anything carrying information faster than light would upset the order of cause and effect. Physicists can describe precisely how this limit works, but not why nature set it where it is. [1] [2]

The Penn State account explains that, in Einstein's calculation, an object gains mass as it speeds up, and the more mass it has, the more energy is needed to speed it further. At the speed of light the mass would be infinite, and so would the energy required. NASA makes the same point: objects with mass cannot reach light speed. The fastest electrons from accelerators travel at about 0.999999 times the speed of light, never at or above it. [1] [2]

Gravity Probe B's Stanford site describes the same behavior in experiments: electrons pushed faster and faster act as though they gain mass, and near light speed the energy needed for each small increase outstrips what a power utility, or even a nation, could supply. [3]

The second reason concerns causality. Relativity says that if two events are so far apart that no signal travelling at light speed or slower can connect them, different observers will disagree about which happened first. Physics World notes that most physicists still believe a cause must precede its effect, so no information can be sent faster than light. Penn State's Donald Schneider gives an example: with a faster-than-light bullet, some observers would see it hit the target before the gun was fired. He calls this a serious problem, since relativity requires physical laws to be the same for all observers. [4] [1]

Some things can look faster than light without breaking the rule. A lighthouse beam sweeping across a distant shore can have a spot moving faster than light, but the spot is not a single object and information still travels from the lighthouse at light speed. Physics World calls these cases the motion of effects. In a 2000 experiment, the peak of a laser pulse seemed to leave a caesium cell before entering it, but this occurred in an amplifying medium that reshapes the pulse, so the peak is not the kind of thing the limit applies to. [4]

Light also slows to about three-fourths of its vacuum speed in water, so charged particles in a reactor can exceed that reduced speed. They never exceed light's speed in a vacuum. Tachyons, wormholes and warp drives remain speculation, and some physicists dismiss their significance. [1]

The Stanford relativity page says the equations explain extremely accurately how the limit behaves, but no one knows why nature chose that particular speed. Penn State's Schneider says no experiment has contradicted special relativity in a century of testing. NASA says no way around the limit is currently known, though some researchers hope for a loophole. A Wolfram Demonstrations summary adds that in quantum field theories, phenomena linking massive particles must occur inside the light cone. [3] [1] [2] [5]