Why Is the Speed of Light the Universe's Speed Limit?

Throw a ball forward from a moving train and someone on the platform sees the ball travel at the train's speed plus the throw. Everyday speeds add. Light does not. In 1887 Albert Michelson and Edward Morley tried to detect the Earth's motion through space by measuring light travelling in different directions and found no difference at all, and every experiment since has agreed: shine a torch from a rocket travelling at half the speed of light and both the rocket's crew and a stationary observer measure the beam moving at exactly the same speed, not one and a half times it.

Einstein's special relativity of 1905 took that result as a law of nature and worked out what else had to be true. If the speed of light is fixed for everyone, then space and time cannot be fixed. Moving clocks must run slow, moving objects must contract along their direction of travel, and events that are simultaneous for one observer are not for another. Each of these has since been confirmed, and the clocks on GPS satellites are corrected for exactly this effect every day.

The limit follows from what happens to energy. Push on an object and it speeds up, and in ordinary physics the same push always adds the same speed. In relativity, as the object approaches the speed of light, each push adds less and less speed and more and more of the energy goes into something else: the object's resistance to being pushed further grows without bound. At 90 percent of light speed it takes more than twice the energy to add each further increment; at 99.9 percent, more than twenty times; reaching the speed itself would take infinite energy, which no source can supply.

Physicists sometimes express this by saying that mass increases with speed, and sometimes prefer to say that kinetic energy increases faster than the classical formula predicts; the two descriptions are the same equation read differently. The particles in the Large Hadron Collider are pushed to 99.9999991 percent of the speed of light and go no faster however much more energy is poured in. Light itself can travel at that speed only because photons have no mass; anything with mass is stuck below it.

The other half of the story is what happens to time. A clock moving at 87 percent of light speed ticks at half the rate of one at rest, as measured by the stationary observer; at 99.5 percent it ticks at a tenth. The effect is real and routinely observed: muons, unstable particles created by cosmic rays high in the atmosphere, should decay long before reaching the ground at the speed they travel, and they reach it in large numbers because, from our point of view, their internal clocks are running slow.

For a traveller, the slowing is a gift. A ship accelerating steadily at one Earth gravity could reach the centre of the galaxy, 26,000 light years away, in about twenty years of the crew's time, while tens of thousands of years passed at home. The speed limit constrains how fast anything moves through space as others see it; it does not limit how far a traveller can go in a lifetime, only what they come back to.

There is a deeper reason the limit cannot be broken, and it is about causality. In relativity, the order of two events that are far apart in space and close in time can differ between observers, and the only events whose order everyone agrees on are those that could be connected by a signal at light speed or slower. If anything could travel faster than light, there would be observers for whom it arrived before it was sent. A message could be received before it was transmitted; an effect could precede its cause. The speed of light is really the speed of causality, the maximum rate at which one part of the universe can influence another, and light happens to be the thing that travels at it.

The nearest star is 4.2 light years away, so a radio message takes 4.2 years each way and a probe travelling at the speed of the fastest spacecraft yet built, about 0.06 percent of light speed, would take seven thousand years. Ideas for going faster within the rules include light sails driven by lasers to perhaps 20 percent of light speed, which would reach the nearest star in a couple of decades. Ideas for going around the rules, warping space so the ship does not move through it, appear in the equations of general relativity but require forms of energy that may not exist. What the limit fixes and what it leaves open:

• •Fixed: no signal, object or influence can cross space faster than light
• •Fixed: reaching light speed would take infinite energy for anything with mass
• •Open: a fast traveller experiences less time, so distance is not the barrier it seems
• •Open: space itself can expand faster than light, which is why the observable universe has an edge
• •Speculative: shortcuts through curved space, allowed on paper and unbuildable so far

The speed of light is a limit because every observer measures light at the same speed, which forces time and space to stretch for anything moving fast, makes each increment of speed cost more energy without bound, and would let effects precede causes if anything went faster. It is the speed of causality, and while nothing with mass can reach it, time dilation means a traveller can still go remarkably far.