Special relativity

Einstein’s special relativity, which he formulated in his “miracle year” of 1905, was a theory that revolutionised our ideas of space and time – and ultimately paved the way for some even bigger surprises.

Its origins stretched back half a century. In the 1860s, the Scottish physicist James Clerk Maxwell succeeded in melding electricity and magnetism into one unified theory of electromagnetism. But his equations turned up a surprise. However Maxwell sliced it, the numbers only made sense if light travelled through space at the same, constant speed, regardless of how fast the source of the light itself is travelling.

This goes against established classical (often called “Galilean”) ideas of the relativity of speed. If someone fires a bullet from a moving car, for instance, to a bystander the bullet travels at the sum of its speed and the car’s speed. Yet when, 20 years after Maxwell, US physicists Albert Michelson and Edward Morley were looking for the luminiferous ether, a medium supposed to carry light, they too concluded that the speed of light must be a constant, unaffected for example by the Earth’s rotation.

Einstein’s genius was to raise the constant speed of light to a principle of nature, and to say it is in fact the ultimate cosmic speed limit. No influence – not matter, not information, not gravity or any other force – may travel faster than it. Reports of cosmic speed breakers, such as faster-than-light neutrinos announced in 2011, have always turned out to be wrong.

This has odd consequences. In everyday experience, two cars approaching each other at 100 kilometres an hour collide at double that, 200 km/h. But imagine you’re sitting in one of two spaceships approaching each other, each travelling at 90 per cent the speed of light (known as “c”). From the perspective of one, what speed is the other approaching at?

Work through the equations that Einstein devised and you’ll find the answer is in fact 99.4 per cent – and the crucial thing is that it can’t be bigger than c. This leads to the core prediction of special relativity: that time and space warp to accommodate light’s speed limit. Moving clocks tick slower and moving rulers appear shorter, so there is no one objective measure of time and space – and you really will age less in a speeding spacecraft. At our normal speeds, these warping effects are negligible, but close to light speed they become hugely significant, and ensure no object can ever cover a given space in a shorter time than light can.

Another unexpected consequence was that mass would not be conserved in the new theory, but could be converted into energy, according to the famous equation E=mc². Mass is just a concentrated form of energy, connected by the constant speed of light. That’s the basis of nuclear bombs, and is also why, when bashing particles together at very high energies, as in CERN’s Large Hadron Collider, you can create other, more massive particles. This path of discovery, by probing newly developed quantum field theories, eventually led to the standard model of particle physics.