## Einstein’s theory of special relativity

Einstein’s theory of special relativity relies on one key fact: the speed of light is the same no matter how you look at it. To put this into practice, imagine you are travelling in a car at 32km/h (20mph), and you drive past a friend who is standing still.

As you pass them, you throw a ball out in front of the car at 16km/h (10mph). To your friend, the ball’s speed combines with that of the car, and so appears to be travelling at 48km/h (30mph). Relative to you, however, the ball travels at only 16km/h (10mph), as you are already travelling at 32km/h (20mph).

Now imagine the same scenario, but this time you pass your stationary friend while travelling at half the speed of light (theoretically, of course). Through some imaginary contraption, your friend can observe you as you travel past. This time you shine a beam of light out of the car windscreen. Previously, we added together the speed of the ball and the car to find out what your friend saw, so in this instance, does your friend see the beam of light travelling at one and a half times the speed of light? According to Einstein, the answer is no. The speed of light always remains constant, and nothing can travel faster than it. Therefore, on this occasion, both you and your friend observe the speed of light travel ling at its universally agreed value c, roughly 299,792,458 metres per second. This is the theory of special relativity.

## What is space-time?

When it was first suggested that the Earth was a sphere in three dimensions as opposed to a flat two-dimensional plane of sorts, the idea was largely scoffed at until experimental evidence proved it to be true. The same can be said with space-time, albeit with an extra dimension involved. It was once thought that space and time were separate, and that the universe was merely an assortment of cosmic bodies arranged in three dimensions. Einstein, however, introduced the concept of a fourth dimension – time – that meant that space and time were inextricably linked. The general theory of relativity suggests that space-time expands and contracts depending on the momentum and mass of nearby matter. The theory was sound, but all that was needed was proof.

That proof came recently courtesy of NASA’s Gravity Probe B, which demonstrated that space and time were indeed linked. Four gyroscopes were pointed in the direction of a distant star and, if gravity did not have an effect on space and time, they would remain locked in the same position. However, scientists clearly observed a ‘frame-dragging’ effect due to the gravity of the Earth that meant the gyroscopes were pulled very slightly out of position. This seems to prove that the fabric of space itself can be altered and, if space and time are as linked, then time itself can be stretched and contracted by gravity.

## What is the world’s most accurate clock?

The most accurate clock in the universe would probably be a rotating star like a pulsar (although this is debatable), but on Earth it’s atomic clocks that provide an accurate track of time.

The entire GPS system in orbit around the Earth uses atomic clocks to accurately track their position and relay data to the Earth, while entire scientific centres are set up to calculate the most accurate measure of time, usually by measuring transitions within a caesium atom. While most atomic clocks rely on magnetic fields, modern clocks are using lasers to more accurately track and detect energy transitions within caesium atoms and keep a more definite measure of time.

Although caesium clocks are currently used to keep time around the world, strontium clocks (strontium atoms inside a laser grid) promise twice as much accuracy, while an experimental design based on charged mercury atoms could reduce discrepancies even further to less than one second lost or gained in 400 million years.