﻿ Physics Evolution - Text Only Version | Interact | physics.org

# Time Twins - Special Relativity

In 1905, at the age of 26, Einstein told us something amazing - that we were completely wrong about the way the universe works. Einstein replaced the laws of Newton and Galileo with his theory of special relativity which showed how space and time are really related. The theory of special relativity is the combination of two ideas and their seemingly weird consequences.

1. The laws of physics are the same wherever you are. This means that an experiment carried out in a moving train will give the same results as when it is performed in a lab. Furthermore, if there were no windows on the train and it was moving at a constant speed, there is no experiment that you could do to see whether or not it was actually moving.
2. The speed of light is the same for everyone. The speed of light being the same wherever you are might not seem strange, but think about how we normally experience speeds. A ball thrown on a moving train will have a greater speed than a ball thrown with the same force by someone standing on the platform. This is because the speed of the train is added to that of the ball to give its total speed. But this isn't the case with light. If you measure the speed of the light produced by torches on a moving train and a stationary platform, you will get the same speed - the speed of the train doesn't matter. When you measure the speed of light it doesn't matter if you are moving or stationary, or if the source of the light is moving - the speed is always the same: 300,000,000 metres per second.

But the only way that the laws of physics and the speed of light can always be the same is for something else to change. Special relativity shows that measurements of distance and time depend on how fast you are travelling - a result that goes against our everyday experiences. If you measured the length of a baguette and the time it took you to eat it, there would be no difference whether you were on a moving train or standing on a platform - but that is only because the speed of the train is so small. As speeds increase towards the speed of light, the so-called relativistic effects of time dilation (clocks running slow) and length contraction (objects getting shorter) become more and more obvious.

Interstellar space travel provides an example of the effects of time dilation. 25 year old twins, Sarah and Jane, have very different careers. Sarah is an astronaut and Jane is a lawyer. Sarah sets off on a round trip to a nearby star, travelling at speeds close to the speed of light. Jane meanwhile stays on Earth and goes to court each day. When Sarah finally returns, Jane is not happy. For Jane, her clock, and face, show that 10 years have passed, whilst Sarah's high speed travels mean that her clock, and face, show that just 8 years have passed. And it's not just distance and time that change. Special relativity also shows that as an object travels faster, its mass increases. But as the mass of an object increases, it takes more and more energy to increase its speed any further. Eventually, as the object gets close to the speed of light, it becomes so massive that no amount of energy will make it go any faster. This means that the speed of light is a universal speed limit which nothing with mass can break.

But the most famous part of special relativity is the equation E=mc2, where E is energy, m is mass and c is the speed of light. The equation stems, in part, from the relationship between energy and momentum that Einstein developed to ensure that the speed of light was the same for everyone no matter what they were doing. The equation tells us that energy and mass can be changed from one to the other - that they are equivalent.

E=mc2 has come to symbolise all of Einstein's work, but the changing space, time and mass of special relativity are only part of the story. 10 years later Einstein filled in the gaps and produced his theory of general relativity which introduced gravity into his view of spacetime. It is not surprising that relativity is hard to understand and even to believe - it goes against our everyday experiences. But relativity has stood the test of time, and like it or not, the universe really is this strange.