The journey towards relativity really started in 1887 with the Michelson-Morley experiment. They were trying to find the aether. The aether was the medium through which electromagnetic waves were believed to propagate, in the same way that sound waves propagate through air.

It was thought that the aether was underpinning the fabric of the universe, and to find it, Michelson and Morley did an experiment shining light in the direction of the Earth's rotation and at right angles to it. The pressure of the Earth pushing through the aether should slow down the light, making light heading towards the Earth's rotation slower than that at right angles to the rotation. The trouble was, it didn't. The speed of light was the same. This didn't make sense, and it was this and other experiments that began laying the aether to rest. The person to finally do this, though was Einstein.

Einstein developed his special theory of relativity in 1905. It contained two ideas:

1.The laws of physics must be the same everywhere in the universe, no matter what the velocity of the observer

2.The speed of light is constant and independent of the motion of the light source.

In other words, it doesn't matter how fast one is travelling, the same speed of light will always be observed.

Imagine yourself in a car doing 65 mph, and a car goes pats you at 70 mph. From your point of view, the other car is moving at 5 mph. Now, imagine a car heading towards you at 70 mph. The car appears to be going at 135 mph. This is easy to see. Watch the traffic when you are in a car or bus, and the cars coming towards you seem to be going much faster than the cars overtaking you.

Now, imagine you are in a spacecraft almost at the speed of light. Someone flashes a light past you. Einstein showed that the light would look like it was traveling at light speed past you. Not lightspeed minus your speed, but at the full light speed. A beam of light coming towards you, would not appear to be going at your speed plus the speed of light, but at the speed of light.

In a really fast vehicle, if light overtook you, it would look just as fast to you as if you were standing still.

How can this be?

The quicker one travels, the slower time gets, until at lightspeed time=0. There is no time at lightspeed. This has been highlighted using the twins paradox. If two people are born at exactly the same time, and one goes on a really long journey to close to light speed, when they are reunited, the one who has travelled will be younger than the other. This is a paradox, because to each other, the other one travelled at near light speed. So there is an asymmetry. This has been tested in reality using synchronised atomic clocks, and the empirical results show that a clock that travels does experience time at a different rate to the stationary one.

E=MC² which came from the special theory also shows that energy and mass are interchangeable, and that the faster one goes, the more energy one has. So, the only way to know a particles real mass is to calculate it at rest, this is known as the rest mass. The total energy of an object at speed is the rest mass plus the kinetic energy. The faster the object goes, the more energy it has, until at lightspeed, if an object starts with a rest mass, the amount of energy it has reaches infinity. Only particles like photons can travel at light speed, because all the energy of a photon is made up of kinetic energy, so, it has no rest mass.

In Newtonian physics, time is a constant. Time is experienced by all people at the same rate. Also forces are propogated instantly. Gravitational effects are felt instantly at all points in the universe. Einsteins special theory of relativity had shown that cannot be true. Nothing can break the light speed barrier, due to the infinite energy and the time for object at lightspeed being zero, so greater than light speed would mean negative time! This contradicts Newton, so Einstein needed a new view of gravity. This he brought out in 1915, and it was called the general theory of relativity.

Newton described gravity as a force. Einstein described it as a form of geometry. To picture this, imagine a rubber sheet stretched out and suspended. That is the universe without any objects, and so without gravity.

If a ball is dropped onto the sheet, a depression is made, and that is what gravity is, a warping of the geometry of space. When light from a distant star is observed passing another star, it looks like it is bent by gravity. The fact is it is going in a straight line through Einsteins geometry, and it is only from our viewpoint does it look bent. Back to Science home page

Back to Home Page