[taxman (Matt)]

Carl beat me to it. Meh, my redundant response is below anyway.

First, let me have a crack at an explanation of relativity in terms of spacetime.

** No maths (well, not much), I promise **

Let's say you are sitting on the nose of a ship traveling 1 meter per second slower than the speed of light and I am watching you from a stationary point. If you point a torch in the direction you are travelling in and switch it on, after one second, you will see the light travel its approximate 300 million meters in front of you, but I will only see it one meter in front of you.

There is the first part of an explanation: A contraction of space in the traveller's frame of reference occurs the faster they travel - 300 million metres to you is only 1 metre to me.

Now, in your frame of reference, to travel 300 million metres from your perception at any single point in your journey at constant speed will take slightly less than 1 second, but for you to travel the same distance in my frame of of reference will take 300 million seconds, as it took you one second to travel that one meter to catch up to the light from the torch.

And here is the second part of the explanation: The closer an object gets to the speed of light, the more their spacetime contracts in comparison to a stationary observer.

As Carl ponted out, relative time contracts to nothing at all if an object travels at the speed of light. This is because in a speed of light frame of reference, there is no distance to form the dimensions of space and without space, time cannot exist. Attributes that exist at sub-light (e.g. mass or charge) cannot exist at any other point than the one point where all faster than light existence occurs.

Again this is because there is no distance, and everything is in the same place at the same time (such as it is). So these attributes if not ceasing to exist entirely, at the least become meaningless.

So to give you an answer: If you stepped into a craft that immediately dropped into and out of light speed, your journey would take no time to you or the ship, but four years to a stationary frame of reference such as your departure or arrival points.

However, any lightspeed message sent by you (say laser or radio transmission) would take another 4 years to get back to Earth, making a total of 8 years after you left before any evidence to those on Earth is available that you got there.