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Say you take a ride in a car. What's truly moving - your car or the road? Or both? Relativity tells us: there is no way of knowing.

Here's a simpler way to visualise this: say you walk into your house. Only that your house isn't there. In fact, nothing is there. It's just you in a void. Of nothingness. Now, before we get into the details of "how do I breathe" and "what the heck is this place TAKE ME BACK THIS INSTANT," let's make you feel more at home. Your computer desk appears next to you. It then flies away from you. Now, what's moving - you or your desk? Or both?

There is no way of knowing.

Now replace your desk with something big. Like the Earth. It flies away from you. Now because the Earth is so big and you are so little, it would seem that you are the one who is moving. But again, like your desk:

There is no way of knowing.

» But that's not the point.

You see, the funny thing about relativity is not how everything is relative to everything else - it is about how one thing is not relative to anything else. That thing being the speed of light.

Say, if you're driving a car at 20 km/h along the roof of a train which is going at 100 km/h in the same direction, your nett velocity would be 120 km/h, yes? Now, if the light from your car's headlights are going at - well, the speed of light (let's round that off to 300,000,000 m/s for the sake of mental arithmetic). So, logically, the beam of light being emitted from your car's headlights as it travels along the roof of the train would be:

(120/3.6) + 300,000,000 = 300,000,033.33 m/s

ERR! Wrong answer.

This is because light is the ultimate speed limit of the universe. Approach near it and weird things start happening. Your spacecraft becomes shorter, fatter and more dense. Time itself slows down. What happens when you travel at the speed of light? Unless you're a little photon, you will never know.

» How relativity relates to time travel

When Albert Einstein first came up with General and Special Relativity, he had no idea of the awesome results and discoveries that his theories would allow. Two of those awesome things were:

» Time Dilation (Special Relativity)

Time dilation states that when objects (presumably rockets or some other scarily-fast thing) move near the speed of light, time for the moving object is dilated, and will appear to be moving slower for stationary (not-moving) observers.

Also, if the object moves closer to the speed of light, it's time will appear to move slower and slower, until it appears to stand still. On board the fast, speedy object, time will pass normally for all the observers that are moving with the object.

This way, a time traveller could simply go in a horrifically speedy rocket, fly around the universe a couple of of times near the speed of light, and then fly back to Earth. That way, only five years would have passed on board the rocket, whist fifty thousand years would have passed on Earth!

» Gravitational Time Dilation (General Relativity)

This type of time dilation is much like the one stated above, only this is affected by high gravitational fields. Basically, the higher the gravitational field (relative to a stationary observer), the slower time will appear to pass.

A time traveller could travel to the future by hanging around a black hole (at a safe distance, of course!) and simply go back the Earth to find that another couple of thousands of years had passed.