# Einstein’s General Relativity – Simplified

Albert Einstein was a man who has inspired many heated arguments, battles and advances in physics. He has one of the most popular, misused and often quoted formulae known to the world outside of physics academia; E=mc^{2}. What Einstein did in seemingly simple formula was give mass and energy equivalence; showing that energy and mass were in fact able to accurately describe each other.

Einstein’s General Theory of Relativity (which is way more complex than one, famous formula) describes how space, time and matter behave relative to each other. This was an expansion of his Special Theory of Relativity where he showed that the speed of light was the same for all observers. This was the first time that the weirdness of physics was exposed.

If you throw a ball from one car to another moving in opposite directions, that ball is going to be moving much faster relative to one car than the other, right? The ball would be moving slower to the thrower than it is to the catcher. The speed of the throw plus the speed of the thrower’s car makes the ball faster than either alone; but now add the speed of the car coming the other way and you know how hard that ball is going to hit the catcher’s hand! This is physics. You count all the variables and then come up with a solution, even if the solution is to get your hand out of the way of the ball!

You throw the ball at 20m/sec. Both cars are also moving at 20m/sec. You, the thrower, are moving at 20m/sec, the ball you threw is your speed plus your throw, or 40m/sec. The catcher is coming the other way, making the ball’s speed 60m/sec from the catcher’s point of view. Whilst this works for balls, it does not work for light and other electromagnetic phenomena, which refuses to follow these logical steps.

If this is still not quite as clear as you'd like, imagine you and a fiend on a fast train, playing catch up and down the corridor of a car on that train. Despite the fact you are both moving on the train at 100m/sec, the relative velocity of the ball you throw to each other is perceived as 20m/sec as if you were both standing still. Since you, your friend and the ball are already moving in the train, your relative speeds cancel each other out when calculating the speed of the ball. It is as if you were both standing still.

The Special Theory of Relativity evidenced that the speed of light was the same for all observers. If you were shining a light on the front of your spaceship moving at 100,000Km/p/second (^{1}/_{3} of light speed) towards another ship coming the other way at the same velocity, the light shining from your torch will still be moving at a smidge under 300,000kM/p/sec whether measured from you in your spacecraft, the oncoming one or someone else in the middle who is not moving at all, regardless of the speed difference between all obeservers!

Now comes the other fun fact; imagine there is a sun behind you and that its light is streaming past you and towards that oncoming spacecraft. The speed of your light and the sun’s light are the same, even though you are moving 100,000Km/sec away from the sun. Even stranger, both your light and that of the sun you are rapidly leaving behind will still measure as the same speed as any other light coming in from anywhere else to the receiving spaceship.

This Special Theory of Relativity was the first big break away from what can be described in science as *regular and logical*. It was the starting point of physics getting weird, in a counter-intuitive way. Light does not present differently, it presents at the same speed, relative to the observer, no matter how the emitter and receiver interacted. Until then science had the ‘ball from a moving vehicle’ understanding, which made perfectly good and rational sense. The only problem is that the universe’s laws do not play ball!

What the Special Theory of Relativity did was establish the speed of light as an absolute, the maximum speed limit in the physical universe. To accelerate to a speed faster than the speed of light, you can’t have mass (the ‘why’ is best left for another discussion at this point). It also showed that light (and all other electromagnetic phenomena) is perceived and measured as having the same speed, no matter what the relative velocities are of the emitter and receiver.

When he expanded this to his General Theory of Relativity, he showed that space can bend, time can deform and everything behaves differently when all these other things are taken into account. This got so weird that one person even got a Nobel Prize after essentially spending more than a decade futilely trying to disprove Einstein’s work. What he in fact did was push Einstein’s theory through the criteria of the scientific method, it accurately predicted events and it couldn’t be disproved! He proved the theory was robust and accurate.

Without plunging too far into these very deep waters, Einstein gave us a way of understanding the framework of how the universe goes together that is still accurate today, except at the far reaches of the tiny, where atoms are big structures and energy is both wave and particle. This is the world of how atoms do what they do, the world of **Quantum Physics**. Here is where stuff gets really weird!