#### Theory of Relativity explained

###### Monday 14th March

It’s Albert Einstein’s birthday on March 14th, so to celebrate we’re going to talk about his best-known work: the theory of relativity.

**Two theories in one
**

What’s commonly referred to as the theory of relativity is actually two theories: “special” relativity and “general” relativity.

**Thought experiments**

Whilst working in a patent office, Einstein who would develop “thought experiments” on questions about physics, came upon a theory that described how light moves at a constant speed. Always.

This means that it is not affected by the speed at which objects move. For example, imagine a space ship moving towards Earth at 1,000 km/h. The speed at which light from the space ship arrives on Earth is:

299,792,458 metres per second.

If the space ship travels away from Earth at 1,000 km/h, the speed at which light from the space ship arrives on Earth is:

299,792,458 metres per second.

The same.

The speed of the space ship does not change the speed of light.

Einstein’s “special theory of relativity” describes the constant speed of light, and also that it’s impossible to determine whether or not your are moving without looking at another object.

Nothing is still. Everything is in movement. Planets, stars, galaxies, they’re all moving in relation to each other.

For example, your car might move at 50 km/h on the surface of the earth. But the Earth moves at 30 kilometres per second, compared to the Sun. And the Sun moves through space in relation to the centre of the Milky Way.

Consider the example of two people (Bethany and Daniel) in an open train carriage throwing a ball back and forth. The train is running west at a steady speed of 100 km/h. When Bethany throws the ball up the carriage towards Daniel, the ball will appear to travel at 110 km/h (100 km/h for the moving train plus 10 km/h for the ball being thrown). When Daniel throws the ball back, it will still be travelling west, but at a speed of 90 km/h (100 km/h minus 10 km/h).

For an observer watching the ball being thrown from outside the train, the ball is always travelling west. For Bethany and Daniel in the carriage, it appears that the ball is changing direction depending on who throws it.

The result is that the speed of the ball depends on the “relative” position of the observer.

Einstein published his “general theory of relativity” a decade after the special theory. It describes gravity using geometry. It also implies the existence of black holes [link?]. What does it mean to describe gravity using geometry? This video explains it nicely: