Einstein's General Theory of Relativity

Gravity - It makes me feel so... down.

On a trampoline we can jump up but are always pulled down due to a force called Gravity. It's the thing that keeps us on the ground.

Interestingly, Einstein looked at Gravity in much the same way as a trampoline - just think about how you make a dip in the fabric when you're jumping. This is a great way to visualize how space and gravity works.

Einstein's theory tells us how much space curves, or dips, according to how much stuff there is in one place: the bigger the object, like the Sun for example, the bigger the dip; the smaller the object, like a planet, the smaller the dip.

A BIG DIP IN SPACE IS CAUSED BY A BIGGER OBJECT LIKE THE SUN

WHILE A SMALLER DIP IS MADE BY A SMALLER OBJECT LIKE THE EARTH

So we can think about space as a curvy landscape of stars, planets and everything out there. Using this idea we can see how planets orbit the Sun. The dip is actually steeper the closer you are to the star, meaning that Gravity is stronger here, and weakens as you move away from the star.

So in order for the each planet to keep the same distance from the Sun, closer ones need to move faster than more distant ones - because closer ones are further down into the dip while further ones sit on a more gentler slope. Mercury orbits the Sun in 88 days, whereas Pluto takes 248 years.

We can roughly do the same thing on our trampoline, but here the friction of the material will slow our marble down, in space there is no friction, so the marble would stay in the same orbit - and the same distance away from the Sun. In fact, any masses will have a gravitational attraction: everything attracts everything else.

Just as planets are deflected by the dip, so is light. Light is just a form of energy, but Einstein's famous equation E=mc2 shows us that this energy E (such as light) is also mass (m). So light has mass, much less so than a planet of course but this still means that it affected by gravity. The eclipse of 1919 proved that Einstein's theory was right, as a star's light from close to the Sun was deflected from its true position by the gravitational dip of the Sun.

Imagine an object getting bigger and bigger, the dip gets deeper and deeper until an incredibly massive object causes a bottomless well, and so the Theory of Relativity also predicts Black Holes.

RELATIVITY IDEAS (just for fun):

Gravity is a geometric phenomenon.
Space is curved, but a planet thinks it's are moving in a straight line.
Equivalence: Gravity in one direction is simply acceleration in the opposite direction.
The theory also predicts and expanding Universe, and the slowing of time (time dilation) near massive objects.
The trampoline is a common sense view of an embedding diagram representing curved spacetime.