# General relativity

A theory of gravity that warps spacetime

### What is it?

General Relativity is a pillar of modern physics. Let’s try and explain it in a few paragraphs.
First, we need to understand Newton’s first law of motion. This states that an object will not move or keep moving forever at a constant speed, unless a force acts on it.

Let’s explain what this means with a simple example.

When a parachutist jumps out of a plane, they accelerate to the ground because the force of gravity is pulling them down.

They don’t feel their weight when this happens but they should feel this effect under Newton’s first law of motion. In a similar way, you’ve probably not aware that you’re moving when you’re going up or down in a lift. Under Newton’s laws, this doesn’t make sense.

So, Einstein came up with the solution: gravity is a product of warped space and time.

What is space and time? It’s like a blanket on which every object sits.

And the heavier you are the more you warp space and time.

In the above picture, Red and Blue warp the fabric of space-time more than a single LEGO brick.

In a similar way, the Earth does not keep the Moon in orbit by exerting a physical force on it, but because its mass distorts the surrounding space and forces the Moon to move that way.

This means something at rest in a gravitational field and another thing that’s accelerating are physically identical. Whether you’re parachuting to the ground or standing on the ground, it doesn’t matter under General Relativity.

### So what?

Einstein made a series of predictions using General Relativity. For example, using his theory, he explained why Mercury’s orbit drifts – because of this distortion of space-time. He also predicted that if you have a really massive object (like the Sun) then even the path of the light passing close of this body will be distorted. He also argued that the wavelength of light emitted close to a massive body will be stretched.

So, under General Relativity, the parachutist isn’t falling to the ground – the ground hits them.

If you have a REALLY massive object, like a Black Hole, then space-time will be distorted so much that not even light can escape it.

General relativity has helped scientists understand the Universe, predicting phenomena that have helped us delve into its deepest reaches. It’s helped scientists unlock everything from the Big Bang to Dark Energy.

What’s more, if we did not take the effects of General Relativity into account, there would be no electromagnets to power our computers, motors and generators, and your GPS system would break. Click here to find out more.

### What else?

Another consequence of General Relativity is that it can slow time. This is an effect called time dilation. It can also make objects shorten, an effect called length contraction. These two things don’t appear in our everyday lives as their effects are only apparent when you move close to the speed of light. We’ll look into these phenomena and Einstein’s infamous E=mc2 equation in our next post. For now, here’s a great visualisation of General Relativity:

#### Einstein’s Masterwork

1915 and the General Theory of Relativity by John Gribbin.

#### General Relativity

Explained as you’ve never seen it before thanks to Science.

#### Relativity at 100

A collection of articles from Nature celebrating the theory’s centenary.

### Black holes

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Dark energy
December 4, 2019

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