### What is a light cone?

// November 28th, 2010 // 1 Comment » // How Things Work

Simultaneous is not simultaneous. Space and time do not exist. These, and other strange and wonderful things, are the hallmarks of Einsteins Theory of Relativity and what we’re talking about today.

A few days ago I got Request a Post which read:

*ahoy there, cap’tun! a prawn from beneath your deck speakin’. could you do a post on Light Cones? yeh know, the stuff Stephen Hawking talks about in the second chapter of his ‘Brief History of Time’? i’d love to read a simplified version. i simply can’t wrap the wiki article around my head! danke!*

Ahoy there Prawn! Danke for requesting and here be yer post.

Something can’t move through space without moving through time as well. The Theory of Relativity dispenses with time and space, and instead describes a new thing called Spacetime.

Imagine a train moving in a straight line to the Eastward. We can map its movement through Space, Time or best of all, Spacetime! Notice how much more epically cool Spacetime looks. This will be important later.

With our Spacetime graph, we map space horizontally and time vertically.

In the realz world, there are three dimensions of space – length, width and height. Thankfully there is only one dimension of time, and this runs from past to future (unless you’re looking backwards.) To graph something using all four dimensions is hard, so instead we just take two dimensions of space and make a horizontal plane, then map time vertically. This will make sense in a second.

Now imagine a flash of light. Say you were standing on a hill on a moonless night and turned on a torch for a second. The light would spread out in every direction, lighting first the bushes near by, then later the trees in the distance. If we think about it in just 2 dimensions we can draw it like this, kind of like ripples in a pond.

If we map time vertically, and stack these pictures up above each other, we get a cone. So a Spacetime graph looks like this. Notice again that it is epically cool.

So a light cone is a flash of light moving through Spacetime. Usually people draw it as two cones. The bottom one is light collapsing into a single event, and the top one is it exploding out again.

You might be thinking – why bother?

Because this is a more accurate way to imagine the world. It shows how the past can influence the present and the future. When light cones overlap, it means two objects or events interact with each other. Every event in the Universe has an associated light cone. It’s a mathematical way to represent the Universe, and the basis for lots of complex physics (such as curved Spacetime, and why simultaneous events are relative to the observer.)

The theory of relativity replaced the absoluteness of space and time with the absoluteness speed of light.

I hope this answers ye question Prawn. Find out more about awesome light cones here.