We started our study of Einstein’s special theory of relativity this week. Special relativity is tricky get your head round, so I’ve put together a collection of videos that help to explain the ideas we’re going to consider. Let’s start with a video about the speed of light.

The next video follows Einstein’s thought process as he worked through his special theory of relativity.

When I walked around the room throwing the tennis ball, I showed that what I saw as purely vertical motion looked to you like the curved path of a projectile. For me (the moving observer) and you (the stationary observer), the ball appeared to travel different distances. Extending this situation to a ray of light, as in the video, we need to consider two aspects of special relativity, **time dilation** and **length contraction**. The speed of light can only be constant for all observers if something happens to time and length (distance) when considering fast moving objects.

We’ll look at time dilation first.

**time dilation**

A Tale of Two Twins from Oliver Luo on Vimeo.

Here is another take on special relativity and the twins paradox

…and the Glesga Physics version

**length contraction**

This video has helpful examples to explain length contraction.

Sometimes it’s easier to imagine we’re a stationary observer watching a fast moving object go whizzing past. For other situations, it’s better to put yourself into the same frame of reference as the moving object, so that ** everything else** appears to be moving quickly, while you sit still. The muon example in this video shows how an alternative perspective can work to our advantage in special relativity.

Another way to think about this alternative frame of reference is that it’s hard to measure distances when you yourself are moving really quickly. Think about it, you’d get tangled up in your measuring tape like an Andrex puppy.

###### image: trotonline.co.uk

It would be far easier to imagine you’re the one sitting still and all the objects are moving relative to your position, as if your train is stationary and it’s everything outside that’s moving. That keeps everything nice and tidy – including your measuring tape. Got to love Einstein’s postulates of special relativity.

###### image: mirror.co.uk