## Celebrating general relativity

Submitted by Marianne on November 16, 2015A hundred years ago, on 25 November 1915, Einstein presented his general theory of relativity to the world. But what exactly is this famous theory and what does it say about the world we live in? To celebrate the centenary of general relativity we bring you a collection of articles, videos and podcasts exploring the theory, Einstein's struggle to find it, and some interesting consequences.

**Einstein and relativity ** — This two-part article traces Einstein's bitter struggle for his greatest achievement and explains what it means in simple terms. The article is based on a talk by the physicist David tong, given to an audience of 16 and 17-year-olds. You can watch a video of the talk here.

**What is general relativity? ** — Physicist David Tong explains the theory and the equation that expresses it. Watch the video or read the article!

**What is a black hole? ** — One of the strangest prediction of general relativity is that the Universe contains black holes. We asked cosmologist Pau Figueras everything you've ever wanted to know about them. Read the articles (What is a black hole – physically? and What is a black hole – mathematically?), listen to the podcasts or watch the video!

**How does gravity work? ** — We explore Newton's gravity, Einstein's gravity and the ripples in space-time called gravitational waves.

**Steady on, Einstein ** — A recent discovery of one of Einstein's unpublished papers shows how he explored the consequences of his general theory of relativity, and that even a genius of Einstein's calibre can occasionally be lured down a blind alley. You can listen to a version of this article as a podcast.

**Problems of gravity ** — Despite the success of general relativity, some physicists are doing what many would consider sacrilege: they are tinkering with the theory, producing modified versions of it. But why?

This image shows one of the most perfect spheres ever created by humans: a fused quartz gyroscope for the *Gravity Probe B* experiment, refracting the image of Albert Einstein in the background. Image courtesy NASA.