My favourite shape
Light cone Matthew Wright
My favourite shape is the light cone. It is a fourdimensional shape lying in space-time, and it is the path travelled by beams of light emied from a single point. Although a simple concept, it turns out to be of fundamental importance: it determines the entire notion of causality. Everything that can be causally aﬀected by an event at one point in space and time must lie within that event’s light cone, since nothing can travel faster than the speed of light. Einstein realised that gravity wasn’t a force in the conventional sense, but rather distorts the structure of space and time, tipping and deforming the light cones in the process. This is why nothing can escape a black hole: the light cones are tipped over so much that everything in the future of the light cone must lie inside the black hole.
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Back cover puzzle answer On the back cover, you were asked to make a cube from five matches. Here is one way to do it: