26.9 The twin paradox

Before you read this, I suggest you read post 26.8. We’re going to think about hypothetical twins. When they become adults, one works in a bank: the other becomes an astronaut. The astronaut goes on a very long journey often at speeds comparable to the speed of light. During this time, the banker twin becomes old and retires from work. Then the astronaut returns home but is much younger than the retired banker. So we have twins who are very different ages! How can this be possible? The previous paragraph describes the twin paradox. It is really a thought experiment based on Einstein’s special theory of relativity. I have two criticisms of the way in which the twin paradox is often presented. One: it is not very important – it’s simply a consequence of time dilation. Two: its name is misleading. So why am I writing a blog post about it? The twin paradox is often presented, without much explanation, to create the idea that physics is mysterious or weird. I want to show that it follows…