Question

Describe the Twin Paradox

Answer 1

The twin paradox is a thought experiment that demonstrates the curious manifestation of time dilation in modern physics, as it was introduced by Albert Einstein through the theory of relativity.

Consider two twins, named Bill and Cliff. On their 20th birthday, Biff decides to get in a spaceship and take off into outer space, traveling at nearly the speed of light. He travels around the cosmos at this speed for around 5 years, returning to the Earth when he is 25 years old. Cliff, on the other hand, remains on the Earth. When Bill returns, it turns out that Cliff is 95 years old.

So what has really happened ?

According to relativity, two frames of reference that move differently from each other experience time differently, a process known as time dilation. Because Biff was moving so rapidly, time was in effect moving slower for him. This can be calculated precisely using Lorentz transformations, which are a standard part of relativity.

But in relativity, what one observer sees as happening to a second one, the second one sees as happening to the first one. To the space-going brother, time moves more slowly on Earth than it does in her spacecraft; when he returns, his Earth-bound brother is the one who is younger. How can the space-going twin be both younger and older than his Earth-bound brother?

The answer is that the paradox is only apparent, for the situation is not appropriately treated by special relativity. To return to Earth the spacecraft must change direction, which violates the condition of steady straight-line motion central to special relativity. A full treatment requires general relativity, which shows that there would be an asymmetrical change in time between the two brothers. Thus, the “paradox” does not cast doubt on how special relativity describes time, which has been confirmed by numerous experiments.