by Matthew Vannette

Created on: January 27, 2007   Last Updated: May 02, 2007

The clock paradox is more often referred to as the twin paradox. It, like all paradoxes in science, comes about due to a lack of applicability of a theory to a particular experiment.

The twin paradox is based on the verified fact that the faster an inertial reference frame (IRF) moves, the slower time passes in said IRF from the point of view of someone outside of the IRF. This can be thought of in the following way. You are standing on the side of the road and a friend is driving past you in a car. The car is moving very fast, say 90% of the speed of light (0.9c, in the parlance of physicists). If you, on the side of the road, look at a clock in the car, you will see that 'one second' in the car takes more than 'one second' on a watch on your wrist.

However, one of the postulates of the special theory of relativity is that the *uniform motion* of a reference frame (that is, constant velocity, so no speeding up or slowing down or even changing direction) will not affect any experiment you do. This boils down to saying there is no way for you to do an experiment to determine if you are moving one way or if the rest of the Universe is moving the other way. So, this means your friend in the car would say (s)he is sitting still and you are moving at 0.9c in the opposite direction. Therefore, from your friend's point of view, 'one second' on your wrist-watch takes more than 'one second' on the car clock. Both of you say the other's clock is moving slower than your own. The thing is, the time dilation, as it is called, is not limited to clocks. It affects everything in the other frame, except the speed of light. In particular for this discussion, all biological processes (breathing, heart rate, thought processes, digestion, etc.) are slowed in the moving frame when viewed from the outside frame.

Now, let's think about a set of twins, one of whom (Terrence) wants to stay on Earth while the other (Estelle) wants to explore the near stars. Estelle gets on a ship and travels to a nearby star system at 0.9c. Once she gets there, she stops, has lunch, looks around, and decides to head home. So, she flies back to Earth at 0.9c. The question is, what is the age difference between the twins when they are reunited?

From Terrence's point of view, Estelle was moving at 0.9c toward the other star for a bit and 0.9c toward the Earth for a bit. As Terrence looks at Estelle's clock he would say her clocks were moving slower. Therefore, she was aging slower and she should be younger

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