 We'll end this segment with a look at the famous twin paradox. It goes like this. Suppose two 20-year-old twins start out together on the Earth. One of them gets into a spaceship for a trip to Vega, traveling at 99% of the speed of light. The person on the Earth sees the trip taking just over 25 years, and the trip back taking the same amount of time. She is over 70 years old when a ship carrying her twin sister arrives back on Earth. But she also observes that her twin's clock ran a good deal slower than hers during the trip. Her twin is aging more slowly than she is. At 99% of the speed of light, time dilation would have the twin at just over 27 years old on her return. But from the point of view of the twin on the spaceship, she is motionless and the twin on the Earth is moving away and back. And like our cosmic ray muons, she sees the distance to Vega at only 3.5 light-years due to space contraction. She also sees the twin on the ground aging slower than her over the 7-year journey. By her observations, her sister will be only one year older on her return due to time dilation. That's 6 years younger than she is, not 27 years older. How can it be that they are both older than the other? This is the paradox. The first thing to notice is that the twin in the spaceship spends several periods of time being accelerated, meaning it is not an inertial frame. This paradox is fully resolved by Einstein's general theory of relativity. We'll cover that in our next segment.