 You may recall from our segment on special relativity that the time dilation due to velocity creates a paradox. It goes like this. Suppose two 20-year-old twins start out together on the Earth. One of them gets into a spaceship or 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 in her own reference frame, and the twin on the Earth is moving away and back. In addition, 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 1 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. But there is at least one point where the twin in the rocket is not in an inertial reference frame. As the spaceship approaches Vega, it decelerates to a stop and then re-accelerates back to Earth. The traveling twin finds that she is in a gravitational field, and gravitational time dilation needs to be taken into account. Let's say her acceleration is 10 Gs, or 98 meters per second squared. At this rate, it would take her 35 days to decelerate to zero, and another 35 days to re-accelerate back to 99% of the speed of light. Ravitational time dilation shows that as her clock ticks 70 days, her twin's clock on Earth will have ticked 18,134 days. That's 48 years. The twin on Earth agrees. So instead of both twins thinking the others should be younger, they both agree that the twin on the rocket to Vega and back is younger. No contradiction is involved, and the paradox is resolved. The general theory of relativity is now 100 years old. All the basic tests have shown it to be an accurate description of nature as we find it. The implications for astronomy have been enormous. In the next chapter, we'll cover gravitational lensing and how it enables us to see deeper into space than anyone ever thought possible.