13.3 Time travel. Ivan's doctor breaks the sad news that Ivan has been diagnosed with an incurable disease and has just over 2 years to live. "Medical researchers are working on a cure right now," the doctor explains, "and it will be available in twelve years. But I'm afraid that this will be too late to help you." Rather than give in to despair, Ivan jumps into a fast rocket ship and travels at high speed. When he returns to Earth he has aged 2 years and is near death. But 12 years have elapsed on Earth, the medical research has been successful, and Ivan is cured. How fast did Ivan travel? (Ignore the brief periods of acceleration.)

13.3 Time travel. Ivan's doctor breaks the sad news that Ivan has been diagnosed with an incurable disease and has just over 2 years to live. "Medical researchers are working on a cure right now," the doctor explains, "and it will be available in twelve years. But I'm afraid that this will be too late to help you." Rather than give in to despair, Ivan jumps into a fast rocket ship and travels at high speed. When he returns to Earth he has aged 2 years and is near death. But 12 years have elapsed on Earth, the medical research has been successful, and Ivan is cured. How fast did Ivan travel? (Ignore the brief periods of acceleration.)