After reading a popular science book, I have a problem that I don't quite understand. It's basically like this: if I start from the earth and fly in a spaceship at a speed close to the speed of light (the unit of light speed is c), (for example, to fly to Pluto, you need to stay in a spacecraft for more than ten years to get there), and you think it has been more than ten years since you started, but at this time, people on earth have been more than twenty years or thirty years. Why? (I don't know if it's wrong, but I can't understand these time problems.)

After reading a popular science book, I have a problem that I don't quite understand. It's basically like this: if I start from the earth and fly in a spaceship at a speed close to the speed of light (the unit of light speed is c), (for example, to fly to Pluto, you need to stay in a spacecraft for more than ten years to get there), and you think it has been more than ten years since you started, but at this time, people on earth have been more than twenty years or thirty years. Why? (I don't know if it's wrong, but I can't understand these time problems.)

According to Einstein's theory of relativity, when the speed reaches 86% or more of the speed of light, the expansion of time can be observed. If the spaceship departing from the earth to Pluto flies at 86 +% of the speed of light, the spaceship's time will slow down relative to the time of the earth when it departs