If you leave Earth at the age of 20 in a spacecraft traveling at the speed of light and spend 5 years roaming space, when you return to Earth you will be 25 years old right? But you’ll find that all your friends you left who were 20 are 70 years old.
Stranger than that could happen if, for example, the trip to a planet in the Serial Woman galaxy, about 2 million light-years away from us. Suppose the spacecraft accelerates with an acceleration of 9,81 meters/square per second, we will find that when the spacecraft returns, 4 million years have passed on Earth, while for you only 65 years have passed. This is a result of Einstein’s theory of relativity and is called the “time slowdown.”