Final answer:
If a 25-year-old person were able to travel at the speed of light and leave the Earth for 50 years, they would be 25 years old when they returned.
Step-by-step explanation:
If a 25-year-old person were able to travel at the speed of light, leaving the Earth for 50 years, they would be 25 years old when they returned to Earth.
According to the concept of time dilation in special relativity, time slows down for objects moving at high velocities relative to an observer at rest. As the person approaches the speed of light, time dilation becomes more significant. This means that while 50 years would have passed on Earth, only a few years would have passed for the person traveling at the speed of light.
However, it is currently impossible for objects with mass to reach or exceed the speed of light. The theory of relativity limits the maximum velocity to the speed of light, and as an object approaches this speed, its mass increases infinitely, making it impossible to accelerate further.