According to the principles of special relativity, as described by Albert Einstein, time dilation occurs when an object or person is moving relative to another observer at a significant fraction of the speed of light. This phenomenon leads to the perception of time passing differently for the moving object compared to the stationary observer.
In the scenario you described, where someone travels in a spaceship at the speed of light for 5 years (from their perspective), a significant time dilation effect would indeed occur. However, it is important to note that traveling at the speed of light is not achievable for objects with mass according to our current understanding of physics. The speed of light is the ultimate speed limit in the universe, and as an object with mass approaches the speed of light, it would require infinite energy to reach that velocity.
Let's assume that instead of traveling at the speed of light, the spaceship is traveling at a significant fraction of the speed of light, such as 99.99% of the speed of light. In this case, time dilation effects would still be present but would be less extreme compared to the speed of light scenario.
From the perspective of the person aboard the spaceship, only 5 years would have passed. This is due to time dilation, where the moving object experiences time more slowly relative to the stationary observer. However, for the people on Earth, a greater amount of time would have passed. The exact amount depends on the velocity of the spaceship and can be calculated using the equations of special relativity.
If we consider the scenario you presented, where the person on the spaceship is 15 years old when they leave and the trip lasts 5 years from their perspective, they would return to Earth at the age of 20. However, during their absence, their friends on Earth would have aged more due to the slower passage of time for the person on the spaceship. The exact age difference would depend on the velocity of the spaceship relative to Earth, but it could result in a significant gap between their ages upon reunion.
It's worth noting that traveling at such high velocities, close to the speed of light, would have other significant effects as well, such as length contraction and changes in mass. These relativistic effects are essential considerations in the theory of special relativity and have been experimentally verified.
While the scenario you presented is an interesting thought experiment, it's important to remember that achieving speeds close to the speed of light with objects of significant mass is currently beyond our technological capabilities. However, the principles of special relativity have been extensively tested and confirmed through various experiments and observations, providing a profound understanding of the nature of time, space, and motion at high speeds.