According to our current understanding of physics, it is not possible for an object with mass, such as a human being, to travel at the speed of light. As an object with mass approaches the speed of light, its energy requirements become infinite, making it impossible to reach or exceed that speed.
However, we can explore the implications of traveling at speeds close to the speed of light, based on the theory of special relativity proposed by Albert Einstein. According to this theory, time dilation and length contraction occur as an object's velocity approaches the speed of light relative to a stationary observer.
If a spacecraft were to travel at a significant fraction of the speed of light, several effects would occur from the perspective of an observer on Earth. Time dilation would cause time to pass more slowly for the people aboard the spacecraft compared to those on Earth. This means that while only a short amount of time would pass for the astronauts, a much longer period would elapse on Earth.
Additionally, length contraction would occur, causing the length of the spacecraft to appear shorter to an external observer as its velocity increases. This contraction would be noticeable in the direction of motion.
From the perspective of the astronauts on the spacecraft, they would experience these effects differently. They would perceive time passing normally for themselves while observing time dilation effects on Earth. The contraction of the spacecraft's length would not be noticeable to them.
It is important to note that reaching speeds even close to the speed of light would require an enormous amount of energy, and currently, we do not possess the technology to achieve such velocities.