According to our current understanding of physics, it is impossible for an object, including an astronaut, to travel faster than the speed of light in a vacuum. The theory of special relativity, proposed by Albert Einstein, tells us that as an object with mass accelerates closer to the speed of light, its mass increases, and its length contracts along the direction of motion. Additionally, time dilation occurs, meaning time appears to pass more slowly for the moving object relative to an observer at rest.
As an object with mass approaches the speed of light, the energy required to accelerate it further increases dramatically. In fact, to reach the speed of light, an infinite amount of energy would be needed, which is not physically possible.
So, it is not feasible for an astronaut to travel faster than the speed of light without violating the laws of physics as we currently understand them.
However, we can consider a thought experiment to explore the hypothetical consequences of traveling at or close to the speed of light. If we assume that somehow the astronaut could reach relativistic speeds without being destroyed by the enormous energy requirements, several effects would occur:
Time dilation: Time would pass much more slowly for the astronaut relative to observers on Earth. A short trip for the astronaut could correspond to a much longer time passing on Earth. For example, a few years of astronaut's travel might correspond to decades or even centuries passing on Earth.
Length contraction: As the astronaut's velocity increases, their length in the direction of motion would contract. This means the astronaut's perception of the distance traveled would be different from that of stationary observers.
Increased mass: As the astronaut's speed approaches the speed of light, their mass would increase significantly. This effect is known as relativistic mass increase. This would make it increasingly difficult to accelerate the astronaut further.
Cosmic radiation: If traveling through space at high speeds, the astronaut would be exposed to high-energy cosmic rays, which could be dangerous to their health over extended periods.
Doppler effect: The light from stars and galaxies would experience a significant blue-shift or red-shift depending on the direction of travel. This would affect the astronaut's perception of the universe.
It's essential to note that this thought experiment is purely hypothetical and doesn't reflect a real-world possibility. As of now, our understanding of the laws of physics prevents any object with mass from reaching or exceeding the speed of light. The theory of special relativity has been confirmed through countless experiments and observations, showing that the speed of light in a vacuum (approximately 299,792,458 meters per second) is an ultimate speed limit in the universe.