According to our current understanding of physics, it is not possible for a massive object with mass to travel at the speed of light. As an object with mass approaches the speed of light, its energy requirement approaches infinity, making it physically impossible to reach or exceed the speed of light.
However, I can provide an explanation based on the principles of special relativity for a thought experiment involving two observers in a spaceship moving at a significant fraction of the speed of light (but not exactly at the speed of light).
In this scenario, if two observers are traveling in a spaceship moving close to the speed of light relative to an observer outside the spaceship (at rest relative to the galaxies), they will experience different observations due to the effects of time dilation and relativistic velocity addition.
Observer A is inside the spaceship, and Observer B is outside the spaceship, at rest relative to the galaxies. Observer A, moving with the spaceship, will perceive distances and times differently compared to Observer B.
According to the principle of time dilation in special relativity, as an object approaches the speed of light, time appears to slow down for that object relative to a stationary observer. Therefore, Observer A will perceive time passing more slowly compared to Observer B.
Additionally, according to the relativistic velocity addition formula, the relative velocity between two objects moving at significant fractions of the speed of light does not simply add up linearly. Instead, it follows a non-linear formula that takes into account the effects of time dilation. As a result, Observer A will observe the galaxies to be moving towards them at a faster rate than Observer B.
So, in this scenario, Observer A inside the spaceship would perceive the galaxies rushing towards them, while Observer B outside the spaceship would not observe any unusual movement of the galaxies.
It's important to note that this scenario is a hypothetical thought experiment, as objects with mass cannot actually reach or exceed the speed of light. The principles of special relativity are used to explain the behavior of objects moving close to the speed of light, but they do not apply directly to objects traveling at the speed of light itself.