Special Relativity is a theory developed by Albert Einstein in the early 20th century to explain the relationship between space and time. It states that the laws of physics are the same for all observers in uniform motion, and the speed of light is constant regardless of the observer's frame of reference.
Special Relativity is especially relevant to spaceships because they often travel at extremely high speeds, close to the speed of light. This theory allows us to understand the effects of time dilation and length contraction on objects moving at such speeds.
Time dilation is the phenomenon in which time appears to pass slower for an object moving at high speeds compared to an object at rest. This means that a clock on a spaceship moving at a high speed will tick slower than a clock on Earth, according to an observer on Earth.
Length contraction is the phenomenon in which an object's length appears to be shorter when it is moving at high speeds compared to when it is at rest. This means that a spaceship traveling at high speeds will appear shorter to an observer on Earth than it actually is.
According to Special Relativity, simultaneity – the concept of two events happening at the same time – is relative and can differ between observers in different frames of reference. This means that what may be considered simultaneous for one observer may not be simultaneous for another observer moving at a different speed. This is because the perception of time and space is relative to the observer's frame of reference.