The constancy of the speed of light is a fundamental principle of special relativity, which states that the speed of light in a vacuum is always the same for all observers, regardless of their relative motion. This principle has been experimentally verified and is a cornerstone of modern physics.
Time dilation and length contraction are counterintuitive phenomena predicted by special relativity, which arise due to the way space and time are intertwined. According to special relativity, space and time are not absolute, but rather form a four-dimensional fabric known as spacetime.
The key idea is that the measurements of space and time intervals are not independent but are linked together in a way that preserves the constancy of the speed of light. In other words, when an observer is in relative motion with respect to another observer, their measurements of space and time intervals will differ, but in such a way that the speed of light remains constant for both observers.
Time dilation occurs when the relative velocity between two observers is significant. If two observers are moving relative to each other, each observer measures the other's time to be running slower than their own. This means that moving clocks appear to tick more slowly from the perspective of a stationary observer.
Length contraction, on the other hand, occurs when an object is moving relative to an observer. From the observer's perspective, the length of the moving object appears to be shorter in the direction of motion compared to its rest length.
These effects might seem counterintuitive because our everyday experiences occur at speeds much lower than the speed of light, where these relativistic effects become noticeable. However, numerous experimental observations have confirmed the predictions of special relativity, including the effects of time dilation and length contraction.