Length contraction and time dilation are both consequences of the theory of relativity, which describes how space and time are intertwined and can change depending on relative motion. While they may appear to be different phenomena, they are actually two sides of the same coin.
In special relativity, length contraction occurs when an object is observed to be shorter along its direction of motion relative to a stationary observer. This contraction is a result of the relativistic effects on the measurement of distances. However, it's important to note that length contraction is not a physical compression of the object itself but rather a consequence of how measurements of length are made in different reference frames.
On the other hand, time dilation refers to the slowing down of time for an object that is moving relative to an observer at rest. This effect means that time appears to pass more slowly for the moving object compared to the stationary observer. Time dilation has been experimentally confirmed and is a fundamental aspect of special relativity.
The reason time dilation is considered "permanent" while length contraction is not is due to the way these effects are perceived in different reference frames. Time dilation is independent of the direction of motion; it applies equally to all objects moving relative to the observer. Therefore, it is a more universal effect and can be considered "permanent" in the sense that it is consistent across different frames of reference.
In contrast, length contraction is direction-dependent. It occurs along the direction of motion but not perpendicular to it. As an object changes its relative velocity or direction, the amount of contraction observed will change accordingly. Therefore, length contraction is not considered "permanent" because it is frame-dependent and can vary depending on the relative motion between the observer and the object being measured.
In summary, time dilation is considered "permanent" because it is a consistent effect experienced by all moving objects, while length contraction is frame-dependent and can vary based on the relative motion between the observer and the object being measured.