An observer on Earth measures the length of a spacecraft travelling at a speed of 0.700c to be 78.0 m long. Determine the proper length of the spacecraft.
If the observer on Earth measures the speed of the spacecraft as 0.700c and its length as 78.0 meters, then engineers aboard the spacecraft measure their vehicle as 142.4 meters long.
They're both correct. There's no such thing as the spacecraft's "real" or "proper" length.
