Apparent length of a moving object
For an object at rest, one simply measures the length with a ruler also at rest.
Example of a moving train.
- To a person on board the train its length can be measured by laying down meter sticks.
- ( Nothing changes! No experiment on the train can detect its uniform motion!)
How can a person on the ground measure the length of the train while it is moving?
- For an object moving wrt the ruler, then one measures the length by the position of the front and the back at the same time.
Thus “simultaneity” in time is part of the measurements of lengths in space!