Editing Time dilation (section)

== Invisibility ==
Time dilation is a relationship between clock readings. Visually observed clock readings involve delays due to the propagation speed of light from the clock to the observer. Thus there is no direct way to observe time dilation. As an example of time dilation, two experimenters measuring a passing train traveling at .86 light speed may see a 2 second difference on their clocks while on the train the engineer reports only one second elapsed when the experimenters went by. Observations of a clock on the front of the train would give completely different results: the light from the train would not reach the second experimenter only 0.27s before the train passed. This effect of moving objects on observations is associated with the [[Relativistic Doppler effect|Doppler effect]].<ref>{{cite journal |last1=Hughes |first1=Theo |last2=Kersting |first2=Magdalena |title=The invisibility of time dilation |journal=Physics Education |date=5 January 2021 |volume=56 |issue=2 |pages=025011 |doi=10.1088/1361-6552/abce02 |bibcode=2021PhyEd..56b5011H |doi-access=free }}</ref>