Editing Time dilation (section)

===Reciprocity===
[[File:Time dilation02.gif|thumb|Transversal time dilation. The blue dots represent a pulse of light. Each pair of dots with light "bouncing" between them is a clock. In the frame of each group of clocks, the other group is measured to tick more slowly, because the moving clock's light pulse has to travel a larger distance than the stationary clock's light pulse. That is so, even though the clocks are identical and their relative motion is perfectly reciprocal.]]
Given a certain frame of reference, and the "stationary" observer described earlier, if a second observer accompanied the "moving" clock, each of the observers would measure the other's clock as ticking at a ''slower'' rate than their own local clock, due to them both measuring the other to be the one that is in motion relative to their own stationary frame of reference.

Common sense would dictate that, if the passage of time has slowed for a moving object, said object would observe the external world's time to be correspondingly sped up. Counterintuitively, special relativity predicts the opposite. When two observers are in motion relative to each other, each will measure the other's clock slowing down, in concordance with them being in motion relative to the observer's frame of reference.

[[Image:Eigenzeit.svg|right|thumb|Time UV of a clock in S is shorter compared to Ux′ in S′, and time UW of a clock in S′ is shorter compared to Ux in S.]]
While this seems self-contradictory, a similar oddity occurs in everyday life. If two persons A and B observe each other from a distance, B will appear small to A, but at the same time, A will appear small to B. Being familiar with the effects of [[Perspective (visual)|perspective]], there is no contradiction or paradox in this situation.<ref>{{Cite book |last=Adams |first=Steve |url=https://books.google.com/books?id=1RV0AysEN4oC&pg=PA54 |title=Relativity: An introduction to space-time physics |publisher=[[CRC Press]] |year=1997 |isbn=978-0-7484-0621-0 |page=54}}</ref>

The reciprocity of the phenomenon also leads to the so-called [[twin paradox]] where the aging of twins, one staying on Earth and the other embarking on space travel, is compared, and where the reciprocity suggests that both persons should have the same age when they reunite. On the contrary, at the end of the round-trip, the traveling twin will be younger than the sibling on Earth. The dilemma posed by the paradox can be explained by the fact that situation is not symmetric. The twin staying on Earth is in a single inertial frame, and the traveling twin is in two different inertial frames: one on the way out and another on the way back. See also {{slink|Twin paradox|Role of acceleration}}.