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Relativistic Speed
Speeds that are a significant fraction of the speed of light, where the effects of time dilation become perceivable.
Lorentz Factor
Measures the factor by which time differs in two moving frames of reference. Gets bigger as relative speeds approach the speed of light.
Twin Paradox
Refers to the logical contradiction that if “moving clocks run slow” and each twin sees the other as moving relative to themselves, each one should expect that the other will be younger than themselves when they reunite.
Light Year
The distance light travels in one year. A distance unit.