I looked around the net and couldn't find an answer to this particular setup: Suppose I am at rest with two friends (friend A and friend B). We all have a clock. All three clocks are synchronized and tick at the same rate. Now one of my two friends (friend A) starts his rocket engine and leave us at 0.5c. At the same time, my other friend (friend B) does the same, in the same direction, but at 0.9c. When I observe my friend's clocks, they will both look to tick slower than mine. Friend B's clock will be more affected as it is going faster. But the opposite is also true. My friends will look at my clock and see it ticking slower. Friend A will see my clock ticking slower. Friend B will see my clock ticking even slower. How could my clock tick at two different rate at once ? How can my clock tick at a different rate for each possible observer out there (with each their own velocity relative to me) ?