Usually, we choose the ‘Earth’ or ‘platform’ perspective as the stationary frame within which we display the coordinate axes of the moving ‘spaceship’ or ‘train’ frame. This demonstration rectifies that asymmetry by showing not only how the spaceship frame looks from the perspective of the Earth frame, but also how the Earth frame looks from the perspective of the spaceship frame. Study these diagrams to convince yourself that aside from $v$ being replaced by $-v$ (since the spaceship says that the Earth frame has velocity $-v$), there is complete symmetry between them.
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