I’m just confused about what the reference point is when we say something like “this object in space is moving at 4% the speed of light”, what is that in relation to?
In: Physics
As long as you specify which reference frame you’re using, when making your measurements, it doesn’t physically matter which frame you choose.
Any reference frame is going to be as valid as any other. That fact is a consequence of special relativity and general relativity.
If you have one rocket moving away from you at 0.99*c*, and one moving toward you at 0.99*c*, then that’s exactly what you measure. From their reference frames, though, looking at each other, effects of length contraction & time dilation will mean that the rockets will see each other as still being less than the speed of light *c*, and not 1.98*c*.
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