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*.
You are right, it needs a reference point. Generally this would be implied as the earth or a destination.
Car 1’s speedometer reads 10 mph. Car 2 is travelling in the opposite direction at the same speed. From the reference frame of someone sitting in car 1, car 2 appears to be going 20 mph. From the reference frame of someone standing on the side of the road, both cars are going 10 mph in opposite directions (10 mph and -10 mph where positive and negative describe direction).
“Reference frame” just means “looking at it from this spot.”
If you look at a rocket from space, the rocket is moving. If you look at space from the rocket, space is moving. Ezpz
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