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harrylin said:One conceptual model that works for SR is that the car and the light travel (or perhaps one should say "propagate") through the same medium. That's one way to make sense of it.
Another conceptual model that works for SR is that the car and the light describe trajectories through Spacetime, interpreted as a 4D physical "medium".
There were never ending debates on this forum in which people, in vain, tried to disprove either or both interpretations, and a stop of such debates is being enforced here. Remains that you can choose the interpretation you like: whichever fits better with how your brain is wired.
You can find the debates including elaborations of how those models work by means of a search on this forum of such terms as "block universe".
Numbers - speed of object
Light from any moving object will arrive at the observer at C. That observer will calculate the speed of light with relation to the moving object as: C minus speed of object.
Example
Light from a spacecraft heading towards Earth at 0.99c will arrive at Earth at speed C. The speed of the spacecraft is 0.99C. The speed of light is C. Therefore the speed of object with relation to the light emitted is 1C - 0.99C = 0.01C according to observers on Earth
For the comet the speed of light with relation to itself would obviously not be 0.01C, it would be C. The comet would calculate that the light arrived at Earth at 1.99C (from the comets perspective)
The formula for adding velocities is not required for the above analysis. It would be required if the comet traveling towards Earth at 0.99C fired a rocket towards Earth at 0.2C (or any velocity) relative to itself. Then the speed of the rocket relative to Earth would be calculated using the velocity addition formula.
Sources
"Spacetime Physics"
http://galileo.phys.virginia.edu/classes/252/time_dil.html
Interpretation
I don't think the special theory of relativity is matter of "interpretation" - particularly for non experts in the field (like me). What difference does it make how one's brain is "wired"? How is that relevant to the facts? How can physics be a matter of personal choice?
Where you describe a "medium" through which light and the car "propogate", would such a medium be analogous to the ether? How can that be "one way to make sense of" the Theory.
How can you describe Spacetime as "another conceptual model that works for special relativity"?
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