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Relativity Principle #4
Simultaneity
Our runner disagrees with us about where the light beam is at any given time.
Suppose the beam hits an asteroid after 3 seconds (for us). 3 seconds for us would be 2.598 seconds for the runner.
Our runner thinks he has been traveling for 2.598 seconds and is 1.229 light seconds from the asteroid, so he calculates that the asteroid got hit 3.897 seconds after the race started. That’s 3.375 seconds of time for the stationary observer.
The two disagree about what time the asteroid got hit!
Notes: