: : "Observer A" is on the x axis at position 0 and "observer B" is on the x axis at position x = 10. : At t = 0 both set off flashbulbs. : "Observer A" will see his flash immediately but will not see "observer B's" flash until the light has had time to reach him. : So the two events are not simultaneous

: Or

: "Observer A" knows what the speed of light is and can calculate the actual time that event B occurred and he will determine that : the time was t = 0 and he will therefore conclude that the two events were simultaneous.

: Thanks

Lau: 'Simulaneousness' is relative to the obsever. (ie. depending on the what frame the observer is in) After OA (observer A) has calculated the time taken for light to travel 10 units of distance, he'd come to conclusion that it's simultaneous. Note if someone say OC who is travelling at a velocity v (v not= 0) relative to OA and OB, then to him the two events are not simultaneous. (Since by the time light reaches OC he has moved a certain distance.)