A "Seriously Heavy Point"
It is very important to consider the addition/subtraction of three velocities. If I am standing at Point A and I see a light ray flashing past past another B I would be interested in the three velocity of the light ray[my own three velocity being the null vector]. This is relevant to the issue in thread#6 --->http://physicsforums.com/showpost.ph...10&postcount=6
In threads #19 and #25 [http://physicsforums.com/showpost.ph...7&postcount=19
"Curved Space-time and the Speed of Light" DaleSpam has tried to counter the concept of Relative Velocity in Curved Space-time in by giving examples in relation to 4-D space,I mean by referring to four vectors.[ Of course these examples have failed in their mission]
He has kept silent on the issue of the addition/subtraction of 3-velocities!
The issue of subtraction of three velocities at a distance,especially when one is null, is extremely relevant to the discussion in thread #6
I am saying all this not to give any extra fortification to what ever I have said in relation to 4D considerations but because these points are seriously heavy. My assertions in relation to 4D concepts are strong enough to stand on their own feet.