I found the following in a locked thread (https://www.physicsforums.com/showthread.php?t=16514), and it perfectly illustrates how the discussion about these things usually starts (it's always the same): - What if two objects (A and B) move away from each other at the speed of 0.7c relative to some third observer C..? Do they (regarding to observer C) have a relative speed of 1.4c? This is called the "closing speed" regarding to Wikipedia. The answer then usually contains something about "you obviously don't understand this stuff", and in the next second you get a standard school-answer proving that the one giving the answer doesn't understand it either. Finally someone says: "the actual relative speed is 0.94c". Now my question is, regarding to who is the speed 0.94c? How is this velocity defined? Remember that there is no absolute motion. So which observer (A, B or C) will measure this speed? A and B will never be able to do any measurements on each other whatsoever, since no light from A will ever reach B and vice versa. (Why? Because the "light cone" says so, or because A actually moves at 1.4c relative to B? Or are both these answers equivalent?) So my guess would be C!? But C measures 1.4c, doesn't he/she? My actual guess would be A or B. We could let them collide with each other and measure the released energy, right? No. The energy released in the collision would be the same as if they actually travelled at 1.4c relative to each other. Hmm.. Any suggestions? And finally about light cones... They are just a workaround to say the forbidden words: "A and B both move at 1.4c relative to each other". They do so by saying that "the space between A and B expands while the light travels between them", which is exactly the same thing. The problem occurs when the light from A actually hits B, in which case B measures the speed of the light to the everlasting constant c. But this is another topic. It is about Lorentz transformation.