I don't understand why >= light speed is impossible. If 2 objects are on a collision course and each is traveling >= 0.5 light speed, aren't they essentially traveling at the speed of light relative to one another? It seems that 0.5 light speed should be achievable......how much energy would it require to propel e.g. 1,000 kg object to 0.5 light speed within 1 light year? Or if it had X amount of continuous thrust for unlimited time, shouldn't it at some point reach at least 0.5 light speed relative to it's starting point? I'm not a physicist or mathematician......just trying to understand the concept of speed....it seems almost like an illusion.....except when things collide and release energy.