Hello everyone, One of the conclusions of relativity is that no massive object can move at v ≥ c, but with respect to what, I don't understand? Let me try making my question clearer with an example: Suppose there are two bodies both moving side by side at v=0.9c relative to earth. The relative speed between the two bodies is 0. So I wonder, is it possible for one of the bodies to accelerate to 0.1c or more with respect to the other body? It would seem perfectly possible as long as the moving bodies are all we're looking at, but there's also earth to consider, relative to which, the speed of the moving body was already 0.9c. So I'm a little confused; would that body be able to accelerate to 0.1c relative to the other body making the relative velocity between it and the earth equal to 1c? I suppose not, but considering that some basic equations of special relativity, like time dilation, have only the relative velocity variable in them between the two moving frames in question... it seems, at first glance, that it just might be possible... So I could definitely use some help here, lol. Thank you!