"at rest" in Einsteinian relativity I'm just wondering what the concept "at rest" means in Einsteinian relativity. The common thought experiment of an observer on a platform (Albert) and an observer on a train (Henry), moving relative to each other, is often used to explain relativity. If we consider things from Albert's perspective, then Albert's co-ordinate labeling system will label him and the platform as having a zero velocity, while attributing all of the relative velocity to Henry. Albert's co-ordinate labeling system will label Albert and the platform as "at rest" but label Henry as "in motion". The question is, what does "at rest" mean in that scenario? I don't think it can simply mean the platform and Albert are "at rest" relative to each other, because Henry is also included - plus Albert can conduct numerous experiments to determine if he is at rest relative to the platform (contrary to the stated consequence of the PoR). We can divest the thought experiment of everything except for just the two observers - putting them in "empty space". In this case Albert's co-ordinate labeling system will label him as "at rest" with a zero velocity, this is despite the relative motion to Henry. What does the "at rest" mean in Einsteinian relativity?