This is a continuation of the thread Derivation of proper time of acceleration in SR We have definitions for proper time, proper speed, proper acceleration, coordinate speed, coordinate time, and coordinate acceleration. 1) What is the definition ofproper distance? 2) If proper speed (or velocity) is [itex] v_p = v* \gamma[/itex] then it is conceivable for [itex] v_p [/itex] to exceed the speed of light. Is that true? If so, what physical meaning does that have??? Is there a limit on [itex] v_p < c?[/itex]. Seems like there should be in the sense that there should be NO FR for a given observation in which [itex] v \geq c [/itex] Likewise, proper acceleration or [itex]a_p = a\gamma^3[/itex] also can be [itex] > c [/itex] but I have been told that there is no limit on acceleration other than that the coordinate velocity be [itex] < c [/itex]. Is that valid?