Superluminal Speeds and All That Jazz A popular prejudice is sometimes expressed as “nothing can travel faster than light”. But Special Relativity (SR), from which this prejudice is derived, in fact teaches only that “nothing can be observed to travel faster than light”. The meaning of ‘observed’ here is technical and can be unpacked by explaining how distances are measured and clocks are synchronized with light signals in SR. It may then come as a relief to realize that SR creates no mysterious speed barrier. SR does not prevent one traveling to a star, measured on Earth to be light-years away, in just a few hours; SR dictates only that travel-time be measured by the traveler on his/her clock, not by an Earth-bound stay-at-home. It would be misleading, though, to call the traveler’s speed ‘superluminal’, say because at home on Earth the distance the that the traveler covered was measured as a few light-years. From an SR point of view, this would be like comparing apples and oranges. In nature as described by SR there are no superluminal speeds that can be measured as such. What about superluminal speeds in General Relativity (GR)? ‘Superluminal speeds’ are certainly talked about when dealing with models of the universe based on GR, in many threads in this forum. Sufficiently distant parts of a model expanding universe are sometimes said to ‘separate at superluminal speeds’. To me, this seems in an inverse sense just as simplistic as saying that “nothing can travel faster than light”. In GR speed is not a simple concept: First, in GR ‘to separate’ is generally taken to mean to increase ‘the proper distance between’. Second, ‘proper distance’ means the sum of a chain of ‘local’ proper distances, measured as in SR, over a span in which spacetime is perceptibly non-Euclidean (‘distance’ could also measured by radar, as in SR, with possibly different results). Third, proper-distance measurements are to be made simultaneously, at the same instant of a defined ‘time’ (in consensus cosmology ‘simultaneous’ means when observers measure the same density of mass/energy, locally, in a model universe.) In GR the concept of ‘speed’ needs a lot of unpacking! If, after all this unpacking, one gets an answer that exceeds c, there is in my view nothing to make it worthwhile to describe a speed which in practice you cannot measure or observe as ‘superluminal’. Using 'superluminal' suggests that something special or exceptional is being uncovered. In cosmology, getting an answer for a speed greater than c in an expanding universe means simply that one is extrapolating a model into unknowable regions beyond the (dynamic) red horizon of the observable universe. In observable nature as described by GR there are also]no superluminal speeds that can be measured as such. Why then bother with the word ‘superluminal’ at all, I ask?