I realize that the understanding of physics and the perferred description or mathematical representation of our understanding evolves over time. So I have a feeling that the answer to my question is strongly a mix of history and physics. Please, when at all possible add some of the historical context into your answers to help flesh out my understanding. -Thank you. My questions involves: It seems to me that what special relativity "actually says and doesn't say" has changed over the years, and this can cause confusion to some new students. How did what physicists consider "Special Relativity" evolve over the years? In particular, in what way has what they feel it predicts/covers change? Is there a definitive "current" source that every phycist would agree on, or once people get nit picky will the number of opinions approximate the number of people asked? (And yes, that last part will probably be inevitably played out on this board. And please no crackpots, there is no place for that in this discussion. People like Herbert Dingle and the opinions of non-mainstream scientists do NOT count. I'm asking about the evolution of (and current) mainstream understanding of SR.) Some people may wonder why I think anything evolved at all. So let me give my understanding on this. Let me start from the end and then see the path from 1905 to the current now. Current: When I speak to theoretical phycists (mostly grads and profs), they seem to view SR as merely a requirement of "global lorentz symmetry" for lagrangians describing a system in an inertial coordinate systems. And what survives in GR is "local lorentz symmetry" for lagrangians describing a system. This view automatically gives an upper speed limit in an inertial coordinate sytem which all such systems agree upon (the fact that it happens to be the speed of light in this case is merely a coincidence based on light being massless, it seems in this view of relativity as just a symmetry requirement, that it doesn't require light to be this speed. But experiment shows that it is.) I'm sure some people will dislike this wording, but it is undoubtably a common view (the distinctions this causes will become more clear in a bit). The beginning 1905 - (English translation, for I can't read german.)http://www.fourmilab.ch/etexts/einstein/specrel/www/ Two postulates 1) ``Principle of Relativity'' - the same laws of electrodynamics and optics will be valid for all frames of reference for which the equations of mechanics hold good 2) light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body Inertial frames seem to be implied for #2 (maybe my bias is causing history revisionism, can someone comment on this), so I will assume he originally meant it as such. It should become instantly clear what I mean by "evolving understanding" just by looking at his original phrasing here. Einstein showed that Newton's laws of mechanics needed revising ... so what frames does he mean by "frames of reference for which the equations of mechanics hold"? This is the first example in many where every one who reads it "knows what he means", and fills in what he feels is the essence of that phrases understanding. In fact, any term which has survived many centuries we treat like this. What is a Force? What is an inertial frame? They are very hard to define precisely without making the use you wish for becoming tautological. This is because when we see these phrases we fill in with the "essence" of what we understand these phrases to mean. I remember reading an article by Nobel laureate Frank Wilczek complaining about the notion of "Forces" for a similar reason. 1907 - The Geometric view Hermann Minkowski introduced a way of viewing special relativity as a 4-dimensional space-time with lorentzian symmetry. For example, the four-vector relating to energy-momentum is a geometrical object - it is invarient. It is only our coordinate representation of it which changes depending on our choice of coordinate labels. This led eventually to a coordinate free way of looking at things. 1915/1916 - General relativity Now the old theory of relativity is just a special case, and that 'principle of relativity' finally becomes "special relativity" (not sure when this phrase was first used). Now any coordinate system is acceptable, and at least in current times, the geometical view becomes dominant (which also shapes how SR is introduced in many cases). Modern "common phrasing" of SR in introductory books (that don't take the symmetry approach I mentioned at the begginning which theorists seem to consider the "modern view") is usually along the line of: First postulate - Special principle of relativity - The laws of physics are the same in all inertial frames of reference. Second postulate - Invariance of c - The speed of light in a vacuum according to an inertial frame of reference is a universal constant, c. So, my large gaps in the history asside, to help show some of the evolution of ideas, let me point out some questions that throughout this history I am not sure everyone would have agreed on depending on their "version" of SR: In the beginning notice that Einstein doesn't say "all laws of physics" will be the same for all inertial frames. He specifically mentions the laws of electrodynamics. So if they had data on the life times of energetic muons appearing longer to us (due to the weak force - not handled by electrodynamics), would physicists of that time considered that support of relativity? Or would they consider that a trivial extension of relativity? In the beginning, Einstein seems to be restricting himself to "all frames of reference for which the equations of mechanics hold good". SR shows the second and third laws of Newton need some adjusting, so is he suggesting we define an inertial frame by Newton's first law alone (frames in which unperturbed objects move in straight lines with constant velocity)? Clearly not (although I have seen people use this definition of inertial frame), for this does not restrict how we synchronize our clocks, so I can easily get frames where the speed of light is not a universal constant. Those last two are just warm ups to show that our understanding of several terms have evolved some over the years. But not much physical content is there. Let's look at something that people probably would really have varied their opinions on over the years. Let's say we have an inertial coordinate system (unprimed) and I define a new coordinate system (primed) as follows: t' = t + A x' = x y' = y z' = z Where A is a constant. I sure everyone would agree that this is still an inertial coordinate system. So the "common phrasing" above seems to require the laws of physics be invarient to this transformation. But time translational symmetry gives energy conservation. Would you say special relativity requires energy conservation? Similarly, I can do this with spatial translation. Would you say special relativity requires momentum conservation? Now consider this one: t' = t x' = -x y' = -y z' = -z I very much believe phycists of 1905 would consider this an inertial coordinate system. Would they believe special relativity requires parity symmetry in the laws of physics? While it was after Einstein's time, in 1957 the laws of physics were experimentally shown to exhibit parity violation. No one currently worries if this violates special relativity (I don't know if anyone ever did, does anyone have some historical info?). Considering special relativity the way the theorists worded it above, there is no worry as special relativity is merely interested in Lorentz symmetry. But why? And how? did this come to be? How do people decide what the "essence" of the theory is, so that it evolves without anyone considering it changing? What really amazes me, is that with the current reduction to just requiring Lorentz symmetry, this understanding of SR could have been true even if light did use a medium. Just as sound propagation in a medium fits fine in SR as the lagrangian still has lorentz symmetry. Although if light did use a medium it would have taken us a lot longer to discover SR, so we should be grateful it doesn't. In my opinion, it seems the theory has evolved quite a bit. It makes it more difficult when people seem to not acknowledge this and there is no precise and definitive current phrasing. I am very interested in hearing your views on this (and especially corrections and additions to the historical context here).