Hello everyone, and thanks for reading. This will probably pretty long despite editing attempts, be forgiving :-) I'm currently reading a book called "Introducing Einstein's relativity" by Ray d'Inverno. This is my hundredth attempt to try and fundamentally understand special relativity. I have never heard of the "K-calculus" until I've read this book and was intrigued. However, I again fail to understand some really basic derivations in the book. I hope I will be able to portray my problems clearly enough. Oh, by the way, I've done a bachelor in Maths and Physics (in Israel) 2 years ago, and planning on doing my master (in Germany) now. Just so you'd know I'm familiar with F=ma and so on :-) However, I found myself encountering very basic questions. I'm either rusty (the questions might sound and even be very stupid), or I've developed a new (to me) form of critical thinking, but that's how it is. The questions are sort of connected. That means, that by fully answering one I might be able to understand the rest. Might. :-) Here goes - 1. When one draws a time-space diagram and world-lines in it, what is the significance of the coordinates on the different world lines and events? Are they the coordinates that I, the drawer, as an observer measure? Would it be correct saying, that every time I see such a diagram, I could draw my own world line simply as the y-axis? I'm bothered, cause somehow, in the book, it's never mentioned who's doing the drawings. For example, the description sounds like "Observer B is moving with the velocity of v in respect to observer A", and the we see their world lines. Are "we" then observer A? Are the coordinates written on the diagram all in respect to observer A's clock and measuring equipment? What if there is no observer at rest? Are we a third observer? --panic-- :-) 2. This is a hard one: Events. Events happen (ideally) at a point in space-time, (t,x). What I don't really get, is how events are defined and what do they portray. 2.a When the event is "A is sending a light signal for a split of a second" I can "dig it". But if it's a moon, for example, that's just there (radiating constantly). Then A wants to "measure" the event, or more appropriately, to fit coordinates to it. He then sends a light signal to that moon, awaits it's reflection and writes the times down. (that's how it's been described in the book, and probably how it is in reality as well). But then - what do get here? The coordinates of the moon? However, the moon has definitely moved 'till we got out signal back - so what do these coordinates describe? Where the moon was as the light signal arrived to him? What are we exactly measuring? On the other hand - how can we measure an event happening in the split of a second? For example, if B smiles to me on the horizon for an instant, how am I to measure that? Are these then two prototypes of events? 2.b The book states that the distance coordinate is determined through the expression: 0.5(t2 - t1), where t1 and t2 are the times of the the sending and the acceptance times of the light signal respectively. That's clear (c being 1), if the space coordinate are meant to describe the location of the "event" when the light signal arrived to him. Same for the time coordinate (0.5(t1 + t2)). Is this then the goal of the coordinates? Why should we want to describe the location (time and spacewise) of objects by determining their positions when our signals hit them? 3. This is the hardest to explain without referring to the book, but I'll try. This is in short a description in the book: A and B are synchronized at the zero point, and B is moving with a velocity of v in respect to A. Accordingly there's a "k" factor defined, which I've pretty much understood, or thought I have. After a time interval of T A sends a signal to B, that is reflected back to him at event P. (P = reflection event) B, in his clock, measures a time interval k*T until he gets the light. A then measures k*(k*T) as the time it took for the light to return (k^2 * T). Then: t1 = T, t2 = k^2 * T. What I don't get (I think that sums up my problem): how can we calculate coordinates measured by A while moving to B's clock and back. I mean, so B gets the light, in his clock, after k*T. Ok - what does it tell us about A? Sigh... does this make any sense? Can someone maybe understand what's vague for me? It's not easy to explain. I hope I didn't exhaust you to death, and I appreciate you reading! That's it for now... Tomer.