HI, I've been mulling over the derivation of the Lorentz factor gamma in my head for some time now & had worried about it a bit. I've now discovered that what was bothering me was something called "frame-jumping". This is where you use the Pythagorean Theorem to derive Gamma. If you're unsure of what I'm talking about I'll point you to a video showing the derivation, it's only around a minute long, watch at time 12:45 to 13:30 or so and you'll see what I'm on about. http://video.google.com/videoplay?docid=-6328514962912264988# [Broken] So I'd just like to hear from an expert WHY every standard college level intro textbook uses an incorrect derivation of the Lorentz Transforms...? If this is true. Also, A VERY IMPORTANT THING I HOPE YOU'LL BE ABLE TO ANSWER. In Kleppner & Kolenkow's Introduction to Mechanics will they use this derivation? Thanks so much for any comments... BTW: I found out about this thing called frame jumping from an online post, here is the link if you're curious, http://www.physforum.com/index.php?showtopic=21780 If you don't want to mull through the page I'll quote one important part of it, the saw tooth thing is a reference to the hypotenuse line of motion that the photon traveling perpendicular to the velocity.