I found this nice book on Google http://tinyurl.com/yh2y2zb that does a great job explaining relativity, however, even though the conclusions make perfect sense to me so far, I'm stuck on a conceptual issue when reading over the classic thought experiments that makes me feel like I'm still missing something. Given that the motion of an inertial frame is indiscernable from within that frame... If I'm in an inertial frame moving "up" at .9c relative to an external observer, who shoots a laser beam towards me perpendicular to my direction of motion from their perspective, and the beam enters my proverbial space-elevator at a point exactly midway up one wall, the external observer should observe it striking a point below the midpoint of the opposite wall. I should observe the same thing, except that I observe that the laser beam was shot slightly at an angle relative to my walls. The second picture midway down this page: http://tinyurl.com/yze2b7v illustrates this concept, and it seems reasonable. Second, if I position a laser pen exactly half way up one wall in my elevator, exactly horizontal to the floor of my elevator, and position a target exactly half way up the other wall, I should observe that the laser beam runs horizontally and hits the target, but an external observer sees that the beam moves at an angle, upwards with the elevator, to hit the target. Fine. Here's the dilemma. If the external observer is looking at my elevator so the beam goes from their left to right, then my elevator floor should look horizontal to them, even as it rises, which means my laser pen should also appear horizontal to them as it shoots. But they would see the laser beam leave my pen at an angle. Even though my laser pen ALWAYS shoots straight as observed from its own reference frame. Does this simply mean that a laser will appear to leave a source at an angle if the source is moving relative to the observer? So a moving light source behaves differently than a relatively motionless one? What's more, if I were to position a straw somewhere between my laser pen and the target, I would think nothing of the beam going straight through and hitting the other side, but wouldn't the observer observe the laser start out at an angle, then apparently change direction to follow the horizontal straw, then exit at an angle to hit the now higher target? Although this might not be a problem if you consider each photon entering the straw at an angle and continuing up with the straw as it rises, rather than the entire beam as a continuous line. But still, overall, how could it appear a straight line? If my laser appears hits the wall slightly below the target from my perspective, then I can tell that I'm moving. If I turn around and move down, I would see the laser hit above the target, thus telling me I'm moving in the other direction, even if I could not tell how fast relative to anything else since the light would always take the same amount of time as measured by me to cross the elevator. So what gives?