I've had my second class of Modern Physics and I have some "obvious" doubts that many of you certainly had, although I didn't find anything answering the first doubt on a quick google search. 1)Say I am in an inertial reference frame. I can see that any photon going around me do it at a constant speed equal to c. Therefore, I'm tempted to say that if I choose a reference frame on a moving photon, it would be inertial. But the problem would be in calculating anything that involve distances and time with respect to the photon's frame of reference. Some Lorentz transformation would involve the Lorentz factor [tex]\gamma[/tex] which would be equal to [tex]\infty[/tex] which is impossible. So it seems that the reference frame of any photon cannot ever be inertial. Is this right? If so, why?! 2)Another question is: If I'm accelerating in a laser's beam (toward the source of the laser), would I see the photons hitting me with a speed faster than c? I know that I would observe a Doppler effect and thus the momentum of the photons would probably be greater than if I wasn't accelerating, but I'm not sure since I've never studied those things yet. 3)How can I know if I'm an inertial reference frame of reference? I'm guessing by looking at any photons hitting my system and measure their speed. 4) In the twin paradox, one man stays on Earth while the other travels with respect to Earth at great speeds. I think that by using Lorentz transformations, one can see that an interval of time of the man in space correspond to a lesser interval of time than the one of the man on Earth. That's why when the man in space comes back to Earth they have aged differently and that the space man is less old than the Earth's man. My question is: Looking only at the 2 men, we can't say that one is moving while the other doesn't. Maybe the space man can say that he's in an inertial reference frame? (or not due to changes in acceleration because he's getting away from Earth and thus the gravitational force is less strong.) I hope these are not too many questions, I'm a newbie in Modern Physics but I'd appreciate any kind of answers (even complicated ones) and also if you answer "only" one question. Thanks!