Okay, I realize that there are difficulties in combining the theory of GR and quantum mechanics, but I thought SR and quantum could be combined alright (relativistic quantum field theories, etc). If that is not correct, please let me know as it makes the rest of my questions pointless. My questions here are based on the fact that I am having trouble understanding two issues of "combining" SR and quantum. Any help you can provide would be much appreciated. 1] Since I'm interested in SR, we will consider a spanse of spacetime that is negligibly curved. Observers moving relative to each other should measure the same speed of light and any other physical constants. This means they will agree on c (a speed) AND a unit of length (the planck length). How can observers agree on a "universal" speed constant AND a "universal" length constant? One explanation I've seen in literature is to slightly change the "constancy of the speed of light postulate" of SR to "there exists one velocity upon which all observers agree" and refer to this velocity as the velocity of light in vaccuum in the limit the energy -> 0. And then allow for light to have a dispersion relation in vaccuum, and a universal length to be agreed on. Is there another explanation that is simpler? And I'm not really sure how this answers the question in the first place. 2] Another issue is that quantum mechanics has that pesky "measurement postulate". All of quantum mechanics involves unitary and local evolution until you throw a measurement in there. How in the world can you define a "wavefunction collapse" in a lorentz invarient manner? It seems to have a built in "simultaneity" to it.