I don't have the Zwiebach's string theory book myself, but I paid a visit to a library, and took a glance on it. The chapter 5 was about relativistic point particle. Now, did I understand correctly, that the string people actually have a technique to quantize a relativistic point particle? I thought that the basic QM and QFT texts are forbidding that procedure, declaring it impossible. The way how Zwiebach was using the proper time [tex]\tau[/tex] seemed dangerous. It's ratio to the time [tex]t=x^0[/tex] depends on the particle's velocity. So if you in quantum mechanics parametrize the wave function with such proper time, [tex]\psi(x,\tau)[/tex], aren't you having different fourier amplitudes propagating with "different speeds in time" then? It looks very messy. I have difficulty seeing what's happening with that kind of approach. Zwiebach seemed to be mainly interested in the operators. There was no discussion about spatial probability densities. Am I wrong to guess that despite the fact that string theorists have a technique to quantize a relativistic point particle, they have nothing to say about the probability densities of relativistic particles?