@article{oriti_2005,  title={The Feynman propagator for quantum gravity: spin foams, proper time, orientation, causality and timeless-ordering},  volume={35},  ISSN={0103-9733},  url={https://doi.org/10.1590/S0103-97332005000300019},  DOI={10.1590/S0103-97332005000300019},  number={2b},  journal={Brazilian Journal of Physics},  publisher={Sociedade Brasileira de Física},  author={Oriti, D.},  year={2005},  month={Jun},  pages={481–488} }