* Direct Observation of Gravitational Waves: Educator's Guide: https://www.ligo.caltech.edu/system/media_files/binaries/303/original/ligo-educators-guide.pdf | * Direct Observation of Gravitational Waves: Educator's Guide: https://www.ligo.caltech.edu/system/media_files/binaries/303/original/ligo-educators-guide.pdf |