Question: what are the basic defining characteristics of a photon
[{'answer': 'zero rest mass'}, {'answer': 'always moves at the speed of light within a vacuum'}, {'provenance': [{'wikipedia_id': '23535', 'title': 'Photon', 'start_paragraph_id': 1, 'start_character': 11, 'end_paragraph_id': 1, 'end_character': 342, 'bleu_score': 0.7808390100068281, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13968593364807850435}, 'section': 'Section::::Abstract.'}]}]