1:48:52Why Light Bends Around Gravity Even Though It Has No Mass
A photon has zero rest mass, so Newton's gravity says nothing should ever be able to pull on it. Yet starlight bends around the Sun by 1.75 arcseconds, exactly twice what any Newtonian calculation allows, and that factor of two is the whole argument. Over an hour and forty eight minutes this calm explainer builds the resolution from the ground up: momentum without mass, the equivalence principle and the accelerating elevator, gravity as geometry rather than force, and the specific claim that photons ride null geodesics at exactly the local speed of light along paths that only look curved from far away. Then it spends the second half on the evidence, from Pound and Rebka in a Harvard stairwell and the Shapiro radar delay to Einstein rings, the Bullet Cluster, a supernova whose reappearance was predicted a year in advance and arrived on schedule, and the shadow of a black hole. The line it earns: the photon is straight, the road is bent.