In Newtonian gravity, masses attract each other based on their magnitude and separation. General relativity reframes gravity as spacetime curvature induced by mass and energy. But what about massless particles? Do they bend to gravity's influence?
Isaac Newton's law of universal gravitation sparked a scientific revolution. It unified the fall of apples on Earth with the majestic orbits of planets and moons across the cosmos. Gravity shapes Earth's spheroid form, keeps moons in planetary thrall, and binds worlds to the Sun.
Newton's elegant equation—F = G(m1 m2)/r²—quantifies attraction using the gravitational constant G, masses m1 and m2, and distance r. This formula illuminated circular, elliptical, parabolic, and hyperbolic orbits, gravitational potential energy, kinetic conversions, and escape velocities. For nearly two centuries, it reigned supreme, acting instantaneously across the universe on all massive objects.
Yet advancing telescopes revealed flaws, like starlight bending near the Sun—evidence incompatible with a mass-only model.
Newton viewed space and time as rigid, absolute arenas. Einstein's 1915 general relativity shattered this, merging them into dynamic spacetime, relative to every observer. Energy—mass merely one manifestation—warps this fabric, with ripples propagating at light speed, not instantly.
Related topic: Spacetime, Curved by Mass or by Energy?
Massless photons carry energy E = hν (h: Planck's constant, ν: frequency), curving spacetime subtly and following its dictates. Thus, even without rest mass, they generate and respond to gravity.
Einstein's field equations link energy-matter distribution to spacetime geometry: matter tells spacetime how to curve; spacetime tells matter how to move. Planets don't 'orbit' via force but trace geodesics—the 'straightest' paths in warped space.
Photons, traveling 'straight' at c, deviate along geodesics bent by stars or galaxies. Observations like gravitational lensing confirm massless particles are profoundly affected by gravity.
