if photons are massless, how can it push something like a light sail, and how can sunlight adds weight to earth?

Question

if photons are massless, how can it push something like a light sail, and how can sunlight adds weight to earth?

In: 2

Answer 1

The idea that *momentum = mass × velocity* (or *p = mv*) is from Newtonian physics—which means that it’s a good approximation for slow-moving objects with mass, but it’s an oversimplification that fails to accurately describe physics at velocities near the speed of light or for particles without mass. Under relativity, pioneered by Einstein, photons have momentum despite having zero mass, and that momentum is calculated as *Planck’s constant ÷ the photon’s wavelength,* or *p = h / λ*.