this post was submitted on 04 Jun 2026
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I think it's neat that Newton is taught first. As in: gravity is a function of mass. Because that works in so many scenarios.
But then you learn that gravity bends light and that photons have no mass.
So.... Gravity isn't a force, it's more like going downhill... in the dimension of time.
If they have no mass, how do they push solar sails?
They have momentum, but not mass (ignore the other explanation, like yes, E=sqrt(m0^2 c^4 + (pc)^2), but so what? m0=0 for photons)
As you can see, momentum, p, is p=E/c, and we know that the energy for light is proportional to its frequency, f, E=hf (h is Plank's constant). So, p=hf/c. When light is absorbed by a material momentum (and energy) conservation apply and it imparts p onto the object. If light is reflected it imparts 2*p, showing this is left as an exercize to the reader
They have mass. Everything that has energy has mass. They don't have inertial mass but it's just part of the equation
They have momentum, not mass, in relativistic physics you need something more complex than p=mv to describe momentum.
Energy=mass always. Theoretically you can make a black hole out of light, or you can turn that photon energy into inertial mass by running light in a closed loop.
No lol, there's a momentum component to that equation which is just conveniently 0 for massive objects at rest, photons don't have a rest reference frame and are governed by E=pc.