2 years ago I was prescribed Ziac to reduce blood cholesterol. But after I lose 35 lbs, my blood pressure becomes too low, I'm off Ziac. Now my diet consists of low fat meat, low carb and salad or as much seafood as I can afford and also bottled water.
Millikan, the famous experimentalist, did his monumental oil-drop experiment to measure the unit electric charge by assuming that the force of gravity is zero such that
in the symmetrical frame of the laboratory, this vector equation became just a scalar equation.
This scalar equation should apply to a photon, since a photon is not affected by gravity because its rest mass at low energy-low speed-weak gravity domain is practically zero. Inside a black hole (very strong gravity field), photon acquired mass due to its energy content as required by the mass-energy equivalence and Einstein field equations of general relativity.