Velocity of light as a constant.
Einstein assumed the low result of the Michelson inteferometer to be experimental error & deduced that light consisted of projectiles. Strangley, the photon he found had inertia but no rest mass. If the photon is a particle in a medium the mass would be difficult to detect - everything floats.
Einstein faced a problem in that light arrives at the same velocity irrespective of the velocity of its source, whereas the velocity of a projectile should be the velocity of the projectile with respective to its source plus the velocity of its source.
He assumed the velocity of light to be a constant in mathematical terms, & working from the Lorentz transformation produced equations that allowed something to be added to the velocvity of light without altering it. Although the maths was general he restricted its use to a vacuum. Outside a vacuum what maths must be used? Are they still projectiles? But experiments have shown that the velocity of light increases passing the Sun, in a vacuum.
The assumption of a constant produces problems with space & time. If space is curved it must consist of something but nothing has been detected.
Experiments with atomic clocks have shown that they run slower near the ground than higher up, the theory being that light is slower near the ground. This implies that light has a physical component, which has not been detected. Common sense dictates that the clocks run slower near the ground because light is slower there, thus bending the light wave round the Earth.