“Just how smart Einstein was?” is same as “just how dumb are we?”.
Einstein was average, we were all dumb.
Then us: we are smart, you are very very smart. Whats your problem with that?
Einstein: Oh ok.
The following represents a simple thought parade to demonstrate
the uncanny physical insight of Einstein.
Everyone was thinking in terms of physics (i.e. force). Einstein knew one part of that is maths, pure maths. Force is physics because of mass. But acceleration is geometry, its the shape of your trajectory and its maths.
Imagine a pipe which is horizontally fitted across a wall. Now water jet is flowing through it, inside of it. Now that wall is on a rotating base. You rotated the wall downwards. The pipe is inclined downward, water streams across downward.
Now imagine you have a torch light fixed to one end cap. You get a horizontal output of light. Now the wall is rotated downward, obviously the light will also stream across downwards.
The mass of the water or light did not matter. Because they are guided by the pipe and wall.
The same thing happens in the universe. The space is what carries mass, and it carries all physical phenomena including light. When that space itself is inclined (warped, rotated or receded or whatever) light would follow a curved trajectory just like any object of mass would. Space is what holds them, matter and light. Space curves under gravity.
The whole question of whether a mass-less object would curve under gravity is dealt. The mass, force or physics isn’t coming into the question.
Since from maths we knew curvature (of space or universe) is acceleration it acts equally on mass-objects as well as mass-less ones. Its acceleration, it can’t distinguish between masses.
Einstein realized that there is no difference between gravity (acceleration due to gravity) and any other type of acceleration. They all are curvature in space and time. He imagined a lift falling through space. If light enters through one hole and hits opposite wall, from the curvature of the light path one could know whether the lift is in uniform motion, or accelerated downwards or upwards. But one wouldn’t know how much of it is acceleration and how much is gravity. In essence this is important because you might hit the ground and die from the mechanical hurt.
Now this is the basis of principle of equivalence: gravity and acceleration have equivalent effects.
Its not about gravitational or inertial mass. But given that we can’t assume them to be equal for any reason whatsoever, lets assume them to be different. In that case due to principle of equivalence (of acc and grav) the effect would show up somewhere.
All our careful experiments have failed to bring out any difference of these masses. We end up thinking principle of equivalence is about gravitational masses and inertial (kinematic) masses. But thats like saying Microsoft was invented in Nepal because one can see them getting applied there.