Einstein's Happiest Thought made him Unhappy

Why was the later Einstein so unhappy?

It started with a young Einstein expressing that "his happiest thought" was the following:

• The gravitational field has only a relative existence... Because for an observer freely falling from the roof of a house – at least in his immediate surroundings – there exists no gravitational field.

It was the pivotal thought to allow Einstein to take the step from his 1905 Special Theory of Relativity SR without gravitation, to his 1916 General Theory of Relativity GR including gravitation. 

Einstein's happy thought was to view free fall of a (small) object as a state without internal forces (stress free) with all parts of the body accelerating the same way subject to a gravitational force being constant over the body, and then draw the conclusion in the absence of internal forces that the body was not subject to any force at all. This brought GR with gravitation back to SR without gravitation. 

Recall that a body in free fall is subject to gravitational force, which is constant for an infinitesimally small body, but for a body with extension gives rise to internal tidal forces, like the gravitational force from the Moon creating tidal waves in the oceans of the Earth. To view free fall as inertial motion without gravitational force lacks reason. 

In any case, Einstein considered a (small) observer O1 with closed eyes under 

• inertial motion (SR)
• free fall (GR)

Einstein noted that in both cases O1 would feel to be stress free without internal forces, and so it would be impossible for O1 to distinguish between inertial motion and free fall from the presence of internal forces, since they would be zero in both cases. But to a stationary observer O2 in a Euclidean coordinate system following the motion of O1, there would be a clear difference between inertial motion as motion with zero acceleration and free fall as accelerated motion. This would also be clear to O1 when opening the eyes.

But Einstein did not take the position of O2 or O1 with open eyes, but instead made a distinction between inertial motion as motion in flat space-time and free fall as motion in curved space-time, which led him to GR. 

So far so good, but Einstein's goal was to form a unified field theory including both gravitation and electromagnetics, which required electromagnetics to also be expressed in curved space-time. Einstein worked on this problem incessantly since 1916 to his death in 1955, however without any success. 

This was the reason Einstein was so unhappy during his later life. 

Is it also the destiny of a modern physicist in quest for a unified field theory including GR, to be unhappy?

Not necessarily, since it is possible to form a unified field theory including Newtonian mechanics and electromagnetics because this possibility was opened with the 2019 SI meter Standard, as exposed in recent posts. Would you be interested in exploring this possibility, and get happy? Then send me a note and I will help to get started, to the best of my knowledge. Ok?

Einstein, the loneliest man in town as smart as a man can be but: He ain't got rythm.