1. Einstein's general relativity is regarded as the greatest intellectual (not just scientific) achievement of our civilization (along with Minkowski's discovery of the spacetime structure of the world).

2. General relativity revealed that gravity is not a force but a manifestation of the non-Euclidean geometry (= curvature) of spacetime.

3. In 1948 Einstein wrote in a letter he disagreed that "the general theory of relativity is geometrizing Physics or the gravitational field." The most probable reason for disagreeing with the essence of his own theory may be that even in 1948 Einstein had doubts about the reality of spacetime. Indeed, if spacetime did not represent a real four-dimensional world (and were nothing more than a mathematical space), then, clearly, gravitational phenomena could not be manifestations of the curvature of something that does not exist.

4. But at least two comments written after 1948 appear to indicate that what Einstein wrote in that letter might not have been his final view of the true meaning of his general relativity.

4.1. In 1952 Einstein added a fifth appendix "Relativity and the Problem of Space" to the fifteenth edition of his book "Relativity: The Special and General Theory," where his doubts that spacetime represents a real four-dimensional world seem to have been overcome:

"It appears therefore more natural to think of physical reality as a four-dimensional existence, instead of, as hitherto, the evolution of a three-dimensional existence."

4.2. In March 1955 Einstein wrote a letter of condolences to the widow of his longtime friend Besso which provided another indication of what seems to be Einstein's full realization of the depth of Minkowski's discovery that reality is an absolute four-dimensional world (where all moments of time form the fourth dimension of the physical world and therefore are given en bloc exactly like the space points of the spatial dimensions are given en bloc):

"Now Besso has departed from this strange world a little ahead of me. That means nothing. People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion."

5. What Einstein wrote is interesting, but no authority can challenge the ultimate judge in physics - the experimental evidence (moreover, Einstein himself wrote "To punish me for my contempt for authority, fate made me an authority myself"). Just one piece of the experimental evidence, which proves that gravity results from the curvature of spacetime and is not a force. Falling bodies do not resist their downwards acceleration (a falling accelerometer, for example, reads zero acceleration, i.e., zero resistance). Every physics student is supposed to see that this experimental fact proves that there is no gravitational force in Nature:

As a falling body does not resist its fall it moves by inertia (non-resistantly), because no force that would be resisted acted on the body. A gravitational force would be required to accelerate the body downwards if and only if the body resisted its acceleration, because only then a gravitational force would be needed to overcome that resistance.

The non-existence of gravitational force demonstrates that there is no gravitational energy (and momentum) either, because gravitational energy should be defined as the work done by gravitational forces, which, however, do not exist.

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