Einstein said who are the three people who understand the theory of relativity? Is it really so hard to understand?

There is a joke in the history of physics. At a banquet shortly after Einstein released the general theory of relativity, the reporter who went to interview and Sir Eddington, a British physicist, joked, "I heard that only three people in the world understand the general theory of relativity." Eddington didn't answer for a long time. The reporter asked him what he was thinking, and he replied, "Who am I thinking?" That is to say, only two people understood at that time, but there was no accurate answer to who these two people were.

However, as time goes on, more and more people understand it. But this does not mean that you have read the formula of relativity, can recite it, can solve simple problems, and just understand relativity. In fact, Einstein also has his own limitations. Look at the following paragraph:

Relativity predicted that stars might collapse into black holes, but Einstein, the creator of relativity, refused to accept it. He objected that matter could not be so dense. Similarly, Einstein also refused to accept the expansion model of the universe derived from the relativistic equation.

These two deductions actually mean that our universe is unstable. Before Einstein, human thought that our universe was always stable. So Einstein was also worried about the unstable universe derived from his equation, and he even tried to add a constant to his equation to ensure the stability of the universe. It was not until 1929 that astronomer Hubble's observation confirmed that galaxies were indeed leaving us. The universe is expanding. Einstein later admitted that the "cosmological term" was the stupidest mistake in his life. More and more people can only understand. The difficulty of understanding varies from person to person, depending on everyone's understanding ability.