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Thus it was that 1915, the capstone year for his theory of relativity, ended with mixed emotions for Einstein. As he put it in a letter to Zangger: Dear friend Zangger, Just now I received the enclosed letter from my Albert, which upset me very much. After this, it's better if I don't take the long trip at all rather than experience new bitter disappointments. The boy's soul is being systematically poisoned to make sure that he doesn't trust me. Under these conditions, by attempting any approaches I harm the boy indirectly. Come, dear old friend, Lady Resignation, and sing me your familiar old song so that I can continue to spin quietly in my corner! ...
Currently I am also having quite a curious experience with my dear colleagues. All but one of them is trying to poke holes in my discovery or to refute the matter, if only so very superficially; just one of them [Hilbert] acknowledges it, insofar as he is seeking to partake in it, with great fanfare, after I had initiated him, with much effort, into the gist of the theory ... Heartfelt greetings, yours, Einstein So Einstein spent Christmas Day in his Berlin apartment. That morning, he took out of his satchel some of the drawings that Hans Albert had sent him and wrote the boy a postcard saying how much they pleased him. He would come for Easter, he promised. To Einstein's delight, his son enjoyed playing piano. "Maybe you can practice something to accompany a violin, and then we can play at Easter when we are together."
Things would eventually improve. When the final version of Hilbert's paper came out, he was both clear and generous in insisting that credit for the theory of relativity belonged to Einstein. They were soon visiting each other's homes once again. "There has been a certain ill-feeling between us, the cause of which I do not want to analyze," Einstein wrote. "I have struggled against the feeling of bitterness attached to it, with complete success. I think of you again with unmixed geniality and ask you to try to do the same with me. It is a shame when two real fellows who have extricated themselves somewhat from this shabby world do not afford each other mutual pleasure."
The situation within his family also got better--in fits and starts. That following Easter, as promised, Einstein went to Zurich to visit his boys. They were delighted to see him, and he wrote a note of thanks to Mileva for making things go smoothly:
My compliments on the good condition of our boys. They are in such excellent physical and mental shape that I could not have wished for more. And I know that this is for the most part due to the proper upbringing you provide them. I am likewise thankful that you have not alienated me from the children. They came to meet me spontaneously and sweetly.
Einstein then took Hans Albert off alone, as the boy wished, for a hiking excursion at a mountain resort overlooking Lake Lucerne. In a postcard to his cousin and future wife Elsa, Einstein described his joy:
My dear Elsa, Yesterday I went on a hike with the boy and am enjoying very much being with him. He is kindhearted, trusting, and surprisingly eager to learn, and intelligent. My relationship with him is becoming very warm. Kisses from your, Albert