In 1915, Albert Einstein was struggling to wrest from nature what would turn out to be his crowning achievement, perhaps the most beautiful theory in all of science. Ten years earlier, he had come up with the special theory of relativity, which said that time and space were each relative for observers moving at different constant velocities. Now he was trying to generalize the theory by conceiving of gravity as a curving of the fabric of something he called space-time.

It was an excruciating period. His marriage to Mileva Maric, an intense and brooding Serbian physicist who had helped him with the math of his 1905 paper, had just exploded. She had left him in Berlin and moved to Zurich with their sons Hans Albert, 11, and Eduard, 5. Suffering from acute stomach pains exacerbated by the food shortages of World War I, he was being nursed by a first cousin, Elsa Einstein, whom he would eventually marry.

His letters, including some made public this week, show how his personal and scientific struggles intertwined in 1915, culminating in his great triumph that fall. The tale begins with two letters written in early April by Hans Albert (known as Adu), begging his father to visit him and his brother (known as Tete) in Zurich for spring vacation: Dear Papa, Imagine, Tete can already multiply and divide, and I am doing gometetry (geometry), as Tete says. Mama assigns me problems; we have a little booklet; I could do the same with you then as well. But why haven't you written us anything lately? I just think: "At Easter you're going to be here and we'll have a Papa again." Yours, Adu!

Dear Papa, Today we told each other our dreams. Tete suddenly said: "I dreamed that Papa was here!" Then I thought: "It really would be much nicer if you were with us." I can tinkle away on the piano much better now already; not long ago I played a Haydn and a Mozart sonata and some sonatinas. In short, I could also play with you. The examination is approaching now; but at the same time, so is Easter. Last Easter we were alone; do we have to spend this Easter alone as well? If you were to write us that you are coming, that would be the finest Easter bunny for us. We can live here quite well, you know, but if Mama gets ill one day, I don't know what to do. Then we would have no one but the maid. Also for this reason it would be better if you were with us. Yours, Adu The war made it impossible for Einstein to visit them, but he responded to the postcards by promising Hans Albert that he would come in July for a hiking vacation in the Swiss Alps. "In the summer I will take a trip with just you alone," he wrote. "This will happen every year, and Tete may also come along when he is old enough for it." He expressed delight that his son had taken a liking to geometry. It had been his "favorite pastime" when he was about the same age, he said, "but I had no one to demonstrate anything to me, so I had to learn it from books."

Einstein wanted to be with his son to teach him math, but that would not always be possible, he lamented. Perhaps they could do it by mail? "If you write me each time what you already know, I'll give you a nice little problem to solve." He sent a toy for each of his sons, along with an admonition to brush their teeth well. "I do the same and am very happy now to have kept enough healthy teeth."

But the tension in the family worsened. Einstein and Mileva exchanged letters arguing about both money and vacation timing, and at the end of June a curt postcard came from Hans Albert in response to his father's request that he be available on a particular date to go on their proposed summer vacation: Dear Papa, You should contact Mama about such things, because I'm not the only one to decide here. But if you're so unfriendly to her, I don't want to go with you either. We have plans for a nice stay that I'd only give up very reluctantly. We are going at the beginning of July and are staying the whole vacation. Yours, A. Einstein The coldness of the letter was evident by the signature. Hans Albert was no longer signing off with the affectionate nickname Adu, but with the same initial and last name that his father used on formal letters.

Einstein was convinced that Mileva was dictating the postcards, both the plaintive ones that had made him feel guilty and now the one asking him not to come for the summer hike. So he decided to go on vacation with his new love, Elsa. He explained his decision in a July 1915 letter, also recently made publicly available, to his friend Heinrich Zangger, a medical professor in Zurich who was trying to mediate between the Einsteins: My dear friend Zangger, My fine boy has been alienated from me for a few years already by my wife, who has a vengeful disposition, but also is so sly that outsiders and particularly men are always deceived by her. If you only knew what I had to live through with her, you would hold it against me that I did not find the energy for so long to separate myself from her. The postcard I received from little [Hans] Albert had been inspired, if not downright dictated, by her ... When I write to him, I get no response. Under these circumstances it appeared as if I couldn't see the children at all if I came now to Zurich in July, as I was firmly resolved to do. So at the last minute I decided, while I was at Göttingen giving talks about the general theory of relativity, to relax here in Sellin, where my cousin [Elsa] had rented lodgings with her children. A. Einstein The trip to Göttingen he referred to was to give some lectures at the invitation of the mathematical physicist David Hilbert. Einstein was particularly eager--too eager, it would turn out--to explain all the intricacies of relativity to him. The visit was a triumph, he exulted to Zangger. "I was able to convince Hilbert of the general theory of relativity."

Amid all of Einstein's personal turmoil, a new scientific anxiety was about to emerge. He was struggling to find the right equations that would describe his new concept of gravity, ones that would define how objects move through space and how space is curved by objects. By the end of the summer, he realized the mathematical approach he had been pursuing for almost three years was flawed. And now there was a competitive pressure. Einstein discovered to his horror that Hilbert had taken what he had learned from Einstein's lectures and was racing to come up with the correct equations first.

It was an enormously complex task. Although Einstein was the better physicist, Hilbert was the better mathematician. So in October 1915 Einstein threw himself into a monthlong frenzy in which he returned to an earlier mathematical strategy and wrestled with tensors, equations, proofs, corrections and updates that he rushed to give as lectures to Berlin's Prussian Academy of Sciences on four successive Thursdays--even as he was struggling to arrange a reconciliation with his sons.

His first lecture was delivered on Nov. 4, 1915, and it explained his new approach, though he admitted he did not yet have the precise mathematical formulation of it. That very afternoon, as soon as he finished his lecture, he wrote an anguished--and poignant--letter to Hans Albert:

I will try to be with you for a month every year so that you will have a father who is close to you and can love you. You can learn a lot of good things from me that no one else can offer you. The things I have gained from so much strenuous work should be of value not only to strangers but especially to my own boys. In the last few days I completed one of the finest papers of my life. When you are older, I will tell you about it ... I am often so engrossed in my work that I forget to eat lunch.

Einstein also took time off from furiously revising his equations to engage in an awkward fandango with his competitor Hilbert. Worried about being scooped, he sent Hilbert a copy of his Nov. 4 lecture. "I am curious whether you will take kindly to this new solution," Einstein noted with a touch of defensiveness.

Einstein presented his second paper the following Thursday, Nov. 11. He still had not found the right equations, but he felt he was getting closer. Once again, he sent the paper to Hilbert. "If my present modification (which does not change the equations) is legitimate, then gravitation must play a fundamental role in the composition of matter," Einstein wrote. "My own curiosity is interfering with my work!"

The reply that Hilbert sent the next day must have unnerved Einstein. He said he was about ready to oblige with a "solution to your great problem." He had planned to hold off discussing it until he had explored it further. "But since you are so interested, I would like to lay out my theory in very complete detail this coming Tuesday," which was Nov. 16. He invited Einstein to come to Göttingen and have the dubious pleasure of personally hearing him lecture. Then, after signing his name, Hilbert felt compelled to add what must surely have been a tantalizing and disconcerting postscript. "As far as I understand your new paper, the solution given by you is entirely different from mine."

Einstein wrote four letters on Nov. 15, a Monday, that suggest that the stress he was under was starting to take its toll. To his son Hans Albert, he wrote that he would like to travel to Switzerland around Christmas and New Year's to visit him. "Maybe it would be better if we were alone somewhere," such as at a secluded inn, he suggested to his son. "What do you think?"

He also wrote his estranged wife a conciliatory letter that thanked her for her willingness not "to undermine my relations with the boys." And he reported to their mutual friend Zangger: "I have modified the theory of gravity, having realized that my earlier proofs had a gap ... I shall be glad to come to Switzerland at the turn of the year in order to see my dear boy."

And finally, he replied to Hilbert and declined his invitation to visit Göttingen the next day. His letter did not hide his anxiety: "Your analysis interests me tremendously ... The hints you gave in your messages awaken the greatest of expectations. Nevertheless, I must refrain from traveling to Göttingen for the moment ... I am tired out and plagued by stomach pains ... If possible, please send me a correction proof of your study to mitigate my impatience."

On Nov. 18, Einstein received Hilbert's new paper. Einstein was dismayed to see how similar it was to his own work. His response to Hilbert was terse and clearly designed to assert the priority of his own work. "The system you furnish agrees--as far as I can see--exactly with what I found in the last few weeks and have presented to the Academy," he wrote.

Hilbert responded kindly and quite generously the following day, claiming no priority for himself. "If I could calculate as rapidly as you," he wrote, "in my equations the electron would have to capitulate and the hydrogen atom would have to produce its note of apology about why it does not radiate." Yet one day later, Hilbert sent a paper to a scientific journal with his own version of the equations for general relativity. The title he picked for his piece was not a modest one. "The Foundations of Physics," he called it.

Einstein's climactic fourth lecture at the Prussian Academy, on Nov. 25, was titled "The Field Equations of Gravitation." It contained the correct set of equations that capped his theory of general relativity. Although it came a few days after Hilbert had sent in his own paper, Einstein's version was more complete, and the underlying concepts were his alone.

The theory was one of history's most imaginative and dramatic revisions of our concepts about the universe. It was, said Paul Dirac, the Nobel laureate pioneer of quantum mechanics, "probably the greatest scientific discovery ever made." Max Born, another giant of 20th century physics, called it "the greatest feat of human thinking about nature, the most amazing combination of philosophical penetration, physical intuition and mathematical skill."

Nevertheless, Einstein's triumph was tempered by his continued struggles with Hans Albert. The boy told a family friend that he wanted to spend the entire Christmas vacation hiking with his father, but he wrote a chilly letter to his father indicating the opposite: Dear Papa, I will come over New Year's, i.e., from the 31st to 2nd. I don't want to stay longer because Christmas is nicest at home. Besides, I got skis and would like to learn how to use them with my colleagues. The ski equipment costs about 70 francs, and Mama bought them for me on condition that you also contribute. I consider them a Christmas present. Yours, Adu So Einstein informed his son that he was canceling the trip. "The unkind tone of your letter dismays me very much," he wrote just days after finishing his last lecture on general relativity. "I see that my visit would bring you little joy, therefore I think it's wrong to sit in a train for two hours and 20 minutes."

There was also the question of paying for the skis. Einstein was not pleased. He replied that he would send Hans Albert a gift in cash, "but I do think that a luxury gift costing 70 francs does not match our modest circumstances," he wrote, underlining the phrase.

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

Einstein's relationship with his family would continue to be intense and volatile, with periods of strain and of affection. In order to dissolve his marriage to Mileva, he offered her a deal: if she agreed to give him a divorce, he would give her the money from the Nobel Prize he fully expected to win someday. She considered the offer for a week, then took the bet. And when he won a few years later, she was able to buy three apartment buildings in Zurich with the money.

Young Eduard (Tete) eventually succumbed to mental illness and was confined to an asylum near Zurich for the rest of his life. Things turned out better for Hans Albert. He went to the Zurich Polytechnic, where his parents had met, studied engineering, and later became a professor at the University of California, Berkeley. He would be at the bedside when his father died, 40 years after the tumultuous year when he conquered his theory of gravity while wrestling with the even more mysterious forces that swirled around his family.

Isaacson, president of the Aspen Institute and a former managing editor of TIME, is writing a biography of Einstein that will be published by Simon & Schuster in April 2007. For more letters, go to time.com/einstein For information about The Collected Papers of Albert Einstein, go to www.pupress.princeton.edu