On November 7, 1915, the London newspaper *The Times* published headlines: “Revolution in science. A new theory of the Universe. Newton’s ideas superseded. A few days later *The New York Times* said that Albert Einstein had invented a new theory that was “one of the greatest successes – if not the greatest – in the history of human thought.”

It had all started years ago, on a day in November 1907. Einstein, sitting at his desk in the patent office in Bern (Switzerland) as Second Class Technical Expert, **was thinking about the implications that the special theory of relativity could have.** , which he had formulated in 1905. It was the answer to a question that had obsessed him since it was asked in 1895 in the Swiss city of Aarau, at the age of 16: **What would the world look like mounted on a ray of light?**

And there he was, two years later, in the patent office, pondering it. Suddenly, an idea came to his mind: “ **If a person falls freely, he does not feel his own weight** … this simple thought deeply impressed me. It pushed me toward a theory of gravitation.” He later told his old friend Michele Angelo Besso, who also worked at the patent office, that it had been “the happiest thought of my life.” **You have just opened the door to your masterpiece: the General Theory of Relativity.**

## The Equivalence Principle

Einstein had stumbled upon the so-called *equivalence principle* : locked in a closet, there is no way to distinguish by any kind of experiment whether we are on a planet or whether we are being carried through space at constant acceleration. That is, **gravity and acceleration are interchangeable** . Einstein had just found the key to building a relativistic theory incorporating gravity.

But like all good stories, this one has a mystery. **For four years, from 1907 to June 1911, Einstein was surprisingly silent** . We do not know if he worked on the problem during those years, in which he published scientific articles on other topics, such as blackbody radiation. In 1911 he moved from Switzerland to Prague to teach at the city’s German university. It was here, in the city of Kafka, the city of nightlife in cafes, the city of eternal intrigues between the Jesuits and the administration of the Austro-Hungarian Empire, where he took the first steps towards his new theory.

## First steps to grand theory

The first step toward a new view of gravity was to formally develop the equivalence principle, which stated that Newton’s laws should be the same whether we were in a gravitational field or traveling at constant acceleration. The next was to show, from the equivalence principle, that a ray of light coming out of a very massive object would be redshifted. How did this happen? **I had no idea because I lacked the mathematical knowledge for it; I just knew it had to exist.** This was his first encounter with mathematics: special relativity had not required very complex techniques, but here he realized that he needed a powerful mathematical apparatus that he knew little about, tensor calculus. Luckily, he had friends who could help him, such as Georg Pick, a mathematician 20 years older than him and to whom history has done little justice.

Shortly after arriving in Prague he received an offer from the Zurich Polytechnic Institute, his *alma mater* , to become a professor there. He didn’t think twice because he didn’t like the attitude of the people he met in the streets and cafes. So in June 1912 he changed Prague for Zurich. But before leaving, he came to a revolutionary conclusion: **the space around a massive body is non-Euclidean** , or to put it another way, space was not flat but curved.

Einstein knew almost nothing about non-Euclidean geometry and this time he did realize its importance. He urgently needed a mathematician and in Switzerland was an old friend from his student days in Zurich, Marcel Grossmann. Perhaps then he remembered Pick’s advice and, after several months of fruitless calculations, wrote to his friend: “Grossmann, you must help me or I will go mad!” And so it was: they worked together to develop a first draft of the equations of general relativity.

But there was still a long way to go.

*References:*

*Hoffmann, B. (1984) Einstein, Salvat*

*Kuznetsov, B. (1990) Einstein. Life, Death, Immortality, Progress*

*Pais, A. (1984), ‘The Lord is subtle…’: The science and life of Albert Einstein, Ariel*