General relativity : everyone has heard about the theory imagined by Albert Einstein. This year, the theory celebrates its 100th birthday. Few people know that it has been successively praised, attacked, rejected, even forgotten and then finally «rediscovered» during the 1960s.
That story, unknown to the general public, is being told to us by Albert Einstein himself, and by some of the scientists who validated and revived the theory. Like Sir Roger Penrose, one of the most brilliant British mathematician who gave visual representations of a black hole, a strange cosmic object that challenged the theory. Or Jocelyn Bell who, as a student in Cambridge during the sixties, discovered a new range of stars – the pulsars – which matches the predictions of general relativity.
Einstein in the sky. Credit: Naji El Mir/LookatSciences.
Einstein is without a doubt the most popular scientist of all time. His name would probably be the first one that come into most people’s mind when asked about a scientist’s name. The great impact of Einstein’s Theory of General Relativity on the understanding of the universe and his advocacy to raise awareness of the social issues of his time made him a cultural icon. No wonder why the prestigious Time magazine named him the most important person of the 20th century. The success of the General Relativity made him appear on the headlines of newspapers around the world and with that it began the countless journeys to give conferences and lectures worldwide to explain his theory. But his audience was not only comprised by scientists, everyone wanted a piece of Einstein’s time, from head of states to celebrities, Einstein’s life became what he called the circus of relativity. Einstein’s galvanizing effect on the popular imagination continued throughout his life, and after it, influencing modern culture from painting to cinema.
Credit: Arthur Sasse.
On 4 November 1915 Einstein wrote to his elder son Hans Albert Einstein, “In the last days I completed one of the finest papers of my life; when you are older I’ll tell you about it” (1). He referred to the first out of four papers Einstein wrote in November 1915 where he finally developed his General Theory of Relativity. Today, November 25th, marks the 100th anniversary of the presentation of the Einstein field equations on the Prussian Academy of Sciences, which is one of the anniversaries celebrated during the International Year of Light and Light-based Technologies 2015 (IYL 2015).
In the summer of 1895, while the rest of the world was locked in the Victorian age, Albert Einstein was pondering the phenomenon of light. Though he was just sixteen, Einstein had renounced his German citizenship, rejected Judaism, and begun “a positively fanatic orgy of free thinking.” Bored in his prep school near Zurich, he took long walks in the Swiss Alps, experimenting in his own private chamber — his mind.
All his life, Einstein would perform Gedankenexperimente — thought experiments. That fall, he did his first. What would the world look like, he wondered, if he could ride on a beam of light? “If a person could run after a light wave with the same speed of light,” he thought, “you would have a wave arrangement which could be completely independent of time.” He knew that “such a thing is impossible,” yet he could not stop thinking about the ride. Viewed from a beam of light, clocks would seem frozen, the light from their moving hands never reaching the retreating passenger. And yet, like someone in a slow elevator, light’s rider would barely know he was moving. All the staid laws of Newton’s universe would be upset.
Credit: Saulo Trento
“The energy of a light ray spreading out from a point source is not continuously distributed over an increasing space [wave theory of light] but consists of a finite number of energy quanta which are localized at points in space, which move without dividing, and which can only be produced and absorbed as complete units.” With these words Albert Einstein (1879-1955) introduced his “heuristic point of view toward the emission and transformation of light” which was presented in his first Annus Mirabilis paper published in 1905, a year that Einstein himself referred to as “very revolutionary”. Einstein introduced the concept of “light quanta”, an indivisible packet, although it was not until 1926 when the term “photon” (coined by Gilbert Lewis (1875-1946) in an article published in Nature) substituted Einstein’s “light quanta” forever.
Of course, this year 2015 marks the centenary of the publication of Einstein’s papers on general relativity and –as the IYL 2015 Resolution points out– “the embedding of light in cosmology thorough general relativity”. However, perhaps it is not so well known that Einstein also made several seminal contributions to the science of light. He did not only introduce in 1905 the concept of “light quanta” and applied it to theoretically study “the emission and transformation of light”, as it has been mentioned before, but he also postulated stimulated emission in 1916, which eventually became the basis of laser operation. Besides these two contributions, Einstein was also one of the pioneers in exploring the wave-particle duality of light in a paper published in 1909. There is no doubt that Einstein gave us a remarkable legacy in light.
Albert Einstein in 1905. Credit: Wikipedia.