Einstein, Albert (1879-1955), American physicist of German origin, best known as creator of the theory and general relativity.
Born in Ulm, Albert Einstein spent his youth in Munich, where his family owned a small workshop producing electrical machinery. From an early age, he showed an intense curiosity, showing a remarkable ability to understand most difficult mathematical concepts. Aged 12, he learned on his own the basics of Euclidean geometry.
When a commercial bankruptcy forces his family to leave Germany to settle in Milan, Einstein follows his parents in Italy for one year, before leaving for Munich to complete his secondary education. He then enters, in 1896, the Swiss Federal Institute of Technology in Zurich, where he does not shine by his results, nor by his attendance to classes. He managed, nevertheless, to pass his examinations and obtained his license in 1900.
Rather poorly regarded by his teachers, Einstein is not recommended for a teaching place at the university. Naturalized Swiss, in 1902 he landed a job at the Federal Patent Office in Berne in Switzerland. He married, the following year, Mileva Marić, a former classmate at the Polytechnic Institute.
In 1905, Einstein obtained his doctorate at the University of Zurich for a thesis on the theoretical dimensions of molecules. He also published the same year four theoretical papers that proved to be of great importance for the development of physics in the twentieth century. Published in the German scientific journal Annalen der Physik his memoirs were titled: On a heuristic point of view on the production and processing of light; On Brownian motion; On the electrodynamics of moving bodies; does Inertia of a body depends on its energy content?
In the first article, Einstein gives an explanation to the photoelectric effect by issuing the assumption that light is made up of grains of energy later called photons. He also assumed that these quanta must have an energy proportional to the frequency of radiation, suggesting the formula E = hu, where E is the energy radiated, h Planck's constant, and u frequency of photon. The existence of these photons will be confirmed only eighteen years later by the American physicist Arthur Compton, in an experiment on the X-ray
Einstein, whose primary interest is to understand the nature of electromagnetic radiation, contributed to the continued development of the theory, developed by Louis de Broglie in 1923, which incorporates and unifies the corpuscular and wave models of light.
The second article relates to the study of Brownian motion, meaning the random movement of particles suspended in fluid. Using the probabilities, Einstein has made a mathematical description of the phenomenon.
In the third article, by far the most famous, Einstein explains the fundamental theory of relativity. Since the time of Newton, scientists tried unsuccessfully to link the laws of motion to the laws of Maxwell in the framework of a unified description of the world. According to the mechanistic conception, the laws of motion should be able to explain all phenomena, while, according to supporters of Maxwell, the laws of electricity should be the foundation of physics. But these two major theories both remained unable to give a coherent explanation of the aspect that takes the interaction of light with matter in different inertial benchmarks, meaning at a constant relative speed to each other.
In the spring of 1905, Einstein realized that the heart of the problem lied not in the theory of matter, but in the measurement theory. He is therefore obliged to revise notions of measurement of space and time, which led him to develop a theory based on two assumptions: the principle of relativity, stipulating that all laws of physics are similar in all inertial reference points , and the principle of invariance of the speed of light, stating that the speed in a vacuum is a universal constant. Thanks to this theory, he was then able to give a logical and correct description of physical events in different inertial reference points without having to make as many assumptions about the nature of the substance or radiation, or how they interact .
The fourth article that Einstein published in 1905 is actually a corollary of the precedent: he there described the new concept of equivalence between mass and energy, introducing the famous formula E = mc^2.
The articles of Einstein soon hold the attention of leading scientists of the time, even though most of them remained highly sceptical. The total rejection of his theories is neither due to their mathematical complexity, or any technical darkness, but rather to the approach of the subject by Einstein.
Adopting, indeed, a very personal point of view on how to capture the experience and theory, he considers that the experience is the only real source of knowledge, scientific theories being only free creations, produced by a deep physical intuition. He believed further that the premises upon which are based the theories can not be linked to the experimentation logically. Hence, a theory is valid in his eyes if it contains the bare minimum of assumptions required to justify a physical evidence. This scarcity of assumptions, very characteristic of the work of Einstein, may explain why his colleagues are so reluctant to admit his theories.
However, Einstein is still supported by leading physicists, starting with the German Max Planck. Quickly acquiring some recognition within the scientific community, he participates in numerous international conferences where he tries to convince on the theory of relativity. He starts finding his place in the German academic system, and won his first university post in 1909, at the University of Zurich. In 1911, he held a post at the University of Prague, before returning the following year in Zurich. In 1913, he accepted a professorship at the Kaiser-Wilhelm Institute of Physics in Berlin.
Before his departure from the Patent Office, Einstein has already begun work on the extension and the generalization of his theory of relativity beyond the inertial reference points. In this context, he establishes the principle of equivalence, postulating that the gravitational field is equivalent to acceleration, following the landmark reference in which the observer is. In addition, he introduces the concept of space-time, four-dimensional space, including the three classic dimensions of space and the time. This mathematical abstraction allows him to study interactions between the bodies in a new context, interactions attributed so far to the gravitational field.
Published in 1916, the theory of general relativity appears to many physicists as a theory more philosophical than scientific, even quasi-mystical. However, this theory allows Einstein to explain the changes in the orbital motion of certain planets, but also to predict the curvature of the light of stars near a body mass as the Sun. The confirmation of this latter phenomenon during a solar eclipse in 1919 accredits the theories of Einstein, who is therefore pushed to the forefront of the scientific scene.
During the rest of his life, he attempted to generalize further his theory, working on the unification of electromagnetism and gravity, but his work was not crowned with success
Between 1915 and 1930, physics is dominated by a new conception of the fundamental nature of matter, the quantum theory. This theory uses the notion of wave-particle duality, already put forward by Einstein in an article published in 1917, explaining that the light has the properties of a particle but also those of a wave. It is also based on the principle of uncertainty, developed by the German physicist Heisenberg, saying it is impossible to know at the same time some physical quantities, for example the position and velocity of a particle. The quantum theory, which calls into question the notion of causality in physics, will never be fully accepted by Einstein, who refuses to give up his determinism: "God does not play dice with the world," he said. However, he contributed to this theory by studying the behaviour of photons, and, in 1924, he published an article written by the Indian physicist Satyendra Nath Bose, on this subject. Working with the latter, he developed the statistical theory of Bose-Einstein, which is used to describe the behaviour of quantum particles called bosons integer spin. Based on this theory, Einstein predicted that in a gas of identical atoms and without mutual interactions, cooled to a low enough temperature, a significant fraction of atoms should accumulate in the same quantum state of minimum energy (state ). This phenomenon, known as condensation of Bose-Einstein, is finally observed in 1995 by Eric A. Cornell, Carl E. Wieman and Wolfgang Ketterle, who jointly get the Nobel Prize for Physics in 2001 for confirmation of the predictions of Einstein and their subsequent studies on the properties of Bose-Einstein condensates.
After 1919, Einstein benefits of an international reputation. he accumulates honours and awards, especially in receiving the 1921 Nobel Prize in physics for his study of the photoelectric effect, not for the theory of relativity, which remains highly controversial. His visit in any part of the world becomes an event, photographers and journalists following him in his travels. he benefits from he reputation to defend his social and political concepts, including the support it gives to pacifism and Zionism.
During the First World War, he is one of the few German intellectuals who publicly denounced the belligerence of Germany. After the war, his commitment to the pacifist and Zionist theses makes him the preferred target of anti-Semitic elements and extreme right in Germany. Even his scientific theories are subject to public attacks, including the theory of relativity.
When Hitler comes to power in 1933, Einstein must leave Germany, migrating first to Paris, then in Belgium, before moving to Princeton (USA), where he held a post at the Institute for Advanced Study.
Continuing his efforts in favour of Zionism, Einstein breaks with pacifism before the terrifying threat to humanity represented by the Nazi regime.
In 1939, at the request of other physicists, Einstein agrees to write a letter to U.S. President Franklin Roosevelt, warning him of the danger the world would be exposed to if the German government committed itself in the path of nuclear energy. This famous letter is the origin of the Manhattan Project, the U.S. program of research aimed at building an atomic bomb. Einstein, however, did not contribute to this project, unlike some of his colleagues like Enrico Fermi and Niels Bohr. In 1945, when he understood that this program would succeed, he took the initiative to write again to pray for Roosevelt to renounce to atomic weapons.
After the war, Einstein calls for international worldwide disarmament, while continuing to actively support the cause of Israel. His commitment to social and political causes is sometimes described as unrealistic. In fact, his proposals are always carefully crafted. Like his scientific theories, they are motivated by a powerful intuition, based on a deep and insightful assessment of the evidence and observation. Even if Einstein devotes much of his time defending political and social causes, science still occupies first place in his work. Indeed, he constantly says that only the discovery of the nature of the universe would have a lasting significance.
Among his popular works, it is worth mentioning Fundamentals for the SRT and widespread (1916); About Zionism (1931); Manufacturers universe (1932); Why war? (1933), in collaboration with Sigmund Freud; How I see the World (1934); Evolution of ideas in physics (1938), co-authored with physicist Polish Leopold Infeld; Conceptions scientific, moral and social (1950).