When Max Plank embarked upon the study of physics in the early 20 th century, his first lesson was that that there was nothing new to be unearthed in physics; Newton had already discovered the only universe in existence. But then, along came Einstein and replaced it with his own. "Newton, forgive me," Einstein wrote in his Autobiographical Notes, "You found the only way, which in your age, was just about possible for a man of highest thought and creative power." While Newton came up with one single system for explaining the universe, Einstein came up with two -- relativity and quantum theory -- both remarkably accurate in their respective domains of the very large and the very small.
The bare essentials Einstein laid the essential ideas of these theories in just five path-breaking papers -- all published in a single year -- that ended classical physics and founded modern physics. Historians call 1905, Einstein's annus mirabilis, the miraculous year.
Now physicists the world over are celebrating the 100 years since the Einstein papers were published, with 2005 decared by the un as the International Year of Physics.
Even today, not too many scientists are able to grasp relativity. A student asked British astrophysicist Arthur Eddington, if it was true that only three people understood relativity. Pausing, he came up with the reply, "I am trying to think who the third person is". Indeed it was Eddington's Total Solar Eclipse experiment in 1919, which provided the experimental confirmation of Einstein's ideas.
Steps to relativity In the 1860's, Maxwell's theory showed that light was oscillating electric and magnetic fields, which immediately raised the question of the medium of oscillation. Oceans had waves in water, sound waves moved through air; it seemed nonsense to imagine that waves could just be. Unable to comprehend waves that were not vibrations in some medium, physicists postulated the existence of ether, an otherwise undetectable substance through which light travels. Like swimming along the current is faster, it was anticipated that speed of light in the direction of the ether wind would be higher than speed of light in a perpendicular direction.
Prompted by this, Albert Michelson and Edward Morley performed a series of ingenious experiments from 1881 to 1887, to detect ether by comparing differences in the speed of light during the earth's rotations around itself with its revolutions around the sun. But they failed, finding that the speed of light was same in all frames of reference.
The failure of this experiment was rationalised by Dutch physicist Hendrik Antoon Lorentz's preposterous proposition that there was a contraction in the direction of the earth's movement, just enough to make the two speeds seem the same, though he could not explain how this contraction occurred.
Abandoning the ether hypothesis as playing no role, Einstein held forth on two basic postulates. One, speed of light is invariant in all frames of reference regardless of the speed of the observer. Two, laws of physics should look the same as long as the observer is in uniform motion (first codified by Galileo). On these, Einstein construed his revolutionary theory of relativity. While everyone thought of time as invariant, Einstein brought in the theory that time was relative.
As a consequence of his new theory, Einstein proposed 'time dilatation': time, analogous to length and mass, is a function of the velocity of a frame of reference and furthermore, that nothing can travel faster than the speed of light.Nonetheless, Einstein did not immediately perceive that the unchanging nature of speed of light also implied that mass and energy are interchangeable, the rate of exchange being defined by the speed of light and governed by perhaps the most famous equation in science: E=mc 2 , in which E represents energy, m is mass and c, speed of light. This was published as notes in November 1905.
Einstein, who had confidently used Maxwell's equations, that considered light as a wave, in his theory of relativity, made a dynamic turnaround and explained the puzzling phenomena of photoelectric effect by arguing that light was a stream of particles (later named as photons). The emission of photoelectric metals was found to have certain paradoxical properties; shining a brighter beam at metal conductors did not increase voltage, although current increased -- bright light produced more electrons, but not more energetic electrons. Turn up the frequency of the beam, however, and the voltage went up. If light were to be a wave, as was thought then, both the energy and the number of the electrons emitted from the metal should increase with an increase in the intensity of light. Observations contradicted this prediction and baffled physicists for many decades.
Einstein argued that light is actually composed of tiny particles (later called as photons) whose energy is proportional to its frequency; therefore increasing the intensity of the light increased the number of photons, while the energy of each individual photon remained the same, as long as the frequency of the light remained the same. Therefore the number of electrons emitted would increase, but the energy transmitted to them by the particles of light would remain the same.
Indeed, Max Plank was the first to advocate quantising radiation (occurring in pockets of energy) but he stopped short of deducing that quantising light means it's composed of particles rather than waves. But Einstein, in his seminal paper published in March 1905, concluded that energy in fact existed only in multiples of quanta. This profound conceptual leap paved the way for quantum mechanics and earned him the Nobel in 1921.
Between Plank's work on quanta of heat and Niels Bohr's later work on quanta of matter, Einstein's work anchors the most shocking idea in 20th century physics: we live in a quantum universe -- built out of tiny, discrete chunks of energy and matter -- that are wave and particle simultaneously!
Reifying atoms and molecules
Until the 1920s, Einstein's two papers, published in April and May 1905, on 'Brownian motion', were the most cited. One (his doctoral thesis) inferred the size of molecules from the speed with which sugar dissolves in water. The other invented a new method of counting and determining the size of atoms or molecules in a given space, where the molecular theory of heat was applied to liquids.
In passing, this also solved the old puzzle of the Brownian motion -- incessant, irregular "swarming" motion of microscopic bits of plant pollen in still water -- first observed in 1828 by English botanist Robert Brown. Einstein showed that molecules hitting the particles caused the motion, so they're real. Theoretical proof that that atoms actually existed -- still an issue at that time.
A man of peace
With the discovery of fission of the uranium nucleus, Einstein and Leo Szilard, warned President Roosevelt in 1939 of probable Nazi weaponry, and also led the Allied development of the atomic bomb (whose theoretical basis was his own equation e=mc2). But as the dangers of atomic weapons dawned, he and many of his fellow scientists became ardent pacifists, opposed nuclear weapons and argued for international civilian control. In May 1946, he became chairman of the Emergency Committee of Atomic Scientists and appealed for nuclear disarmament. An early and firm supporter of the un, he was convinced that the solution to international conflict was world law, world government, and a strong world police.
Upset at deployment of science and technology for war, he reiterated, " Concern for man himself must always constitute the chief objective of all technological effort". His outspokenness extended to a denouncement of McCarthyism, bigotry and racism. Piqued by Germany's growing anti-Semitism, he became a passionate Zionist, yet he expressed concern about the rights of Arabs in any Jewish state. His liberal expression even included support for socialism. He said, "uncontrolled competition results in an excessive waste of labour and the crippling of the social consciousness of individuals." His views irked the us establishment and he was under surveillance.
The legacy endures
But Einstein's courageous stand on the issues of our times should not come as a surprise to any student of physics. He showed the same spirit while reshaping our cosmos, as we knew it, by replacing long-cherished concepts of physics with his counterintuitive ideas about the nature of space and time. Time magazine's 'man of the century', his eminence as a scientist is matched by his enduring humanism.
T V Venkateswaran is principal scientific officer, Vigyan Prasar, New Delhi
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