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By Root 500 0

length and time were not absolute and unchanging. Spatial distances and time intervals depended on the relative motion of observers. Compared to his earth-bound twin, for an astronaut travelling at near light-speed, time slows down (the hands on a moving clock are slower), space contracts (the length of a moving object shrinks), and a moving object gains mass. These were the consequences of 'special' relativity, and each would be confirmed by experiments in the twentieth century, but the theory did not incorporate acceleration. 'General' relativity did. In the midst of his struggle to construct it, Einstein said that it made special relativity look like 'child's play'.28 Just as the quantum was challenging the accepted view of reality in the atomic realm, Einstein took humanity closer to understanding the true nature of space and time. General relativity was his theory of gravity, and it would lead others to the big bang origin of the universe.

In Newton's theory of gravity, the force of attraction between two objects, such as the sun and the earth, is proportional to the product of their respective masses and inversely proportional to the square of the distance separating their centres of mass. With no contact between the masses, in Newtonian physics gravity is a mysterious 'action-at-a-distance' force. In general relativity, however, gravity is due to the warping of space caused by the presence of a large mass. The earth moves around the sun not because some mysterious invisible force pulls it, but because of the warping of space due to the sun's enormous mass. In short, matter warps space and warped space tells matter how to move.

In November 1915, Einstein tested general relativity by applying it to a feature of Mercury's orbit that could not be explained using Newton's gravitational theory. In its journey around the sun, Mercury does not trace out exactly the same path every orbit. Astronomers had precise measurements that revealed that the planet's orbit rotated slightly. Einstein used general relativity to calculate this orbital shift. When he saw that the number matched the data within the margins of error, he had palpitations of the heart and felt as if something had snapped inside. 'The theory is beautiful beyond comparison', he wrote.29 With his boldest dreams fulfilled, Einstein was content but the Herculean effort left him worn out. When he recovered he turned to the quantum.

Even as he worked on the general theory, in May 1914, Einstein was among the first to grasp that the Franck-Hertz experiment was a confirmation of the existence of energy levels in atoms and 'a striking verification of the quantum hypothesis'.30 By the summer of 1916, Einstein had 'a brilliant idea' of his own about an atom's emission and absorption of light.31 It led him to an 'astonishingly simple derivation, I should say, the derivation of Planck's formula'.32 Soon Einstein was convinced that 'light-quanta are as good as established'.33 However, it came at a price. He had to abandon the strict causality of classical physics and introduce probability into the atomic domain.

Einstein had offered alternatives before, but this time he could derive Planck's law from Bohr's quantum atom. Starting with a simplified Bohr atom with only two energy levels, he identified three ways in which an electron could jump from one level to the other. When an electron jumps from a higher to a lower energy level and emits a quantum of light, Einstein called this 'spontaneous emission'. It occurs only when an atom is in an excited state. The second type of quantum leap happens when an atom becomes excited as an electron absorbs a light-quantum and jumps from a lower to a higher energy level. Bohr had invoked both types of quantum leap to explain the origin of atomic emission and absorption spectra, but Einstein now revealed a third: 'stimulated emission'. It occurs when a light-quantum strikes an electron in an atom that is already in an excited state. Instead of absorbing the incoming light-quantum, the electron is 'stimulated', nudged, to leap to a lower