einstein

Atoms emit radiation in a spontaneous fashion, but Einstein theorized that this process could also be stimulated. A roughly simplified way to
picture this is to suppose that an atom is already in a high-energy state from having absorbed a photon. If another photon with a particular
wavelength is then fired into it, two photons of the same wavelength and direction can be emitted.
What Einstein discovered was slightly more complex. Suppose there is a gas of atoms with energy being pumped into it, say by pulses of
electricity or light. Many of the atoms will absorb energy and go into a higher energy state, and they will begin to emit photons. Einstein argued that
the presence of this cloud of photons made it even more likely that a photon of the same wavelength and direction as the other photons in the cloud
would be emitted. 35 This process of stimulated emission would, almost forty years later, be the basis for the invention of the laser, an acronym for
“light amplification by the stimulated emission of radiation.”
There was one part of Einstein’s quantum theory of radiation that had strange ramifications. “It can be demonstrated convincingly,” he told Besso,
“that the elementary processes of emission and absorption are directed processes.” 36 In other words, when a photon pulses out of an atom, it does
not do so (as the classical wave theory would have it) in all directions at once. Instead, a photon has momentum. In other words, the equations work
only if each quantum of radiation is emitted in some particular direction.
That was not necessarily a problem. But here was the rub:there was no way to determine which direction an emitted photon might go. In
addition, there was no way to determine when it would happen. If an atom was in a state of higher energy, it was possible to calculate the
probability that it would emit a photon at any specific moment. But it was not possible to determine the moment of emission precisely. Nor was it
possible to determine the direction. No matter how much information you had. It was all a matter of chance, like the roll of dice.
That was a problem. It threatened the strict determinism of Newton’s mechanics. It undermined the certainty of classical physics and the faith that
if you knew all the positions and velocities in a system you could determine its future. Relativity may have seemed like a radical idea, but at least it
preserved rigid cause-and-effect rules. The quirky and unpredictable behavior of pesky quanta, however, was messing with this causality.
“It is a weakness of the theory,” Einstein conceded, “that it leaves the time and direction of the elementary process to ‘chance.’ ” The whole
concept of chance—“Zufall” was the word he used—was so disconcerting to him, so odd, that he put the word in quotation marks, as if to distance
himself from it. 37
For Einstein, and indeed for most classical physicists, the idea that there could be a fundamental randomness in the universe—that events could
just happen without a cause—was not only a cause of discomfort, it undermined the entire program of physics. Indeed, he never would become
reconciled to it. “The thing about causality plagues me very much,” he wrote Max Born in 1920. “Is the quantumlike absorption and emission of light
ever conceivable in terms of complete causality?” 38
For the rest of his life, Einstein would remain resistant to the notion that probabilities and uncertainties ruled nature in the realm of quantum
mechanics. “I find the idea quite intolerable that an electron exposed to radiation should choose of its own free will not only its moment to jump off
but also its direction,” he despaired to Born a few years later. “In that case, I would rather be a cobbler, or even an employee of a gaming house,
than a physicist.” 39
Philosophically, Einstein’s reaction seemed to be an echo of the attitude displayed by the antirelativists, who interpreted (or misinterpreted)
Einstein’s relativity theory as meaning an end to the certainties and absolutes in nature. In fact, Einstein saw relativity theory as leading to a deeper
description of certainties and absolutes—what he called invariances—based on the combination of space and time into one four-dimensional
fabric. Quantum mechanics, on the other hand, would be based on true underlying uncertainties in nature, events that could be described only in
terms of probabilities.
On a visit to Berlin in 1920, Niels Bohr, who had become the Copenhagen-based ringleader of the quantum mechanics movement, met Einstein
for the first time. Bohr arrived at Einstein’s apartment bearing Danish cheese and butter, and then he launched into a discussion of the role that
chance and probability played in quantum mechanics. Einstein expressed his wariness of “abandoning continuity and causality.” Bohr was bolder
about going into that misty realm. Abandoning strict causality, he countered to Einstein, was “the only way open” given the evidence.
Einstein admitted that he was impressed, but also worried, by Bohr’s breakthroughs on the structure of the atom and the randomness it implied
for the quantum nature of radiation. “I could probably have arrived at something like this myself,” Einstein lamented, “but if all this is true then it
means the end of physics.” 40
Although Einstein found Bohr’s ideas disconcerting, he found the gangly and informal Dane personally endearing. “Not often in life has a human
being caused me such joy by his mere presence as you did,” he wrote Bohr right after the visit, adding that he took pleasure in picturing “your
cheerful boyish face.” He was equally effusive behind Bohr’s back.“Bohr was here, and I am just as keen on him as you are,” he wrote their mutual
friend Ehrenfest in Leiden. “He is an extremely sensitive lad and moves around in this world as if in a trance.” 41
Bohr, for his part, revered Einstein. When it was announced in 1922 that they had won sequential Nobel Prizes, Bohr wrote that his own joy had
been heightened by the fact that Einstein had been recognized first for “the fundamental contribution that you made to the special field in which I am
working.” 42
On his journey home from delivering his acceptance speech in Sweden the following summer, Einstein stopped in Copenhagen to see Bohr, who
met him at the train station to take him home by streetcar. On the ride, they got into a debate. “We took the streetcar and talked so animatedly that
we went much too far,” Bohr recalled. “We got off and traveled back, but again rode too far.” Neither seemed to mind, for the conversation was so
engrossing. “We rode to and fro,” according to Bohr, “and I can well imagine what the people thought about us.” 43
More than just a friendship, their relationship became an intellectual entanglement that began with divergent views about quantum mechanics but
then expanded into related issues of science, knowledge, and philosophy. “In all the history of human thought, there is no greater dialogue than that
which took place over the years between Niels Bohr and Albert Einstein about the meaning of the quantum,” says the physicist John Wheeler, who
studied under Bohr. The social philosopher C. P. Snow went further. “No more profound intellectual debate has ever been conducted,” he
proclaimed. 44
Their dispute went to the fundamental heart of the design of the cosmos: Was there an objective reality that existed whether or not we could ever
observe it? Were there laws that restored strict causality to phenomena that seemed inherently random? Was everything in the universe
predetermined?
For the rest of their lives, Bohr would sputter and fret at his repeated failures to convert Einstein to quantum mechanics.Einstein, Einstein,
Einstein, he would mutter after each infuriating encounter. But it was a discussion that was conducted with deep affection and even great humor.
On one of the many occasions when Einstein declared that God would not play dice, it was Bohr who countered with the famous rejoinder: Einstein,
stop telling God what to do! 45
Quantum Leaps