Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [4]
PART I
THE QUANTUM
'Briefly summarized, what I did can be described as simply an act of desperation.'
—MAX PLANCK
'It was as if the ground had been pulled out from under one, with no firm foundation to be seen anywhere, upon which one could have built.'
— ALBERT EINSTEIN
'For those who are not shocked when they first come across quantum theory cannot possibly have understood it.'
— NIELS BOHR
Chapter 1
THE RELUCTANT REVOLUTIONARY
'A new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it', wrote Max Planck towards the end of his long life.1 Bordering on cliché, it could easily have served as his own scientific obituary had he not as an 'act of desperation' abandoned ideas that he had long held dear.2 Wearing his dark suit, starched white shirt and black bow tie, Planck looked the archetypal late nineteenth-century Prussian civil servant but 'for the penetrating eyes under the huge dome of his bald head'.3 In characteristic mandarin fashion he exercised extreme caution before committing himself on matters of science or anything else. 'My maxim is always this,' he once told a student, 'consider every step carefully in advance, but then, if you believe you can take responsibility for it, let nothing stop you.'4 Planck was not a man to change his mind easily.
His manner and appearance had hardly changed when to students in the 1920s, as one recalled later, 'it seemed inconceivable that this was the man who had ushered in the revolution'.5 The reluctant revolutionary could scarcely believe it himself. By his own admission he was 'peacefully inclined' and avoided 'all doubtful adventures'.6 He confessed that he lacked 'the capacity to react quickly to intellectual stimulation'.7 It often took him years to reconcile new ideas with his deep-rooted conservatism. Yet at the age of 42, it was Planck who unwittingly started the quantum revolution in December 1900 when he discovered the equation for the distribution of radiation emitted by a blackbody.
All objects, if hot enough, radiate a mixture of heat and light, with the intensity and colour changing with the temperature. The tip of an iron poker left in a fire will start to glow a faint dull red; as its temperature rises it becomes a cherry red, then a bright yellowish-orange, and finally a bluish-white. Once taken out of the fire the poker cools down, running through this spectrum of colours backwards until it is no longer hot enough to emit any visible light. Even then it still gives off an invisible glow of heat radiation. After a time this too stops as the poker continues to cool and finally becomes cold enough to touch.
It was the 23-year-old Isaac Newton who, in 1666, showed that a beam of white light was woven from different threads of coloured light and that passing it through a prism simply unpicked the seven separate strands: red, orange, yellow, green, blue, indigo, and violet.8 Whether red and violet represented the limits of the light spectrum or just those of the human eye was answered in 1800. It was only then, with the advent of sufficiently sensitive and accurate mercury thermometers, that the astronomer William Herschel placed one in front of a spectrum of light and found that as he moved it across the bands of different colours from violet to red, the temperature rose. To his surprise it continued to rise when he accidentally left the thermometer up to an inch past the region of red light. Herschel had detected what was later called infrared radiation, light that was invisible to human eyes from the heat that it generated.9 In 1801, using the fact that silver nitrate darkens when exposed to light, Johann Ritter discovered invisible light at the other end of the spectrum beyond the