Quantum_ Einstein, Bohr and the Great Debate About the Nature of Reality - Manjit Kumar [74]
In February 1923 Bohr received a letter dated 21 January, from Arnold Sommerfeld, alerting him to the 'most interesting thing that I have experienced scientifically in America'.95 He had swapped Munich, Bavaria for Madison, Wisconsin for a year and managed to escape the worst of the hyperinflation about to engulf Germany. It had been a shrewd financial move for Sommerfeld. To get an early glimpse of the work of Arthur Holly Compton before his European colleagues was an unexpected bonus.
Compton had made a discovery that challenged the validity of the wave theory of X-rays. Since X-rays were electromagnetic waves, a form of short-wavelength invisible light, Sommerfeld was saying that the wave nature of light, contrary to all the evidence in its favour, was in serious trouble. 'I do not know if I should mention his results', wrote Sommerfeld somewhat coyly, since Compton's paper had not yet been published. 'I want to call your attention to the fact that eventually we may expect a completely fundamental and new lesson.'96 It was a lesson that Einstein had been trying to teach with varying degrees of enthusiasm since 1905. Light was quantised.
Compton was one of America's leading young experimenters. He had been appointed professor and head of physics at the University of Washington in St Louis, Missouri in 1920 at just 27. His investigations into the scattering of X-rays conducted two years later would be described as 'the turning point in twentieth-century physics'.97 What Compton did was fire a beam of X-rays at a variety of elements such as carbon (in the form of graphite) and measure the 'secondary radiation'. When the X-rays slammed into the target most of them passed straight through, but some were scattered at a variety of angles. It was these 'secondary' or scattered X-rays that interested Compton. He wanted to find out if there was any change in their wavelength compared to the X-rays that had struck the target.
He found that the wavelengths of the scattered X-rays were always slightly longer than those of the 'primary' or incident X-rays. According to the wave theory they should have been exactly the same. Compton understood that the difference in wavelength (and therefore frequency) meant the secondary X-rays were not the same as the ones that had been fired at the target. It was as strange as shining a beam of red light at a metal surface and finding blue light being reflected.98 Unable to make his scattering data tally with the predictions of a wavelike theory of X-rays, Compton turned to Einstein's light-quanta. Almost at once he found 'that the wavelength and the intensity of the scattered rays are what they should be if a quantum of radiation bounced from an electron, just as one billiard ball bounces from another'.99
If X-rays came in quanta, then a beam of X-rays would be similar to a collection of microscopic billiard balls slamming into the target. Although some would pass through without hitting anything, others would collide with electrons inside atoms of the target. During such a collision an X-ray quantum would lose energy as it was scattered and the electron sent recoiling from the impact. Since the energy of an X-ray quantum is given by E=hv, where h is Planck's constant and v its frequency, then any loss of energy must result in a drop in frequency. Given that frequency is inversely proportional to wavelength, the wavelength associated with a scattered X-ray quantum increases. Compton constructed a detailed mathematical analysis of how the energy lost by the incoming X-ray and the resulting change in the wavelength (frequency) of the scattered X-ray was dependent upon the angle of scattering.