Guest post by Maria Maragkou, Associate Editor at Nature Materials
This week’s entries for the poll of the most beautiful experiments with light occurred around the turn of the 19th century.
Wilhelm Roentgen, a physics professor in Wurzburg, changed the course of medicine when he accidentally discovered X-rays, energy waves at frequencies from 0.1 – 10 nanometres. In November 1895, while experimenting with an electron-discharge tube covered with black cardboard, he noticed that a fluorescent screen further away was illuminated. Eventually he realised that the tube emitted a type of ray – marked X for unknown – that was blocked by dense material, such as lead or bones, but could penetrate other objects. As he held a piece of lead in front of the X-rays, he could see the contrast between bones and flesh on the fluorescent screen. A few weeks later, Roentgen took an X-ray picture of his wife’s hand, who allegedly claimed “I have seen my death” upon seeing it. With the discovery of X-rays, it became possible to look inside the human body without surgery and Roentgen earned the first ever Nobel Prize in Physics in 1901 for this remarkable invention.
The beginning of the 20th century also marked the beginning of the era of the theory of general relativity. Albert Einstein first conceived the theory in 1915 but he rose to stardom overnight in 1919, when Sir Arthur Eddington and his team of astronomers confirmed his theory. Einstein had predicted that light should bend slightly when travelling through a gravitational field with a deflection angle larger than the one due to gravity.
The total solar eclipse of May 29, 1919 was the perfect occasion to test this hypothesis; it was supposed to last almost seven minutes and as it coincided with the Sun crossing the Hyades star cluster, the stars near the Sun could be visible during the eclipse. The true positions of the stars were determined during February and March and were then measured in May simultaneously during the eclipse from two sub-teams located in the island of Príncipe, in West Africa and Sobral, Brazil. Eddington’s team indeed confirmed that light was bent by 1.61 seconds of an arc, close to the value of 1.75, calculated by Einstein. Although many questioned the accuracy of the measurements at the time, numerous astrophysical measurements carried out since then have confirmed Einstein’s calculations.
Another aspect of Einstein’s theory would be confirmed shortly after. At that time, the debate about the wave-particle nature of light was at its peak. Einstein had only recently (1905) proposed the existence of light quanta (or photons as we know them now), small particle-like objects that constitute light, based on Max Planck’s interpretation of black body radiation. Of course, this challenged the by then well-established wave behaviour of light. Arthur Compton observed what happens when X-ray and gamma ray photons collide with electrons. If light is a wave, it should be scattered in all directions, similar to waves caused by a stone thrown in a lake. If however the quantities involved can be seen as billiard balls, their collision is governed by the rules of energy and momentum conservation. It was the latter picture that could explain and predict Compton’s observations of the wavelength change of the scattered photons and the speed of electrons, at different angles. With his famous Physical Review article, he unambiguously showed for the first time, that the assumption that light is made of quanta is valid, earning the Nobel Prize in Physics 4 years later.
Experiments covered this week:
1895 Roentgen’s first X-ray image
1919 General relativity put to the test: measuring light bending during a solar eclipse
1923 Observation of the Compton effect (inelastic scattering of photons by charged particles)
New week Leonie Mueck will look at Gabor’s invention of holograms and Hubble’s discovery of an expanding universe from Doppler shift.