In the last decades of the 19th century, there was a general feeling that all the major discoveries in physics had been made and all that remained was to pursue the values of the fundamental constants to extra decimal places. In reality, this could not have been further from the truth as subsequent events were about to prove. One area of research which was to bring surprising results was the electrical conductivity of gases.

Following the invention of the incandescent filament lamp by Joseph Swan in 1878, the cathode-ray tube, developed in the 1880s, became a major tool for studying electrical conduction in gases. It was found that the cathode rays could be deflected by electric and magnetic fields and it was suspected that the "rays" were in fact negatively charged particles, a fact that was established in 1897 by Joseph John Thomson (no relation to the author) at the Cavendish laboratory in Cambridge. Thomson correctly identified the electron as a particle which could be knocked out of atoms in the gas, leading to the production of positively charged ions which, when accelerated by the high voltage, produced so-called positive rays. This line of research would eventually lead to the invention of the mass spectrograph by Aston in 1919 and to the discovery of isotopes - atoms of the same element but which have different nuclear masses.

Meanwhile in Wurtzburg , a German physicist called Wilhelm Roentgen was studying fluorescence in minerals. One day in November 1895, he noticed that a fluorescent screen glowed when at a considerable distance from a cathode ray tube and even when the tube was completely covered by black cardboard. Roentgen had discovered X-rays and in 1902 he was awarded the first Nobel Prize for physics. Following Roentgen's line of research into fluorescence, Henri Becquerel working at the Sorbonne, reported in February 1896 that he had discovered an " emission of rays" from uranium compounds which fogged wrapped photographic plates even when stored in a dark cupboard.

The nature of these rays eluded Becquerel, who incorrectly associated them with X-rays. It was left to his Polish student Maria Slodowska, soon to be married to Pierre Curie, to correctly identify the source of "Becquerel rays" which she renamed radioactivity. Marie and Pierre Curie realised that the radiation was being emitted from the uranium itself and over the next few years, the same property was found in thorium, as well as in the previously unidentified elements they named radium and polonium. They shared the 1903 Nobel Prize with Becquerel for the discovery of radioactive elements. Marie Curie was the first female recipient of the prize and also the first person to win it twice, for her work on radium and its compounds. She died of anaemia caused by overexposure to radiation in 1934.

Back in England, a young New Zealander named Ernest Rutherford was working with Thomson to identify the exact nature of the radiation emitted by uranium. Between 1896 and 1900, sharing a corner of the Cavendish laboratory in Cambridge with Thomson and his cathode tubes, Rutherford identified three types of radiation. A heavy particle which could be stopped by sheets of paper he named alpha and identified it as the core of a helium atom. A lighter particle, named beta was found to be an energetic electron which could penetrate thin sheets of aluminium. The third type of radiation was found to be not a particle at all but a more powerful form of electromagnetic wave similar to X-rays, called gamma-rays. He was awarded the Nobel Prize for chemistry in 1908. Rutherford went on to discover the nucleus of the atom, in Manchester in 1911, and he produced the first artificial nuclear transmutation in 1919 when he bombarded nitrogen gas with alpha particles to produce oxygen nuclei and protons.

Models of the Atom

In ancient Greek philosophy the word 'atom' was used to describe the smallest bit of matter that could be conceived of; the word 'atomos' means indivisible. Early in the 19th century, John Dalton (1766-1844), the Manchester schoolmaster and chemist, made studies of the way in which the various elements combine to form compounds. Dalton's most important contribution to science was his theory that matter is composed of atoms of differing weights ( we would say masses !) and combine in simple ratios by weight. [In his book A New System of Chemical Philosophy, published in 1808, he listed the atomic weights of a number of the known elements which formed the basis for the periodic table .] Dalton's model of the atom was that of a rigid sphere, like a billiard ball.

Following Rutherford's discovery of the nucleus, a new model of the atom emerged. Thomson's idea of the atom as a kind of 'plum-pudding' ( in which electrons and protons were mixed together in a fuzzy ball ) was replaced by Rutherford's in which a small, dense nucleus, 10 000 times smaller than the atom as a whole, was orbited by a cloud of negatively-charged electrons, like a swarm of bees around a hive. However, the orbits of the electrons were still thought to be circular, like those of the planets around the Sun, and to obey the laws of Newtonian mechanics.

The problem with this model was that the classical theory of electromagnetism, developed by Scottish physicist James Clerk Maxwell (1831-79), predicted that an electron moving in a circular orbit would radiate energy continuously until it fell into the nucleus. Rutherford's atom would therefore be unstable. This difficulty led Danish physicist Neils Bohr, also working in Manchester in 1913, to postulate an alternative approach; that of 'quantized' orbits with fixed energies, but no longer precisely circular. This helped to explain the observed emission of discrete wavelengths of light emitted by the atoms of gases [ See hydrogen spectrum ] but the theory proved to be too difficult to apply to atoms with more electrons than helium and then it was only in approximate agreement with observations.
 
 
 
 

Enter the Quantum . . .

Problems arising from differences between the predictions of 'classical' theory and observations ( reminiscent of the problems encountered with the Ptolemaic system in the 16th century ) led the German physicist Max Planck ( 1858-1947 ) to postulate in 1900 that energy can be emitted or absorbed by matter only in small, discrete units called quanta. This energy is in the form of electromagnetic radiation and the energy associated with it is related to its frequency by the formula E = hf, where h is Planck's constant ( 6.626 x 10 - 34 Js ).
[ See Energy Levels ]

Einstein used Planck's concept of the quantum to explain the photoelectric effect, in which electrons are 'knocked-out' of some metals when light of a certain wavelength shines on their surfaces. Classically, the energy of the light should be proportional to its intensity or brightness. Einstein showed, however, that the correct explanation depended on the energy being proportional to the frequency of the radiation as Planck had proposed. Unlikely as it seemed, the light waves were behaving more like particles which were named photons. It was for this breakthrough in quantum theory that Einstein was awarded his Nobel Prize in 1921 - not for his rather more famous theory of Relativity, which was 'relatively' poorly understood by the scientific community at the time.

Further evidence for the particle-like nature of the photon came from the experiments of the American physicist Arthur Holly Compton (1892-1962) who in 1923, measured the change in wavelength of X-rays scattered by electrons. He showed that the X-rays behaved in collisions with electrons as though they were particles and that they had momentum. He also succeeded in measuring the wavelength associated with the recoiling electrons, demonstrating that particles also had wave-like properties.
 

Wave-particle duality

After the discovery and interpretation of the Compton effect, the existence of quanta could no longer be questioned. This left physics in an uncomfortable position. On the one hand, electromagnetic radiation manifests distinctly wave-like properties such as diffraction and interference. The classic experiment which established the wave nature of light was performed in 1801 by Thomas Young, a British physicist, physician and Egyptologist ( who helped to decipher the hieroglyphs  on the Rosetta Stone  found by French troops in 1799 ). In his 'double-slit' experiment, Young showed how light from two sources produces a pattern of bright and dark fringes, which can only be satisfactorily explained by the wave theory.

On the other hand, some properties of light, such as the photoelectric effect and Compton scattering, could only be explained by regarding light as quanta, which have characteristics more like those of (localised) particles. In his doctoral dissertation in 1924, French physicist Count Louis Victor de Broglie suggested that, under certain circumstances, particles might exhibit wave-like properties. This suggestion was verified experimentally  within a few years by G P Thomson ( son of JJ ! ) and Clinton Davisson, who shared the 1937 Nobel Prize for their discovery that a beam of electrons, scattered by the atoms in a crystal, produces a wave-like diffraction pattern similar to that produced by X-rays [as shown in 1912 by Sir William and his son, Lawrence Bragg who shared the 1915 Nobel prize for their efforts. ] The wavelength of a beam of electrons is given by de Broglie's wave equation as  where p is the momentum of the electrons (mv) and h is of course Planck's constant. [ see electron diffraction ]
 

"Curiouser and curioser !"  cried Alice ( she was so much surprised . . .)

The strange and wonderful idea that particles can behave like waves and vice-versa led the Austrian physicist Erwin Schroedinger (1887-1961) to develop an equation to describe the wave-properties of particles, in particular the behaviour of the electron in the hydrogen atom. The solutions of Schroedinger's wave-equation were successful in describing both the hydrogen and helium atoms in terms of quantum numbers and they confirmed the validity of the rule, established empirically in 1925 by Wolfgang Pauli, that no two electrons could simultaneously occupy the same energy state. This is known as the Pauli exclusion principle.

Another strange consequence of the new science of quantum mechanics was the statement in 1927 by the German physicist Werner Heisenberg (1901-1976) that it is impossible to know simultaneously the precise position and the exact momentum of any particle. It is called the Uncertainty Principle and it is an underlying feature of all modern physics. It is not just that we cannot measure these quantities accurately enough ( although that is difficult enough ) but that the Universe conspires to prevent us from knowing too much about a particle because it also has a wave-like nature. That we cannot be sure exactly where it is and what it is going to do next. This unpredictability or randomness in the laws of nature led Einstein to comment, " I am convinced that (God) is not playing at dice." Despite his own part in bringing the subject into the light of scientific curiosity, Einstein remained convinced in the ultimate simplicity of the laws of nature and their eventual understanding by the minds of human beings; "All of these endeavours are based on the belief that existence should have a completely harmonious structure."

Modern physicists are well aware that much about quantum mechanics does not make sense; they have a term for it - quantum weirdness. Richard P Feynman (1918-1988) who was arguably this century's greatest physicist since Albert Einstein, wrote in his book The Character of Physical Law; " Do not keep saying to yourself, if you can possibly avoid it, 'But how can it be like that ?' because you will get 'down the drain', into a blind alley from which nobody has yet escaped. Nobody knows how it can be like that."