1895 – Germany
‘X-Rays are high energy radiation given off when fast-moving electrons lose energy very rapidly’
’Not only was matter composed of particles not visible even with the modern microscope, as scientists from DEMOCRITUS to DALTON had surmised, but those particles were themselves composed of even smaller components’
By the end of the nineteenth century scientists had cleared up much of the confusion surrounding atomic theory. The discovery of the sub-atomic particle was made in April 1897. They believed that they now largely understood the properties and sizes of the atoms of elements; without question, hydrogen was the smallest of all.
When JJ Thomson announced the discovery of a particle one thousandth the mass of the hydrogen atom the particles were named ‘electrons’ and have been a fundamental part of the understanding of atomic science ever since.
Thomson was investigating the properties of cathode rays, now known to be a simple stream of electrons, but at the time the cause of widespread debate. The rays were known to be visible, like normal light, but they were quite clearly not normal light. He devised a series of experiments, which would apply measurements to the cathode rays and clarify their nature. The rays were created by passing an electric charge through an airless or gasless discharge tube.
By improving the vacuüm in the tube, it was demonstrated that the rays could be deflected by electric and magnetic fields. Thomson drilled a hole in the anode of the tube to allow the mysterious rays from the cathode to pass through. In the space after the anode, he arranged that a magnetic force field from a magnet would tug the cathode rays in one direction, and an electric force field between two electrically charged metal plates would tug them in the opposite direction. The rays would eventually strike the glass wall of the tube to create a familiar greenish spot of light on the phosphor-coated tube.
Thomson concluded that the rays were made up of particles, not waves. He saw that the properties of the particles were negative in charge and didn’t seem to be specific to any one element; they were the same regardless of the gas used to transport the electric discharge, or the metal used at the cathode. From his findings he concluded that cathode rays were made up of a jet of ‘corpuscles’ and, more importantly, that these corpuscles were present in all elements. Thomson devised a method of measuring the mass of the particles and found them to be a fraction of the weight of the hydrogen atom.
The position of the spot indicated how much the beam of cathode rays had been deflected. The deflection could be made zero by adjusting the magnetic and electric forces so that they perfectly balanced. In such a situation, Thomson could read off the strength of the electric force. He knew in theory how the magnetic force on a charged particle depends on its speed. By equating the two forces, he was able to deduce the speed of the cathode rays. The deflection was also influenced by the electric charge carried by the cathode ray particles, and their mass. The larger the charge, the greater the force the particles felt and the greater their deflection, the smaller the mass, the easier it was for any force to push the particles about and again, the greater their deflection.
Independent evidence from electrolysis (passing electricity through liquids) that electric charge came in discreet chunks, which he assumed to be carried by individual cathode ray particles, enabled Thomson to calculate their mass.
He arrived at a figure that was a thousand billion billion billionth of a kilogram – a 1000th of the mass of a hydrogen atom.
Knowing the deflection of the dot and the velocity of the particles (the slower the particles, the longer they were exposed to the electric force and the greater the deflection of the glowing dot), Thomson expected to be able to deduce their charge and mass. What he actually deduced was a combination of their charge and mass.
Atoms were made of smaller things, but the fundamental building-blocks were not hydrogen atoms, as had been maintained by PROUT.
Thomson’s particles were christened ‘electrons’ and were the first subatomic entities. Thomson visualized a multitude of tiny electrons embedded ‘like raisins in a plum pudding’ in a diffuse ball of positive charge.
‘The atom is a sphere of positively charged protons in which negatively charged electrons are embedded in just sufficient quantity to neutralise the positive charge’
This was the accepted picture of the atom at the start of the twentieth century until RUTHERFORD found a way to probe inside the atom in 1911.
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1898 – France
‘1903 – Awarded the Nobel-Prize for Physics jointly with Marie and Pierre Curie’
Stimulated by WILHELM CONRAD ROENTGEN’s discovery of X-rays in 1895, Becquerel chanced upon the phenomenon that is now known as radioactivity in 1896. The Frenchman believed that Röntgen’s X-rays were responsible for the fluorescence displayed by some substances after being placed in sunlight. Although he was wrong to assume that fluorescence had anything to do with X-rays, he tested large numbers of fluorescent minerals.
He found that uranium, the heaviest element, caused an impression on a covered photographic plate, even after being kept in the dark for several days, and concluded that a phenomenon independent of sunlight induced luminescence.
Investigation isolated the uranium as the source of ‘radioactivity’, a name given to the occurrence by Mme. Curie.
The SI unit of radioactivity, the becquerel is named in his honour.
1898-1902 – France
‘Pitchblende, the ore from which uranium is extracted, is much more radioactive than pure uranium. The ore must therefore contain unknown radioactive elements’
Following the discovery of radioactivity by HENRI BECQUEREL (1852-1908) in 1896, Marie Curie conclusively proved that radioactivity is an intrinsic property of the element in question and is not a condition caused by outside factors.
She correctly concluded that pitchblende contained other, more radioactive elements than uranium.
The Curies isolated two new radioactive elements, polonium and radium, from pitchblende. The discovery of new elements by their radioactivity was proof that radioactivity was a property of atoms.
Even today, Marie Curie’s notebooks of her studies remain too radioactive to handle.
1900 – Germany
‘Energy is not a continuous quantity but it is quantised; it flows in discrete packets or quanta. When particles emit energy they do so only in quanta’
According to Quantum theory, the energy (E) of one quantum (photon) is given by E = hf where f is the frequency of radiation and h is Planck’s constant.
Its value is 6.63 x 10-34 joules per second
h is a tiny number, close to zero, but it is has a finite value. This implies energy is released in discrete chunks, a revolutionary notion.
By the late 1800s the science of thermodynamics was developing to the point that people were beginning to understand the nature of energy.
The traditional view was that energy was released in a continuous stream and that any amount of energy could be indefinitely divided into smaller and smaller ‘lumps’. Planck’s work on the laws of thermodynamics and black body radiation led him to abandon this classical notion of the dynamic principles of energy and formulate the quantum theory, which assumes that energy changes take place in distinct packages, or quanta, that cannot be subdivided. This successfully accounted for certain phenomena that Newtonian theory could not explain.
The basic laws of thermodynamics recognised that energy could not be created or destroyed, but was always conserved. The second law was drawn from an understanding that heat would not pass from a colder body to a hotter body.
The study of thermodynamics was based on the assumption that matter was ultimately composed of particles. LUDWIG BOLTZMAN had proposed an explanation of thermodynamics, saying the energy contained in a system is the collective result of the movements of many tiny particles rattling around. He believed the second law was only valid in a statistical sense; it only worked if you added up all the bits of energy in all the little particles.
Among his detractors was Max Karl Ernst Ludwig Planck.
Planck began his work on the second law of thermodynamics and the concept of entropy. He investigated how materials transform between solid, liquid and gaseous states. In doing so he found explanations for the laws governing the differing freezing and boiling points of various substances.
He also looked at the conduction of electricity through liquid solutions (electrolysis).
In the mid 1890s Planck turned his attention to the question of how heated substances radiate energy. Physicists were aware that all bodies radiate heat at all frequencies – although maximum radiation is emitted only at a certain frequency, which depends on the temperature of the body. The hotter the body, the higher the frequency for maximum radiation. (Frequency is the rate per second of a wave of any form of radiation).
Planck had been considering formulae for the radiation released by a body at high temperature. Using ideas developed by ROBERT KIRCHOFF, he knew it should be expressible as a combination of wavelength frequency and temperature. For a theoretical ‘black body’, physicists could not predict expressions that were in line with the behaviour of hot bodies at high frequencies and were in agreement with other equations showing their nature at low frequencies. Thus no law could be found which fitted all frequencies and obeyed the laws of classical physics simultaneously.
Plank resolved to find a theoretical formula that would work mathematically, even if it did not reflect known physical laws. His first attempts were partially successful, but did not take into account any notion of particles or quanta of energy, as he was certain of the continuous nature of energy. In an ‘act of despair’ he renounced classical physics and embraced quanta.
The final straw had been a concept developed by John Rayleigh and James Jeans that became known as the ‘ultraviolet catastrophe’ theory. They had developed a formula that predicted values for radiation distribution and worked at low frequencies, but not at high frequencies. It was at odds with Planck’s formula, which worked for high frequencies but broke down at low frequencies. In June 1900 Rayleigh had pointed out that classical mechanics, when applied to the oscillators of a black-body, leads to an energy distribution that increases in proportion to the square of the frequency. This conflicted with all known data.
Planck’s answer was to introduce what he called ‘energy elements’ or quanta and to express the energy emitted as a straightforward multiplication of frequency by a constant, which became known as ‘Planck’s constant’ (6.6256 x 10-34 Jsec-1). This only works with whole number multiples which means for the formula to have any practical use one must accept the radical theory that energy is only released in distinct, non-divisible chunks, known as ‘quanta’, or for a single chunk of energy, a ‘quantum’. This completely contradicts classical physics, which assumed that energy is emitted in a continuous stream. The individual quanta of energy were so small that when emitted at the everyday large levels observed, it appears that energy could seem to be flowing in a continuous stream.
Thus classical physics was cast into doubt and quantum theory was born.
Planck announced his theory on December 14 1900 in his paper ‘On the Theory of the Law of Energy Distribution in the Continuous Spectrum’. Planck said ‘energy is made up of a completely determinate number of finite equal parts, and for this purpose I use the constant of nature h = 6.55 x 10-27(erg sec)’
When ALBERT EINSTEIN was able to explain the ‘photoelectric’ effect in 1905, suggesting that light is emitted in quanta called ‘photons’, by applying Planck’s theory – and likewise NIELS BOHR in his explanation of atomic theory in 1913 – the abstract idea was shown to explain physical phenomena.
Planck was awarded the Nobel Prize for Physics in 1918.
1901 – Sweden
1866 – Invents dynamite
1876 – Invents blasting gelatin
1886 – Invents ballistite
1896 – Nobel Foundation set-up to comply with the terms of Nobel’s will
1901 – First Nobel Prizes awarded
A Swede educated in Russia, France and the United States, Alfred Nobel was a chemist who set up a factory to manufacture the relatively unstable nitro-glycerine to serve the civil-engineering market. After a disaster in 1864, Nobel found a way to stabilise the liquid explosive with kieselguhr, which he called dynamite. He went on to develop blasting gelatin, ballistite and a series of detonators.
The success of Nobel’s dynamites was compounded by his holdings in oil, leading to vast personal wealth. He left much of his fortune to funding the establishment of a series of awards, one of which included an accolade for peace.
There was another dedicated to literature, with the remaining three presented for achievements in the sciences. The first Nobel prizes for medicine (or physiology), physics and chemistry were awarded in 1901.
The prizes are awarded annually, according to the terms of Nobel’s will; ‘to those who, during the preceding year, shall have conferred the greatest benefit on mankind.’
1905 – Switzerland
- ‘the relativity principle: All laws of science are the same in all frames of reference.
- constancy of the speed of light: The speed of light in a vacuüm is constant and is independent of the speed of the observer’
The laws of physics are identical to different spectators, regardless of their position, as long as they are moving at a constant speed in relation to each other. Above all the speed of light is constant. Classical laws of mechanics seem to be obeyed in our normal lives because the speeds involved are insignificant.
Newton’s recipe for measuring the speed of a body moving through space involved simply timing it as it passed between two fixed points. This is based on the assumptions that time is flowing at the same rate for everyone – that there is such a thing as ‘absolute’ time, and that two observers would always agree on the distance between any two points in space.
The implications of this principle if the observers are moving at different speeds are bizarre and normal indicators of velocity such as distance and time become warped. Absolute space and time do not exist. The faster an object is moving the slower time moves. Objects appear to become shorter in the direction of travel. Mass increases as the speed of an object increases. Ultimately nothing may move faster than or equal to the speed of light because at that point it would have infinite mass, no length and time would stand still.
‘The energy (E) of a body equals its mass (m) times the speed of light (c) squared’
This equation shows that mass and energy are mutually convertible under certain conditions.
The mass-energy equation is a consequence of Einstein’s theory of special relativity and declares that only a small amount of atomic mass could unleash huge amounts of energy.
1915 – Germany
‘Objects do not attract each other by exerting pull, but the presence of matter in space causes space to curve in such a manner that a gravitational field is set up. Gravity is the property of space itself’
From 1907 to 1915 Einstein developed his special theory into a general theory that included equating accelerating forces and gravitational forces. This implies light rays would be bent by gravitational attraction and electromagnetic radiation wavelengths would be increased under gravity. Moreover, mass and the resultant gravity, warps space and time, which would otherwise be ‘flat’, into curved paths that other masses (e.g. the moons of planets) caught within the field of the distortion follow. The predictions from special and general relativity were gradually proven by experimental evidence.
Einstein spent much of the rest of his life trying to devise a unified theory of electromagnetic, gravitational and nuclear fields.
1909 – USA
The charge on the electron’
Millikan measured the charge on the electron.
His experiment showed that the electron is the fundamental unit of electricity; that is, electricity is the flow of electrons.
From his experiment Millikan calculated the basic charge on an electron to be 1.6 × 10-19 coulomb.
This charge cannot be subdivided – by convention this charge is called unit negative, -1, charge.
Millikan also determined that the electron has only about 1/1837 the mass of a proton, or 9.1 × 10-31 kilogram.
1911 – Holland
‘At very low temperatures, some materials conduct electricity without any resistance: that is, virtually without any loss of energy’
These materials are called superconductors. In 1908 Kamerlingh-Onnes found that metals such as mercury, lead and tin become superconductors at very low temperatures.
It is now known that about 24 elements and hundreds of compounds become superconductors near absolute zero.
Superconducting technology advanced little until 1986, when scientists developed a metallic ceramic compound that becomes superconductive at around the temperature of liquid nitrogen – minus 196 degrees Celsius.
1911 – Manchester, England
‘The atom contains a core or nucleus of very high density and very concentrated positive charge. Most of the atom is empty space, with the electrons moving about the tiny central nucleus’
Working under JJ THOMSON (1856-1940) at the Cambridge Cavendish Laboratory and later at the McGill University in Montreal, in 1898 Rutherford put forward his observation that radioactive elements give off at least two types of ray with distinct properties, ‘alpha’ and ‘beta’ rays.
In 1900 he confirmed the existence of ‘gamma’ rays, which remained unaffected by a magnetic force, whilst alpha and beta rays were both deflected in different directions by such an influence. Although both displayed the ability to stab through solid matter, alpha rays were far less penetrating than beta rays.
He proved through experimental results that they were helium atoms missing two electrons.
Alpha rays are in fact positively charged helium atoms that become true helium when they slow down and their charge is neutralised by picking up electrons.
Beta rays were later shown to be made up of electrons, and gamma rays to have a shorter wavelength than X-rays.
In Montreal, Rutherford worked with Frederick Soddy and showed that over a period of time, half of the atoms of a radioactive substance could disintegrate. During the process the substance spontaneously transmuted to other elements. During radioactive decay, one kind of atom (radium) was ejecting another kind of atom (helium).
Working with other elements, Rutherford and Soddy found that each radioactive element had its own characteristic ‘half-life’. After one half-life, a sample retained only half its original radioactivity, after two half-lives a quarter, after three half-lives an eighth. The half-life of thorium emanation, now known as radon, was close to a minute. The half-lives of other radioactive elements ranged from a split-second to many billions of years. That of radium was 1620 years, while uranium had a half-life of 4.5 billion years.
The concept of half-life provides a way of measuring the age of rocks. As radioactive atoms decay they emit alpha particles. As these are essentially helium atoms, the amount of helium gas accumulates within the pores and fissures of a sample of a uranium mineral as a measure of how many atoms have decayed. Heating samples to drive off their helium and measuring the amount gives an indication of their age.
In order to provide more reliable dates, measuring the amount of lead, the ultimate decay product, compared with the amount of uranium, eliminates the errors introduced by the escape of some of the helium decay product to the air.
Dating rocks in this way gives an estimate of the age of the Earth, and by implication also the Sun, of around 4.5 billion years.
A radioactive atom is simply a heavy atom, which happens to be unstable. Eventually it disintegrates by expelling an alpha, beta or gamma ray. What remains is an atom of a slightly lighter element. A radioactive atom may decay more than once. Uranium, for instance, transforms itself into a succession of lighter and lighter atoms, one of which is radium, until it achieves stability as a non-radioactive atom of lead.
Working with HANS GEIGER (1882-1945), Rutherford developed the Geiger counter at Manchester University in 1908. This device measured radiation and was used in Rutherford’s work on identifying the make-up of alpha rays.
While he was at McGill, Rutherford had experimented firing alpha particles at a photographic plate. He had noticed that, while the image produced was sharp; if he passed the alpha particles through thin plates of mica, the resulting image on the photographic plate was diffuse. The particles were clearly being deflected through small angles as they passed close to the atoms of mica.
In 1910 his team undertook work to examine the results of directing a stream of alpha particles at a piece of platinum foil. While most passed through, about one in eight thousand bounced back – that is, deflected through an angle of more than 90 degrees.
In 1911 he put forward the theory that the reason for the rate of deflection was because atoms contained a minute nucleus that bore most of the weight, while the rest of the atom was largely ’empty space’ in which electrons orbited the nucleus much as planets orbit the Sun. The reason that one in eight thousand alpha particles bounced back was because they were striking the positively charged nucleus of an atom, whereas the rest simply passed through the spacious part.
But what was an atomic nucleus made of?
At 100,000th the size of the atom, it would take decades of painstaking experiments to discover.
In 1919, working in collaboration with other scientists, Rutherford artificially induced the disintegration of atoms by collision with alpha particles. In the process the atomic make-up of the element changed as protons were forced out of the nucleus. He transmuted nitrogen into oxygen (and hydrogen) and went on to repeat the process with other elements.
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1912 – England
X-rays scattered from a crystal will show constructive interference provided their wavelength ( λ ) fits the equation
2d sin θ = n λ
where d is the spacing between atoms of the crystal, θ the angle through which the rays have scattered and n is any whole number
This is the cornerstone of the science of X-ray crystallography.
1913 – Denmark
‘Electrons in atoms are restricted to certain orbits but they can move from one orbit to another’
Bohr’s was the first quantum model for the internal structure of the atom.
Bohr worked with RUTHERFORD in Manchester and improved upon Rutherford’s model, which said that electrons were free to orbit the nucleus at random.
Classical physics insisted that electrons moving around the nucleus would eventually expire and collapse into the nucleus as they radiated energy. Bohr resolved the issue surrounding Rutherford’s atomic structure by applying the concept of quantum physics set out by MAX PLANCK in 1900.
He suggested that the electrons would have to exist in one of a number of specific orbits, each being defined by specific levels of energy. From the perspective of quantum theory, electrons only existed in these fixed orbits where they did not radiate energy. The electrons could move to higher-level orbits if energy was added, or fall to lower ones if they gave out energy. The innermost orbit contains up to two electrons. The next may contain up to eight electrons. If an inner orbit is not full, an electron from an outer orbit can jump into it. Energy is released as light (a photon) when this happens. The energy that is released is a fixed amount, a quantum.
Quanta of radiation would only ever be emitted as an atom made the transition between states and released energy. Electrons could not exist in between these definite steps. This quantised theory of the electrons’ orbits had the benefits of explaining why atoms always emitted or absorbed specific frequencies of electromagnetic radiation and of providing an understanding of why atoms are stable.
Bohr calculated the amount of radiation emitted during these transitions using Planck’s constant. It fitted physical observations and made sense of the spectral lines of a hydrogen atom, observed when the electromagnetic radiation (caused by the vibrations of electrons) of the element was passed through a prism.
The prism breaks it up into spectral lines, which show the intensities and frequencies of the radiation – and therefore the energy emissions and absorptions of the electrons.
Each of the elements has an atomic number, starting with hydrogen, with an atomic number of one. The atomic number corresponds to the number of protons in the element’s atoms. Bohr had already shown that electrons inhabit fixed orbits around the nucleus of the atom.
Atoms strive to have a full outer shell (allowed orbit), which gives a stable structure. They may share, give away or receive extra electrons to achieve stability. The way that atoms will form bonds with others, and the ease with which they will do it, is determined by the configuration of electrons.
As elements are ordered in the periodic table by atomic number, it can be seen that their position in the table can be used to predict how they will react.
In addition to showing that electrons are restricted to orbits, Bohr’s model also suggested that
Bohr called the jump to another orbit a quantum leap.
Although it contained elements of quantum theory, the Bohr model had its flaws. It ignored the wave character of the electron. Work by WERNER KARL HEISENBERG later tackled these weaknesses.
Bohr’s theory of complementarity states that electrons may be both a wave and a particle, but that we can only experience them as one or the other at any given time. He showed that contradictory characteristics of an electron could be proved in separate experiments and none of the results can be accepted singly – we need to hold all the possibilities in mind at once. This requires a slight adjustment to the original model of atomic structure, we can no longer say that an electron occupies a particular orbit, but can only give the probability that it is there.
In 1939 he developed a theory of nuclear fission with Jon Archibald Wheeler (b.1911) and realised that the 235uranium isotope would be more susceptible to fission than the more commonly used 238uranium.
The element bohrium is named after him.
1914 – Manchester, England
‘Moseley’s law – the principle outlining the link between the X-ray frequency of an element and its atomic number’
Working with ERNEST RUTHERFORD’s team in Manchester trying to better understand radiation, particularly of radium, Moseley became interested in X-rays and learning new techniques to measure their frequencies.
A technique had been devised using crystals to diffract the emitted radiation, which had a wavelength specific to the element being experimented upon.
In 1913, Moseley recorded the frequencies of the X-ray spectra of over thirty metallic elements and deduced that the frequencies of the radiation emitted were related to the squares of certain incremental whole numbers. These integers were indicative of the atomic number of the element, and its position in the periodic table. This number was the same as the positive charge of the nucleus of the atom (and by implication also the number of electrons with corresponding negative charge).
By uniting the charge in the nucleus with an atomic number, a vital link had been found between the physical atomic make up of an element and its chemical properties, as indicated by where it sits in the periodic table.
This meant that the properties of an element could now be considered in terms of atomic number rather than atomic weight, as had previously been the case – certain inconsistencies in the MENDELEEV version of the periodic table could be ironed out. In addition, the atomic numbers and weights of several missing elements could be predicted and other properties deduced from their expected position in the table.
1923 – Toronto, Canada
‘Discovery of insulin’
Early research had shown that there was almost certainly a link between the pancreas and diabetes, but at the time it was not understood what it was.
We now know a hormone from the pancreas controls the flow of sugar into the blood stream. Diabetics lack this function and are gradually killed by uncontrolled glucose input into the body’s systems.
Banting believed that the islets of Langerhans might be the most likely site for the production of this hormone and began a series of tests using laboratory animals.
After successfully treating dogs – showing signs of diabetes after the pancreas had been removed – with a solution prepared from an extract from the islets of Langerhans, Banting’s team (Best, MacLeod and Collip) purified their extract and named it insulin.
Human trials successfully took place in 1923 and dying patients were restored to health. The same year, industrial production of insulin from pigs’ pancreas began.
In the Second World War Banting undertook dangerous research into poisonous gas and was killed in an air crash while flying from Canada to the United Kingdom.
1924 – France
‘The wave-particle duality of matter.
Like photons, particles such as electrons also show wave-particle duality, that is, they also behave like light waves’
Einstein had suggested in one of his 1905 papers that the ‘photoelectric’ effect could be explained by an interpretation that included electromagnetic waves behaving like particles. De Broglie simply reversed the argument and asked: ‘if waves can behave like particles (a stream of quanta or photons), why should particles not behave like waves?’
By applying quantum theory de Broglie was able to show that an electron could act as if it were a wave with its wavelength calculated by dividing PLANCK‘s constant by the electron’s momentum at any given instant. His proposal was found to be plausible by experimental evidence shortly afterwards.
BORN, SCHRODINGER and HEISENBERG offered arguments to the debate. NIELS BOHR provided some context in 1927 by pointing out that the equipment used in experiments to prove the case one way or another greatly influenced the outcome of the results. A principle of ‘complementarity’ had to be applied suggesting the experimental proof to be a series of partially correct answers, which have to be interpreted side by side for the most complete picture. Uncertainty and Complementarity together became known as the ‘Copenhagen interpretation’ of quantum mechanics.
Eventually, the ‘probabilistic’ theories of Heisenberg and Born largely won out. At this juncture, cause and effect had logically been removed from atomic physics and de Broglie, like Einstein and Schrödinger, began to question the direction quantum theory was taking and rejected many of its findings.
‘At temperatures close to absolute zero atoms and molecules lose their separate identities and merge into a single ‘super-atom’. This ‘super-atom’ is known as Bose-Einstein condensate’
Like solid, liquid, gas and plasma (hot ionized gas), Bose-Einstein condensate is a state of matter.
In quantum mechanics, elementary particles can, in some circumstances, behave like waves. The waves – which are waves of probability – describe where a particle is most likely to be at a given moment. The uncertainty principle dictates that it is impossible to know the exact position of a particle. In 1924, while in Germany, Einstein predicted, based on ideas originally suggested by Indian-born Bose, that when atoms approach absolute zero the waves would expand and finally overlap; the elementary particles of which they are composed all merge into a single quantum state.
This state is now known as Bose-Einstein condensate.
1925 – Austria
‘No two electrons in an atom can have the same quantum number’
A quantum number describes certain properties of a particle such as its charge and spin.
An orbital or energy level cannot hold more than two electrons, one spinning clockwise, the other anti-clockwise.
Electrons are grouped in shells, which contain orbitals. The shells are numbered ( n = 1,2,3 etc. ) outwards from the nucleus. These numbers are the ‘principle quantum numbers’.
An increase in n indicates an increase in energy associated with the shell, and an increase in the distance of the shell from the nucleus. The number of electrons allowed in a shell is 2n2. Each shell contains sub-shells or energy sub-levels. A shell can only have n sub-shells. A shell is given a number and a letter ( s,p,d,f,g,etc. ). For example, the electron shell structure of lithium is 1s22s1 (two electrons in ‘s’ sub-shell of the first shell, and one electron in ‘s’ sub-shell of the second shell; the superscript indicates the number of electrons in the shell).
The Pauli principle provided a theoretical basis for the modern periodic table.
1930 – Austria
‘The radioactive beta decay of an atomic nucleus in which a neutron turns into a proton and emits an electron does not seem to follow the law of conservation of energy.’
To account for the missing energy, Pauli postulated that a particle of zero charge and zero mass is released in such reactions.
A few years later ENRICO FERMI named the new particle a neutrino.
There are three known types of neutrino – muon, tau and electron.
1926 – Austria
‘The complex mathematical equation describing the changing wave pattern of a particle such as an electron in an atom. The solution of the equation gives the probability of finding the particle at a particular place’
This equation provides a mathematical description of the wave-like properties of particles.
Schrödinger developed what became known as ‘wave mechanics, although like others, including EINSTEIN, he later became uncomfortable with the direction quantum theory took. His own proposal was built upon that of LOUIS DE BROGLIE – that particles could, in quantum theory, behave like waves. Schrödinger felt that de Broglie’s equations were too simplistic and did not offer a detailed enough analysis of the behaviour of matter, particularly at the sub-atomic level. He removed the idea of the particle completely and argued that everything is a form of wave.
PLANCK’s work had shown that light came in different colours because the photons had different amounts of energy. If you divided that energy by the frequency at which that colour of light was known to oscillate, you always arrived at the same value, the so-called Planck’s constant.
Between 1925 and 1926 Schrödinger calculated a ‘wave equation’ that mathematically underpinned his argument. When the theory was applied against known values for the hydrogen atom, for example in calculating the level of energy in an electron, it overcame some of the elements of earlier quantum theory developed by NIELS BOHR and addressed the weaknesses of de Broglie’s thesis.
Schrödinger stated that the quantum energies of electrons did not correspond to fixed orbits, as Bohr had stated, but to the vibration frequency of the ‘electron-wave’ around the nucleus. Just as a piano string has a fixed tone, so an electron wave has a fixed quantum of energy.
Having done away with particles, it was required that a physical explanation for the properties and nature of matter be found. The Austrian came up with the concept of ‘wave packets’ which would give the impression of the particle as seen in classical physics, but would actually be a wave.
The probabilistic interpretation of quantum theory based on the ideas of HEISENBERG and BORN proposed that matter did not exist in any particular place at all, being everywhere at the same time until one attempted to measure it. At that point, the equations offered the best ‘probability’ of finding the matter in a given location. Wave mechanics used much simpler mathematics than Heisenberg’s matrix mechanics, and was easier to visualise.
Schrödinger showed that in mathematical terms, both theories were the same and the rival theories together formed the basis for quantum mechanics.
Schrödinger joined Einstein and others in condemning the probabilistic view of physics where nothing was explainable for certain and cause and effect did not exist.
Ironically, PAUL ADRIAN MAURICE DIRAC went on to prove that Schrödinger’s wave thesis and the probabilistic interpretation were, mathematically at least, the equivalent of each other. Schrödinger shared a Nobel Prize for Physics with Dirac in 1933.
1927 – Germany
‘It is impossible to determine exactly both the position and momentum of a particle (such as an electron) simultaneously’
The principle excludes the existence of a particle that is stationary.
To measure both the position and momentum ( momentum = mass × velocity ) of a particle simultaneously requires two measurements: the act of performing the first measurement will disturb a particle and so create uncertainty in the second measurement. Thus the more accurately a position is known; the less accurately can the momentum be determined. The disturbance is so small it can be ignored in the macroscopic world, but is quite dramatic for particles in the microscopic world. MAX BORN’S ‘probabilistic’ interpretation, expressed at about the same time, concerning the likelihood of finding a particle at any point through probability defined by the amplitude of its associated wave, led to similar conclusions.
The uncertainty principle also applies to energy and time. A particle’s kinetic energy cannot be measured with complete precision either.
Heisenberg suggested the model of the proton and neutron being held together in the nucleus of the atom after the work of JAMES CHADWICK who discovered the neutron in 1932.
Heisenberg decided to try to develop a new model of the atom, more fundamentally based on quantum theory that worked for all atoms. He believed the approach of trying to visualise a physical model of the atom was destined to fail because of the paradoxical wave-particle nature of electrons.
Every particle has an associated wave. The position of a particle can be precisely located where the wave’s undulations are most intense. But where the wave’s undulations are most intense, the wavelength is also at its most ill-defined, and the velocity of the associated particle is impossible to determine. Similarly, a particle with a well-defined wavelength has a precise velocity but a very ill-defined position.
Since the orbits of electrons could not be observed, he decided to ignore them and focus instead on what could be observed and measured, namely, the energy they emitted and absorbed as shown in the spectral lines. He tried to devise a mathematical way of representing the orbits of electrons, and to use this as a way of predicting the atomic features shown up in the spectral lines. He showed that matrix mechanics could account for many of the properties of atoms, including those with more than one electron.
Together with PAUL DIRAC, Pascual Jordan created a new set of equations based on the rival theories of Schrödinger and Heisenberg, which they called ‘transformation theory’. Whilst studying these equations, Heisenberg noticed the paradox that measurements of position and velocity (speed and direction) of particles taken at the same time gave imprecise results. He believed that this uncertainty was a part of the nature of the sub-atomic world.
The act of measuring the velocity of a subatomic particle will change it, making the simultaneous measurement of its position invalid.
An unobserved object is both a particle and a wave. If an experimenter chooses to measure the object’s velocity, the object will transform itself into a wave. If an experimenter chooses to measure its position, it will become a particle. By choosing to observe either one thing or the other, the observer is actually affecting the form the object takes.
The practical implication of this is that one can never predict where an electron will be at a precise moment, one can only predict the probability of its being there.
1928 – UK
‘Every fundamental particle has an antiparticle – a mirror twin with the same mass but opposite charge’
‘It appears that the simplest Hamiltonian for a point-charge electron satisfying the requirements of both relativity and the general transformation theory leads to an explanation of all duplexity phenomena without further assumption’
1931 – UK
‘A magnetic monopole is analogous to electric charge’
A magnetic monopole is a hypothetical particle that carries a basic magnetic charge – in effect, a single north or south magnetic pole acting as a free particle.
1929 – England
‘First identification of an antibiotic – the discovery of penicillin’
The chance discovery of a mould in 1928 led to the development of a non-toxic drug, which is used to combat the bacteria that infect wounds.
Whilst Paul Erlich (1854-1915) worked in Germany to produce a ‘magic-bullet’, a compound or dye that could stick to bacteria and damage them, Alexander Fleming’s chance discovery of the antibacterial properties of the mould Penicillium notatum led him to conclude there was a chemical produced by the mould that would attack the bacterial agents of disease.
Whilst searching for a naturally occurring bacteria-killer, Fleming’s experiments were concentrated on the body’s own sources, tears, saliva and nasal mucus.
The chance discovery of the anti-bacterial properties of Penicillium notatum was not developed commercially until World War Two over a decade later.