Matter, Light and Energy

• see also:
  • electromagnetic radiation
  • the origin of the Universe
  • electromagnetic radiation & optics - a historical account

The quest to discover the basic fundamental substance that is matter:

• ancient Jews:
  • Genesis creation story:
    • first things created were earth, heavens, light & water
• ancient Greeks & Romans:
  • were fascinated by the concept of matter & how it was formed & structured.
  • the 4 elements:
    • Empedocles (b. 492BC), noting that when wood burnt, it gave off fire, smoke ( a form of air), & ash (a kind of earth), devised a theory that all matter could be broken down into the essential 4 components of earth, air, fire & water. This was accepted by the western world for the next 2000 yrs, although competed with the "atomists" theory of Leucippus (450BC), which although present, was over-powered by the 4 element view of matter.
  • Zeno (470-460BC) "all matter was like a huge jelly that filled all space"
  • the "atomists":
    • the term 'atom' is derived from the Greek atomos meaning 'that which cannot be divided or cut';
    • Leucippus (450BC): "all matter was broken into minute particles with space between them"
    • Lucretius (c50BC): added to Leucippus' theory the idea that the minute particles were indestructible & eternal
• ancient Indians:
  • in 500AD, had developed a complex theory of atomism which included the notion of atomic size & that atoms were 10^-9 cm in size which is remarkably close to modern measurements!!
• medieval alchemists:
  • medieval alchemists believed that it would be possible to transmute common base  metals into noble metals such as gold & silver, unfortunately for them they failed
• modern chemistry:
  • by the time of Robert Boyle in 2nd half of the 17thC, when the foundations of modern chemistry were being laid, it became increasingly established in belief that changing an element from one metal to another was impossible.
  • 1658: Gassendi revived the interest in atomic theory of the ancient Greeks & discussed the possible applications of atomic theory as well as coining the term 'molecule' for groups of atoms.
  • Boyle & Newton applied atomic theory to such scientific problems as the nature of sound & light & the principles of motion.
  • work on atomic weights & combinations set the scene for Dalton to start the development of the modern atomic theory.
• the concept of the modern atom as a billiard ball with kinetic energy:
  • 1808: Dalton's publishes his New System of Chemical Philosophy and established the concepts:
    • that all atoms of one element are the same & atoms in one element are different from atoms in other elements
    • different atoms have different atomic weights
    • all atomic weights should be measured against a base of hydrogen equalling one
    • atoms join each other on a one-to-one basis although this idea was discarded
  • 1808: Gay-Lussac established that atoms combine in different ratios
  • 1811: Avogadro suggested that molecules or clusters of atoms may be the smallest units of many elements & that equal volumes contained equal numbers of molecules (these were ignored until re-stated in 1858 by Cannizzaro).
  • Clerk Maxwell "atoms were and always had been, in all circumstances unchangeable"
• the periodic table of elements:
  • 1812: early expts in electrolysis leads to Berzelius assigned certain atoms a positive charge (eg. sodium) and others a negative charge (eg. chlorine) leading to an early theory of valency & the concept that atoms combine because they attract each other electrically.
  • relative atomic masses determined
  • 1829: Dobereiner noticed that certain groups of 3 chemically similar elements had values which were in approx. arithmetic progression, forming Dobereiner's triads 
  • 1838: Liebig idea of acids being compounds containing hydrogen replaceable by metals to form a "salt"
  • 1839: Dumas proposed there were certain fundamental types of chemical compound & that any element or group of elements in these types could be replaced, equivalent for equivalent, by another element or group of elements. This theory was successful in organic chemistry & eventually developed the idea of homologous series.
  • 1864: Newland's law of octaves - by arranging elements in ascending order of relative atomic mass & assigning the elements a series of ordinal numbers which he called atomic numbers, he noticed that similar elements had atomic numbers that differed by 7 or some multiple of seven.
  • 1869: Mendeleef's law of periodicity, which essentially restated Newland's law, was supported by the periodic relationships noticed by Lothar Meyer in plotting atomic volumes ( relative atomic masses / densities ) of the elements against their relative atomic masses.
• the sub-divisible atom:
  • 1895: X-rays were discovered by Rontgen forming the starting point of modern physics as well as revolutionising diagnostics in medicine.
  • 1895: Ramsay showed that that a gas liberated from uranium had the same spectral characteristics as helium
  • 1896 radioactivity of uranium was discovered by Becquerel, then in 1898, Curie discovered radium
  • Rutherford discovered alpha & beta rays and by 1899, the electron - the 1st sub-atomic particle - had been discovered.
  • 1897: J.J.Thomson discovered that atoms were actually made of electrons, suggesting that atoms could be divided.
  • these, along with research into the photo-electric effect in 1898, were to set the scene for Rutherford to change our perception of matter with his vision of the atom & its structure, and for Einstein to change our perception of space and time.
  • 1903: Rutherford renames "long penetrating rays" as gamma rays but their nature was uncertain
  • 1903: Rutherford finally shows he can deflect alpha rays with both an electric field & a magnetic field & showed that the ratio of its charge to mass was approx. one half that for the hydrogen atom
  • 1903: Soddy & Ramsay show that that a gas liberated from radium had the same spectral characteristics as helium
  • 1903: Lenard using a cathode ray tube shows that an atom had an open structure & was mostly empty space
  • 1904: Rutherford shows that the charge of an alpha particle is twice that of an electron & thus its mass must be 4 x that of hydrogen atom, therefore an alpha particle was probably an helium atom which was expelled during the process of disintegration
  • 1904: the transmutation of radioactive elements:
    • Rutherford in his Bakerian lecture proposes that as the half period of radium is of the order of a thousand years, any radium that had been there 100,000yrs ago would have vanished, thus radium must be renewed by some very long-lived radioactive substance which he thought would be uranium. His theory was later supported when he showed that uranium ores from various parts of the world all had the same proportion of radium to uranium. He also predicted that the final breakdown element of many of these would be lead.
    • Rutherford also stated that the heat generated from radioactivity would slow down the cooling of the earth which would make Lord Kelvin's calculation of the age of the earth based on its current temperature of 20-40 million years to be too short. 
  • 1905: Einstein postulates his quantum theory to explain the photo-electric effect:
    • he interpreted results which showed that for each metal there is a critical wavelength above which no photo-electrons are emitted, as meaning that radiation could be regarded as made up of small 'packets' of energy, known as photons, when a metal is irradiated by such photons, some of the energy was used in ejecting electrons from the metal whilst the remainder was given up to the electrons.
    • the energy of a photon is dependent on the wavelength, or frequency, of the radiation concerned according to the basic equation of the quantum theory E = hv
  • 1905: Rutherford discovers that thorium's radioactivity decays with a half-life => "disintegration theory"
  • 1908: Rutherford proves that alpha particles are helium atoms and working with Geiger, use the scintillation method for measuring the scattering when a narrow beam of alpha rays passes through a thin sheet of metal 
  • 1909: Geiger & Marsden complete their scattering expts which surprisingly showed that occasionally, an alpha particle travelling at 10,000 miles/sec is bounced back, which suggested to Rutherford that atoms may have a small dense nucleus from which the particles bounced if hit directly.
  • 1911: Rutherford's atom with a nucleus surrounded by electricity - his view of the atom as having a nucleus containing alpha particles, so well protected from, so inaccessible to, ordinary physical & chemical action, that it cannot be broken up by ordinary chemical or physical forces. The nucleus had a charge of magnitude Ne, where e = unit electronic charge & N is a whole number, and was surrounded by a sphere of electricity of the opposite kind. He could not say if the nucleus was positive or negative as either would account for the scatter paths of alpha rays.
  • 1912: von Laue proves the wave nature of X-rays by passing them through a crystal & Moseley showed that different elements produce different X-ray spectra & that the square root of the characteristic frequency increased by a constant amount as one passed from one element to the next when the elements were arranged in order of atomic weight, but that it was the atomic number itself that correlated, not the weight. To Moseley, this proved that there is a fundamental quantity in an atom which increases by regular steps as we pass from one element to the next. This quantity can only be the charge on the central positive nucleus. Thus, the N in Rutherfords Ne nuclear charge was equivalent to the atomic number. He then predicted any missing elements could be identified using this & indeed 4 gaps were found & later elements were discovered to fill these gaps.
  • 1912: Wilson devises his cloud chamber for photographing tracks of alpha particles
  • 1913: after working with Rutherford, Geiger invents the Geiger counter
  • 1913: discovery of isotopes - elements existing having identical chemical properties but different masses - suggested that nuclei have the same charge but different masses
  • 1914: Rutherford and Andrade using X-ray crystal analysis methods, showed that gamma rays were of the same nature as X-rays but of shorter wave length
• Bohr's atom based on quantum theory:
  • the problem of the hydrogen spectrum:
    • whilst solid objects when heated give out visible light of all frequencies due to closely packed atoms interfering with each other, a gas at low pressure through which an electric discharge is passed, gives out sharp spectral lines which represent certain well-defined frequencies.
    • as hydrogen is the simplest atom, its spectra was studied and Balmer showed that its frequencies formed a series - the Balmer series - that obeyed a very simple law being proportional to (1/2² - 1/n²) & later Ryder showed that to get the wave numbers from these for use in wave equations, a multiplier must be used - the Ryder constant
    • the problem was that if a moving charged particle produced this light, it should do so at a single frequency & produce a monochromatic light, and if it lost energy in giving out the light, its orbit should gradually diminish producing a continuous range of frequencies, but here was hydrogen with a single electron producing a number of discrete frequencies! 
  • 1913: Bohr solves the problem of the hydrogen spectrum using quantum theory by postulating that:
    • electrons move around the nucleus & those moving in an atomic orbit did not radiate
    • the laws of electromagnetism that explained large-scale physics did not apply to atoms which had their own laws
    • of all the infinite variety of orbits that were permitted by classical laws, only certain widely separated ones were actually possible, these being determined by a special quantum condition
    • if all electrons were circulating in these orbits then the atom was in a "stationary state" & to each stationary state would belong a certain energy, which could be calculated by Bohr's method.
    • the atom sent out radiation only when it passed from one stationary state to another (eg. when an electron moved from one legal quantum orbit to another), the frequency of the radiation was given by the difference in energy between the two states divided by h (Planck's constant)
    • it is the electrons that are responsible for the chemical properties of an element, leading in 1916 to the study of ionic & covalent bonds, electronegativity of elements
  • Rutherford shows that that particles which alphas knocked out of bombarded nitrogen were hydrogen nuclei to which he gave a special name, proton. This presumably led to Bronsted & Lowry in 1923 formulating their concept of acids & bases as being proton donors & proton acceptors respectively.
  • 1920: Rutherford in his 2nd Bakerian lecture, predicts the existence of heavy hydrogen (deuterium - discovered in 1931) and also light helium, but most remarkable of all was his anticipation of the existence of a particle with zero nuclear charge, the neutron which was discovered in 1932 by Chadwick
  • 1925: Uhlenbeck & Goudsmit postulate electron spin to account for the splitting of many single spectral lines into double lines when examined under a spectroscope of high resolving power.
  • Pauli exclusion principle - if two electrons occupy the same orbital, they must have different spins thus no orbital can contain more than 2 electrons.
  • 1930: Ernest Lawrence invents the cyclotron, a machine to accelerate a particle to energy of tens of MeV.
• quantum mechanics:
  • in the 1920's, Heisenberg, Dirac & Schrodinger developed a new picture of reality called quantum mechanics
  • no longer did tiny particles have a definite position & speed but introduced the Uncertainty Principle which stated that the more is known about the position of a particle, the less can be known about its speed & vice versa
  • these became the foundation of modern developments in chemistry, molecular biology & electronics.
  • 1924 de Broglie suggested that moving electrons had waves of definite wavelength which was demonstrated in 1927 and their wavelength = h/mv where h is Planck's constant, m is mass of electron & v is the velocity of the electron & if this is written as momentum x wavelength = h & thus relates the particle-like aspect of an electron ie. its momentum, to the wave-like aspect, ie. wavelength.
  • 1927, Schrodinger postulated based on his intuition without proof, that the wave pattern of an electron could be expressed as an equation relating a wave function, the total energy & potential energy of the system, mass of the electron, Planck's constant & the coordinates of the system.
  • 1927, Heisenberg put forward the Uncertainty Principle as a long wavelength can be measured with greater fractional accuracy than a short one, thus according to de Broglie's equation, a particle with small momentum has a correspondingly large wavelength which can be measured with some accuracy but at the expense of a relatively inaccurate determination of the small momentum.
• artificial transmutation of elements:
  • 1934: Joliot-Curies found that by bombarding light elements such as boron, magnesium & aluminium with alpha particles, radioactive isotopes could be produced, some of which gave out positrons in the course of their rather rapid decay, a new feature.
  • Fermi showed a special effectiveness of neutron particles in penetrating heavy elements with large nuclear charges which otherwise repel alphas so strongly that they cannot get near enough to cause any disintegration. He was able to produce, out of 63 elements investigated, 37 new elements showing radioactive properties, among these were new elements produced from thorium & uranium, which were heavier than any natural elements & they were radioactive in a different way by giving out beta particles (electrons) instead of alpha particles. He also showed that neutrons that had been slowed down by repeatedly hitting protons, were more effective in producing atomic transmutations than fast neutrons. 
  • 1936: Rutherford predicts the possibility of producing energy on an industrial scale from nuclear transmutation.
  • 1937: Rutherford dies.
  • 1938: in response to Mussolini adopting  the Nazi code, Fermi leaves Italy to go to US
  • Einstein predicts the possibility of releasing enormous amounts of energy by splitting the atom & thus nuclear explosions.
• the splitting of the atom:
  • 1939: Hahn & Strassmann in Germany stumble upon the process of fission when they bombarded uranium & produced barium and krypton thus splitting the atom "nuclear fission" with the final mass being lower than the initial & thus the difference in mass was the energy released. Scientists now realised the potential energy source they had discovered. Joliot-Curies now suggested the possibility of a chain reaction using the expelled neutrons which are faster than the ones used to create the reaction, to release an enormous amount of energy. The smallest amount of material needed to sustain a chain reaction is called the critical mass. Similarly:
    • U²³⁵ + neutron => U²³⁶ => La¹⁴⁸ + Br⁸⁵ + 3neutrons + 0.20 amu which becomes kinetic energy
  • 1940: Seaborg & McMillan discover plutonium;
  • 2 December 1942: Fermi et al at Chicago demonstrated that the chain reaction created by 'splitting the atom' could be contained & was thus self-sustaining. This was the vital experiment that ensured that all the previous work would eventually lead to the atomic bomb.
  • it took 2.5yrs, the labour of 500,000 people & $1.4b before the Manhattan Project as it was called, successfully converted Fermi's expt into a bomb. Much of the expense entailed taking quantities of raw uranium & producing adequate amounts of the rare but highly fissionable material U-235.
  • 16 July 1945: at Alamogordo in New Mexico, the 1st atom bomb was successfully detonated.
  • 6 August 1945, an atom bomb using uranium was dropped on Hiroshima, and 3 days later an atom bomb using plutonium was dropped on Nagasaki, the resulting explosions killing 120,000 people but resulted in an end to the war with Japan.
  • If the chain reaction is uncontrolled, an explosion will result - an atomic bomb, however, if suitable materials are introduced to absorb and slow down the neutrons, a controlled reaction can be established with energy produced at a predicted rate - this is the principle of the nuclear reactor power station, the typical components of which are:
    • fissionable fuel (uranium or plutonium)
    • the moderator (graphite or D2O to slow down the fission-producing neutrons)
    • the control rods (usually Cd strips, whose insertion captures neutrons & slows the  fission rate)
    • a coolant (water, air, hydrogen, or liquid metal such as sodium)
  • nuclear energy may also be produced by nuclear fusion which is the fusion of small nuclei into larger nuclei with loss of total mass in the system & thus release of energy. Nuclear fusion is the process that occurs on the sun as it converts hydrogen atoms to helium, but as this requires temperatures found in stars whilst the reactants must be confined within walls which wont melt (perhaps a magnetic containment field), it has yet to be made possible by man. The initial steps would be to heat the hydrogen atoms sufficiently to strip them of electrons & thus produce a "fourth state of matter" - a fully ionised gas or plasma.
  • the hydrogen bomb is large scale fusion of deuterium using a fission bomb in the centre to produce the high temperatures required for fusion.
• the structure of crystals and polyatomic molecules:
  • 1939: Sidgwick-Powell rule - the approximately spherical charge clouds within a particular electronic shell will stay as far away from each other as possible 
  • 1951: Bethe & van Vleck introduce crystal or ligand field theory, initially to ionic crystals, but Orgel developed its more general chemical aspects.
  • 1957: Gillespie & Nyholm fully develop use of the Sidgwick-Powell rule to predict molecular shapes
• more sub-atomic particles:
  • 3 technologies combined in the early 1960's to enable the discovery of many more sub-atomic particles:
    • more powerful particle accelerators
    • the bubble chamber
    • computers to analyse data from the bubble chamber
  • matter consists of fundamental matter particles quarks & leptons (eg. electron, muon, tauon & neutrinos) with 6 flavours of each
  • all subatomic particles have spin: 
    • fermions (1/2 spin) eg.  quarks, leptons
    • bosons (0, 1 or 2 spin): eg. photons, gluon
  • matter particles acted upon only by strong nuclear force are called hadrons:
    • baryons (3 quarks, fermion-type): proton, neutron
    • mesons (1 quark, 1 antiquark, boson-type, unstable): pion
  • 4 known fundamental forces, each mediated by a fundamental particle (quantum, known as a carrier particle):

    • Force	Particle/quantum	relative strength	range (meters)
      strong nuclear	gluon	1	10-15
      electromagnetic	photon	7 x 10-3	infinite
      weak nuclear	W+, W-, Z	10-5	10-17
      gravitation	gravitron (tentative)	6 x 10-39	infinite

  •  
• not particles but strings:
  • an extension of quantum physics using strings (one-dimensional extended objects that vibrate) to represent the "particles" form the super-string theories of the mid-1980's
  • see the origin of the Universe
• anti-matter:
  • 1928: a young physicist named Paul Dirac solved the problem of combining quantum theory with special relativity by creating a strange mathematical equation but this also in some way, predicted the existence of an antiworld identical to ours but made out of antimatter. Dirac speculated on the existence of a completely new Universe made out of antimatter!
  • 1930: the invention of the cyclotron - when a particle is accelerated to the speed of light then is rapidly stopped, it gives off energy in the form of incredible heat which in turn creates a particle and an anti-particle, these if they collide, will annihilate each other and give off energy in the form of light.
  • 1932: Carl Anderson discovers the anti-electron which he called a positron which was confirmed by Blackett in 1933.
  • 1954: Lawrence invents the Bevatron, a particle accelerator optimised to produce anti-protons by colliding two protons at 6.2GeV. The anti-proton was discovered in 1955 by Segre & his group.
  • 1960: the anti-neutron is discovered.
  • 1965: an anti-deuteron (a nucleus of antimatter made out of an anti-proton & anti-neutron) was created at Cern.
  • 1980's: NASA consider antimatter as a propulsion as it is the perfect fuel with all its matter converted into energy - 1kg converted to energy equates to 25,000,000,000 kWh energy - 1g would supply a medium sized town for 1 day. But antimatter proved to be very difficult to create & requires enormous amounts of energy!
  • 1990's: the PET scan for scanning brains:
    • When electron and positron meet, they annihilate, turning into energy which, at high energies, can rematerialise as new particles and antiparticles. This is what happens at machines such as the Large Electron Positron (LEP) collider at CERN.
    • At low energies, however, the electron-positron annihilations can be put to different uses, for example to reveal the workings of the brain in the technique called Positron Emission Tomography (PET).
    • In PET, the positrons come from the decay of radioactive nuclei incorporated in a special fluid injected into the patient. The positrons then annihilate with electrons in nearby atoms. As the electron and positron are almost at rest when they annihilate, there is not enough annihilation energy to make even the lightest particle and antiparticle (the electron and the positron), so the energy emerges as two gamma-rays which shoot off in opposite directions to conserve momentum.
  • 1995: Cern's unique Low Energy Antiproton Ring (LEAR) that slows down anti-protons creates 9 high energy anti-hydrogen atoms. CERN can produce 50 millions antiprotons in each cycle (about once a minute), that allows them to make a few hundred antihydrogen atoms (ie. a billionth of a gram per year). Cern thus decided to create the 1st "self-contained antiproton factory" - the Antiproton Decelerator (AD) which produces the low energy antiprotons needed to make antimatter.
  •  2002: Cern create 50,000 low energy anti-hydrogen atoms.
  •  
•