HPS 200Y Lecture #49: Quantum Physics: New Uncertainties
ROOTS OF ATOMIC BOMB
- discovery of radioactivity in France by Henri Becquerel and Pierre and Marie Curie
- modern model of the atom at McGill University by Ernest Rutherford and F. Soddy
- Einsteins quantum theory of light and matter
- unified energy and matter in quantum physics
- wave theory of light and matter not predictive like Newtonian physics
- the big push was to make it predictive
- resulted in in-depth study of the nature of matter
- the scientific outcome was knowledge of how to make the atomic bomb
RADIOACTIVITY
- x-rays discovered 1895 by W. K. Roentgen (first Nobel Prize winner for discovery)
- 1896 Henri Becquerel discovered uranium salts gave off radiation (radioactivity)
- 1898 Marie and Pierre Curie discovered radioactive elements of radium and Polonium
- shared 1903 Nobel Prize with Becquerel for these discoveries
- radium most radioactive substance found: glowed in dark; stayed warmer than surroundings
- gave hint of energy at heart of matter
ATOMIC STRUCTURE: first models of atoms
- New Zealander Ernest Rutherford trained at Cavendish Laboratory, Cambridge, where electrons were discovered by J.J. Thompson , with whom he worked
- first academic job at McGill - studied radioactivity from radium penetrating aluminum metal
- theory of radioactive disintegration 1903 (still right in its fundamentals)
- radioactive atoms tear themselves apart, emitting alpha rays ( ionized helium); Beta rays (electrons); and gamma rays (powerful x-rays)
- as radioactivity proceeds final product from radium is lead
- at U. of Manchester, Rutherford revised Thompsons plum pudding model of atom (electrons embedded in atomic ball)
- new model: positively charged nucleus; negatively charged electrons fly around nucleus at relatively large distance like planets in solar system
- necessary since beta particles can pass through atoms (1908 Nobel Prize for Chem.)
NIELS BOHR - Danish scientist - electrons must move in strictly specified orbits
- electrons move from one orbital level to another in quantum jumps
- jumps occur when a photon (quantum packet) is absorbed or released as a black body is being heated or cooled (explained meaning of Plancks constant)
THE NEED FOR EINSTEINS RELATIVITY IN ATOMIC MODELS
SPECTRAL EMISSIONS DIFFERED FROM THOSE PREDICTED
- German theorist A. Sommerfeld added relativity to atomic calculations in 1915
- Einsteins relativity predicts the faster something moves the heavier it becomes
- new relativity formula got excellent agreement with experiment
. . . ( ) ( ) ( ) . . BOHRS CORRESPONDENCE PRINCIPLE
- more energy needed for electrons closest to nucleus to jump to next orbital level, even though electron orbital distances greater in outer orbital paths
- but no accurate predictions of energy frequencies until atomic model included controversial wave-particle duality
WERNER HEISENBERG: HEISENBERGS PRINCIPLE OF UNCERTAINTY
- German physicist at Gttingen University stated we must stop trying to picture atoms
- if we know momentum of electron we cannot work out its position
- if we know position of electron we cannot work out its speed
- p (position) x q (mass times speed) gives a different answer from q x p
- electron is a smear of probability
PRINCE LOUIS DE BROGLIE - MATTER IS WAVE-LIKE
- 1923 showed that since light and matter are linked by Einsteins relativity, then matter must have a wave-like nature (Nobel Prize for this in 1929)
- electrons are like photons, which were admitted to be waves
- so electrons also are waves
- Plancks constant h acts like bridge between wave and particle
- only Einstein understood De Broglie until C.J. Davisson at Bell Telephone Laboratories found experimental proof of unity of energy and matter since electrons gave diffraction patterns as do X-rays, which are waves (Nobel Prize 37)
ERWIN SCHRODINGER - wave model of the atom
- spectrum frequencies of hydrogen atoms correspond to the difference of frequencies between the electron orbits and not the frequencies themselves
- electron orbits closed rings with x number of waves per ring
PAUL ADRIEN MAURICE DIRAC - electron spin (1927)
- English Mathematics professor at Cambridge used quantum analysis to show that photons could be treated as electrons; electrons orbit AND have a spin induced by relativity
J.R. OPPENHEIMER - American physicist in 1926 stated
-"electron orbits are waves of probability; no mental image of Heisenberg-Schrdinger atoms possible
JOHN VON NEUMANN
- great mathematician (gave us Von Neumann architecture of computers)
-worked out in 1930s that quantum theory is a complete system in itself
- fabric of space and time is contained in the fundamental particles of energy and matter itself
. . . ( ) ( ) ( ) . . JAMES CHADWICK - 1932 discovered the NEUTRON (Nobel prize 1935)
- former pupil of Rutherford discovered new heavy particle with no electrical charge
- Harold C Urey discovered heavy water same year - heavy hydrogen contains neutron in nucleus (deuterium)
- neutron radiation needed to bombard uranium to create fission reaction; led to fission