a jfk original inspired by john gribbin

This is a modest attempt (albeit incomplete) to present some of the most important experimental discoveries and theoretical models that have helped to shape the framework of Physics in the 20th Century, in a chronological order. Although it may argued that this is by no means the best way to appreciate the development of the subject, since this was not always so straightforward as a list of dates might suggest, it does however at least provide a backdrop against which to compare our changing ideas and conceptions of the physical world.

Ozzie       Phoebe

1846 Michael Faraday speculated on the existence of waves produced by oscillating electric currents and varying magnetic fields.

1856 Sir William Crookes researched electrical discharges in gases observing a dark space near the cathode that bears his name & the tube’s ability to produce phosphorescence.

1858 Plücker showed that the fluorescence obtained in a discharge tube could be moved around with a magnet held close to the cathode.

1859 First great paper on the dynamical theory of gases that paved the way for kinetic theory was published by Maxwell explaining Boyle’s law diffusion and viscosity.

1864 James Clerk Maxwell published his theory and equations of the electromagnetic field predicting the velocity of such waves as c = 1/ Ö e _{om o.}

1871 Mendeleev published a periodic table of the elements based on their atomic weights, recognising similarities between the elements he predicted the existence of Ge (dis.1886)

1876 Goldstein coined the word Kathodenstrahlung-cathode rays when describing the well-defined shadow of objects placed within the fluorescent area of the rays.

1885 Balmer found an empirical formula 1/l = R_h (1/2² - 1/n²) to describe the wavelengths in the visible spectrum of Hydrogen, hinting at the non-continuous nature of radiation.

1888 Heinrich Hertz published results on the production of e-m radio waves by oscillating charges noting sparks when transmitter and detector were in line (photoelectric effect.).

1893 Lummer and Pringsheim made the first measurements of the distribution of radiant energy in the spectra of hot solids the so-called black body radiation curves.

1893 Wein used R (l ) = ƒ (l T)/l ⁵ to account for the short wavelength end of the black body curves avoiding the ultraviolet catastrophe predicted by the Rayleigh Jeans law.

1895 Wilhelm Röntgen experimenting on a discharge tube saw the fluorescence of nearby barium platinocyanide discovering X rays in the process (Awarded 1st Nobel prize).

1896 Henri Becquerel discovered natural radioactivity researching the phosphorescence of compounds of uranium salts wrapped in photographic plates not exposed to sunlight.

1897 Discovery of the electron by J.J Thompson in cathode rays, using electric & magnetic fields he obtained an e/m value showing they were negatively charged particles.

1897 C.T.R. Wilson reported that gaseous ions could act as nuclei for the condensation of a cloud, resulted in the invention of the cloud chamber used for observing atomic particles.

1898 Marie Curie discovered radium (1000x more radioactive than uranium) while chemically separating tonnes of pitchblende and isolated polonium in the process.

1898 Experiments at the Cavendish Laboratory on radium radiation, indicated two components:a was absorbed by a sheet of paper while b was only stopped by @ mm’s Al.

1900 Villard showed even after absorption a component g of the radiation remained that could not be deflected by electric/magnetic fields and could easily pass through @ cm’s Pb.

1900 Lenard investigated the factors effecting the production of a current in a vacuum tube when the anode was irradiated with ultra violet radiation, the so photoelectric effect.

1901 In order to overcome the problems posed by the emission of electromagnetic radiation by matter Max Planck proposed an ad hoc solution:Energy was quantised E=h n .

1905 Einstein used Planck’s quanta idea in the absorption of radiation by matter to explain the photoelectric effect for which he received the 1921 Nobel Prize hc/l = ej +½mv².

1908 Charles Barkla showed that the non-linearity of a Log Intensity-Thickness graph of X rays scattered by various materials indicated a component characteristic of the element (Ka lines).

1909 Royds and Rutherford showed a ‘s were He ions by obtaining the spectrum from an evacuated discharge tube containing a glass vessel with a radioactive radon gas emitter inside.

1909 Geiger and Marsden experiment suggested by Rutherford to test whether a particles could be deflected from a thin gold foil by observing the scintillation of a ZnS screen.

1911 Ernest Rutherford equated the energy of a ‘s with the coulomb force to calculate the distance s of closest approach the size of the nucleus (s= Z_AuZa e²/4p e _o(6.3 MeV)@ 10^-15m).

1912 Gaede & Langmuir’s vacuum diffusion pump encouraged Coolidge of GEC to design an efficient X ray tube now widely used in medical and industrial equipment.

1912 Friedich & Knipping obtained the first X ray diffraction photograph from ZnS after Laue had suggested that crystals might act as small diffraction gratings for X rays.

1913 WH and WL Bragg developed the X ray spectrometer enabling crystal lattice spacings to be measured deducing the structure of NaCl (Bragg’s Law nl = 2d sinq ).

1913 Soddy suggested the name Isotopes for substances with different physical properties but identical chemistry ie a common atomic number Z but differing mass number A.

1913 Niels Bohr’s model of the H atom with electrons of quantised angular momentum mvr=nh/2p radiating energy E₂-E₁ between circular orbits predicted discrete spectral lines.

1913 Moseley’s measurement of the characteristic X ray spectrum from various elements identified the atomic number Z with position in the periodic table n = 0.248 10¹⁶(Z-1)².

1914 Classic Frank-Hertz experiment performed confirming the quantisation of atomic energy levels-observing at discrete voltages sharp drops in the current from a triode valve.

1916 Sommerfeld extended the Bohr model to explain the hyperfine splitting of spectral lines by introducing two quantum numbers in the condition p_q d_q= n_q h.

1916 Millikan verified experimentally Einstein’s photoelectric equation and had previously in 1909 accurately measured the charge on the electron e as 1.6 10 ^-19 C.

1919 First artificial transmutation of matter carried out by Rutherford via the reaction:

¹⁴N + ^4a Þ ¹ p + ¹⁷O thereby discovering the proton.

1923 Compton investigated the spectrum of monochromatic X rays passing through a thin target. He observed an increase in X ray l due to momentum loss to the target electrons.

1924 In his doctoral thesis De Broglie postulated that electrons might possess wavelike properties relating momentum p and wavelength l by l = h/p (E = hn = hc/l = p/c).

1925 Wolfgang Pauli enunciated his famous Exclusion Principle for fermions (Fermi Dirac statistics): No two electrons in the same atom may have identical quantum numbers.

1925 Born Jordan and Heisenberg developed the idea of matrix mechanics & stressed the importance of the noncommutativity of quantum variables via the relation pq - qp ¹ h/2p i.

1926 Erwin Schrödinger’s wave equation to describe the motion of particles on an atomic scale promptrd the famous cat paradox ( i h/2p { ¶ y /¶ t} = -h²/8mp ^{2{ Ñ 2y }} +Vy ).

1926 P.A.M. Dirac developed another more complete formulation of quantum mechanics in terms of Hamiltonians, postulated electron spin & predicted the existence of antimatter.

1927 Werner Heisenberg introduced his now infamous uncertainty principle that set limits on the accuracy of knowledge of the conjugate variables: D x. D p ³ h/2p , D E. D t ³ h/2p .

1927 Davisson and Germer confirmed De Broglie’s ideas by observing that accelerated electrons exhibited diffraction effects when reflected from single crystal Ni (eV = hc/l ).

1927 Bohr introduced the concept of complementarity into quantum mechanics in what became known as the Copenhagen interpretation.

1927 Max Born @ Göttingen interpreted Schrödingers wave function in terms of probability amplitudes, y .y * is the probability of finding an electron (the wave collapses).

1928 Geiger and Müller devised a radiation detector from a cylinder containing a low pressure gas and co-axial wire with a pd between them, sparking on being ionized.

1932 James Chadwick discovered the neutron by bombarding a Be disc with a ‘s and capturing protons ejected by the particles from paraffin wax placed before his detector.

1932 The antielectron (positron) was discovered by Carl Anderson from measurements on the curvature and direction of tracks of cosmic rays in a cloud chamber ( r = mv/Be ).

1935 The "EPR Paradox" formulated by Einstein, Podolsky and Rosen