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The history of physics is closely linked to the unification of seemingly disparate phenomena. (See Figure 1505a.) Each stage of unification in turn advanced a new theoretical framework, which provides a deeper understanding of nature. (see Figure 1505b.) The followings provides a history of the development of physics and the many unifications as depicted in Figure 1505a. It also contains a brief description of the theoretical concept at each stage. For those who wish to learn or relearn elementary physics, there is a website, which uses animations to illustrate many of the subjects commonly taught in high school.  
Figure 1505a Unifications 
Figure 1505b Theories 
of the distance between the two "gravitating" objects concerned. Newton introduced a new fundamental constant of nature, G, which characterizes the strength of the gravitational force. Gravity is always attractive in contrast to other forces of nature. Newtonian mechanics works in a three dimensional Euclid space (flat space). Time is independent of the system of reference  it is just a running variable. An invariance in such space is the length. It is the same in any inertial frames of reference. Other consequences derived from Newton's formalism include action at a distance, i.e., there is no time delay for the two "gravitating" objects to interact; and determinism, which assumes that events are entirely determined by other, earlier events.  
Figure 1505c Newtonian Mechanics [view large image] 
Figure 1505d Electromag netism [view large image] 
Figure 1505e Steam Engine [view large image] 
of reference. In particular, time ticks slower and length becomes shorter in a fast moving reference frame (with respect to the observer). Another consequence is the finite propagation of interaction, the signal can travel from the past to object A only with a speed less than or equal to the velocity of light. The light cones in Figure 1505f define the space within which communication is possible. They form the event horizons for a particular time in the past or future. These ideas are in complete variance with the Newtonian Mechanics. However, classical mechanics is still a good approximation for phenomena involving low velocity (in comparing to the velocity of light).  
Figure 1505f Light Cones [view large image] 
[view large image]  
Figure 1505g Principle of Equivalence 
Figure 1505h Electron Cloud [view large image] 
energy". The particle required to do the job must have zero mass and no electric charge to escape detection by the experimenters. In 1933 Enrico Fermi took up Pauli's idea and put it on a respectable footing by introducing a new force called "weak" interaction (manifested by long mean lives of decay in the order of minutes). The hypothetical particle is called neutrino. He proposed that when a neutron changes into a proton it emits a mediating boson called W^{}, which carries off the negative electric charge and excess energy, while the neutron changes into a proton and recoils. The W^{} boson then quickly decays into an electron and an antineutrino (see Figure 1505i). Evidence for the existence of the neutrino came in 1953. The W^{} boson was discovered in 1983.  
Figure 1505i Weak Interaction [view large image] 
Hideki Yukawa suggested that the strong nuclear force (Figure 1505j) must be mediated by the exchange of another kind of forcecarrying particle, which became known as pion. His explanation for the short range of the force is related to the uncertainty principle. If the pion has mass, then its virtual existence can last only for a short time. He estimated  
Figure 1505j Nuclear Energy [view large image] 
the mass of the pion to be 150 Mev. The actual mass is 140 Mev when it was discovered in 1947. It is now known that the pion is a composite boson, it is not truly a fundamental force carrier. 
1940s. QED makes predictions about the scattering of photons and electrons and other charged particles that agree with experiment to an accuracy of eleven decimal places. All the computations are perfromed by a procedure called perturbation theory, which seems to be very promising for small coupling constant. Each term in the perturbative series can be represented by a graph known as Feynman diagram (Figure 1505k2). The K_{+} and _{} factors in the mathematical expression represent the probobility amplitude of the process between points (s_{56} is the distance travels by the photon). At the vertex (intersection), the likelihood that an electron would emit or absorb a photon is e_{}, where e is the electron's charge and _{} a vector called Dirac matrices to keep track of the electron's spin. Conversely, a process can be computed by drawing graphs and then applying the  
Figure 1505k1 A MiniConference in QED 
Feynman rules. The trouble with Feynman's method is that it always leads to infinite expressions for the loop diagrams such as those in the fourth order electronelectron scattering (diagrams a  i in Figure 1505k2). However, Feynman and others discovered 
that these are related only to quantities involving mass and charge. It is then figured out that if one simply corsses out these infinite answeres wherever they appear, and substitutes the right, finite answer, all the calculations become sensible again. This procedure is called renormalization. When it works for a theory, that theory is said to be renormalizable. The electroweak interaction and quantum chromodynamics are the other examples of renormalizable theory. Unfortunately, quantum gravity is not renoramlizable. New theories are being developed to allow the merger of quantum theroy and the gravitational force.  
Figure 1505k2 Feynman Diagrams [view large image] 
See some "animated Feyman Diagrams". 
different in the mirror image (+ 180^{o} rotation) world. With some hints from experimental results, T. D. Lee and C. N. Yang pointed out that conservation of parity may be violated in weak interaction. A test was arranged by C. S. Wu to observe the beta decay of cobalt60 in a magnetic field. It shows a preferred direction for the emitting electrons (the lefthanded electrons) and thus validates the hypothesis of parity violation for weak interaction  the mirror world behaves differently from the real world (see Figure 1505l). The three Chinese physicists shared the 1957 Nobel Prize for their efforts in identifying this peculiar behaviour in weak interaction.  
Figure 1505l Parity Violation [view large image] 
Note that parity will be conserved if there are equal number of electrons in both directions. It is a useful tool to predict permissible process when parity is conserved. 
mathematical formalism, as a single force  the electroweak interaction (based in part on works developed previously by Sheldon Glashow and others). The theory requires three intermediate vector bosons with mass to explain the weak interaction. The predicted masses of these bosons were duly observed in experiments at CERN in the early 1980s. A scalar field called the Higgs field is introduced in this formalism to endow mass to the gauge bosons Z_{o}, W^{+}, and W^{}. The lower limit on the mass of the Higgs boson is estimated to be 113 Gev. At present, there is no experimental evidence in favor of a Higgs boson, nor is there any against (see LHC updates, also "Discovery of Higgs" in July, 2012; and the subsequent Nobel Prize award in 2013). Figure 1505m shows the Feynman diagrams for interactions in the Standard model including the electroweak and strong interactions. Development of the Standard Model was in limbo for many years until 1971 when Gerard 't Hooft showed that the theory is renormalizable  
Figure 1505m Interactions in Standard Model 
by employing mathematical techniques such as path integral, gauge swapping, dimensional regularization, and numerical computation. 
Figure 1505n QCD 
composite particle (Figure 1505n). Together, the electroweak theory and QCD constitute what has become known as the "Standard Model" of elementary particles. 
simply another particle field in Quantum Gravity) runs quickly into problem of infinite Feynman diagrams. Figure 1505o summarizes the steps in the evolution of the theory of gravitation. Each step in this chart builds on the successes of the previous one. Newton thought gravity was a force that acted instantly over a distance. Einstein proposed that gravity is just the manifestion of spacetime curvature. Quantum gravity assumes that gravitation is caused by the exchange of particlelike gravitons. Superstring theory identifies gravitation as the exchange of closed strings.  
Figure 1505o Theories of Gravity [view large image] 
According to Lee Smolin, there are three roads leading to the domain of qunatum gravity: 
A few of the other approaches to quantum gravity may turn out to play significant roles in the final synthesis. Among them will be the twistor theory and the noncommutative geometry. They will provide essential insights into the nature of the quantum geometry of spacetime. Quantum gravity will emerge as a more fundamental theory since it will possess more explanatory and predictive powers. Figure 1505p shows the relationship between quantum gravity and the other branches of physics at the limit of the various universal constants, where the gravitational constant G is associated with gravity, the Planck constant is for quantum, and the velocity of light c comes with special relativity. In quantum gravity, all the fundamental units are expressed in terms of G, , and c: Planck length = (G/c^{3})^{1/2} = 1.62x10^{33} cm, Planck time = (G/c^{5})^{1/2} = 5.39x10^{44} sec, Planck mass = (c/G)^{1/2} = 2.17x10^{5} gm, Planck energy = (c^{5}/G)^{1/2} = 1.22x10^{19} Gev, and Planck temperature = (c^{5}/Gk_{B}^{2})^{1/2} = 1.42x10^{32} ^{o}K, where k_{B} is the Boltzmann's constant, which relates energy to absolute temperature on the Kelvin scale.  
Figure 1505p Quantum Gravity [view large image] 
Figure 1505q is a group photo for the physicists of yesterday. It was taken in 1927 at the Solvay Conference on Quantum Mechanics, Belgium. Most of the physicists are European males with 2 exceptions from the U.S. and one lady (Madam M. Curie), whoes entry was secured by the fame of pioneering the investigations into radioactivity. Table 1501b lists all the participants with their nationality, field(s) of study, and the year when the Nobel prize was conferred (if any). Most of their names are linked to various kinds of theory, equation, and formula. It is very difficult to avoid them in a text book for physics. The year 1927 was within a relatively quiet period between the end of World War I (1919) and the onset of Great Depression in 1929. It was the heyday for the developments of quantum theory and relativity. However, all was not well.  
Figure 1505q 1927 Solvay Conference [view large image] 
Hitler was on the way to seize power in Germany. Nightmare would soon begin in 1933 when he became Chancellor of the Reich. 
Name  Nationality  Nobel  Field(s) of Study 

Auguste Piccard* (18841962)  Switzerland  Stratosphere, Ocean Floor  
Émile Henriot (18851961)  France  Radioactive Elements, Highspeed Spin  
Paul Ehrenfest* (18801933)  Austria  Electron Microscope  
Édouard Herzen (18771931)  Belgium  Quantum Statistical Mechanics  
Théophile de Donder (18721957)  Belgium  Thermal Irreversible Process  
Erwin Schrödinger* (18871961)  Austria  1933  Schrödinger Equation 
JulesÉmile Verschaffelt (18701955)  Belgium  Secretary of the Solvay Institute of Physics  
Wolfgang Pauli* (19001958)  Austria  1945  Exclusion Principle 
Werner Heisenberg* (19011976)  Germany  1932  Uncertainty Principle, Particle Physics, QFT 
Ralph Howard Fowler (18891944)  Britain  Stellar Structure  
Léon Brillouin* (18891969)  France  Solid State Physics, Information Theory  
Peter Debye* (18841966)  Netherland  1936  Low Temperature Specific Heat, Physical Chemistry 
Martin Knudsen (18711949)  Denmark  Kinetic Theory of Gases, Knudsen Number  
William Lawrence Bragg* (18901971)  Britain  1915  Xray Diffraction 
Hendrik A. Kramers* (18941952)  Netherland  Dispersion Theory, Atomic Transitions  
Paul Dirac* (19021984)  Britain  1933  Dirac Equation 
Arthur Compton* (18921962)  U.S.A.  1927  Compton Scattering 
Louis de Broglie* (18921987)  France  1929  Waveparticle Duality 
Max Born* (18821970)  Germany  1954  Probability Interpretation of Wave Function, Born's Rule 
Niels Bohr* (18851962)  Denmark  1922  Semiclassical H atom, Copenhagen Interpretation 
Irving Langmuir* (18811957)  U.S.A.  1932  Atomic and Molecular Structures 
Max Planck* (18581947)  Germany  1918  Quanta of Light, Planck's Constant 
Marie Curie* (18671934)  Poland  1911  Radioactive Elements 
Hendrik Lorentz* (18531928)  Netherland  1902  Lorentz Transformation 
Albert Einstein* (18791955)  Germany  1921  Theories of Relativity 
Paul Langevin* (18721946)  France  Statistical Physics  
Charles E. Guye (18661942)  Switzerland  Mathematics  
Charles T. R. Wilson (18691959)  Britain  1927  Cloud Chamber 
Owen W. Richardson* (18791959)  Britain  1928  Vacuum Tubes 
Year  Author  Discipline  Subject  Equation(s) 

1687  Isaac Newton  Classical Mechanics  Motion of Partilce  
1865  J. C. Maxwell  Electrodynamics  Electricity and Magnetism  
1872  L. Boltzmann  Thermodynamics  Tendency toward Disorder  
1905  Albert Einstein  Special Relativity  Constant Velocity of Light  
1915  Albert Einstein  General Relativity  Gravity  Warpped Spacetime  
1927  W. Heisenerg  Quantum Theory  Microscopic Particle  
1928  P. A. M. Dirac  Quantum Field Theory  Free Field Equation for Fermion 
"Source", "Metric Generator", and "Mediating Particle" conspire together to produce each of the "Property Space". The "Source" is a specific property of matter responsible for generating the "Property Space". The "Metric Generator" is the rule (the mathematical equations  the distorting medium) for its production. The "Mediating Paricle" is the messenger to implement the rule. The "Property Space" is the final product, which could show up in various configurations or states to  
Figure 1505r Property Space [view large image] 
Figure 1505s Distorted Image 
direct the movement of other matter with similar "Source" in that particular "Property Space". 
Interaction  Sources / Fermions  Metric Generator  Mediating Particle(s)  Property Space  

Gravity  EnergyMomentum  General Relativity  Graviton (?) 


Electromagnetism  Electric charges / e^{1}  Quantum Electrodynamics  Photon 
 
Weak Interaction  Hypercharge, 3rd Isospin Component / _{}, e^{1}  WeinbergSalam Model  Z^{0}, W^{} mesons 
 
Strong Interaction  Color charges / 6 lefthanded u_{r,b,g} , d_{r,b,g} + 6 righthanded u_{r,b,g} , d_{r,b,g} 
Quantum Chromodynamics  8 Gluons 

gluon states together with a diagram to illustrate the swapping process (exchange of the blue and green color charges). The numerical factor of 1/_{} signifies 50% probability for each of the possible state.  
Figure 1505t Gluon States [view large image] 
Figure 1505u Fermion Table 
Figure 1505v Dodecahedron Analogy [view large image] 
Anyway, the situation is now likened to a dodecahedron (corresponding to the central entity) broken into unrecognizable pieces (Figure 1505v)  the various property spaces. 
tried to show that all the interactions would merge in the tiny region (corresponding to high probing energy) within which the "central entity" resides. The attempt is not very successful (see left diagram of Figure 1505w). However, the improvement is drastic (including gravity as shown in the right diagram of Figure 1505w) if supersymmetry (SUSY) is introduced into the consideration. But none of the partner particles predicted by SUSY has ever been detected despite the deployment of many modern technologies (see dark matter as SUSY/WIMP or some LHC updates).  
Figure 1505w Domain of Unification [view large image] 
BTW, MSSM = Minimal Supersymmetric Standard Model, and see more beauty in "Conservation Rules". 