Most Important Discoveries in Physics: A Personal View

This article is my personal view of the most important discoveries in Physics. However, in light of some of the great mysteries mentioned at the end of this article, I believe that perhaps the greatest discoveries in physics are yet to be discovered. There are of course other items that could easily been included in this list, e.g., Dmitri Mendeleev of the Periodic Table and major discoveries regarding the origin and development of the universe.

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I – Mechanical Laws of Motion:  The study of how objects move or do not move when forces act upon them is known as mechanics.  In 1687 Isaac Newton published his three laws of motion in the “Mathematical Principles of Natural Philosophy.”  Newton’s First Law of Motion: an object’s motion will not change unless there is an external force on it. (2) Newton’s Second Law of Motion: the force on an object is equal to its mass times its acceleration. (3) Newton’s Third Law of Motion: when two objects interact, they apply forces to each other of equal magnitude and opposite direction.

Newton’s Mechanical Laws of Motions describes mechanics completely and correctly for over 200 years until the early 1900s when Albert Einstein proposed his Theory of Relativity that led to some corrections to Newton’s classical theory of mechanics when objects are moving very rapidly (closed to the speed of light).

Besides Newton, other key contributors included Galileo Galilei,

II – Theory of Gravitation:  Because of an object’s mass, every object affects all other objects.  Because of their masses, predicting how do these objects affect each other is known as gravitation theory.  Newton created the theory of gravity around 1666 by proposing that every particle attracts every other particle in the universe with a force that is directly proportional to the product of the masses and inversely proportional to the square of the distance between them.

Just like Newton’s mechanical laws of motion, Newton’s classical theory of gravitation described the physical world very accurately for over 200 years until Einstein’s Theory of Relativity (in particular, Einstein’s General Theory of Relativity) that require some modifications when massive objects are involved.

Besides Newton, key contributors include Johannes Kepler and Nicolaus Copenicus.

III – Thermodynamics:  Thermodynamics is the study of the relations between heat, work, temperature, and energy. The laws of thermodynamics describe how the energy in a system changes and whether the system can perform useful work on its surroundings.

First law of thermodynamics: one of the most fundamental laws of nature is the conservation of energy principle. It simply states that during an interaction, energy can change from one form to another but the total amount of energy remains constant.

One such scientist was Sadi Carnot, the “father of thermodynamics”, who in 1824 published “Reflections on the Motive Power of Fire,” a discourse on heat, power, and engine efficiency. Most cite this book as the starting point for thermodynamics as a modern science.

Entropy is a scientific concept that is most commonly associated with a state of disorder, randomness, or uncertainty. The term and the concept entropy are used in diverse fields, from classical thermodynamics where it was first recognized, to the microscopic description of nature in statistical physics, and to the principles of information theory. It has found far-ranging applications in chemistry and physics, in biological systems and their relation to life, in cosmology, economics, sociology, weather science, climate change, and information systems including the transmission of information in telecommunication.

Besides the first law of thermodynamics, there are several other laws of thermodynamics, such as the second law of thermodynamics that states that the entropy of a system always increases, or heat does not spontaneously pass from a colder to a hotter body. or the third law of thermodynamics that states that a system’s entropy approaches a constant value as its temperature approaches absolute zero.

Key contributors to thermodynamics include Francis Bacon, Robert Boyle, Sadi Carnot, Rudolf Clausius, Lars Onsager, Benjamin Thompson, and others.

IV – Electrodynamics or Theory of Electricity and Magnetism:  Before the invention of electromagnetism, people or scientists used to think electricity and magnetism are two different topics. The view has changed after James Clerk Maxwell published A Treatise on Electricity and Magnetism in the year 1873. The publication states that the interaction of positive and negative charges is mediated by one force. This observation laid a foundation for Electromagnetism which describes electrodynamics by Maxwell’s four equations.

Classical electrodynamics can be completely described by the four Maxwell’s equations which can be found in any undergraduate textbook on electrodynamics or electricity and magnetism.

Beside James Clerk Maxwell, other contributors to electrodynamics include Andre-Marie Ampere, Charles-Augustin de Coulomb, Michael Faraday, Hans Christian Oersted, Alexandro Volta.

This is now known as Classical Electrodynamics, which describes electrodynamics accurately until we have to take into account quantum mechanical impacts when we consider electrodynamics involving subatomic particles.

At the end of the 19th century and the beginning of the 20th century, we have seen two revolutionary developments which have completely revolutionized physics, and that is the development of Quantum Mechanics (or Quantum Physics) and Theory of Relativity.  We will discuss Theory of Relativity in Item 5 and Quantum Physics in Item 7.

V – Theory of Relativity:  Special Theory and General Theory:  The Theory of Relativity has two parts:  The Special Theory of Relativity that does not take into account the gravitation force, and the General Theory of Relativity takes into account the gravitation force. 

In Newtonian physics, different observers in different reference frames may observe different laws, but in the Special Theory of Relativity, the same laws of physics hold true in all reference frames; furthermore, the speed of light is the same for all observers, which is different from classical mechanics that states that it depends on  the speed of the observers doing the measurement.  One of the consequences of the Theory of Special Relativity is that the mass of an observer depends on the velocity of the object, and there is a relation between the energy of an object of mass m and its velocity v via the famous relationship of E = mc2.

Because the mass of an object increases with increasing velocity and it will take an infinite amount of energy to increase its velocity to pass the speed of light, the speed of light is the maximum speed of any object.

In the Theory of General Relativity where gravitation is taken into account, it states that any object will distort the geometry around that object by creating a geometric field around that object, the force that is around that object follows the geometry of the field created by that object.

There have been many confirmations of the predictions of General Relativity. The earliest ones were done in 1919 by Arthur Eddington during the solar eclipse of the sun on the bending of light due to the sun’s gravitation.

The Theory of General Relativity has deep significance in determining the behavior of objects, especially involving very massive objects involving large velocities.  That is why there are many significant implications in astronomy and massive objects such as neutron stars and back holes.

Besides Albert Einstein, other contributors to Theory of Relativity include Peter Bergmann, Herman Bondi, Arthur Eddington, Marcel Grossman, Steve Hawking, Leopold Infeld, Albert Michalson, Hendrik Lorentz, Herman Minkowski, Edward Morley, Robert Oppenheimer, Roger Penrose, Max Planck, Henri Poincare, Karl Schwarzschild, John Wheeler.

VI – Theory of the Nucleus and the Atoms:  The theory of the atom and the nucleus involved several discoveries.  It was John Alton who proposed that matter is made up of indivisible small atoms and all atoms of the same element are identical.  It was Ernest Rutherford who proposed that the core of an atom is made up of a nucleus consisted of electrically positive protons and electrically neutral neutrons.  It was Niels Bohr who proposed a theory of the hydrogen atom based on quantum theory that electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation.

Key contributors include: John Alton, Ernest Rutherford, Niels Bohr, Max Planck.

VII – Quantum Physics:  A big revolution occurred at the end of the 19th century  and the beginning of the 20th century with the discovery and formulation of Quantum Physics.  In 1900 Max Planck first proposed that atoms and molecules can emit or absorb energy in discrete quantities only. The smallest amount of energy that can be emitted or absorbed in the form of electromagnetic radiation is known as quantum. Then in 1913 Niels Bohr made use of Planck’s idea that some physical quantities only take discrete values, and electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation.

Several other people built on the quantum idea and made additional discoveries that led to the full development of quantum physics that revolutionized the world with all kinds of electronic gadgets build on quantum physics, such as the transistors, radar, computers, worldwide web.

Although Quantum Physics revolutionized physics, science, and introduced many wonders to the world, at the same time, Quantum Physics also introduced many mysteries to the world, such as wave-particle duality, the uncertainty principle, the act of observing nature can change what is being observed, the probability interpretation instead of the deterministic interpretation in classical physics.  Many people question whether Quantum Physics can be the real theory describing the world.

Besides Max Planck and Niels Bohr, there were many major contributors to Quantum Physics, including Albert Einstein, Louis de Broglie, Max Born, Werner Heisenberg, Wolfgang Pauli, Erwin Schrödinger, Paul Dirac, Arthur Compton, Clinton Davisson, Lester Germer, George Paget Thomson, John Bardeen, Walter Brattain, and William Shockley, John Wheeler.

VIII – Quantum Electrodynamics (QED):  QED is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum physics and special relativity is achieved. QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of photons and represents the quantum counterpart of classical electromagnetism giving a complete account of matter and light interaction.  QED has made the theoretical prediction of the magnetic moment of the electron to an accuracy of about one part in a trillion,making QED the most accurate theory in the history of science.

Key contributors to Quantum Electrodynamics include:  Paul Dirac, Richard Feynman, Julian Schwinger, Shinichiro Tomonaga, Freeman Dyson.

IX – Discovery of the Weak Force and Formulation of the Electroweak Force:  The weak force is the force that is involved in radioactive decay such as when a neutron inside an atomic nucleus undergoing an interactive decay and transforms into a proton plus an electron and an anti-neutrino. The weak force was discovered by Enrico Fermi in 1933 based on earlier work by Marie Curie. Weak interactions were involved in most of the reactions in the very early Universe by which particles changed from one sort to another. They are therefore largely responsible for the overall mixture of particles from which the current Universe is made.

in the mid-1950s, T. D. Lee and C. N. Yang proposed that weak interaction does not conserve parity, which shortly after was confirmed experimentally by C. S. Wu.  In the 1960s, Sheldon Glashow, Abdus Salam and Steven Weinberg unified the electromagnetic force and the weak force by showing them to be two aspects of a single force, now termed the electroweak force.

Key contributors include Marie Curie, Enrico Fermi, T. D. Lee, C. N. Yang, C. S. Wu, Yoichiro Nambu, Sheldon Glashow, Leon Lederman, Martin Perl, Abdus Salam, Steven Weinberg, Gerardus ’t Hooft , Martinus J. G. Veltman.

X – Building Blocks of MatterThis isa long standing problem for physicists.  However, great progress seems tp have been achieved in the last half century plus, both theoretically and experimentally.  It is somewhat premature to claim that this is among the greatest discoveries in Physics until they have been completely proven.  At this time we only declare that this Item 10 and the next Item 11 “Standard Model of Particle Physics” to be potential candidates to be included as part of the greatest discoveries in Physics.  One reason our current understanding is incomplete is because the so-called “Standard Model of Particle Physics” (see next Item 11) addresses only the 3 forces (the strong force, the weak force, and the electromagnetic force), but it does not address the gravitation force.

Our current understanding is that matter is made up of atoms, consisting of a nucleus and electrons orbiting the nucleus.  The nucleus is made up of protons and neutrons.  Protons and neutrons are made up of quarks, and there are 6 quarks:  up quark (u), down quark (d), strange quark (s), charm quark (c), top quark (t), and bottom quark (b). The nuclear matter made up of quarks interact with each other by:

  • exchanging gluons when interacting via the strong force
  • exchanging photons when interacting via the electromagnetic force,
  • exchanging the W-boson or the Z-boson when interacting via the weak force.

Besides the electron neutrino, there are also the muon neutrino, and the tau neutrino.

A missing piece of the building block of matter is the Higgs boson, which was proposed in 1964 as a mechanism for some particles acquiring mass, and was finally discovered in 2012.

Key contributors include Murray Gell Mann, George Zweig, Yuval Ne’eman, Robert Brout-François Englert-Peter Higgs, Gerald Guralnik-Carl Hagen-Tom Kibble, Leo Lederman, Martin Perl, Federick Federick Reines, Melvin Schwartz, Jack Steinberger, Burton Richter, Samuel C. C. Ting, and many others (especially experimentalists).

XI – Standard Model of Particle Physics:  The strong force was proposed in 1935 by Hideki Yukawa, that governs the interaction of protons and neutrons inside an atomic nucleus (or interactions between different quarks which are the components of protons and neutrons).

The current theory of strong interactions is described by Quantum Chromodynamics (QCD), which describes how particles called quarks (which are constituents of protons and neutrons) and leptons (which include electrons) interact to make up matter. It also explains how force carrying particles, which belong to a broader group of bosons influence the quarks and leptons.  QCD has two features:  One called “Asymptotic Freedom” which says that at high energies (or short distances) the interaction of quarks are essentially free, another called “Color confinement” that says that when the quarks are separated by large distances, there is a force confining them so that they cannot be separated, i.e. individual quarks cannot be found in nature.” 

The Standard Model of Particle Physics refers to the combination of Quantum Chromodynamics (QCD) and the Electroweak Force theory of Glashow-Salam-Wainberg (as discussed in Section 9), and is used to explain three of the four fundamental forces of nature that govern the universe: electromagnetism, the strong force, and the weak force.

Quantum Chromodynamics (QCD) is the theory of the strong interaction between quarks mediated by gluons.  The strong force, which is carried by gluons, binds together atomic nuclei to make them stable.  The strong force also governs the interactions inside a sun known as nuclear fusion. It is nuclear fusion that creates the energy from the sun that governs our solar system and the whole universe.  The weak force, carried by W and Z bosons, causes nuclear reactions that have powered our Sun and other stars for billions of years.  Electromagnetism is carried by photons and involves the interaction of electric fields and magnetic fields. The fourth fundamental force is gravity, but it is not adequately explained by the Standard Model of Particle Physics.

Both the Electroweak Theory and Quantum Chromodynamics belong to a type of quantum field theory that is based on the Yang-Mills non-Abelian gauge theory first proposed by C. N. Yang and Robert Mills in 1954.

Key contributors include Hideki Yukawa, Sheldon Glashow, Abdus Salam, Steve Wainberg, David Gross, David Politzer, Frank Wilczek, Robert Mills, C. N. Yang, the people mentioned in Item 10 “Building Blocks of Matter,” and many others, especially experimentalists (since modern experiments are carried out by large experimental teams).

XII – Bell’s Theorem and Experimental Confirmation of Quantum Physics:  Because of many mysteries surrounding Quantum Physics (QP), many people believe that Quantum Physics cannot be correct and it will be replaced in the future by a more realistic theory.  These people included Einstein, who made remarks like God does not play dice, or spooky action at a distance.   Many people thought that a “local hidden variable theory” (LHVT) will replace Quantum Physics.  However, in 1964 James Bell proved a very remarkable theorem that shows any LHVT cannot always have the same prediction as Quantum Physics, thus allowing experiments to determine whether QP or LHVT is correct.

Many experiments in the last half a century have been done, and they have all shown that QP is correct, and LHVT is incorrect, thus confirming that QP may be correct in spite of all its mysteries.

This has led to the beginning of building quantum computers, which with its fantastic speed of calculation, will revolutionize computers and lead to another revolution in industry and in our daily lives just like Quantum Physics revolutionized our world in the 20th century.

Albert Einstein, Edwin Schrodinger, Alan Aspect, John Clauser, Stuart Freedman, Antonio Zeilinger, John Wheeler, and others.

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In spite of all the discoveries in the last half century, however, there are still several major mysteries:

  • Dark matter:  Dark matter is matter in our universe which cannot be seen, because they don’t interact electromagnetically, and they don’t interact through the strong force and maybe also through the weak force, but they interact through the gravitational force.  Ordinary matter make up only about 5% of the universe, but dark matter consist of about 27%.
  • Dark energy:  Dark energy is a theoretical repulsive force that counteracts gravity and causes the universe to expand at an accelerating rate, and it makes up 68% of our universe.
  • Matter-antimatter asymmetry:  Almost all of the matter we see in the universe is made up of matter, but matter and antimatter should have been created in equal amounts when the original universe is made up of energy.

Therefore, major discoveries are waiting to be discovered to answer these questions.

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2 Responses to “Most Important Discoveries in Physics: A Personal View”

  1. Aridaman Jain says:

    Thanks for writing and sending this comprehensive article

  2. Qingyan Zhao says:

    Wow! Thanks for the article. It is so very interesting and amazing! It seemed that all the discoveries beyond the classic models are first done by “imagination” and “math”? and later verified by experiments? If that is true, math is truly God’s language. Thanks again! Please excuse my naive questions. But these are sooooo interesting…

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