When Einstein was developing his relativity, he was inspired by the philosophers David Hume and Ernst Mach, and abandoned the idea of absolute simultaneity (Norton, John D.); this decision gave him the courage to move forward with his theory.  Einstein was also heavily influenced by the work of James Clerk Maxwell, who a little more than half a century earlier had unified electricity and magnetism in his theory of electromagnetism which continues to consistently explain electric and magnetic interactions.  This unification of different phenomena into one theory began what Einstein and others would continue into the 20th and 21st centuries:  the search for a single Grand Unified Theory (GUT) which explained all physical existence in this Universe.  Einstein’s motivation for finding a unified field theory combining electromagnetism and gravity was to be able to explain the currently known physical Universe without invoking quantum theory.  His quest consumed the last 30 years of his life with his work papers scattered next to him on his death bed.  Einstein firmly believed in a rational and deterministic set of rules by which the Universe operated; he also believed in an infinite and static steady-state Universe until the discovery by Edwin Hubble of an expanding Universe in 1929.  Einstein’s reluctance to open his mind to quantum possibilities may have been the cause of his failure to find a unified field theory. 

     The next milestone in the unification movement came during the 1940’s with the development of quantum electrodynamics (QED) which successfully brought together under one set of rules (quantum field theory) relativistic and quantum mechanical ideas (Britannica.com).  QED explains all interactions between light and matter (as first noted in Einstein’s photo-electric effect) and between all charged particles.  First begun by Paul Dirac in 1928 with his discovery of the quantum wave equation, QED was refined by the independent work of Richard Feynman, Julian Schwinger and Tomonaga Shin’ichiro (ibid).  A curious idea in QED is that photons, which transfer electromagnetic (e-m) forces, are treated as virtual particles which cannot be detected–to do so would violate the law of conservation of energy and mass which states that these can neither be created nor destroyed only transformed (ibid).

    So far I have discussed only a small part of the currently known foundation of how our Universe works.  All the physics summarized so far relates only (as far as we know) to about 4% of what makes up our Universe. What about the other 96%?  That is a really big question I will explore later.  For now, suffice it to say that the 96% is made up of “dark” matter (23%) and “dark” energy (73%).  The term dark is used because astrophysicists have no clue what they are since they don’t absorb or emit any known form of radiation (information).  There is speculation that dark matter could be “hiding” inside of supermassive black holes.  Astronomers also have no idea what’s going on inside of blackholes.  The laws of physics (as we currently understand them) may not even apply there.  The proposed existence of these phenomena is based on astronomical evidence which can’t be explained in any other way (i. e. by the 4% ordinary matter/energy). 

   Getting back to unification (of the 4%), so far only two forces have been discussed:  gravity and electromagnetism.  There are however two other forces which need to be mentioned–the strong and weak nuclear forces.  The strong nuclear force is responsible for holding the nucleus (which is made up of neutrons and protons) together.  Since the existence of protons had already been been discovered by Goldstein and Rutherford (wikipedia) and Chadwick had discovered the neutron in 1932 (scholarpedia), there had to be an explanation of why atomic nuclei (where the protons and neutron are located) are not blown apart by the mutual repulsion of the positively charged protons.  In 1935 Hideki Yukawa proposed a strong nuclear force that was controlled (mediated) by the exchange of particles (mesons) between nucleons (protons and neutrons)(ibid).  His idea was later (1970’s) refined in the theory of quantum chromodynamics (QCD) which proposes that the strong nuclear force is mediated by particles called quarks through the exchange of particles called gluons (because they “glue” the protons and neutrons together in stable nuclei (except for very massive elements such as uranium and other radioactive elements)(ibid).

     The phenomenon of radioactivity leads us to the fourth of the four fundamental forces in nature:  the weak nuclear force.  Radioactivity had been known and described through the work of Ernest Rutherford, Enrico Fermi, Marie Curie, Henri Becquerel, Otto Hahn and Lise Meitner (Britannica).  Radioactivity involves the interactions (decay) within large, massive and unstable nuclei (polonium being the smallest, least massive) characterized by the spontaneous emission of various particles and e-m radiation.  

     The cause of radioactivity is the instability of nuclei that results from the size of massive nuclei (more protons and neutrons crammed into the very small nucleus).  The strong nuclear force (QCD) is only effective at very small distances within atomic nuclei.  With larger and larger elements beginning with polonium, these nuclear distances become too great for the effective “gluing” together of protons and neutrons (which are made of quarks) by the exchange of gluons.  

    In the 1960’s, the independent work of Sheldon Glashow, Abdus Salam and Steven Weinberg provided satisfactory explanation of the weak nuclear force only if they included the well known electromagnetic force in their theoretical framework (Brittanica).  In this electroweak theory, four particles were required for mediation of the weak force:  two charged (+,-) W particles and two neutral Z particles (ibid).  These particles were experimentally observed (1983) in proton-antiproton collisions at the European Organization for Nuclear Research (CERN).  The electroweak theory also postulated the existence of another particle (and field) to make it conceptually consistent.  This particle was observed on July 4, 2012 in CERN’s Large Hadron Collider (LHC) and is known as the Higgs boson or so-called “God particle.”  This is a reference to the concept that this particle (and field) give mass to all other known particles (CERN). 

     This summary of the physical nature of our Universe covers much of what is known that is based on experimental evidence, but many details are left out.  So far as we now know, four fundamental forces exist:  gravity, electromagnetism, the strong nuclear force and the weak nuclear force.  But what happened to the so-called unification idea mentioned earlier?  The currently accepted framework for explaining all of this is called the Standard Model of Particle Physics.  This model includes the electroweak theory and quantum chromodynamics.  QCD can be be linked with quantum electrodynamics (QED) under the umbrella of quantum field theory (Britannica and Britannica).  There are alternative models to explain these four forces and the particles involved.  One model, known as string theory, lacks the support of experimental observations.  Critics say that the necessary observations would be impossible to obtain, but this a weak argument against it.  Brian Greene is the primary proponent of string theory.  Another theory, called loop quantum gravity is a variation of string theory based more closely on Einstein’s space-time geometry.

     As challenging as relativity and quantum theory have been to understand and appreciate by the vast majority of people on this planet, they have nevertheless exerted a significant and broad influence in modern technology and popular culture.  The current focus in theoretical physics is to bring gravity under the quantum umbrella or to find another theoretical framework which explains ALL known forces, particles and their interactions.  As complicated as this sounds, most physicists agree that a truly Grand Unified-field Theory should be very simple and elegant (at least to them anyway)!