Updated: Nov 9
Author: Huzaifa Irshad (view email)
The ultimate quest of modern physicists is to discover the Unified Theory of the Universe. A theory in which all fundamental forces of nature come together in one elegant system of equations. Using those equations physicists could then delve into the realm of the unknown to solve the biggest mysteries of the universe such as black hole and dark matter. If this could be achieved, it would be the greatest triumph of human knowledge. [Stephen Hawking: A Brief History Of Time]
Beginnings of Unification
By early 19th century many electrical and magnetic phenomenon had been discovered but most physicists believed that electricity and magnetism were separate forces. On 21st April 1820, Hans Christian Oersted, a professor at the University of Copenhagen, while giving a demonstration to his students noticed that when he turned on an electric current by connecting the wire to both ends of the battery, a compass needle nearby deflected away from magnetic north. 
On July 21, 1820, Oersted published his results in a pamphlet, which was circulated privately to scientific societies.  Oersted’s discovery was the first experimental evidence of the relationship between electricity and magnetism, the importance of which was rapidly recognized by other physicists. 
André-Marie Ampère, a member of French Academy of Sciences in Paris, was fascinated by Orested’s discovery and began studying the relationship between electricity and magnetism.  By the end of September 1820, he had made a discovery of his own. He had found that two parallel wires with flowing current in the close vicinity of each other, exert of force of attraction or repulsion on each other. 
Ampere had electrically generated magnetic attraction and repulsion in the complete absence of any magnets which was an amazing result. He then applied mathematics in generalizing physical laws from these experimental results and brilliantly found an equation as connecting the size of a magnetic field to the electric current that produces it, known as the Ampère’s Law. In 1827 Ampère published Memoir on the Mathematical Theory of Electrodynamic Phenomena.  In a letter to his son, Ampere wrote,
“Ever since I first heard of Oersted’s great discovery…All my time has been dedicated to writing a great theory about these phenomena… and attempting the experiments…, all of which succeeded.”
The quest of unification had begun. Michael Faraday who worked as a laboratory assistant at the Royal Institution in London took the work of Oersted and Ampère on the magnetic properties of electrical currents as a starting point and in August 1831 achieved an electrical current from a changing magnetic field.  He found that when an electrical current was passed through a coil, another very short current was generated in a nearby coil. It wasn’t long before Michael Faraday discovered the concept of the ‘field’. He wrote in his notes,
“By magnetic curves I mean lines of magnetic forces which would be depicted by iron filings.”
Faraday began to mention the magnetic ‘field’ in his publications presented to the Royal Society in 1845. Faraday believed in the unity of all the forces of nature, and wanted to determine whether magnetic fields had an effect on optical phenomena. In September 1845 he found that the plane of polarization of linearly polarized light is rotated when this light travels through a material to which a strong magnetic field is applied in the direction of propagation of the light. Faraday wrote in his diary,
“Today worked with lines of magnetic force… passing a polarized ray of light through them …there was an effect produced on the polarized ray, and thus magnetic force and light were proved to have relation to each other”.
The First Unification
Faraday’s lines of force were not accepted until after mid-1800s when James Clerk Maxwell entered the picture. He discovered four equations that built a rigorous theoretical framework for studying electricity, magnetism, and their relationship. Maxwell developed these equations by carefully studying the work of Michael Faraday. He realized that electricity and magnetism, although once thought to be two completely separate forces, are actually different facets of the same force - the electromagnetic force. 
In the 1860s, Maxwell, when he was a professor of Natural Philosophy at King’s College London, worked out a dynamical model of Faraday’s fields capable of representing most known electrical and magnetic phenomena.  While previous theories had assumed that the energy was located at or on magnets or electrically charged bodies, in Maxwell’s theory, however, the magnetic energy was in the surrounding space, or “field”. The energy was, in other words, the kinetic energy of the vortices. 
Figure of Maxwell's molecular vortex model. 
Maxwell developed his theory on the different aspects of Faraday’s thinking, devising in his paper of 1861 an “ether” full of tiny “molecular vortices” aligned with the lines of force, Maxwell reasoned, each vortex shrinks axially and expands sideways, giving just the stress patterns that Faraday had hypothesized. Maxwell’s vortex-ether was an attempt to give a mechanical explanation of Faraday’s magnetic stresses. 
Maxwell’s model also gave rise to a corresponding “displacement current” in the dielectric medium, which could then be the seat of transverse waves.  Nothing could have been more gratifying to the grand-unifying physicist that to realize that Faraday’s fields and the luminiferous ether were one and the same. 
In 1863 Maxwell looked up the ratio for magnetic to electric forces, which had been determined experimentally by the German physicist Wilhelm Weber and plugged Weber’s force ratio into his equations. He calculated that the speed with which the electromagnetic disturbances predicted by his model travel through space came close to the latest measurements of the speed of light.  He wrote in excitement of this discovery,
“We can scarcely avoid the inference that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena.” 
Maxwell’s new kind of theory unified the three different realms of physics – electricity, magnetism and light in a unique system of equations.  In 1864 Maxwell presented the paper Dynamical theory of the electromagnetic field, to the Royal Society of London.  He published another paper in 1868, containing the four equations that we now know as Maxwell’s equations. In 1873 he also published the Treatise on Electricity and Magnetism, the best exposition of his theory. 
Maxwell’s revolutionary research on electromagnetism established him among the great scientists of history.  In 1887, Heinrich Hertz generated and detected the electromagnetic waves that Maxwell had foreseen.  Einstein later said about Maxwell’s discovery,
“One scientific epoch ended and another began with James Clerk Maxwell.” 
Legacy of Unification
The discoveries about the radioactivity of certain heavy elements towards the end of 19th century, and the ensuing development in physics on the atomic nucleus, led to the introduction of two new forces or interactions: the strong and the weak nuclear forces. Unlike gravity and electromagnetism these forces act only at very short distances of the order of nuclear diameters. While the strong interaction keeps protons and neutrons together in the nucleus, the weak interaction causes the so-called radioactive beta-decay. In the 1950s, C.N. Yang and Robert Mills, worked out the theoretical underpinning of these two forces of nature. 
Around the same time, Peter Higgs and others were trying to understand the origin of mass, which was thought to be a fundamental property of matter. In 1964, Higgs submitted a paper to a prominent physics journal in which he had formulated the idea that all of space is uniformly filled with an invisible substance—now called the Higgs field—that exerts a drag force on particles when they accelerate through it.  The resistance or drag on particulate constituents, contributes to what we perceive as the mass. 
Other physicists who studied Higgs’s proposal realized that his idea was a stroke of genius. In 1967, Pakistani physicist Abdus Salam at Imperial College, London, and American physicist Steven Weinberg at Harvard both independently incorporated Higgs’s ideas into what later became known as electroweak theory.  They suggested that just as electromagnetic fields are believed to be composed of photons, the weak fields also have particulate constituents, called W and Z particles. 
The Weinberg-Salam theory described the W and Z particles interactions to be very similar to photon interactions except one major difference. While photons had no mass at all, the W and Z particles were proposed to have enormous masses; more than 80 times the mass of a proton. This was a puzzle for both physicists which could only be solved by taking the Higgs field into account. 
The researchers believed that just as steam condenses into water as its temperature drops sufficiently, the Higgs field also condensed into a particular nonzero value throughout all of space after the Big Bang. They realized that before Higgs field condensed into what can be referred to as Higgs Ocean, not only did all the force particles had identical masses - zero - but the photons and W and Z particles were identical in essentially every other ways as well.
The symmetry between the electromagnetic and the weak forces is not apparent today because as the universe cooled, the Higgs Ocean formed and photons and W and Z particles interact with the condensed Higgs field differently. That’s why the electromagnetic and weak nuclear forces appear so different in the world around us. The underlying symmetry between them is broken or obscured, by the Higgs Ocean. 
At the time when Salam and Weinberg proposed this theory, few people believed them, and particle accelerators were not powerful enough to reach the energies of 100 GeV required to produce those particles.  Sheldon Lee Glashow, a professor at Harvard University found a way to extend the Weinberg-Salam theory to other classes of elementary particles.  Over the next decade, new predictions of the theory at lower energies agreed well with experiments and the electroweak theory was confirmed. 
Salam, Weinberg and Glashow had looked beyond superficial appearances, they had peered through the obscuring force of nothingness - to reveal a deep subtle symmetry entwining two of nature’s four forces.  Their proposed theories had unified the weak interaction with the electromagnetic force, just as Maxwell had unified electricity and magnetism a hundred years earlier.  They were awarded the 1979 Nobel Prize for the successful unification of the weak nuclear force and electromagnetism into what is now called the electroweak-force. 
In 1983 at CERN, the W and Z massive partners of the photon, with correct predicted masses as well as other properties were discovered.  After the discovery of the W and Z particles, the only remaining part of electroweak theory that needed confirmation was the Higgs field.  More precisely, Higgs boson, the particle quantized manifestation of the Higgs field that generates mass through its interaction with other particles. 
The importance of this particle made it the primary object of enquiry for Large Hadron Collider (LHC) at CERN. In July 2012, CERN announced with much fanfare that the LHC in Geneva had discovered a particle with the right properties to be the Higgs boson, which signified that researchers had confirmed a fundamental theory of mass. 
Theory of Everything
The successful struggle to unify the fundamental forces of the universe has opened the ground for optimism that we may now be near the end of the search for the ultimate laws of nature. After the electroweak theory, there have been attempts to also incorporate the strong nuclear force into so-called Grand Unified Theory.
In the late 20th century many physicists including Glashow put forward the first theory to go partway towards the goal of total unity. Their grand unified theory, suggested that three of the four forces-the strong, weak and electromagnetism-were all part of one unified force during the extreme conditions that existed just after the Big Bang.  Glashow put forward the idea that, gluons, the assumed constituents of the strong interaction, could be freely interchanged with photons as well as W and Z particles at very high temperatures, showing a more robust symmetry than that of the electroweak theory, without any observable consequence. Thus suggesting that at high energies there is complete symmetry among the three force particle and hence among the three forces. However, the grand unification unlike electroweak unification has not been confirmed experimentally. 
...if we do discover a complete theory, it should in time be understandable in broad principle by everyone, not just a few scientists. Then we shall all, philosophers, scientists, and just ordinary people, be able to take part in the discussion of the question of why it is that we and the universe exist. If we find the answer to that, it would be the ultimate triumph of human reason – for then we would know the mind of God. [Stephen Hawking: A Brief History Of Time]
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