Updated: May 14, 2021
Albert Einstein published his Theory of General Relativity in 1916. In 1917, Einstein applied his theory to model the universe as physicists accept it today. Newton's explanation of gravity was that there is a force of mutual attraction between 2 objects with mass separated by a finite distance. Newton's theory had some discrepancies, both theoretical and experimental. Newton himself was sceptical about the fact that a force should exist between two objects only because they were separated by a finite distance.
Newtonian space was flat and an absolute entity. Time was considered to be absolute. Every observer observes the flow of time the same way. By the end of the 19th century, it was proven to be a faulty description of space. In 1908, Hermann Minkowski developed a concept known as "spacetime". Spacetime was a concept which unified the three spatial dimensions with the dimension of time. It was developed as an extension of Einstein's Theory of Special Relativity (a theory regarding the relationship between space and time for objects moving at a constant speed in a straight line).
Minkowski's spacetime explains the universe with Special relativity holding. What it did not account for was gravity. The presence of gravity made Minkowski's spacetime an incomplete description of the universe. Einstein described spacetime taking gravity into account. The presence of a mass in space causes spacetime to curve. The difference between Minkowski's spacetime and Einstein's spacetime was the Metric Tensor. In simple words, the Metric Tensor describes the way spacetime curves. The contents of the universe determine the curvature of spacetime. At the same time, the degree of curvature of spacetime is an indication of the amount of matter present at a point in space. Therefore, gravity is a consequence of curved spacetime. Objects moving in curved spacetime follow paths known as geodesic that is the shortest distance between 2 points in a curved space. An apple thrown into the air comes back down since it follows the curvature of spacetime due to the Earth. The curvature is towards the core of the Earth.
Curvature of Spacetime
Therefore, gravity is a consequence of the curvature of spacetime rather than an actual force between objects. For some time now theoretical physicists have been trying to formulate a theory of quantum gravity, which is the manifestation of gravity at the quantum level. The leading theories for this are string theory and loop quantum gravity.
Particle physics is a branch of physics that studies the most fundamental particles which make up matter and radiations. The Standard Model describes the known particles under one model. Particle physicists explore the extension of the Standard Model to newly discovered particles and even gravity. The Standard Model has discrepancies like the mass of neutrinos.
There are four fundamental forces in nature, the weak force, the strong force, electromagnetic force and gravitational force. The weak force is responsible for radioactive decay(conversion of one subatomic particle into another). The strong force is responsible for binding subatomic particles to form larger atoms and also keep them from physically disintegrating. The electromagnetic force is responsible for interactions between particles that carry a charge. The Standard Model explains three of the four fundamental forces(weak, strong and electromagnetic) of nature and the particles associated with these forces. The known particles are either fermions or bosons. Fermions can be subdivided into quarks (with their anti-particle, the anti-quark) and leptons(with their anti-particles, the anti-lepton). There are six quarks, six leptons, three gauge bosons and the Higgs boson.
Fermions are the elementary particles with spin 1/2. Each fermion has a corresponding anti-particle, which is a particle with an opposite charge. For example, electrons have a negative charge and their corresponding anti-particles are called positrons. If a particle and anti-particle happen to collide, they would annihilate. Annihilation is a process where the two particles are spontaneously converted into a large amount of energy with a blinding flash of light.
Quarks are the particles that makeup protons and neutrons. Three quarks make up a proton or a neutron. Particles containing three quarks are known as baryons. Particles that are made of a quark and an anti-quark are called mesons. Quarks carry a charge of a fractional magnitude of electrons. Quarks interact with each other by the strong force and with other fermions through the weak and electromagnetic interaction. The six types of quarks are up, down, top, bottom, charm and strange.
The other six fermions are known as leptons. Three of these are electrons, muons and taus. Electrons, muons and taus carry the same charge. The other three are electron neutrinos, muon neutrinos and tau neutrinos. Electrons are the lightest, muons are intermediate, while taus are the heaviest. These three particles interact by the electromagnetic force due to the charge they carry. According to the Standard Model, neutrinos are charge-less and massless particles that do not usually interact with other matter and are only influenced by the weak force. This makes them very hard to detect. A fun fact about neutrinos is that almost 100 trillion of them pass through each of our bodies every second. These come from the Sun as a result of nuclear fusion reactions occurring in the core. The probability of neutrinos interacting with matter is so low that almost 50000 tons of water are present in the tank of the underground Super-Kamiokande experiment in Japan.
According to the Standard Model, gauge bosons mediate the weak, strong and electromagnetic interactions. Interactions are the ways different particles influence each other. The value of spin for gauge bosons is 1. There are three types of bosons-photons, W and Z bosons and gluons.
Photons are packets of energy and are the most fundamental constituent of all electromagnetic waves. Photons, which are massless and chargeless particles, mediate the electromagnetic force. The W and Z bosons are the particles that mediate the weak nuclear interaction between other fundamental particles. The W bosons have either a unit negative or positive charge, while Z bosons are electrically neutral. They are very massive on the quantum scale. The weak force is not so weak in magnitude; it is almost equivalent to the electromagnetic force. Rather, the extent of the effect of the weak force is much lesser. It is due to the mass of the W and Z bosons that this happens. The weak force is effective up to a distance of 10^-18 metres and practically disappears beyond the radius of a proton. Gluons are particles that mediate the strong force between quarks. Unlike photons, gluons have charge as a result of which they participate in strong force interactions.
So far, we have seen various particles which constitute matter. What we have not seen is how matter came to exist in the first place. When the Big Bang happened, the Universe was a hot dense plate of these elementary particles flying around at the speed of light. According to special relativity, any object moving at the speed of light cannot have mass. Essentially, this means that had the elementary particles not slowed down a bit at the beginning, the Universe as we know it would not exist. Now imagine a scenario where there were no Higgs bosons. We would not have existed at all!
In 1962 and 1963, Peter Higgs hypothesized the presence of a field called the Higgs field which exists throughout the Universe. At the very beginning, the electromagnetic and weak forces were very similar. When the Standard Model was first developed, it was discovered that W and Z bosons have a mass that set the weak force apart from the electromagnetic force. Hence, there should be a reason why the W and Z bosons became massive. The reason is the Higgs field, which is an unusual field existing throughout the Universe. It causes spontaneous symmetry breaking of the electroweak interaction, triggering the Higgs mechanism. This Higgs mechanism is why W and Z bosons have mass and can mediate the weak force.
Apart from gravity, every field in Physics has an associated particle. Similarly, the Higgs field has the Higgs boson related to it. It is assumed that Higgs bosons stuck in space all around the Universe. Higgs bosons are chargeless and have no spin. These particles are the reason why fundamental particles acquired mass at the beginning of the Universe. The Higgs bosons interacted with quarks and electrons at the beginning of the Universe, slowing them down from the speed of light, thus allowing these particles to acquire mass and bind together to form atoms. Hence our very existence is owed to the Higgs bosons.
Discovery of the Higgs boson
The Large Hadron Collider at CERN(an international research institute) has been trying to simulate the Big Bang for some years now. In 2012, they announced the discovery of a Higgs-like particle followed by a statement in 2013 that highlighted the discovery of a Higgs boson. Essentially, this means that the particle they discovered in 2013 has characteristics very similar to the one described above. The discovery of "the" Higgs boson is yet to happen.
Large Hadron Collider at CERN
What we do not know about the Higgs boson
Even though the existence of a Higgs boson has been confirmed there are still some questions about it.
Does it have any related particles?
Does the Higgs boson spin in extra dimensions which cannot be perceived by humans?
Is the Higgs boson a fundamental particle or does it have even more elementary components?
Why did it freeze into outer space in the first place?
We have seen how our perception about Newtonian gravity is wrong, how gravity is a consequence rather than an actual force, we have explored the "quantum realm" and we have seen how our very existence is owed to a subatomic particle and the scope for research in the foreseeable future. Stay tuned for more interesting and fresh content on The Theory of Everything.