Ancient people thought we are made up of four ‘elements’ – water, earth, fire and air; which we now know are not elements. Presently, there are some 118 elements. All elements are made up of atoms which were once thought to be indivisible. By early twentieth century following JJ Thompson’s and Rutherford’s discoveries, atoms were known to be constituted of nuclei (made of protons and neutrons) at the centre and electrons orbiting around. By 1970s, it was known that protons and neutrons are not fundamental either but are made up of ‘up quarks’ and ‘down quarks’ thus making ‘electrons’, ‘up quarks’ and ‘down quarks’ the three most fundamental constituents of everything in the universe. With the pathbreaking developments in quantum physics, we learnt that particles are actually derivatives, the bundles or packets of energy in the fields implying particles are not fundamental. What is fundamental is field that underlies them. We can now say quantum fields are the fundamental building blocks of everything in the universe (including advanced biological systems like us). We are all made up of quantum fields. Properties of particles like electric charge and mass, are statements about how their fields interact with other fields. For example, the property that we call electric charge of an electron is a statement about how the electrons field interacts with the electromagnetic field. And. the property of its mass is the statement about how it interacts with the Higgs field.
Since the ancient times, people have wondered what we are made up of? What is the universe made up of? What are the fundamental building blocks of nature? And, what are the basic laws of nature governing everything in the universe? Standard model of science is the theory that answers these questions. This is said to be the successful theory of science ever built over the last centuries, one single theory that explains most of the things in the universe.
People knew early that we are made up of elements. Each element, in turn, is made up of atoms. Initially, it was thought that atoms are indivisible. However, in 1897 JJ Thompson discovered electrons using electric discharge through cathode ray tube. Soon after, in 1908, his successor Rutherford proved through his famous gold foil experiment that an atom has a tiny positively charged nucleus at the centre around which negatively charged electrons circle in orbits. Subsequently, it was found that nuclei are made up of protons and neutrons.
In 1970s, it was discovered that neutrons and protons are not indivisible hence not fundamental, but each proton and neutron are made up of three smaller particles called quarks which are of two types – “up quarks’’ and “down quarks’’ (“up quark” and “down quark” are merely different quarks. The terms ‘up’ and ‘down’ do not imply any relationship to direction or time). Protons are made up of two “up quarks’’ and a “down quark’’ while a neutron is made up of two “down quarks’’ and an “up quark’’. Thus, “electrons”, “up quarks” and “down quarks” are three most fundamental particles that are building blocks of everything in the universe. However, with advances in science, this understanding too has seen changes. Fields are found to be fundamental and not particles.
Particles are not fundamental. What is fundamental is the field that underlies them. We are all made up of quantum fields.
As per current understanding of science, everything in the universe is made up of invisible abstract entities called ‘fields’ that represent the fundamental building blocks of nature. A field is something that is spread across universe and takes a particular value at every point in space which can change with time. It is like ripples of fluid that sways throughout the universe, for example, magnetic and electrical fields are spread across the universe. Although we cannot see electric or magnetic fields, they are real and physical as evidenced by the force we feel when two magnets are brought closer. According to quantum mechanics, fields are thought to be continuous unlike energy that is always parcelled up in some discrete lumps.
Quantum field theory is the idea of combining quantum mechanics to fields. According to this, the electron fluid (viz. ripples of the waves of this fluid) get tied into little bundles of energy. These bundles of energy are what we call electrons. Thus, electrons are not fundamental. They are the waves of the same underlying field. Similarly, ripples of the two quark fields give rise to “up quarks” and “down quarks”. And the same is true of every other particle in the universe. Fields underlie everything. What we think of as particles are actually waves of the fields tied up into little bundles of energy. The basic fundamental building blocks of our universe are these fluid-like substances that we call fields. Particles are merely derivatives of these fields. In pure vacuum, when particles are taken out completely, fields still exist.
Three most basic quantum fields in the nature are “electron”, “up quark”, and “down quark”. There is a fourth one called neutrino, however, they do not constitute us but play an important role elsewhere in the universe. Neutrinos are everywhere, they stream through everything everywhere without interacting.
Matter fields: The four basic quantum fields and their associated particles (viz., “electron”, “up quark”, “down quark” and “neutrino”) form the bedrock of the universe. For unknown reasons, these four fundamental particles reproduce themselves twice over. Electrons reproduce “muon” and “tau” (which are 200 times and 3000 times heavier than electrons respectively); up quarks give rise to “strange quark” and “bottom quark”; down quarks give rise to “charm quark” and “top quark”; while neutrino give rise to “muon neutrino” and “tau neutrino”.
Thus, there are 12 fields that give rise to particles, we call them matter fields.
Below is the list of 12 matter fields that make up 12 particles in the universe.
Force fields: The 12 matter fields interact with each other through four different forces – gravity, electromagnetism, strong nuclear forces (operate only at small scale of nucleus, hold quarks together inside protons and neutrons) and weak nuclear forces (operate only at small scale of nucleus, responsible for radioactive decay and initiate nuclear fusion). Each of these forces is associated to a field – electromagnetic force is associated to gluon field, fields associated with strong and weak nuclear forces are W and Z boson field and the field associated to gravity is space-time itself.
Below is the list of four force fields associated with four forces.
| electromagnetic force | gluon field |
| Strong and weak nuclear forces | w & z boson field |
| gravity | space time |
The universe is filled with these 16 fields (12 matter fields plus 4 fields associated with four forces). These fields interact together in harmonious ways. For example, when the electron field (one of the matter fields), starts to wave up and down (because there is an electron there), that kicks off one of the other fields, say electro-magnetic field which, in turn, will also oscillate and ripple. There will be light which is emitted so that will oscillate a little. At some point, it will start interacting with the quark field, which in turn, will oscillate and ripple. The final picture we end up with, is the harmonious dance between all these fields, interlocking each other.
Higgs field
In 1960s, one other field was predicted by Peter Higgs. By 1970s, this became integral part of our understanding about the universe. But there was no experimental evidence (meaning, if we make Higgs field ripple, we should see associated particle) until 2012 when CERN researchers at LHC reported its discovery. The particle behaved exactly the way predicted by the model. The Higgs particle has very short life, of about 10–22 seconds.
This was the final building block of the universe. This discovery was important because this field is responsible for what we call mass in the universe.
Properties of particles (like electric charge and mass) are statements about how their fields interact with other fields.
It is the interaction of the fields present in the universe that give rise to properties such as mass, charge etc. of different particles experienced by us. For example, the property that we call electric charge of an electron is a statement about how the electrons field interacts with the electromagnetic field. Similarly, the property of its mass is the statement about how it interacts with the Higgs field.
An understanding of Higgs field was really needed so that we understood the meaning of mass in the universe. Discovery of Higgs’s field was also confirmation of the Standard Model which was in place since 1970s.
Quantum fields and particle physics are dynamic fields of study. Since the discovery of Higgs’s field, several developments have taken place that have bearings on the Standard model. Quest for answers for the limitations of the Standard model continues.
Standard model of elementary particles: the 12 fundamental fermions and 5 fundamental bosons.
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Sources:
The Royal Institution 2017. Quantum Fields: The Real Building Blocks of the Universe – with David Tong. Available online at https://www.youtube.com/watch?v=zNVQfWC_evg
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FUNDAMENTAL PARTICLES – A Quick look
| Fundamental particles are classified into Fermions and Bosons based on spin. |
| [A]. FERMIONS |
| Fermions have spin in odd half integer values (½, 3/2, 5/2, ….). These are matter particles comprising of all quarks and leptons. – follow Fermi–Dirac statistics, – have a half-odd-integer spin – obey the Pauli exclusion principle, i.,e, two identical fermions cannot occupy the same quantum state or the same location in space with the same quantum number. They cannot both spin in the same direction, but they can spin in opposite direction. – The fermions include all quarks and leptons, and all composite particles made of an odd number of these. – Quarks = six quarks (up, down, strange, charm, bottom and top quarks). – Combine to form hadrons such as protons and neutrons. – Can not be observed outside of hadrons. – Leptons = electrons + muons + tau + neutrino + muon neutrino + tau neutrino. – ‘Electrons’, ‘up quarks’ and ‘down quarks’ the three most fundamental constituents of everything in the universe. – Protons and neutrons are not fundamental but are made up of ‘up quarks’ and ‘down quarks’ hence are composite particles. Protons and neutrons are each made of three quarks – a proton consists of two “up” quarks and one “down” quark whereas a neutron contains two” down” and one “up.” “Up” and “down” are two “Flavors,” or varieties, of quarks. – Baryons are composite fermions made of three quarks, e.g., protons and neutrons are baryons – Hadrons are composed of quarks only, e.g., baryons (made of two or more quarks) and mesons (made of a quark and an antiquark). |
| [B]. BOSONS |
| Bosons have spin in integer values (0, 1, 2, 3, ….) – Bosons follow Bose-Einstein statistics; have integer spin. – named after Satyendra Nath Bose (1894–1974), who, along with Einstein, developed the main ideas behind the statistical thermodynamics of a boson gas. – do not obey the Pauli exclusion principle, i.,e, two identical bosons can occupy the same quantum state or the same location in space with the same quantum number. They can both spin in the same direction, – Elementary bosons are the photon, the gluon, the Z boson, W boson and the Higgs boson. Higgs boson has spin=0 while the gauge bosons (i.e., photon, the gluon, the Z boson, and W boson) have spin=1. |
| [C]. COMPOSITE PARTICLES |
| Composite particles can be bosons or fermions depending on their constituents. – All composite particles made up of an even number of fermions is a boson (because bosons have integer spin and fermions have odd half-integer spin). – All mesons are bosons (because all mesons are made of an equal number of quarks and antiquarks). – Stable nuclei with even mass numbers are bosons e.g., deuterium, helium-4, Carbon -12 etc. – The composite bosons also do not obey Pauli exclusion principle. – Several bosons in the same quantum state coalesce to form “Bose-Einstein Condensate (BEC).” |
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