Here is All the Physics you Missed
Listing Physics based on Length scale
Prologue
This article is a comprehensive overview of the diversity of physics segregated by length scales. A note of caution before proceeding. Like any other discipline, there is no strict demarcation of what comes under a certain field and what doesn’t. Some areas of study that I have mentioned separately may even be considered as just subfields of others at similar scales. Given the significant overlaps with engineering and other natural sciences, I have, whenever possible, taken arXiv as the reference for distinction.
Moreover, when I refer to the various ‘kinds of physics’ or dare I say ‘branches of physics’, I am referring to the things that the particular niche is focusing upon. This, in no way suggests or claims the fields being fundamentally different themselves or in their approach to studies. On the contrary, it should bring to one’s attention the wide assortment of interesting investigations that are being conducted under the same umbrella. Let us enjoy the fruit of efforts that the persual of curiosity by generations of physicists who came before us has led to.
Orders of Magnitude
Before we discuss the different scales and the phenomenon occuring at those respective scales, it imperative to mention the idea of an order of magnitude. So, what is an order of magnitude ? And why do we need it ? Simply put, an order of magnitude refers to the exponent of 10 which lies nearest to a given value. For instance, 8 is closest to 10¹ and 8000 is closest to 10⁴. Hence the respective orders of magnitude of 8 and 8000 are 1 and 4. An estimation of such measure is by no means suitable for calculation purposes and fortunately, we don’t use orders of magnitude for it either. We can however, get a sense of the size (or the mass, length and duration) of the objects we are studying using this concept. But what help does this size distinction do ?
Well, suppose you are to collect timber for the evening campfire. You enter the woods with a knife and an axe. An axe, no matter how sharp, is unsuitable for cutting up the branches of the tree. Even if you do manage to use it for the same, the efforts put in could have been well utilised for some other task. Similarly, a knife will take an eternity to bring down and chop up a whole tree, unless you know what are doing. Analogous to the situation above, phenomenon at different scales may require applying the same principle (here, repeatedly striking with a sharp object), but the applications of these rules pertaining to a particular situation may not translate well enough to that of the other. So, given the different requirements for dealing with the ‘trunk’ and the ‘branch’, we fine-tune our tools to minimize efforts and maximize efficiency.
In physics, this would mean using separate theories for separate orders of magnitude. That is to say, we deal with different scales differently. This benefits us in a way that is specifically helpful for doing physics - it provides a way of to maintain a separation of concerns. We can create theories at a certain scale and willingly neglect the contributions of smaller scale phenomenon, fullfilling the dual purpose of making sensible predictions and obtaining a bound on the possible errors produced due to the same, hence not compromising on the predictive power of the theory.
With this out of the way, we can now look at the various scales and the wonderful physics that awaits us.
Cosmic Scale (10²⁷ m)
Physical Cosmology
The word ‘cosmos’ is derived from the Greek work ‘kosmos’ meaning ‘order’ or world’. Hence, the study of the origin, development, structure, history, and future of the entire universe is called cosmology.
Physical cosmology is the branch of cosmology that is concerned with the study of cosmological models which describe the large scale structures and dynamics of the universe. The study of cosmological models help us explore fundamental questions and deduce information about its origin, structure, evolution, and most importantly, its destiny.
Physical cosmology as we know it today, is quite recent and began only after the development of Einstein’s theory of General Relativity in 1915. Later, research by Edwin Hubble and others revealed a whole new world of galaxies out there in space, moving away from us. This has led us to conclude that the universe must have existed in a state of high density and temperature. Big Bang is the theory we use to describe the expansion of universe from this inital state.
We will not talk of the moments just after the Big Bang (till about 10-³² seconds) as it is expected that quantum effects of gravity dominate physical interactions at this scale and without any concrete theory of the same, there is not much scope of description. However, there seems to be considerable consensus on the happenings in the universe from about 10-³⁶ to 10-³² seconds after the Big Bang. This phase of the universe’s expansion is believed to be one where it went totally bezerk, expanding exponentially in a process known as inflation. There are different mechanisms by which inflationary models account for this phenomenon, which we will also not go into details of here. But the key takeaway is that inflationary theories solve 2 major problems in cosmology - the flatness problem and the horizon/homogeneity problem.
The horizon/homogenetiy problem was first mentioned by Rindler in 1956. As per the Cosmic Mirowave Background (CMB), there exist large and distant regions of space between which, (assuming the standard Big Bang expansion) the light would not have had the time to travel yet have the same Microwave Background Temperature. Despite never being in the causal influence of each other, how can such far away regions be in thermal equilibrium. Similar to the horizon/homogeneity problem, we have the
flatness problem. WMAP has determined the geometry of the universe to be nearly flat. However, under Big Bang cosmology, curvature grows with time and given the current status of our universe, it must have had a very specific amount of matter at the very specific moments of time to result in such a perfect geometry. These problem are what are called as cosmological fine-tuning problems.
Current observations in the field are best explained by the ΛCDM model of Big Bang cosmology which is also referred to as the ‘Standard Model of Big Bang Cosmology’. However, as mentioned earlier, physics currently lacks a widely accepted theory of quantum gravity that can successfully model the earliest conditions of the Big Bang.
Some interesting problems puzzling modern day cosmologists include the identity of Dark Matter and Dark Energy, Cosmological Constant problem, Baryon Asymmetry problem, Horizons problem and the Hubble and S8 Tension.
Galactic Scale (10²¹ m)
Galactic Astronomy
Astronomy studies objects and related phenomenon which is beyond Earth’s atmosphere. Galactic astronomy, hence, is a subfield of astronomy aimed at studies of the Milky Way galaxy and all its constituents. Given our presence in the Milky Way, clearly it is relatively easier to observe the contents of the galaxy as compared to their extragalactic counterparts.
Galactic astronomy employs numerous techniques to study our galaxy which involve different radiations from the electromagnetic spectrum such as the infrared, radio waves and visible light. Each radiation has corresponding strengths and weaknesses, and usage of several of them for distinct yet related purposes gives us a holistic overview of the Milky Way. Studies can thus, be subdivided into specific regions or characteristics of the Milky Way such as the bulge near its centre, its disk, its formation and evolution, the nucleus (Active Galactic Nucleus - AGN), and the neighbourhood of the solar system.
Important problems in the field include the history of Milky Way, the effects of Sagittarius A*, discrepancy between the predicted and observed rotation curve of Milky Way, and the warping of Milky Way’s disk as revealed by Gaia.
Extragalactic Astronomy
From the prefix ‘Extra-’ it is evident that Extragalactic astronomy studies everything which the Milky Way fails to encompass.
It is interesting to note that Extragalactic studies provide us an opportunity to see Einstein’s General Relativity in action as well as the interaction of galaxies over cosmic time scales.
At this scale, higher levels of structure in the universe emerge, some bound gravitionally such as galaxy groups and clusters and others being groups of such clusters themseleves called superclusters. Moreover, an extensive body of research is dedicated to Active Galactic Nuclei (AGN), an extremely bright central region of a galaxy that is dominated by the light emitted by dust and gas as it falls into a black hole. Galaxies containing an Active Nucleus as called Active galaxies.
AGNs are the most luminous and persistent sources of electromagnetic radiation in the Universe. Active galaxies can be classified based on the kind of existing AGN which are themselves differentiated based on observed characteristics. For instance, an AGN with a jet of light and energy pointing towards the Earth is called a blazar whereas the most powerful of the AGNs are called quasars which emit so much radiation that they can single-handedly outshine the entire galaxy altogether.
Current investigations include explaning the Ultraluminous X-ray sources (ULXs) that are not associated with active galactic nuclei and the Infrared Background TeV Gamma-Ray crisis - significant deformations suffered by the spectra of very high energy γ-radiation from distant extragalactic objects during the passage of primary γ-rays through the intergalactic medium.
Planetary Scale (10⁷ to 10¹³ m)
Stellar and Solar Physics
Stellar astronomy is dedicated to studying the way stars are born, change over their lifetimes and die along with the way they influence the chemistry and structure of their environment, and provide a home for any planets in orbit during the process, treating stars both as individuals and as members of a population. Solar physics, specializes in the study of the Sun i.e. the processes that make the Sun shine, produce its magnetic field, shape its atmosphere, and send particles across the Solar System (solar wind), etc.
Solar wind, radio wave flux, solar flares, coronal mass ejections, coronal heating and sunspots are some of the common solar phenomenon occuring in the Sun’s atmosphere. As a consequence of Sun’s activity, the environment of Solar System varies with time. This variation in the space environment is referred to as space weather.
Studying solar phenomenon and space weather involves understanding the internal processes of the Sun, which produce its magnetic field and the dynamics of its atmosphere. The study of solar physics not only sheds light (pun intended) on the Sun’s effect on Earth and the other planets, but also serves as a basis for the study of plasma physics, since Sun is nothing but a hot ball of plasma floating around in space.
Solar physicists tackle challenges such as space weather prediction, explaining the generation of Sun’s magnetic field and the study of Solar cycle - an almost periodic 11-year change in the Sun’s activity measured in terms of variations in the number of observed sunspots on the Sun’s surface.
Planetary Astronomy
Planetary science is the scientific study of planets and their planetary systems which includes moons, ring systems, gas clouds, and magnetospheres. It involves understanding the formation and interaction of these components of the planetary systems. Planetary astronomy is a part of planetary science concerned with using astronomy to study planets and other small bodies. These may be the part of the Solar system, any other Extrasolar system or be free objects floating unbound in galaxies.
Exoplanets are planets which are beyond the Solar system. Some of the unfortunate ones have no place to call home i.e. have no star to orbit. These untethered masses as called rogue planets. The composition of exoplanets cover a wide spectrum, from the rocky ones like Earth to gas giants like Jupiter. Each exoplanet is an alien world, just like the ones we encounter in science fiction, except they are for real. From raining diamonds to being Super Earths, these exoplanets exhibit a myriad of phenomenon, opening our mind to the endless possibilites that exist beyond our aquatic home.
But even in our Solar system, there are several observations yet to be explained. For instance, the clustering of orbits for a group of extreme trans-Neptunian objects (ETNOs), which orbit the Sun in peculiar yet definitive ways, the semi major of their orbit being at least 150–250 AU. This has led to the hypothesis of a Planet Nine being present in the outer Solar system.
Regardless, planetary studies open door to fascinating physics taking place in distant corners of the universe, which perhaps some day might even lead us to extraterrestial life.
Geophysics
Coming back to our beloved Blue Marble, Geophysics refers to the study of Earth’s shape, its gravitational and electromagnetic fields, its internal structure and composition, its dynamics and their surface expression in plate tectonics and the generation of magmas, volcanism and rock formation. Geophysical investigations are made using mathematical and physical methods. This includes everything from an understanding of the microscopic properties of minerals and rocks, to an understanding of global processes such as earthquakes.
Interesting geophysical phenomenon include geomagnetic reversals, telluric currents and seismic waves. Furthermore, Earth’s outer core and mantle can flow (the former being a liquid and the latter behaving like a viscous liquid on geological time scales despite being predominantly solid), hence making geophysical fluid dynamics an essential tool in other Earth sciences such as oceanography and meteorology. The rotation of the Earth has to be taken into account as well, as it has considerable effects on the Earth’s fluid flow.
Geophysics is not important only from a physical consideration, but also from societal and economic point of view. With regards to prediction and prevention of geohazards, it plays an important role in the risk assessment of decisions made at a global scale. The energy sector which includes hydropower, thermal, wind industries and mining industry owe their rapid progess and successes to this science. Equally benifitted are the companies involved in the construction of tunnels, bridges, and other large construction projects.
Atmospheric Physics
Another one of the Earth sciences, atmospheric physics investigates and attempt to model Earth’s atmosphere and the atmospheres of the other planets using fluid flow equations and energy transfer processes in the atmosphere as well as boundary systems such as the oceans. It takes into account the Earth’s energy budget which describes the balance between the radiant energy that reaches Earth from the Sun and the energy that flows from Earth back out to space.
Atmosphere can be divided into layers on the basis of temperature - troposphere, the stratosphere, the mesosphere and the thermosphere. A common term exosphere is used from the part of atmosphere that starts from 500 km above the Earth’s surface that gradually fades into space though its inclusion as a separate layer is not agreed upon much. While each layer of the atmosphere boasts its own set of amazing phenomenon, arguably none is as important as the region of Earth’s atmosphers called the ionosphere.
The ionosphere is the ionized part of the upper atmosphere of Earth, from about 48 to 965 km above sea level, that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation and plays an important role in atmospheric electricity. The reasons for the claim above is its practical applicability in radio communication technology.
Cloud physics is also a part of atmospheric physics and is concerned with the formation and growth of the same. Clouds are extremely important in regulating the Earth’s climate and play a key role in the water cycle. Weather forecasting, thus, requires knowledge of cloud dynamics to accurately assess the conditions leading to better and more reliable forecasts.
Human Scale (1 m)
Medical Physics
Medical physics deals with medical applications that are built upon a foundation of physics for the prevention, diagnosis and treatment of diseases. It includes areas such as Radiotherapy physics, Diagnostic Radiology physics, Nuclear Medicine Physics, and Radiation Protection. Diagnostic Radiology and Nuclear Medicine are often grouped in what is termed as ‘Diagnostic Imaging’.
In 1943, a Hungarian radiochemist George Charles de Hevesy was awarded the Nobel Prize in Chemistry for his key role in the development of radioactive tracers to study chemical processes such as in the metabolism of animals. This approach is now widely used in Medical Physics particularly is Nuclear Medicine, which is a medical specialty that uses radioactive tracers (radiopharmaceuticals) to assess bodily functions and to diagnose and treat disease. Functional imaging differentiates nuclear medicine from traditional X-ray imaging that targets specific areas. This technique provides a comprehensive view of the entire body and its processes over time.
Radiotherapy is a life-saver techonology for cancer treatment. Typical modalities dealt by radiation therapy physicists are linear accelerator (Linac) systems and kilovoltage X-ray treatment units. Megavoltage X-ray therapy matured after the 1950’s with the production of high-energy sealed sources and the development of medical linacs, which facilitated more effective treatment of deeper lesions.
With its powerful and effective techniques, Medical Physics continues to fulfil its goal of improving human health and well-being.
Applied Physics
Applied physics refers to the applications of physics to new technology used for solving both scientific and engineering problems. Thus, it is considered a link between the ‘pure’ and ‘applied’ sciences. Research in applied physics includes fluid mechanics, laser physics, liquids and glasses, optics, photonics, plasma physics, micro- and nano-fabrication and microfluidics, quantum phenomena, semiconductors, thermodynamics and statistical mechanics, and transport phenomena. Technically, Medical physics and Biophysics come under Applied physics as well, but given their independence with other engineering disciplines, we shall learn about them separately.
Let us consider Optical physics in a bit more detail. Optical physics is the study of the generation of electromagnetic radiation, the properties of that radiation, and the interaction of that radiation with matter, especially its manipulation and control. It applies the principles of geometrical/ray and physical/wave optics to the discover and study new phenomenon.
A large chunk of research in this field revolves around quantum optics and attosecond and femtosecond optics. On 16 May 1960, Theodore Maiman revolutionized optical science by demostrating the first working laser at the Hughes Research Laboratories. Besides optical studies, the invention of lasers has led to advancements in numerous disciplines and have become essential to some such as astronomy where they are used in various techniques to improve the imaging quality and capabilities of large astronomical telescopes.
Other important areas of research in optical physics include the development of novel optical techniques for nano-optical measurements, diffractive optics, low-coherence interferometry, optical coherence tomography, and near-field microscopy.
Biological Scale (10-⁷ to 10-⁴ m)
Biophysics
Biophysics applies the principles of physics and chemistry and the methods of mathematical analysis and computer modeling to biological systems, with the ultimate goal of understanding the structure, dynamics, interactions, and ultimately the function of biological systems at a fundamental level. It seeks to explain biological function in terms of the physical properties of specific molecules.
Karl Pearson, an English mathematician who is considered to be the father of statistics, first introduced the term biophysics. This emerged from his work in mathematical biology.
Determining the structure and dynamics of specific biological molecules and of the larger architecture which they assemble into is a common theme in biophysics, hence research in biophysics shares significant overlap with biochemistry, molecular biology, physical chemistry, physiology, nanotechnology, bioengineering, computational biology, biomechanics, developmental biology and systems biology. So, biophysical studies are quite interdisciplinary in nature, often having research groups spanning multiple departments.
Questions fascinating biophysicists include the precise folding of protein required for proper functioning, replication of the insanely long DNA molecule during cell division, working of RNA. Also, the harvesting of chemical energy of ATP hydrolysis for movement of molecules around the cells and selective transportation of cell membrane interest the researchers in the field.
Molecular Scale (10-⁹)
Molecular and Chemical Physics
Molecular physics studies physical properties of molecules as well as their dynamics. It addresses the occurence of phenomena due to both the molecular structure and individual atoms of molecules. Molecular physics overlaps significantly with physical chemistry, chemical physics, and quantum chemistry.
Chemical Physics studies chemical processes from the point of view of physics. It relies on both experiments and computer simulations of molecular processes to probe the structure and dynamics of ions, free radicals and other chemical species.
Experiment techniques used in both the fields are often derived from related fields such as atomic and condensed matter physics. So much so that some researchers consider them as mainly subfields of existing disciplines and the research groups are mostly located in physics or chemistry departments.
Chemical physicists also study the formation of nanoparticles. Nanoparticles are spherical, polymeric particles composed of natural or artificial polymers.The prefix ‘nano-’ is used for naming these particles as their diameter generally lies between 1 to 100 nm (nanometer), though some may be as large as 500 nm. Tucked between the sizes ‘normal’ size objects (10⁰ m) and or molecular structures, they exhibit phenomena that are not observed at either scale. Unlike bulk matter, the surface area/volume ratio (which depends on the nucleation stage of nanoparticle formation) impacts certain properties of the nanoparticles and as a consequence of their spherical shape and high surface area to volume ratio, these particles have a wide range of potential applications.
Even quantum mechanical effects become noticable for these nanoscale particles. Infact, in 2023, Moungi G. Bawendi, Louis E. Brus and Aleksey Yekimov were awarded the Nobel Prize in Chemistry for the discovery and development of quantum dots, which are nanoparticles of semiconducting material that have quantized electronic energy levels.
Molecular and Chemical Physics aim to answer a wide range of intriguing questions such as redistribution of energy between different quantum states of a vibrationally excited molecule, testing of quantum mechanical predictions in simple and complex molecules and determining the mechanism of molecular processes such as solvation, and nucleation and growth of nanoparticles.
Condensed Matter Physics
Condensed matter physics deals with the macroscopic properties of materials. It uses the well-established laws of microscopic physics to predict the collective properties of very large numbers of electrons, atoms or molecules. As the name suggests, it studies condensed phases of matter such as solids and liquids and even the more exotic ones like Bose–Einstein condensates, fermionic condensates, nuclear matter, quantum spin liquid, string-net liquid, supercritical fluid, color-glass condensate, quark–gluon plasma, Rydberg matter, Rydberg polaron, photonic matter, and time crystal.
Theoretical research in condensed matter physics preceded its experimental counterpart, starting with Einstein who with his seminal 1905 paper on the photoelectric effect and photoluminescence, opened the fields of photoelectron spectroscopy and photoluminescence spectroscopy. Condensed matter physics started out as Solid-State physics, studying topics like crystallography, metallurgy and elasticity. It was for the inclusion of the physics of liquids, plasmas and other complex states of matter that the term was introduced.
Theoretical condensed matter physicists model the electronic properties of these states and phase transitions using numerical methods used in particle physics. Interestingly, some physicists hint at the connections between quantum chromodynamics (a theory of strong force) and condensed matter physics. According to them, features of QCD can be seen qualitatively in certain condensed matter systems and that recently some of the analyses that originated in condensed matter physics have found applications in QCD.
Meanwhile, experimentalists try to discover new properties of materials using experimental method involving diffraction with X-rays, electrons or neutrons, and various forms of spectroscopy. This leads to an overlap of interests with materials science and materials engineering.
Such research in condensed matter physics, besides providing an enchanced understanding of systems of several particles with strong interactions among them, has helped the development of the semiconductor transistor, laser technology, and several phenomena pertaining to nanotechnology.
Atomic Scale (10-¹⁰ m)
Atomic physics
We have seen systems of atoms such as molecules and condensed matter in molecular and condensed matter physics. Next up is the analysis of atoms as isolated beings. Atomic physics is that field of physics that studies atoms as an isolated system of electrons and the atomic nucleus. It is concerned with the atomic structure, intra-atomic processes of ionisation, exicitation and de-excitation of electrons by photons and the interaction between.
Well, I have been lying to you for quite a while - sort of but only slightly. Given the similarities in research interest and methodology, atomic, molecular, and optical physics are often considered a single discipline, innovatively named the
atomic, molecular and optical physics (AMO). It refers to the study of matter–matter and light–matter interactions, at the scale of atoms i.e. from Angstroms to nanometers in space and attoseconds to femtoseconds in time.
This is the field that propelled physics for much of the early 20th century. With the discovery of electron by JJ Thompson in 1897, it was clear that atoms were more than an indivisble entity (a property they owed their name to), but their exact structure was yet to be found. Following the discovery, Thompson propose the ‘plum-pudding model’ of the atom which suceeded in explaining the charge neutrality of atoms. In wake of this proposal, his former student Ernest Rutherford investigated atomic structure and with his iconic gold foil experiment, disproved his mentor’s model and proposed an alternate one, seeming to explain the observations of the experiment. However, this model was at odds with Maxwell’s Theory of Electromagnetism and failed to explain the stability of atom.
Just as Einstein had applied Planck’s idea of quantisation to Photoelectric effect, Neils Bohr followed suit and did the same to angular momentum of electrons. What emerged was a novel description of atom which came to be known as Bohr’s Model of Hydrogen Atom. Given its inability of accounting for multi-electron systems (as is the case with most atoms) and the splitting of lines in Hydrogen spectrum, Bohr’s Model, eventually gave way to a Quantum Mechanical Model of the atom.
The advent of quantum mechanics has, in a certain sense, unified AMO physics as we see it today and continues to be the backbone of all investigations in the field.
Nuclear Scale (10-¹⁵ to 10-¹⁴ m)
Nuclear Physics
Having detailed the developments on the structure of atom, we must zoom onto the nucleus. Atomic nuclei, their constituents and interactions come under the purview of Nuclear Physics. It is important to make a disctinction between the 2 fields of nuclear physics and atomic physics here. Atomic physics studies the atom as a whole, i.e. a nucleus and electrons around that nucleus. It is not concerned with the specifics of the electrons or the nucleus themselves, rather only with their interaction. On the contrary, nuclear physics is all about the nucleus and its structure.
The atomic nucleus consists of protons and neutrons bound together in a confined space at the center of atoms. As in other disciplines, Nuclear physicists use theoretical and expertimental approaches to investigate the inner workings of their object of interest. Theoretical approaches are based on calculations of the interactions of quarks and gluons (which we will come to in particle physics), which we believe form the protons and neutrons, using the help of physicist’s best friend - the computer. Experiments use particle accelerators to collide particles moving at nearly the speed of light. Comparison of experimental observations and theoretical predictions tests the limits of our understanding of nuclear matter and suggests new directions for experimental and theoretical research.
A major area of research in the field is the radioactive decay. Radioactivity is the release of energy from the decay of the nuclei of certain kinds of atoms and isotopes. Radioactive nuclei are nuclei that are unstable and that decay by emitting energetic particles. Depending on the kind of decay, the nucleus may even spontaneously break up into several smaller nuclei.
Quantum theory says that it is impossible to pin-point the exact moment a particular nucleus will decay. However, for a significant number of identical atoms (and their nuclei), the decay can be understood in terms of a First-Order reaction viz. the rate of decay of the group as a whole is proportional to the number of nuclei present at a given time. It is easier to talk of radioactivity in terms of the half-life of a radioactive specie. Half-life is the time in which about N/2 out of a set of N nuclei (N»1) decay. These half-lives can range from a mere instant to far longer than the age of the universe. The decays come in all shapes and sizes, some releasing an whole independent nucleus (α decay - He nucleus), others just a small energetic particle (β decay - electron) and others still pure energy (γ decay). There exist more exotic forms of decay and mind-boggling effects related to them which we will not get into here.
Radioactivity is used for medical imaging and targeted cancer treatments. Carbon dating helps us determine the age of fossils and other old rocks. Also, we cannot forget the Nuclear processes of fission and fusion on which we rely for endless energy and owe national defence programmes, covert intelligence operations and Oppenheimer’s fame to. Regardless of the geopolitical tensions caused due to the same, nuclear physics has truly done wonders for us.
Subatomic Scale (<10-¹⁸)
Particle Physics
Starting from the biggest in cosmology, we have finally reached the other extreme - the world of particle physics and its particle zoo. Like the name suggests, here we will find that everything around us is made up of particles. These particles and the forces with which they interact with one another account for most of the physical phenomenon in the universe.
Though, not all of these particles are the smallest building blocks of reality. Some are composite i.e. made of other particles, for instance proton and neutron are kinds of baryon - a composite particle made of 3 quarks while other are elementary particles such as the electron.
This classfication of fundamental and composite particles that we use today is known as the Standard Model of Particle Physics, more commonly referred to as just the Standard Model. For us, the assembly of the table of 3 generations of quarks (up-down, top-bottom, strange-charm) and the 3 leptons (electron, muon, tau and their neutrinos) [matter constituting particles collectively called the Fermions] and the Bosons [force carrying particles] may be nothing more than a compact representation of all there is to study in particle physics, but for the pioneers, the genesis of the field could not have been more difficult.
While the nuclear physicists were busy developing national defence programmes, the post war decade found particle physicists in much confusion over the myriad of flavours in which the particles were being produced in their particle accelerators. In an attempt to organise the ‘mess’ they had created, these physicists were looking for ways classify the particles. It took Gell Mann and Zweig’s genius to postulate the existence of quarks. Hence, the Standard Model was born, though it wasn’t until much later that the term came into use.
A decade of wait brought with it the first verifications of the quark model and finally a quantum theory of the strong force - Quantum Chromodynamics (QCD) was fully formulated. This was followed by development of the Electroweak theory - a unified description of 2 of the 4 fundamental forces of nature, proposed by Abdus Salam and Steven Weinberg. The Standard Model was being completed, albeit in bits and pieces, but nonetheless, each one was as essential to the framework as other. By 1990, only three of the fundamental particles remained to be found - the top quark, tau neutrino and Higgs boson. Fermilab found the top quark in April 1995 and the tau neutrino in July 2000. But the ‘God particle’ eluded us for yet another decade until CERN was able to get hold of it on 4 July 2012.
Till date, the Standard Model is the most successful theory in physics and remains the most rigorously tested as well, even more so than Einstein’s General Theory of Relativity which is currently untested in regions with strong gravitational influence.
The theory describes all the fields and interactions known by us other than gravity and improves our understanding of the fundamental forces and particles of the universe. Still, there remain unanswered questions, primarily pertaining to the exclusion of gravity from such a ground breaking model of nature which has lead to speculations of physics Beyond the Standard Model. Searches for both conventional and unconventional resolutions to the lapses of the Standard Model continue to surprise us in new and interesting ways, giving us insight into the symmetries of fundamental particles, heavy flavor physics, matter-antimatter asymmetry, neutrino behaviour and dark matter and energy among other things.
Conclusion
Firstly, congratulations for reaching the end section of this article. I took me a long while to do so as well. But when I did, I realised something astonishing. We started off with Cosmology and came to know about its issue of quantum gravity. And you know what, this is exactly what the particle physicists are after as well !
With the exculsion of gravity from the Standard Model, a theory of quantum gravity is the biggest frontier for today’s physics. According to Brian Greene, the search for this ‘final theory’ or ‘master equation’ is the holy grail for physicists. We, having explored all these orders of magnitude, have completed a full circle !
And this circle is the circle of reality and that of the pheonix of knowledge that obtains new life by rising from the ashes of its predecessors.
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- https://www.energy.gov/science/np/nuclear-physics
- https://www.energy.gov/science/doe-explainsradioactivity
- https://www.iop.org/about/iop-history/100th-anniversary/100-incredible-years/particle-physics#gref