# Physics

##### Physics and Me

I am a physicist by training and research. I did my PhD in physics from Wayne State University, Detroit, MI and MS also in physics from Bowling Green State University, Bowling Green, OH. I did all my previous college and high school from my native country, Nepal.

I am also active in  research. Currently I am working on the the topic related to my PhD work.

Currently, I am working as a Lecturer at Lawrence Tech University, Southfield, MI. I teach physics at Natural Science Department.

Beside my research, I love teaching physics. In fact, I love to share my knowledge to anyone interested in the subject not just teaching my students. Presenting basic physics in a language understood by general public is one of my hobbies. There is much more physics everywhere in our daily life than most realize. Physics is constitute the strong foundation for other branch of science, engineering and technology. Many view physics as kind of something like you see in science fiction movies and hard and beyond the reach of general public. One of the things I love is to dispel that misconception. The most basics thing physics talks about are space, time and matter. Its laws are simple and talks about symmetries. You might have heard about laws of conservation of energy and conservation of 'momentum' but very few in public know that they are basically the statement about symmetry in time and symmetry is space translation! Laws of physics are general. The same law explains why an ice skater spins faster when she brings her arm closer to her body,  and  why planets revolve around the sun the way they do, or why neutron stars spins extremely fast after it is formed from a dead star! (The particular law in these cases is called the law of conservation of angular momentum.) There are no separate laws for 'earthly' objects and the object in 'heaven'. This last statement seems trivial now but it was not that trivial at the time of Galileo and Newton.

Education in physics and science is not just about getting jobs or making discoveries. It nurtures the use of reasons in your own and the public discourse rather than making people depend on some irrational arguments and mob mentality.  It enriches deep inside you and make you able to solve problems with reasons and facts. It tells you 'wait does this actually make sense?' Physics nurtures and builds on the kind of curiosity and plain thinking you had when you were small kids.

My area of expertise in physics is in the broad sense theoretical nuclear physics. It is about extreme nuclear matter called quark-gluon plasma or QGP. QGP is the extreme phase of matter where consisting of obtained, in an extremely small space and time scale, in high energy heavy ion collision experiments. These experiments were carried out and still being done at RHIC (Relativistic Heavy Ion Collider) in the US and at the LHC (Large Hadron Colliders) in Europe. RHIC is a facility at Brookhaven National Lab in the Long Island, NY while the LHC is one at CERN (European Organization for Nuclear Research).

In order to get some general idea what this is all about I need to go over some basics of the the structures of nucleus and nucleons and the fundamental interactions involved.

##### The Nucleus and its Contents

We know matter is made up of atoms. Atom consists of electrons and the nucleus. The nucleus is extremely small core of the atom. The size of nucleus is about 1/100,000 of the size of the atom. A few electrons, depending on the atomic number of the atom in question, occupy all this intervening space. It is to be noted that all chemical reactions, light, fire, electricity, TNT or RDX explosions are the games of those electrons, or more accurately, the games of the outer electrons, called the valence electrons! The nuclear level involves entirely different level of energy scale. This is comparison between a TNT explosive and an atomic or hydrogen bomb! Other examples of energy involved in nuclear level is particles coming out of a radio active substance, the energy source of a star, etc.

For some time it was believed that electrons, protons and neutrons are the most fundamental particles. But the discovery of plethora other elementary particles put a break on that simplistic picture long time ago.

Current understanding is something like this. It has been decades when physicists realized that protons and neutrons are not fundamental particles as they were thought to be in accordance with the findings of Ernst Rutherford, Niels Bohr and so on.  They are composite, consisting of quarks. Each particles has its negative counterpart called antiparticle. Thus we have antiquarks, anti proton, antielectron (always called positron). However, because, lack of experimental evidence of existence of quarks despite various attempts, the theory was shelved for some time until the the discovery of asymptotic freedom in the 70's (the Nobel prize of 2004). Quarks have been established as fundamental particles. They cannot be detected as independent particles. They however resides inside proton and neutrons freely. Beyond that space scale of less than $10^{-15}$ m, you cannot separate quarks due to strong 'color' force, which means explains why it had been possible to observe quarks all along.

Gluons are the 'messenger particles' or the mediators of strong force or strong interaction. It may be noted that all forces in nature can be reduced to four fundamental forces or interaction. They are gravitational, electromagnetic, strong and weak interactions. We know gravitational and electromagnetic forces. The weak interaction is a force responsible for the $\beta$-decay, a kind of nuclear decay that changes the the atom from one element to another. The strong nuclear interaction is the force that holds the nuclei in a nucleus together despite the tremendous repulsive force between protons in that small space.

Particles interact with another in the field of those forces by exchanging 'mediators'. In case of an electron repelling another electron the the mediator are photons - the quantum of electromagnetic radiation. The messenger of weak interactions are what we call the W and Z bosons. And, the messenger of strong interaction is called gluon. It may be helpful to view gluon as a kind of elastic bond holding quarks together in a proton or neutrons as indicated in the figure above. Inside the nucleons (protons or neutrons), however, the quarks are relatively free however. On this aspect, the color forces (strong forces) are just opposite from electromagnetic or gravitational force, where force between the interacting bodies decreases when their separation gets bigger. In case of the strong force, if the distance scale gets smaller (or, equivalently, energy scale gets bigger) the force between interacting quarks gets smaller! This is the idea of asymptotic freedom, mentioned earlier.

Quarks and gluons come in 'colors' and 'flavors' in additions to charge and spin. The word color here has nothing to do with the literal meaning - it is an intrinsic property in a similar way charge is. You may find people refer it as 'color charge'. A charge can be negative or positive,  and in the same way there are three different type of color charges of quarks and gluons. They are named 'red', 'green' and 'blue'. These quarks have antiparticles. Flavors are basically types of quarks. They are 'up', 'down', 'strange', 'charm', 'bottom' and 'top' in the order of increasing mass. They are respectively denoted by u, d, s, c, b and t.

Quarks have fractional charges. u, c, t have a charge of 2/3 units while d, s and b have a charge of - 1/3.  Their antiparticles have opposite charges. All quarks are spin half particles and belong to a statistically inspired group of particles called fermions. The other statistical group are bosons, which have integral spins. Pions, which consists of a quark and and antiquark are bosons. The messenger particles - photon, W and Z are also bosons. Fermions and bosons have different statistical properties that dictate how they interact with their buddies.

##### The Standard Model

The fundamental particles and interactions, including that of quarks and gluons described by a general theoretical model called the Standard Model. It includes all interactions except the gravitation, which has defies any attempt of unification with others so far.

The electromagnetic interaction consists of exchange of virtual photos photons. Virtual photons are the photons that are waived to violate energy conservation for a extremely brief amount of time dictated by Heisenberg uncertainty principle (here the relevant relation is in terms of energy and time $\Delta E \Delta t \geq \hbar/2\pi$

[Protons and neutronare composed to quarks and gluons (illustrated in the figure on the left). Gluons are the mediators of the strong color force, like a photon is mediator of electromagnetic force. Unlike photons, gluons there are eight kind of gluons and they interact with themselves as well! This makes the strong nuclear interaction described by quantum chromodynamics (QCD) much more complicated than quantum electrodynamics (QED). Another thing that makes QCD different from QED is the coupling strength. Coupling strength in QED is small (~1/137) and one can apply perturbation series truncating off the higher order terms. In QCD we cannon do that since the coupling strength is not so small. At extremely high temperature and pressure, the the coupling strength gets smaller and ultimately we may get quasi free or free quarks and gluons (figure in the right). This is according to the idea of ‘asymptotic freedom’, the discovery of which in early seventies was recognized by 2004 Nobel in physics. Image source: BNL]

Quarks, gluons, leptons, neutrinos, etc. are the most fundamental units of matter. Quarks and gluons do not exists freely in nature under normal conditions. This is because of the strong ‘color force’. Color is name given to a degree of freedom besides charge, spin, etc., and has nothing to do with the literal meaning of color. Only ‘colorless‘ objects can exists freely. For example, a proton (charge = +1) is made up of three quarks - two ups (u, charge = +2/3) and a down (d, charge = -1/3) and combination of three color charges (the three quarks in proton must be ‘red’, ‘green’ and ‘blue’) to make it color neutral. We can say similar thing with neutron, which also has three quarks (u, d, d). Protons, neutrons, delta, etc. contain three quarks and they fall into a sub-category of hadrons called baryons. The other hadrons are mesons and they contain two quarks. Mesons are color neutral bound states of a quark and an antiquark. The most common examples of mesons are pions, which comes in three types: pi plus, pi minus, and pi zero.  Pi plus is a bound state of u and anti d. A meson named J/psi is a bound state of charm and anti-charm quarks. In heavy ion collision experiments most abundant of the detected particles are pions.

[iPhone image taken by my PhD adviser Sean Gavin, 2009]

$\mu$