Dr. N.L. Kachhara
Isaac Newton presented the earliest scientific definition of mass in 1687: "The quantity of matter is the measure of the same, arising from its density and bulk conjointly." That very basic definition was good enough for Newton and other scientists for more than 200 years. They understood that science should proceed first by describing how things work and later by understanding why. In recent years, however, the why of mass has become a research topic in physics.
The foundation of our modern understanding of mass is far more intricate than Newton's definition and is based on the standard model of particle physics, the well-established theory that describes the known elementary particles and their interactions.
Fundamental particles have an intrinsic mass known as their rest mass (those with zero rest mass are called mass less). For a compound particle, the constituent's rest mass and also their kinetic energies of motion and potential energies of interactions contribute to the particle's total mass. The Standard Model lets us calculate that nearly all the mass of protons and neutrons is from the kinetic energy of their constituent quarks and gluons (the remainder is from the quarks rest mass). Thus, about 4 to 5 percent of the entire universe - almost all the familiar matter around us - comes from the energy of motion of quarks and gluons in protons and neutrons.
Unlike protons and neutrons, truly elementary particles - such as quarks and electrons - are not made up of smaller pieces. The explanation of how they acquire their rest masses gets to the very heart of the problem of the origin of mass. The account proposed by contemporary theoretical physics is that fundamental particle masses arise from interaction with the Higgs field. But why is the Higgs field present throughout the universe? Why isn't its strength essentially zero on cosmic scale, like the electromagnetic filed? The Higgs field is a quantum field.
Particles that interact with the Higgs field behave as if they have mass, proportional to the strength of the field times the strength of the interaction. Our understanding of all this is not yet complete, and we are not sure how many kinds of Higgs fields there are. With the super symmetric standard model, at least two different kinds of Higgs fields are needed. The two Higgs field, give rise to five species of Higgs boson, three that are electrically neutral and two that are charged. The masses of neutrinos could arise rather indirectly from these interactions or from yet a third kind of Higgs field. The neutrino masses are less than a millionth the size of the next smallest mass, the electron mass.
The theory of the Higgs field explains how elementary particles acquire the mass. But the Higgs mechanism is not the only source of mass-energy of the universe. About 70 percent of the mass - energy of the universe is in the form of so-called dark energy, which is not directly associated with particles. The chief signs of the existence of dark energy are that the universe's expansion is accelerating. The precise nature of dark energy is one of the most profound open questions in physics. The remaining 30 percent of the universe's mass-energy comes from matter, particles with mass. The most familiar kinds of matter are protons, neutrons and electrons provide about one sixth of the matter of the universe or 4 to 5 percent of the entire universe. A smaller contribution comes from neutrinos, which is less than half a percent of the universe.
Almost all the rest of matter - around 25 percent of the universe's total mass-energy is matter we do not see, called dark matter. We deduce its existence from its gravitational effects on what we do see. We do not yet know what this dark matter actually is. Experiments indicate that the dark matter should be composed of massive particles because it forms galaxy - sized dumps under the effects of the gravitational force. A variety of arguments have let us concluded that the dark matter cannot be composed of any of the normal Standard Model particles. The leading candidate particle for dark matter is the lightest super partner. The mass of lightest super partner (LSP) is thought to be about 100 proton masses.
Thus we have understood the three ways that mass arises. The main form of mass we are familiar with - that of protons and neutrons and therefore of atoms - comes from the motion of quarks - bound into protons and neutrons. The proton mass would be about what it is even without the Higgs field. The masses of quark, themselves, however, and also the mass of the electron, are entirely caused by the Higgs field. Those masses would vanish without the Higgs. Most of the amount of super partner masses and therefore the mass of the dark matter particle comes from additional interactions beyond the basic Higgs one.
