Mass is the basic concept of physics but it is one of the basic fundamental quantity of physics, which is hardest to define. Mass is something which measures quantity or amount of matter in an object. You must be already knowing about all these. Here I’d like to describe the mass generation, forces, the world of gravity and gravity-free models, casts brilliant new light on one such child-like question: What is mass? and What is the origin of mass?

To explore more about mass at a quantum level and relativity, let’s dive deeper.

Origin of Mass

The origin of mass is one of the mysterious and unknown questions of particle physics. Since a long time, physicists are looking for a reason that why mass exhibits inertial behaviour. There is a fundamental and simple way to explain inertial mass.

If two objects are bound to each other in such a way that the binding field enforces a specific separation, then each time the position of one of them changes, a finite time elapses before the other particle moves, due to the finite speed of light. This delay is the cause of the inertial behaviour of mass.

Mass Generation

Mass generation mechanism is a theory in theoretical physics which describes the origin of mass from most fundamental laws of physics.

Physicists have proposed many models that explain different views of the origin of mass. This problem is complicated because the primary role of mass is to mediate gravitational interactions between bodies, and no theory of gravitational interaction reconciles with the currently popular Standard Model of particle physics.

There are two types of mass generation models: gravity-free model and gravitational models that involve gravity.

Gravity-free models

Technicolor Technicolor models break electroweak symmetry through gauge interactions, which were originally modelled on quantum chromodynamics.

Coleman-Weinberg mechanism This model generates mass through spontaneous symmetry breaking through radiative corrections.

Other theories Unparticle Physics and the unhiggs model, UV-Completion by Classicalization, Symmetry breaking, Asymptotically safe weak interactions, Models of Composite W and Z vector bosons, Top quark condensate.

Gravitational Models

Extra-dimensional Higgsless models use the fifth component of the gauge fields in place of the Higgs fields.

Unitary Weyl gauge. If one adds a suitable gravitational term to the standard model action with gravitational coupling, the theory becomes locally scale-invariant (i.e. Weyl-invariant) in the unitary gauge for the local SU(2).

Preon and models inspired by preons such as the Ribbon model of Standard Model particles by Sundance Bilson-Thompson, based in braid theory and compatible with loop quantum gravity and similar theories.

In the theory of superfluid vacuum, masses of elementary particles arise from interaction with a physical vacuum, similarly to the gap generation mechanism in superfluids.

Newtonian Mass

In 1674, Robert Hooke, published his concept of gravitational forces, stating that all celestial bodies have an attraction or gravitating power towards their own centres, and also attract all the other celestial bodies that are within the sphere of their activity, formulated as Universal Law of Gravitation.

Mass in hence is defined in terms of Gravitation as:

Active Gravitational Mass Active gravitational mass measures the gravitational force exerted by an object.

Passive Gravitational Mass Passive gravitational mass measures the gravitational force exerted on an object in a known gravitational field.

Inertial Mass

Inertial mass is the mass of an object measured by its resistance to acceleration. The definition of mass in classical mechanics is slightly different than the definition of mass in special relativity, but the basic and essential meaning is same.

In classical mechanics, according to Newton’s second law of motion, let the body has mass ‘m’, at any instant of time, it obeys the following equation: [ F = ma ], where ‘F’ is the force acting on body in the direction of acceleration and ‘a’ is the acceleration of the center of mass of the body.

The above equation defines how mass relates to the inertia of a body. If we look at this equation [ m = F/a ], where ‘F’ is the constant force acting on the body in the direction of acceleration. We see that mass of the object is inversely proportional to the acceleration. Which means that if we apply an identical force, the object with bigger mass will experience a smaller acceleration, and the object with smaller mass will experience a bigger acceleration. Which means that larger mass exerts a greater resistance to changing its state of motion in response to the force.

From the above equation, we can deduce an another equation for mass in terms of energy.

Atomic Mass

Usually, the mass of an object is measured in relation to that of the kilogram, which is defined as the mass of an International Prototype Kilogram (IPK), which is a platinum alloy cylinder stored in an environmentally monitored safe secured in a vault at the International Bureau of Weights and Measures in France.

But, International Prototype Kilogram (IPK) is not convenient for measuring the masses of small particles like atoms and molecules and other of similar scales, as it contains trillions of trillions of atoms. Therefore, to overcome this problem, we measure the mass of atom with respect to another atom, thus scientist developed the atomic mass unit (amu). By definition, 1u is exactly 1/12 of the mass of carbon-12 atom, and by extension, a carbon-12 has a mass of exactly 12u.

Mass in Relativity

Special Relativity

There are two kinds of mass in special relativity: rest mass (invariant mass) and relativistic mass (mass which increases as its velocity increases).

Rest mass is the Newtonian mass which is measured by an observer moving along with the object.

Relativistic mass is the total quantity of energy in a body or system divided by c2. The two are related by the following equation: where ‘y’ (gamma) is Lorentz factor.

The invariant mass of systems is the same for observers in all inertial frames, while the relativistic mass depends on the observer’s frame of reference.

General Relativity

The equivalence principle in general relativity is any of the several related concepts dealing with the equivalence of gravitational and inertial mass.

Albert Einstein’s idea is that the gravitational force as experienced locally while standing on a massive body is the same as the pseudo-force experienced by the observer in a non-inertial i.e. accelerated frame of reference.

Negative Mass and Exotic Matter

The negative mass exists in the model to describe dark energy and radiation in negative-index metamaterial in a unified way. And thus in this way, the negative mass is associated with negative momentum, negative pressure, negative kinetic energy and FTL (faster-than-light).