Webmaster & Author: Antonino Cucinotta
Graduate in Physics

Copyright 2002 - All rights reserved

I believe in Almighty God, our Father,the Creator of Heaven and Earth, of all the visible and invisible things...


Billions of billions of billions
of invisible microcosms.
Ninety-two are the natural ones,
made of empty space, mass and electric charge,
now light sources now light holes.
Our body is made of them,
not our soul.
Death of our body doesn't change them,
as they have to form other lives and things,
on earth, in sea and sky,
as long as God wills it.

By Webmaster

Matter and antimatter

A mysterious mirror separates world from anti-world.
On this side of the mirror
a proton sees its negative anti-twin,
an electron its positive anti-twin,
as well as a neutron its symmetrical anti-twin.
Both an atom and its anti-atom emit the same light.
A thing sees its anti-thing.
Twins and anti-twins are going toward the mirror
and reach it.
Now we don't see them any more.
They have annihilated each other into gamma-photon pairs.

By Webmaster

Q. What is space-time?

A. Space-time is the union of the ordinary three-dimensional space together with time, that represents the fourth dimension of the physical world.
Space-time is like an immense roll of ribbon, that at the moment of the creation of the universe was all wound.
During the evolution of the universe, the ribbon,that represents the space,unrolled in the past,is unrolling in the present and will unroll in the future,until the universe will exist.
Even if the ribbon has only two dimensions, length and breadth, this model gives us well the image of the time that passes, while the ribbon is unrolling.
To represent more clearly the evolving of the universe in space-time, we can think to a sphere of an elastic material, that, being point-like at the moment of the creation (the big bang) of the universe, expanded uniformly, until it represents the universe as we see it today, with some hundreds of billions of galaxies,whose distances are increasing continously, making the universe always less dense and always less bright (the universe expansion was discovered by the astronomer Hubble in twenties ).
The whole history of the universe is written in space-time, that has the same age of the universe (this age is esteemed currently to be 15 billions of years ).

Q. Why do we speak of space-time and not of space and time, separately?

A. The measure of the duration of any physical phenomenon, for example of the flash emitted by a light source, when is determinated, independently, from an observer who is at rest with respect to the source and by an observer who is moving with respect to it (for example an observer aboard an airplane ), furnishes values that are the more different, the greater is the velocity of the moving observer .
In particular, the duration measured by the observer in motion with reference to the source results greater of the duration measured by the observer stationary with respect to it.
According to Galileo and Newton's theory, the measure of a time interval is absolute, since it is independent from the motion of anyone observing a physical phenomenon, whereas in the context of Einstein' relativity theory, the measure of a time interval depends from the velocity of the observer ; therefore it is spoken of a one's own time (or local time) Dtp ,that is measured by an observer at rest, and of a non-local time Dtnl, if it is measured by a moving observer .
Obviously the differences among the measured values result meaningful only if the velocity v of the observer is not too little with respect to the one of light in vacuum
(c = 300000 km/s): Dtnl = Dtp : square root of (1 - v2/c2). .
Because the measure of a time interval depends on the observer's frame of reference,one cannot consider a time measure is absolute, as in the prerelativistic physics ; therefore, are introduced the concepts of space-time and of space-time event, which is a point individuated in space-time by 4 parameters, that are the 3 spatial co-ordinates and the temporal coordinate.
The measures of length and time made in an experiment (for example, the measure of the length and of the oscillation period of a pendulum ), we plane to performe in a railway carriage, are valid with respect to the train, that represents our frame of reference.
Another observer, standing on a car in motion with respect to the train, could observe the experiment at a distance by binoculars and measure the length and the period of the pendulum with respect to his own frame of reference, the car,which is in moving with respect to the train.
The observer on the car would notice a slightly smaller length ( the so-called contraction of lengths ) and a slightly greater period (the so-called expansion of time) in comparison with the values measured by the observer on the train.
In fact the observer aboard the car, to effect the measures used the light rays coming from the pendulum, and the delay associated with the path of the light rays moving with velocity c = 300000 Km/s, even if it is too little, influenced both the evaluation of the length and the one of the period of oscillation.
The dependence of the measure of a time interval and of a length from the frame of reference couldn't take place if the velocity of light were infinite, that is, if the electromagnetic waves and the light, that consists of electromagnetic waves, were propagating instantly.
In such a case a stationary observer and another one in motion would measure the same times and lenghts relating to a physical phenomenon .

Q. What is the mass of a body?

A. A body or a living creature is made of matter, and the matter is made of microsystems, the atoms of the 92 chemical elements existing in nature, from hydrogen atom,which is the most light one, to uranium atom,which is the most heavy one .
Atoms are bound among them according to the laws of chemistry to form the molecules and crystals of all the chemical compounds of which are made all the bodies and the living creatures.
The molecules of elements, that is of metals ( copper, silver, zinc, etc...) and metalloids (carbon, phosphorus, oxygen, sulphur, etc...), consist of one or more atoms, according to the chemical element to be considered; for example the molecules of gases (hydrogen, chlorine, nitrogen, etc...) consist of two atoms, whereas the ones of metals consist of only one atom.
Atoms consist of a central nucleus, made of positively charged particles, the protons, and electrically neutral particles,the neutrons, and of a cloud of negatively charged electrons, that are distributed around the nucleus with a density depending on the own structure of the chemical element, which is considered .
The number of electrons (the so-called atomic number Z), that in the non-ionized atoms coincides with the one of the protons of the nucleus, determines the chemical properties characterizing an element.
The quantity of matter that forms an inert body or a living creature,defines a basic physical property: the mass or inertia.
A lorry or an elephant have a mass much greater of the one of a little tennis ball, and that involves a different effect (the acceleration) determined by a force of the same intensity .
In fact the second fundamental law of mechanics ( discovered by Galileo and formulated by Isaac Newton in his "Principia mathematica philosophiae naturalis" ) affirms that the variation of velocity per second, impressed by a force to a body, initially at rest or moving with a rectilinear motion with a certain initial velocity, is directly proportional to the intensity of the force applied to the body, and that, on the other hand, by applying the same force to bodies of different mass, variations of velocity per second happen that are inversely proportional to the mass of the body.


When applying a thrust of 200 kg to a body with a mass of 1000 kg, and after to a body with a double mass, in the second case the variation of velocity per second is halved.
The mass of a body, said more precisely its inertial mass, represents the aptitude of a body to withstand to the acceleration (that is the variation of velocity per second) produced by a force.
How much smaller is the mass of a body, much more the body is accelerated (or decelerated ) by a force.
The second fundamental law of mechanics can be expressed by the following approximate formula, since we consider the mean acceleration of the body instead of the instantaneous one:
F = M a = M ( Vfin-Vin )/( Tfin-Tin ),
where a is the mean acceleration, that is expressed by the ratio between the variation ( increase or diminution ) of velocity ( final velocity - initial velocity ) and the duration
( Tfin-Tin ) that this variation is referred to.
It can be observed that, by keeping constant the force F applied to the body, if the mass of the body doubles, then the acceleration a halves and if, viceversa, M halves, a duplicates.
A practical example that is a direct consequence of this law, can be furnished by thinking about the force F necessary to brake the motion of a vehicle: with constant values of velocity and of braking time, the friction force (the resistant force ) the brakes have to effect, is directly proportional to the mass M of the vehicle.
Likewise, the motive force that has to be produced by the engine to accelerate the vehicle from the rest state to a certain velocity in an assigned time, is directly proportional to the mass M of the vehicle.
Likewise the impulsive forces acting in the collision of the vehicle against an obstacle or another vehicle, is directly proportional to M, with an assigned velocity .
It is said commonly that a vehicle, in the acceleration or deceleration phase , is subjected to the inertial force F = Ma, which is directly proportional to the mass (inertia) M and has, respectively, the same direction of the motion or the opposite one.
If are required smaller time and space to brake the vehicle, it is necessary to increase proportionally the braking force F to get an equal variation of the velocity
v ( and of the momentum Mv) within a smaller time interval.
With the same considerations can be explained the protection that is provided by an elastic body interposed between a moving body and a rigid obstacle (let's think to a rubber pneumatic mattress used to reduce the effects of the bump produced from the free fall of a person to the ground from an height of a few meters).
Although the free fall velocity is too high, the elasticity and the softness of the medium determine an increase of the time necessary to slow down the velocity during the bump; consequently, during the bump it is possible to decrease considerably the deceleration a (the negative acceleration) and the impulsive force F = Ma, which is directly proportional to a.

Q. What is energy ?

A. When we work manually, for example lifting a much heavy object, we are getting tired and feeling that our body has spent "something" that has been necessary to lift the body; that "something" is energy.
In this case the manual work has been performed by using the muscles of our body, that are perfect biochemical engines that convert into mechanical work the energy stored by means of complex biochemical reactions that take place every time we eat to feed ourselves.
When we make a mechanical work on a body, we do nothing but apply to it a force (a motive force) increasing its velocity and having an opposite direction to the one of the sum of the resistant forces that make the velocity be decreasing.
In the case of a body that is being lifted to a certain height, the force opposing to the motion is the weight P (the gravitational force), that has to be exceeded by the muscular force so that the body can be lifted.
Also, the greater is the height h (vertical displacement of the body ), the greater are the energy and the mechanical work L = Fh that must be spent, by taking account, in this particular case, that L must be greater than the product of the weight P of the body by the vertical displacement h.
If, for instance, we lift a body of 50 kg from the ground to an height of 2 meters,
we execute a work greater than 50 kg x 2 meters = 100 kgm (1 kgm = 1 kg x 1 meter), by spending an energy greater than 100 kgm.
Therefore a body or a system ( in our case the system is our body ) have an amount of energy, if they are able to execute some work on one or several bodies or on another system.
Vice-versa, if on a body or on a system is performed some mechanical work by one or several bodies or by another system, some energy is accumulated, that can be used subsequently.

Q. How does energy accumulate?

A. The most simple way to accumulate energy consists in making a body gain velocity ; we speak in this case of kinetic or motion energy , because any mass M, accelerated by a force until reach a certain velocity V, when collides against one or several bodies, it is able to execute some work , by accelerating them during the collision and giving up them a share of its kinetic energy, according to the masses and of the type of collision.
The kinetic energy Ec stored in a moving body is directly proportional to its velocity V and is given by the formula
Ec = ( 1/2 ) M V2 .
If the velocity of a body duplicates, the kinetic energy quadruples.
So we can explaine why it is necessary to slow down a vehicle to limit the damages in collisions against other vehicles.
The collisions are defined elastic, if is preserved the total kinetic energy stored in the bodies before the collision, that is if the total kinetic energy after a collision is equal to the initial one.
Practically the collisions are never entirely elastic, because a share of the total kinetic energy of the bodies before the collision, is spent to deform the structure of the bodies in an irreversible mode, as it takes place in the head-on violent crashes between two vehicles or in the crash of a vehicle against a wall.
The work made by the intense impulsive forces acting during an inelastic collision is converted into heat according to the first principle of thermodynamics, as when we beat a nail with an hammer or fold up several times a piece of metal; the deformation work transforms into heat.
Approximately elastic collisions are considered the ones among the billiard-balls or among the molecules of a gas.
Let us remember that the kinetic energy of winds and of the water bodies has been largely exploited in past centuries to produce motive power, before the industrial use of the heat and electric engines.

The energy can be accumulated in several forms:

gravitational energy

It accumulates as potential energy ( or position energy ) in all the systems that consist of masses keeped at a distance each other in the space (systems of heavenly bodies, solid masses and liquid masses raised above the ground);

elastic energy

It accumulates as potential energy in a spring or in a cylinder containing a compressed gas;

chemical energy

It is released by chemical reactions, because of the bond energy among the atoms interacting each other to form chemical compounds;

thermal energy (heat)

It corresponds to the kinetic energy of the molecules of matter, and it determines the energy exchanges that take place among solids, liquids, gases and vapors because of temperature differences, that correspond to differences of both molecular velocity and kinetic energy.
In particular the heat energy that accumulates in a fluid ( liquid, gas or vapor ), that functions as a thermal accumulator, can be transformed into mechanical energy in the thermal engines ( that is the vapor and internal combustion engines ), exploiting the difference of temperature between the thermal source heating the fluid and the environment.
In refrigerators and in air-conditioning plants is exploited the principle of the heat pump, that consists in transferring heat from an environment to another at an higher temperature, by means of the compression of a gas or of a vapor, obtained at expenses of some mechanical energy (furnished by an electric motor ).
In the first phase of the thermodynamic cycle, the environment at a lower temperature, across the serpentine of the evaporator releases heat to a liquefied gas or vapor, causing its transition in the gaseous state; subsequently the gas or the vapor is liquefied while it is being compressed, at expenses of the absorbed electric power necessary to make operate the compresser, with an heat release to the environment at a greater temperature, across the serpentine of the condenser.
The heat transferred to the environment at an higher temperature is equal to the sum of the heat absorbed from the environment at a lower temperature and of the electric work trasformed into mechanical work, neglecting the losses of the system caused by the friction forces and by the heat generated (Joule effect) inside the windings of the electric motor ( electric work that hasn't converted into mechanical energy ).

electric energy

It is produced by the electric generators exploiting various forms of primary energy (kinetic, gravitational, thermal, chemical, nuclear, solar ).
It is stored by accumulators (secondary batteries ) or capacitors.

magnetic energy

It accumulates in permanent (natural or artificial) magnets and in electromagnets by using electric power.

nuclear energy

It is produced by fission and fusion nuclear reactions.
In the unstable nuclei of the heavy elements (uranium, plutonium, thorium ) the electric forces by which the protons ( nuclei of hydrogen atoms with one positive elementary electric charge ) repulse each other causing the nucleus to break up, are balanced precariously by the attractive nuclear forces that act among all the nucleons ( protons and neutrons ), keeping compact the nucleus.
Therefore, provided that the nucleus is excited by absorption of a slow neutron, the repulsive electric forces among protons prevail on the attractive ones, causing the fission of the nucleus in two fragments ( less heavy nuclei ) more 1 or 2 neutrons, with the emission of a considerable quantity of kinetic energy, that subsequently is converted into heat.
The nuclei of the light elements ( hydrogen, deuterium, tritium ) undergo instead , at very high temperatures ( at least about tens of millions of C degrees), the thermonuclear fusion to form neutrons and helium nuclei and release a very high energy, corresponding to the bond energy of the helium nuclei that are formed.

Q. What is the meaning of the well-known
Einstein equation E = mc 2?

A. Before the formulation of Einstein's theory, were considered separately the fundamental principles of conservation of the mass and the one of the energy.
The conservation principle of the mass, formulated in the eighteenth century by the chemist Lavoisier and already expressed by Lucretius in the poem "De rerum natura", consists in considering constant and indestructible the quantity of matter existing in nature; that is, in any system and, by extension, in the universe the total quantity of matter is always the same.
Matter is subjected to physical-chemical transformations, but it is not destroied ("in nature nothing is destroied, everything transforms " ).
The energy conservation principle, formulated in the nineteenth century, affirms that, whatever is the form of energy that is considered ( mechanical, thermal, electric, etc...) in any system and, by extension, in the universe, the total energy is constant.
Einstein showed that both principles are nothing but two faces of the same medal, the conservation principle of both the mass and the energy, that affirms that in any system and, by extension, in the universe, the mass transforms into energy and, vice-versa the energy transforms into mass, as in practice it happens in the nuclear reactions and in the ones among subnuclear particles.
In short, in the energy balancing of any system it is necessary to include the term mc2, where m is the sum of the rest-masses , that is the masses, measured at a zero velocity, of all the bodies forming the system.
The rest-mass term mo derives from the fact that Einstein showed that the mass of a body depends on its velocity, referred to a particular frame of reference, that is the frame in which the mass is at rest.


If we consider the mass of a body keeped at rest in a moving car, the mass of the body, measured aboard the car is the rest-mass mo, being the body stationary in comparison with the car; if instead we consider the mass m(v) of the body in comparison with the road, we speak of motion-mass, that is the greater than the rest-mass, the greater is the velocity of the body, that in this case coincides with the velocity of the car.
Practically, the difference between both masses is so little to result meaningless for a macroscopic body, whose velocity is much smaller of the velocity (c) of the light in vacuum (300000 Km/s), unless we consider microscopic bodies, as the elementary particles ( electrons, protons, neutrons ), that are able to reach, because of their extremely small rest-mass, velocities nearly equal to c, such that the relativistic effects are much meaningful.
If during a transformation of a physical system, takes place a diminution of the total mass of the bodies constituting the system, it means that the difference Dm (the so-called mass deficiency) transforms into energy according to the law E = Dm c2, where Dm represents in this case the difference between the initial mass and the final one.
The great value of c involves that is very huge the energy that is released, even if is very small the mass that disappears, as it happens in the nuclear fission reactors and in the thermonuclear fusion reactions that take place inside the Sun and the other stars.

Q. Why the velocity of a body cannot ever reach the limit value of the velocity of light in the vacuum ?

A. Because the motion mass m(v) of a body grows with the increasing velocity v, becoming very huge, theoretically infinite, while the velocity is approaching to c.
The formula giving the dependence of the motion-mass m(v) on
the velocity v is: m(v) = mo : square root of (1 - v2/c2 ) .
Moreover, since mass and energy are equivalent, it would be necessary to furnish to the body an infinite energy, and that is physically impossible.

Q. Which are the "fundamental charges" of  matter ?

A. The electric charge , the weak subnuclear charge and the strong subnuclear charge.
They correspond to as many fundamental forces (interactions) of nature, provided we consider separately the gravitational force, that acts on the mass of any body, independently from the type of charge that it possesses.
It is easy to speak of the electric forces that, together with the gravitational ones, are well known to everyone, because they determine the chemical reactions, the existence of any form of life, the natural electric phenomena, and the many applications of the electromagnetism that permitted to realize the technological wonders of today world.
Much less known are the weak subnuclear forces, that determine the radioactive b decay phenomena of the natural and artificial radioelements, and that control the velocity of the thermonuclear fusion reactions.
We quote finally the strong subnuclear forces, from which depend the operation of the nuclear fission reactors and the stability of atomic nuclei, that otherwise would disintegrate, if the intense repulsive electric forces among the protons weren't counterbalanced by the intense attractive nuclear forces among the nucleons
( protons and neutrons).

Q. What is the meaning of field ?

A. The wide diffusion of cellular telephones has involved the frequent use of the term "field" in common speechs, because we often hear people say "in that zone there is no field ", in relation to the impossibility to receive electromagnetic signals of intensity enough to the good operation of a cellular telephone.
In the case of the cellular telephones and of the radio and telediffusion systems we speak of electromagnetic fields always with reference to the current laws preventing environmental electromagnetic pollution.
Practically,what is a field ?
If we start from the example concerning the cellular telephones, we associate to an electromagnetic field something invisible that is moving from the transmitter to the receiver, "imbuing" the space, as in effect it happens.
A field is a physical quantity, as temperature, pressure, force of gravity , velocity, whose value is variable in a certain region of the space.
A force field (electric, magnetic or gravitational ) acts on the bodies that are sensitive to it, because they have charges that are able to evidence the forces owing to the field.
The "fundamental charge" sensitive to the electromagnetic field is the electric charge, that in the macroscopic world is always equivalent to a multiple of the elementary electric charge, the negative electric charge of electron.
Likewise, the nucleons ( protons and neutrons ) are carriers of a "strong subnuclear charge" that makes them sensitive to the field of the so-called "strong force", that is a rough expression used to point out strong nuclear force, that is the most intense one of all the natural forces.
Similarly we speak of the "weak subnuclear charge" of subnuclear particles (electrons, positrons, muons and neutrinos ) sensitive to the field of the weak subnuclear force, that is less intense than the other two, but more intense of the gravitational one, that is the most weak of all the forces of nature and becomes very intense only on a very large scale, when are considered the quantities of matter that form heavenly bodies (asteroids, planets, satellites, stars and galaxies ).
On the Earth the gravity force is very intense because of the big mass of our planet and the relatively little distance (~6400 km) between the bodies subjected to the gravity and the Earth center, in which we consider to be concentrated all the mass of the planet, as it is shown in Newton's gravitation theory.

Q. What do the physicists mean when they speak of quantum fields?

A. In the first three decades of the twentieth century the physicists succeeded to elaborate a theory much sophisticated, the quantum mechanics or wave-mechanics, that explains entirely all the atomic phenomena that the classical physics, founded on Galileo and Newton's mechanics and on Maxwell's electromagnetism, was not able to explain.
The atomic-molecular microcosm evidences that all the elementary particles behave as waves with a De Broglie wavelength l = h/( mv ) , which is inversely proportional to the momentum p = mv, and they obey to Heisenberg's uncertainty principle ( 1927 ), by which, when a particle is confined in a space region with dimensions comparable with its associated wavelength l, the uncertainty (error) that characterizes the measure of its momentum p, varies in inverse relation to the uncertainty which characterizes the measure of its position in the region of confinement.
This is equivalent to affirm that elementary particles, taking account of their wave-like behaviour, are as less locatable in the space, as better is known their velocity , and then their linear momentum.
Therefore there is no meaning for the trajectory concept of a particle,which is fundamental instead in classical physics, within which, when a body is moving,it is possible to determine at the same time its position and its velocity ,with all the precision that is required.
The impossibility to define the trajectory is replaced by the possibility to define in every point of the space the probability to find the particle.
On the other hand, it can be shown theoretically that, if a particle is confined in a well delimited region of the space, as in the case of an atomic electron, the request of no probability to find the particle outside the zone of confinement, implicates the impossibility to assign to the energy of the particle any value: from this request derives in fact the quantization of energy, already introduced in the primitive version of Planck (1900) and Bohr's (1913) quantum physics.
Not all the values of the energy are permitted to an electron inside an atom, but only some well determinate values that represent the energetic levels of the atom.
Likewise we speak of quantization of the momentum of a particle confined to move in a well-delimited region of the space.
We ask how to explain the origin of the values of the masses and the electric charges of the elementary particles (electrons, protons and neutrons) ? They have well-determinated values, but still we don't know why it is so.
We are able only to say that the masses and the electric charges of the particles depend on the structure of the universe, and precisely from the values of the three fundamental constants that determine the characteristics of the physical world: the Newton's universal gravitation constant G, the velocity c of light in the vacuum and the Planck constant h.
In this context, even the fields are quantized,since their energy is always multiple of an elementary energy quantity that they exchange with the particles that are able to interacting with them.

Example 1

Einstein expressed the hypothesis of photons (said even quanta of light ) to explain the photoelectric effect.
By considering that a photon is a quantum of the electromagnetic field that transports an energy E = hf, where f is the frequency of the electromagnetic waves, it can be observed that the energy of the electromagnetic radiation, in the interaction with the particles of the microcosm, is divided in a flux of packets, each containing the same energy, that are just the quanta or photons.
The quanta of the electromagnetic field can be considered as special "particles" that, having a rest-mass equal to zero, are always moving with the velocity of light in the vacuum.
In quantum electrodynamics, that has been developed subsequently to the formulation of the relativistic quantum mechanics by Paul Dirac ( 1928 ), the attractive or repulsive force between two electric charges is determined by the fact that a charged particle is entirely screened by a cloud of virtual photons that are continually emitted and absorbed by the particle.
The attractive or repulsive force is the more intense, the more the particles are near each other (according to the Coulomb law ), because, with the decreasing distance, decreases the time D t requested to photons pass from a particle to another.
If, to give an intuitive elementary explanation, we consider the second fundamental law of dynamics  F = DP/ D t ,then , with the decreasing Dt , increases the variation of the momentum per second; then the force increases.

Example 2

In 1935 the Japanese physicist Yukawa proposed to extend this mechanism to a nucleon pair ( protons and neutrons ), expressing the hypothesis that strong subnuclear attraction forces between two nucleons, that is between two protons, between two neutrons and between proton and neutron ( Heisenberg-Maiorana's forces ), are determined by the continuous exchange of virtual particles, the mesons, that are positively or negatively charged or neutral and with a rest-mass between the rest-mass of proton and the one of electron (~270 times the electronic mass ).
When in 1947 Powell and Occhialini discovered in cosmic radiation some charged particles with a rest-mass equal to 270 times the one of electron, that were identified with the p mesons (pions), Yukawa's theory was definitely confirmed.
The exchange of a virtual pion between two nucleons implicates a temporary violation of the energy conservation principle, since, during the flight of the pion, the total mass-energy of the system consisting of both nucleons and the virtual pion, is greater than the total mass-energy of the system before and after the flight, when it is made of only two nucleons.
However, this violation can be allowed, provided it is compatible with Heisenberg's uncertainty principle, that states that, the shorter is the flight time, the heavier is the mass-energy of the exchanged particle.
Because of the very short flight time t ( the time characteristic of the strong subnuclear interactions is 10-23 seconds), the distance R = c t at which two nucleons are subjected to the strong subnuclear force, is very small:
R = 3 .1010cm/s x 10-23 s = 3 .10-13cm = 3 fermi .
Therefore, Heisenberg's principle requires that must be exchanged a virtual particle that, differently from the virtual photon in electromagnetic interactions, has a non-zero rest-mass.
The strong subnuclear force has a very short range and is 1000 times more intense than the electromagnetic one, which instead is extended to a huge distance, theoretically infinite.
To the strong subnuclear force are sensitive not only protons and neutrons, but all the other hadrons ( barions and mesons ) that were discovered in cosmic radiation and were produced artificially in particle accelerators.
The quanta of the strong subnuclear field are just the pions, whose role is like the one of photons for the electromagnetic field.

Example 3

In 1968 the model based on the "ping pong" of virtual particles was extended by Salam, Weinberg and Glashow to the weak subnuclear forces, that determine the phenomena of radioactive decay (beta decay), for which Enrico Fermi in 1934 formulated the first theory, founded on the hypothesis of the existence of neutrino, introduced by Pauli.
Salam, Weinberg and Glashow formulated a theory of the weak interactions, introducing the particles W ( positive and negative ) and Zo, with masses much greater than the one of pion and a very short interaction range (10-15 cm).
Besides they succeeded to show that the weak subnuclear quantum and electromagnetic fields, studied at high energy, trend to identify in an only type of quantum field "the electro-weak field" , for effect of which the W and Zo particles, denominated intermediary vectorial bosons, produce an unified interaction with the same intensity of the electromagnetic one.

Q. What is the spin of a particle?

A. The elementary particles that constitute matter can be represented as spheres of ultramicroscopic dimensions, with a radius of the order of tenths of millionth of millionth of centimetre, and they can be neutral or with a positive or negative electric charge.
Besides exists another property, typical of the microcosm, that differentiates a particle from another: the spin.
A particle can have a zero or a non-zero spin, if it is characterized or not by an intrinsic rotation around an its axis; if a particle has an angular momentum is said to have the spin, and behaves as an ultramicroscopic spinning top.
Besides, an electrically charged particle as, for example, electron or proton, or a neutral one, as neutron, can be considered, provided it has the spin, as a spinning top containing virtual charged particles that are rotating inside it around its axis, so that the particle is equivalent, because of the Ampere equivalence theorem, to an elementary magnet of microscopic dimensions.
The particles behave in different modes, according to the value of the spin: if the particles have spin 0, as the pions ( p mesons),or an integer spin, as,for example, the photon, that has spin 1, they are defined bosons, from the name of the Indian physicist Bose that studied, independently from Einstein, the statistic of these particles (the Bose-Einstein statistics) ; if they have instead a fractional spin ( 1/2, 3/2, 5/2, etc...), as it happens for electrons, positrons, protons and neutrons, whose spin is 1/2, they are defined fermions, from the name of Enrico Fermi that studied their statistic, independently from Dirac (the Fermi-Dirac statistics).
The difference of behaviour between bosons and fermions is in the fact that, while two or many bosons, even in a great number, are able to occupy the same quantum state, individuated for every particle by a combination of quantum numbers, in relation to the energy and the angular momentum , two fermions instead, for the exclusion principle of Pauli, aren't be able to have the same quantum numbers.
A notable example of the behaviour of the bosons ( condensation in an only quantum state ) is shown by the liquid helium, that, keeped at temperatures smaller or equal to 4 K above the absolute zero ( -273,16 C or 0 K ),evidences a behaviour much unusual: the superfluidity, what consists in the fact that the superfluid helium is able to go up again in the capillary pipes and in the containers, against the gravity , to give rise to the so called "fountain effect".
In fact the helium nucleus, formed by 2 protons and 2 neutrons, whose totale spin is equal to zero, behave as zero-spin bosons (scalar bosons).
The fermions instead, since aren't able to be in the same quantum state, spread their energy in an interval of quantum energetic levels, that is the wider, the greater is the number of particles.


If for the atomic electrons, that have spin 1/2, weren't worth the Pauli exclusion principle, they would condense in an only quantum state, in proximity of the atomic nucleus, with a consequent compression of whole the matter.
The density of matter would increase enormously, reaching values nearly equal to the ones that are typical of nuclear matter ( collapse of the atomic structure ).

Q. Which are the "essences" of matter which form all the bodies and the living creatures ?

A. All the bodies and the living creatures, that originated and are evolving in the space-time, are made of :


It is the mass-energy of the protons, neutrons and electrons of which are made atoms of matter.


The fundamental charges of the particles forming atoms are of three types and determine the stability of matter with respect to the conversion of mass into energy:
1) Electric charges:
Leptons (electron, muon,tauon), baryons (proton,hyperon and other particles heavier than proton ), mesons (pions, kaons,etc...) and the respective antiparticles, have an electric charge with the absolute value of 1,6*10-19C (the elementary electric charge e).
To quarks and antiquarks is assigned instead a fractional electric charge with the values,depending on the quark type, of +/-( 2/3 )e or +/- ( 1/3 )e.
Electric charge are the sources of the electromagnetic forces determining the atomic structure,the binding energy among atoms forming molecules and crystals, and the absorption and emission phenomena of electromagnetic energy (by photons ) in atoms, molecules and crystals.
2) Weak subnuclear charges (flavours) :
-six for quarks (up,down,strange,charm,bottom and top )
-six for leptons (electron,electron neutrino, muon, muon neutrino,tauon, tauon neutrino).
They are the sources of the weak subnuclear forces ( Fermi forces ) and of the electroweak ones ( of Salam, Weinberg, Glashow ), that produce as the change of flavour of quarks ( for instance, from charm into strange or from down into up ) and of leptons ( for instance, from electron into electronic neutrino by the exchange of a W boson ), as the b radioactive decay phenomena that take place in atomic nuclei and consist in the transformation of a proton into a neutron, with the emission of a positron ( positive electron ) and a neutrino, or in the transformation of a neutron into a proton, with the emission of an electron and an antineutrino.
3) Strong subnuclear charges
They are the sources of the strong subnuclear forces determining the binding energy of protons and neutrons inside the atomic nucleus, and the strong interaction of quarks , antiquarks and of all the hadrons ( mesons and baryons ).
According to the standard model ( quantum cromo-dynamics ,Q.C.D ), the strong subnuclear charges are the colour charges (red, green,blue) of quarks, of the respective antiquarks and of gluons (the quanta of gluon field determining the strong subnuclear forces to which are subjected quarks and antiquarks.


The quantum fields associated to the forces acting among the particles forming atoms, are of three types:
1) Electromagnetic field , mediated by the exchange of virtual photons ( that cannot be detected experimentally ) among particles having an electric charge> .
Since the photons, without electric charge and with zero mass, that determine electromagnetic forces, have spin = 1, their field is said boson quantum field.
2) Electroweak field (of Salam, Weinberg, Glashow ) , mediated by the exchange of virtual bosons ( W+ and W- electrically charged and Z neutral ) with non-zero mass, among leptons and among quarks and antiquarks forming hadrons.
Since the bosons producing weak forces have spin = 1, their field is said boson quantum field.
3) Gluon field ( from the Latin glus = glue ), mediated by the exchange of eight types of virtual gluons.
Gluons, that determine the strong subnuclear forces to which are subjected quarks and antiquarks, have zero mass and zero electric charge, but six of them are furnished with two color charges.
Even in this case, since the exchanged gluons have spin = 1, their field is said boson quantum field.