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An apple falling onto the ground, a lightning hitting a tree, an iron filing becoming incandescent, a flame changing colour: apparently simple phenomena that physics tries to explain and nevertheless the scientists have not yet succeeded in conceiving and formalizing a coherent and complete picture of the reality. But what appears evident is that the perspective of this objective is quite distant.

Historically, physics has never before endured such a critical period as today's, which began some time in the '80s.

The unification of theories apparently independent is one of the objectives eagerly sought for by the theoretical physicists of whatever historical period, so much as to push them to firmly believe in the oneness of Nature. One of the many to share such an opinion was Herman Weil:

I am audacious enough to believe that all physical phenomena could derive from one universal law of the greatest mathematical simplicity. The universe where we live is interconnected, as every thing interacts with every other thing. It is absolutely impossible to have two theories concerning Nature that explain different phenomena as if one didn't have anything to do with the other.

The discoveries of the last thirty years are notably fewer in comparison with the fruitful period that began with Newton. Around 1780, the formulation of the laws of the motion and the gravitation were followed by Antoine Lavoisier's experiments of quantitative chemistry , subsequently, at the turn of the century, Dalton formulated his atomic theory, and, shortly after, the experiments of Thomas Young disproved Newton's particle theory of the light, confirming its undulating nature; lastly, Michael's Faraday studies on the electricity and the magnetism introduced the concept of field.

In the second half the XIX century James Clerk Maxwell worked on Faraday's studies and formulated the theory of electromagnetism, describing the light as an electromagnetic wave. In the last decades of the XIX century the discovery of the electrons and the phenomenon of radioactivity opened the way to contemporary physics.

At the beginning of the XX century physics was totally upturned by the formulation of the restricted and general relativity and, at the same time, by the birth of the quantum physics, passing through the description of the thermal properties of Planck’s radiation, Einstein’s photoelectric effect, which described the energy in small quantities, called quanta, Bohr’s atomic theory, De Broglie's undulating nature of the matter , Heisenberg’s principle of indetermination, Schrodinger’s wave equation and ending with the prediction of an anti-particle by Paul Dirac. The span of time between 1930 and 1980 has been marked ,instead, by the discovery of innumerable particles and by the partial formulation of a theory that unified all the forces and the particles, called standard model.

With the discovery of J. J. Thomson the atom was no longer considered as a true elementary particle, so there followed a series of models implying the existence of subatomic particles, that is the electron, the proton and the neutron.

The physics of the elementary particles not only studies the fundamental constituents of the matter, but also the fundamental forces that govern its changes both at microscopic and macroscopic levels. Right in this field physics has recorded the greatest progress: Newton universalized the gravitation law, unifying the terrestrial and celestial mechanics; Maxwell formulated the equations of the electromagnetic field, unifying the electricity and the magnetism as two sides of the same coin; Einstein, at first, unified space and time in a four-dimensional world, but then he changed the universal gravitation law into the geometry of space-time. Each of these theories has deeply transformed our conception of Nature. Newton believed that space and time had some eternal and unchangeable properties, independently from the phenomena occurring in them. With the theory of the relativity this conception was radically supplanted by a four-dimensional mutable reality, dependent from the nature, in which the gravity force shows through ripples of space-time, according to a geometry defined by Einstein’s equation, which refers neither to Euclid's nor to others'.

Already in the first decades of the '900 the physicists had taken the pioneer work of Maxwell as a model trying then to unify the two forces known so far: gravity and electromagnetism. If indeed Nature is unique why do two different fields and not only one exist? The idea to consider gravity and electromagnetism as aspects of a single phenomenon of one field began to spread. Two main theories were formulated: one proposed by the Finnish physicist Gunnar Nordstrom, who discovered that only a spatial dimension has to be added to the equations that describe electromagnetism to be able to calculate its gravity; the other proposed by Einstein, that is the theory of the general relativity, in which the gravitation law is incorporated to the restricted relativity. Only the latter was approved by the scientific community, because it was supported by forecasts validly confirmed, but not as a unitary theory. In the same period the quantum physics was developed and two new forces were discovered : the weak and strong interaction. Until his death Einstein tried to formulate a unified theory of the fields, but his remained only a dream.

Presently, too, the unification of the infinitely great and of the infinitely small is one of fundamental problems of the theoretical physics, that is the problem of the quantum gravity. Particularly both the quantum and the relativistic physics introduce a problem of endless values: for the former the endless values coincide with the field (which has a value for every point of the space, which means an endless number of variables, subject to unpredictable fluctuations); general relativity, instead, presents endless values for the gravity force and the density of matter inside a black hole. The problem is that the latter is also a field theory which, applied to the quantum mechanics, will present the problem of the infinites so much as to be incompatible with the reality. With the discovery of the strong force, which bonds the quarks to form a nucleon and the nucleons themselves to form an atomic nucleus, and of the weak strength, responsible of the transmutation of the particles, as the beta decay, the theorists neglected the gravitational force, which interacts much more weakly with the matter in comparison to the others, concentrating on the other three. Electromagnetism had to be re formulated according to the new world of the quantum physics; the sought-after unification is a quantum theory of the fields that, unlike the classical theory, states that two bodies don't instantly communicate without any mean of transmission, but that they interact by means of particles acting as mediators, called field particles, which, in the case of the electromagnetic force, travel at light speed.

The field particle of the electromagnetic force is the photon that is sent forth and electrically absorbed by the charged particles, by a precise time span, determined by the principle of indetermination. The theory that describes the interaction of the electromagnetic field with the matter is the QED (Quantum Electrodynamics), independently developed, at the end the ’40s, by Philip Feynman, Sin-Itiro Tomonaga and Julian Schwinger.

The standard model had progressed some way, but it was further questioned by a series of interrogative concerning the zoo of particles and anti-particles that were observed. The scientists didn't succeed in drawing a logical picture, until enough particles were discovered so as to group them in two series: the leptons, the lightest particles, as the electron; and the hadrons, the heaviest particles, as the proton and the neutron. In the early '60s, a new theory was formulated by Murray Gell-Mann and George Zweig according to which the hadrons were not elementary particles but were made of other particles that were called quarks. These new particles should have had a fractional charge but also another property, as compared to the others, the colour, which could be of three types but not comparable to a real colour. The quarks had to combine so as to form a colourless particle and with a whole charge. In fact a colourless particle could be got from the combination of three quarks, each of different colour, or from the combination of a quark and an anti-quark, so that one had the anti-colour of the other. All of them were combinations dictated by the new law of the conservation of the colour. The theory of the quarks was splendidly confirmed by an experiment that spotted the new particle, as foreseen by Gell-Mann and Zweig, and by another one confirming the presence of other particles inside a nucleon.

After the success of the QED, the physicists tried to apply the quantum physics to the two nuclear interactions. The strong interaction, which bonds the quarks as the electromagnetic force does, has some particles field, too, called gluons. The quantum field theory of the strong force is the QCD (Quantum Chromodynamics).

The four known forces had each enormously different characteristics in terms of intensity: the strong interaction is much more intense than the others, about 10^-38 as compared to the gravitational force, 10^-6 in comparison to the weak interaction and 10-2 in comparison to the electromagnetic force. Besides, only two of the four, the electromagnetic and gravitational force, are perceived in the macroscopic world because they act on a field of infinite extension, unlike the nuclear forces that can be perceived only on subatomic scale. The reason of these enormous differences has been the object of deep reflections of the best theoretical minds of the time, who, in order to solve the problem, reapplied an elegant mathematical theory of Herman Weil, formulated in 1918. In fact, borrowing two principles that currently represent the corner stone of the standard model, fundamental physics has been able to make enormous progress: the simple principle of symmetry (known also as gauge symmetry) and its consequent spontaneous breakup. A symmetry is a transformation that preserves the properties (which can be mathematical, physical, geometric, etc…) of an object, its spontaneous breakup is that asymmetrical state in which the object degrades and is no longer symmetrical accordingly to those transformations. It seems that in nature a symmetrical system is not stable, for instance a needle in balance on its point is a symmetrical system because of rotational transformations as compared to its axle, in whatever directions we observe it always introduces the same geometrical properties. But because of its instability it will fall in any direction that will make it asymmetrical because of rotational transformations. In the '60s Stevens Weinberg, Sheldon Glashow and Abdus Salam formalized these principles applying them to the electromagnetic and weak force, elaborating a quantum theory in which the latter are considered demonstrations of one only electroweak force. The state in which electromagnetic force is symmetrical to the weak force, or rather it is equivalent, occurs after one determined level of energy, of 10² Gigaelectronvolts, which are reached in particle accelerators. A spontaneous breakup of the symmetry follows it, which occurs below the same threshold of energy and which makes the system unstable degrading the two forces in an asymmetrical state. This justifies the different characteristics of the two forces we observe to the relatively low level of energy in which we live. The quantum theory of the electroweak field enjoyed great success because it foresaw the existence of particles not yet observed, the weak gauge bosons (bosons W⁺, W^-, Z), that is the electroweak field particles, observed at the CERN in Geneva in 1983 in an experiment carried out by Carlo Rubbia.

The road of the fundamental physics leading to the complete unification of the nature had been glimpsed by the scientists who, with great enthusiasm, wanted to go on unifying the strong interaction to the electroweak force.

They formalized a theory, called GUT (Great Unified Theories), which unified, according to the same criteria used by Weinberg, Salam and Glashow, the QCD and the electroweak force.

The theory intended not only to unify the three fields of force, but also the different types of elementary particles (the leptons interacting electroweakly and the quarks interacting strongly), so that to turn the ones into the others.

Despite the impossibility of a direct verification, as the level of energy required went well beyond 10¹⁶ Gigaelectronvolts, not attainable even in the particle accelerators of our times, the GUT was welcomed with great enthusiasm because not only it solved some problems till then unsolved, but also because it envisaged new phenomena as the one, long sought after, in which a quark turns into leptons, an electron and a neutrino.

In order to confirm this possibility, the physicists have tried to show the instability of the proton presuming an average life of 10³¹ years.

Many experiments have tried to confirm the new promising theory, one of the most famous took place 600 meters under the Lake Erie, where 8000 tons of purified water were checked thanks to some photomultiplicators.

So far, none of the experiments has produced a single decadence, which has greatly disappointed the scientists.

Since the '80s on, after Rubbia's latest great discovery, no meaningful progress has been made, especially in experiments.

The theorists have formulated different theories, alternative to the standard model, introducing deeply innovative and creative ideas.

One of them, which is gaining greater and greater credit, is the string theory, which considers that every elementary particle is constituted by a string, and its way of waving defines its property. The cause of its success is the amazing theoretical unification of all the forces and the particles. Unfortunately, these alternative theories are far from being experimented.

The crisis of the physics in the last thirty years has been caused by its progressive leaving the empirical approach and by the constant degradation in pure speculation.

The string theory, as the others, proposes to twist our conception of the nature, but until it does not foresee some phenomena, so that it can be verified or disproved, it must be considered just a fascinating mathematical theory.

The nature has so far proved its uniqueness; we need only to walk the correct road leading to the one fundamental truth.

Certainly, the innovative theories developed so far have not proved methodologically correct.

It is necessary to find again the road of the science, that road that has allowed the standard model to make great progress, that road that surely, once it takes us to the final goal, will upset our conception of the reality.

Bibliography

· Davies Paul C. W., Superforce, Touchstone Books, 1985;

· The forces of nature, Cambridge University Press, Cambridge, 1979;

· Hawking Stephen & Leonard Mlodinow, A brief history of time, Bantam Books, 2008;

· Halliday David, Resnick Robert, Krane Kennet S., Fisica 2, 5^ edizione, Ambrosiana, 2004;

· Flegel Ilka, The standard model of particle physics, Super Microscope Hera, Gazing into the Heart of Matter, The Desy Research Center, 2002;

· Hera shows the way to the unification of the forces of nature, Super Microscope Hera, Gazing into the Heart of Matter, The Desy Research Center, 2002;

· Georgi Howard, Una teoria unificata delle particelle e delle forze, in Le Scienze n° 154 06/1981;

· Klanner Robert, Dentro la vita del protone, in Le Scienze n°395 07/2001;

· Quigg Chris, The coming revolutions in particle physics, in Scientific American, 02/2008;

· Particelle elementari e forze, Le Scienze n° 202 06/1985;

· Rith Klaus e Schafer Andreas, Il mistero dello spin dei nucleoni, in Le Scienze n° 373 09/1999;

· Smolin Lee, The Trouble with Physics: The Rise of String Theory, the Fall of a Science, and What Comes Next, Mariner Books, 2007;

· Weinberg Steven, A unified physics by 2050?, in Scientific American, 12/1999;

· Teorie unificate dell’interazione tra particelle elementari, in Le Scienze n°75 11/1974;

· Il decadimento del protone, in Le Scienze n°156 08/1981.

 

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