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> The forces of nature Issue: 2008-2 Section: Science



Since ancient times the man has been keen on interpreting and understanding the phenomena that surround him, always trying to give an explanation to the unknown.

Many questions have arisen in the course of times: Why do we see the Sun move in the sky? What is it made of? What happens inside it?

So far the forces we know of are four: gravity, electromagnetism, the weak interaction and the strong interaction.

It is thought that these four forces are responsible for all the phenomena that we perceive and we don’t perceive, they are the source of every change.

Since the discovery of each of them a long process of understanding and description of their properties started and is still going on.

The man has always been aware of the force of gravity, but only Newton, at the end of the 17th century, studied it in a scientific way, explaining his theory according to which the force is directly proportional to the product of the mass and inversely proportional to the square of the distances.

As all the matter of the known universe is made by non negative mass, the force is always attractive, which explains why, even if it is much weaker than the other forces, it is easily observable.

It acts on a field of infinite extension, it is the weakest, as it has an intensity of 10-38 as compared to the strong interaction.

In 1797 its existence was experimentally confirmed by Henry Cavendish, who calculated the attraction between two big spheres of lead and other two small balls set in an appropriate way.


According to Newton, each body, provided with mass, changes the property of the surrounding space, producing a field of force in the same space; therefore, if other bodies happen to enter this field, they are submitted to its force due to an action at immediate distance, that is without any means of transmission.

This conception of Newton’s universal gravitation was challenged by Einstein's theory of the general relativity, which affirms that no physic means is able to propagate at a speed higher than the light.

Einstein changed the concept of the gravitational field, not considering it as a force but as a manifestation of the curvature of the space-time.

Electricity and magnetism were already known to the ancient Greeks, who discovered that, rubbing a piece of amber, it attracted small objects and that some minerals attracted and repelled one another even at a certain distance.

Around 1850s Maxwell inferred, after Faraday and Oersted had observed respectively that a changeable magnetic field gives origin to an electric stream and that an electric stream origins from a magnetic field, that electricity and magnetism are two forces closely bound and they can be considered two different aspects of a single electromagnetic force.

The electromagnetic force is found among charged particles, it obeys the law of the inversely of the square, like gravity, and acts on a field of infinite extension.

It is not easily observable because, unlike gravity, which does not have particles with negative mass, electromagnetism interacts with particles both positive and negative; all the charged particles of the universe tend to annul one another; for this reason there is not a prevailing manifestation of its force.

The weak interaction was discovered after the discovery of the emission of electrons and antineutrins from the nucleus of radioactive atoms, that is the beta decay.

Fermi asserted that these particles do not exist before they are issued, but that they are created by the energy of the radioactive nucleus, in the course of the transmutation of the neutron into proton.

Fermi’s hypothesis was confirmed by the verification of the decay of the free neutron, which decays into a proton, an electron and an antineutrin.

The force that rules the phenomenon, the weak interaction, is much stronger than the gravitational and much weaker than the electromagnetic one.

Generally it is the cause of the transmutation of the identity of the particles and it supplies the energy necessary to the change of the identity of the particles.

Moreover, the action range on which it acts differs it from the others, because it is null at a distance greater than 10-16 cm; therefore it can act only on subatomic particles.

The other force that works at this distance is the strong interaction, which acts up to 10-13 cm.

It is the force that binds the particles constituting the nucleus.

Only the heavier particles are combined by this force, therefore the electrons and the neutrins are not included.

Since the theory of the quark by Gell-Mann and Zweig, in 1964, and then with the following confirmation of the particle accelerators, the nature of this force was better understood; the quark theory states that the proton and neutron are each formed by three quarks, each with a fractional charge.

The strong interaction is the force that binds closely the three quarks present in the nucleon and with weaker intensity links a nucleon with another one.

Many people might wonder about such questions as: How does a body like the Sun subdue another body like the Earth to its gravitational force? How does the Sun know about the presence of the Earth? Or how does an electron know of the presence of another electron?

A family of particles exists, called field particles, which is intimately associated to the four forces; these particles are the cause of the transmission of the force among the others.

Each of the two electrons creates an electromagnetic field, if they approach at a certain distance, and deviate through the emission and absorption of photons, which are the particles messangers of the electromagnetic field.

As everyone knows, to every particles corresponds an antiparticle, therefore theoretically we should consider also the antiparticles of a field, but practically we cannot distinguish the photons from the antiphotons, the gluons from the antigluons and so on.

This unimportance is due to the fact that the particles and the antiparticles of a field have the same values of the quantic numbers that identify them, such as the charge, the spin, the leptonic and the barionic number, strangeness etc.

To every force a messenger particle is associated: for the electromagnetic force we have the photons, for the gravitational we have the gravitons (whose existence so far has not been ascertained), for the weak interaction we have the W- Z0 W+ particles and, lastly, for the strong interaction we have the gluons.

The gluons and the W- Z0 W+ particles, unlike the photons and the gravitons, have a big quiet mass, a property that justifies their short action range.

The photons and the gravitons have a null quiet mass, consequently they interact on an infinite field.

The force that links the quarks, the strong interaction, is the strongest of all the others, so strong that the scientists have not been able to destroy a nucleon up to now, or any particle which contains some quarks.

In the powerful particle accelerator the scientists have never observed some free quarks, but they were able to observe the quarks bound together.

Six types of quarks, or flavours, have been identified so far: up (u), down (d), charm (c), strange (s), bottom (b), and top (t), the existence of which was proved only in 1997.

Many scientists have wondered if these four forces could be connected all together in one only , similarly to what Maxwell had done with electricity and magnetism.

In the course of the time many theories have been formulated to try a partial unification, but without any experimental proof.

Around the '60s the theoretical physics did not have a clear picture of all the four forces, except electromagnetism, which, with the formulation, in the '20s, of the quantistic electrodinamics (EDQ: it describes the interaction of the electromagnetic field with the matter) has been understood more than sufficiently.

They were not able to formulate a quantistic gravitational theory, because of the mathematic complexity; as regards the weak interaction, the particles messanger of their own field had not been discovered yet and some aspects could not be explained; as for the strong interaction, the situation was even more critic because the quark theory was not much accredited and understood yet.

Later a quantistic theory of the strong interaction was formulated, the quantistic chromodynamics (QCD) which describes its properties and the way in which it acts with the particles, and one of Weinberg and Salam's theories connecting the electromagnetic force with the weak force in the so called electroweak force.

Weinberg-Salam’s theory asserts that the electromagnetic force and the weak one are two different aspects of a single force.

At higher energy these two forces work in the same way; we observe two different forces because we live in a world with relatively low energy.

Under the supervision of Carlo Rubbia and Van der Meer this theory was proved in 1983 in the CERN particle accelerator making protons and antiprotons collide, and noticing the particles messangers of the weak force.

The electromagnetic force and the weak force were unified and the physicists started to hope to find a theory that unified the electroweak force with the strong one and later even with the gravitational one, obtaining in this way the native force, the superforce.

Different hypotheses were presented to try to unify the electroweak force with the strong one.

They were called Great Unified Theory (GUT) and they all differ just for some details.

So far, though, none has gained relevant success, also because presently we do not possess certain proofs confirming one of the GUTs.

In order to have a direct proof we should observe a force that acts in the same way as the electroweak force and the strong one and to obtain this fusion very high energies have to be obtained, which nowadays is not possible.

However, the theory can be verified through indirect proofs.

In fact, the GUT predicts that the proton is not stable, but that it has an average life of about 1031 years, a figure that depends, even if not much, from the GUT adopted and that this age is 1021 greater than that of the universe.

However we don’t know exactly the moment when a proton decays during its average life, because it is unforeseeable.

Scientists have carried out several experiments to find out if the proton is stable or not : the most famous took place in a salt mine at -600 metres under the Lake Erie, where 8000 tons of purified water were checked, thanks to some photomoltiplicator which revealed light impulses.

The experiments revealed no decay of the proton. This result disproofs the simplest GUT .

Other more complicate GUTs hypothesise a longer average life of the proton, but so far no experiments confirming their truthfulness have been carried out.

At the moment a partial unification of the forces has been generally hypothesised, mainly because it is impossible to obtain such high energy as to single out a superforce enclosing them all, but may be one day it will be possible to state that they are closely connected.

We live, as I said before, at relatively low energy, for this reason we see four different forces; if we lived at an energy high enough to fuse the weak force with the electromagnetic one, we would not be able to point out the difference, but we would be aware of three forces.

Instead, if we lived at an energy lower than the present one, we would probably be able to observe some more forces, perhaps discovering that the gravitational force is the fusion of two different forces.

In conclusion, we have walked a long way in understanding the nature of our world and its origin, but a longer one lies ahead of us.



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Il sapere attuale, il Modello Standard

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A. Pennisi, Bernardelli, D. Di Giovine, M. Scapinello, Dall’atomo ai quark,'atomop11.htm, Istituto statale di istruzione secondaria superiore Carlo Anti, Villafranca, 30/05/2001

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· Prof. Michelini, Il Modello Standard Delle Particelle E Delle Interazioni,, Dipartimento di Fisica dell’Università di Udine

· Massimo Pietroni, Nel Mondo Delle Particelle,, Istituto Nazionale Fisica Nucleare, Sezione di Padova



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