la scienza e gli ostacoli di ieri e di oggi

Di

Valentina Vergani

Liceo linguistico G. Parini Barzanò

Anno scolastico 2008-2009

Indice

• Introduzione pag. 3-4
• Astronomy and its limits pag. 5-11
• Il limite euclideo pag. 12-19
• Epistemologia nel XX secolo pag. 20-27
• La bomba atomica pag. 28-32
• Bioetica pag. 33-34
• Cellule staminali pag. 34-38
• Conclusione pag. 39
• Bibliografia & sitografia pag. 40

Introduzione

“La scienza e gli ostacoli di ieri e di oggi” è il titolo che ho scelto per poter parlare dei limiti che questa disciplina ha dovuto affrontare nel corso della storia e che tuttora ha sul suo cammino.

La parola limite sta ad indicare sia il punto o il grado estremo che può essere raggiunto, sia il punto di passaggio ad una condizione diversa da quella considerata normale. È proprio su questa duplicità di significato che si vuole concentrare la mia tesina, riferendo però il concetto di limite ad un ambito ben preciso, che è quello della scienza. Questa disciplina, infatti, è stata la compagna infaticabile dell’uomo nel corso della storia intera. L’ha accompagnato durante tutto il suo cammino verso il progresso, un cammino di cui tuttora non vediamo il traguardo. La scienza è la materia che più di tutte le altre è stata capace di spiegare all’uomo i misteri del mondo che lo circonda e allo stesso tempo di svelare il mistero del corpo e della mente umani, è stata in grado di sperimentare tutte quelle tecnologie senza le quali la vita di oggi non sarebbe come la conosciamo.

Nonostante tutto ciò, sempre più spesso, sentiamo parlare dei limiti che la scienza si trova ad affrontare. A volte si tratta di limiti che la scienza stessa deve porsi, riconoscendo la sua quantomeno momentanea incapacità di andare oltre. Altre volte, invece, sono dei limiti posti da discipline diverse come la morale o la religione, che difendendo i propri valori, impediscono il proseguire della ricerca scientifica.

Il limite, in generale, è qualcosa che deve spronare gli uomini a fare del proprio meglio per superarsi e la scienza in questo senso non fa eccezione, al contrario è la disciplina che forse meglio delle altre spinge ad un continuo superamento di se stessa. La storia ci insegna come tanti risultati siano stati raggiunti proprio grazie a grandi uomini che non si sono fermati di fronte a un limite, che altri prima di loro avevano posto, cosa che disse Einstein stesso in uno dei suoi più famosi aforismi: “Tutti sanno che una cosa è impossibile da realizzare, finché non arriva uno sprovveduto che non lo sa e la inventa”. Queste grandi scoperte hanno permesso indubbiamente di fare dei grandi passi in avanti. È il caso dell’introduzione delle geometrie non euclidee, le quali nascono come superamento della geometria di Euclide che per secoli era stata considerata l’unica geometria possibile. È anche il caso dell’introduzione del sistema eliocentrico, a scapito di quello geocentrico sostenuto per secoli da grandi studiosi fra cui Aristotele e in seguito dalla dottrina cristiana stessa. Ed è proprio su questo aspetto che si è concentrata la speculazione filosofica di molti autori del ‘900 fra cui Popper, Kuhn, Lakatos.

Ci sono però dei limiti che, se superati, possono portare l’uomo e la scienza stessa alla distruzione. Ciò accade quando gli uomini, riconoscendo l’enorme potere che deriva dalla ricerca, pretendono di utilizzarlo esclusivamente a proprio vantaggio, disinteressandosi degli effetti che si potrebbero causare alla vita degli altri. È il caso della bomba atomica che pur essendo una delle più grandi scoperte scientifiche mai avvenute, è da tutti considerata come una delle più grandi disgrazie mai provocate dall’uomo. È il caso oggi di molte ricerche fra cui quella inerente le cellule staminali, grande speranza del futuro, ma anche grande timore del presente. Ed è proprio questo il messaggio contenuto nella lettera scritta da Einstein sia agli scienziati a lui contemporanei, che ai posteri:

“...Vediamo oggi delinearsi, per l’uomo di scienza, un tragico destino. Sostenuto dalle sue aspirazioni alla chiarezza e all’indipendenza esteriore, egli ha, con uno sforzo quasi sovrumano, forgiato da se stesso le armi del suo asservimento sociale e dell’annientamento della sua personalità.

Egli deve piegarsi al silenzio di chi detiene il potere politico, ed è costretto, come un soldato, a sacrificare la propria vita, e ciò che è peggio a distruggere quella degli altri, anche se è convinto dell’assurdità di un tale sacrificio. È tutto concatenato, in questo sinistro svolgersi d’avvenimenti. Ogni passo si presenta come inevitabile conseguenza del precedente. Al termine del cammino, si profila sempre più distinto lo spettro della distruzione completa. Noi non possiamo cessare di ammonire ancora e sempre; non possiamo rallentare i nostri sforzi per dare coscienza alle nazioni del mondo, e soprattutto ai loro governi, dell’immagine del disastro che essi debbono esser certi di provocare se non cambieranno atteggiamento gli uni verso gli altri, e la loro maniera di concepire il futuro. Il nostro mondo è minacciato da una crisi la cui ampiezza sembra sfuggire a coloro che hanno il potere di prendere grandi decisioni per il bene e per il male. La potenza scatenata dall’atomo ha tutto cambiato, salvo il nostro modo di pensare, e noi stiamo scivolando così verso una catastrofe senza precedenti. Perché l’umanità sopravviva un nuovo modo di pensare è indispensabile. Allontanare questa minaccia è divenuto il problema più urgente del nostro tempo”.

Perciò, il mio percorso si pone come obiettivo quello di analizzare alcuni esempi di limiti della scienza e le conseguenze che il loro superamento, in bene o in male, hanno avuto nel corso della storia per poi affrontare un esempio di limite odierno della scienza, di cui ancora non è possibile prevedere gli effetti.

Astronomy and its limits: from the myth to heliocentrism

The first hypothesis about the structure and the origin of the universe are to be found in mythology: in fact the relationship between astronomy and religion, that will last for ages, is justified by the mystery that the universe represented for the primitive men. The sky was considered the reign of the supernatural and divine because it was not accessible. So, the first civilizations tried to establish links among the heavenly phenomena and the human ones in order to explain some of the mysterious events that surrounded them.

The first astronomers were the Sumerian and Babylonian priests that 5000 years ago observed the movements of the Sun, the Moon and the planets and their differences with the stars. They found out that the eclipses repeat periodically but they could not make their own cosmological model. Also the Egyptians were interested in astronomy and they formulated a solar calendar, dividing the day in 24 hours and the hours in 60 minutes, but they produced only myths without real basis.

The Greeks have been great astronomers because they improved both the instruments to observe the Universe and their mathematical knowledge. They elaborated few hypotheses about the origin and the structure of the Universe but since the VIIth century B.C. they created and believed in a perfect and ordinate system in which the Earth was the centre and the dominant part. At that time, the Greeks still thought that the Earth was a plate disk and that the Sun had the dimension of the Peloponnesus. Pythagoras and his school in the Vth century B.C. found out that the Earth is spherical but they also believed that the Universe was formed by ten (perfect number) celestial bodies: the Earth, the Moon, the Sun, the five planets already known (Mercury, Venus, Jupiter, Mars and Saturn), the fixed stars and they added an invisible planet called Anti-Earth to obtain 10 as a result. However, a real model was created and theorised only a few centuries after, in the IInd century A.D. by Claudius Ptolemy; it was described in his work “Almagesto”. This was the geocentric model.

Geocentric model

The geocentric model (in ancient Greek: geo = earth and centron = centre) is a theory which places the Earth at the centre of the universe. Even if nowadays modern science rejects this model, it has dominated ancient but also medieval astronomy.

This theory became more credible after the discovery of the approximately spherical shape of the Earth and great intellectuals like Aristotle and Ptolemy believed in that.

According to the early astronomers, it was evident that the “rest of the universe”, as for example the Sun, the Moon and the others planets… moved around the motionless Earth. In fact stars, the Sun and the Moon could be seen moving around the Earth following circular paths each day. So, it was perfectly reasonable for them to say that also the Earth was stable and motionless, because nothing could make it move. Then, the fact that objects, like meteorites, fall on the Earth was seen as another proof of this model: everything is “attracted” by the centre of the universe.

This model, that was theorised by the Greeks, continued to be accepted also during the Middle Ages because it was in accordance with the theocentric (god-centred) world view dominant in that period.

The geocentric model asserts that all the bodies of the Universe follow perfect circular paths. This choice was not done accidentally. In fact the circle was for the mathematicians and philosophers of the ancient Greece the perfect geometric figure, and for this reason it was the only figure appropriate to describe celestial motions. They theorised the existence of 55 spheres around which the Sun, the Moon and all the others stars move (called deferents).

However, astronomers observed that the patterns covered by celestial bodies were not constant. For example the path of the Moon changed each month and the configuration of the constellations changed according to the season. These changes were explained by the variation of speed at which the celestial bodies revolved around the Earth.

However there was another problem: the planets behaved in ways that were difficult to describe and they obtained the name planets because of this reason (in ancient Greek the word “planetes” meant wanderer). In fact, sometimes they showed retrograde motions; that means that they seemed to stop and start to move in reverse direction if compared to the position of fixed stars, which did not move.

To overcome this new problem, Greek astronomers increased their efforts, that culminated in the work of Claudius Ptolemy (90-168 A.D.). They managed to create new complicated models in which planets moved on circular paths called epicycles that were superimposed on circular orbits about the Earth, that are the deferents. So, a “personal” deferent and epicycles was given to each planet. They were able to describe the particular motion and speed of the single planet.

Mercury and Venus were placed between the Moon and the Sun; instead, Mars, Jupiter and Saturn were placed beyond the Sun.

Ptolemy changed another important thing: he displaced the Earth from the centre of the universe and he said that the spheres revolved around a point called eccentric, the geometric centre of the Universe, that was near the Earth but not on it. Thanks to the improving in the methods of observation and measurement, the models continued to become more and more complicated, in order to overcome the contradictions that appeared constantly between the theory and the observations.

This model has survived despite all the problems and the continuous variations for 14 centuries and there are some reasons for that: the first is the fact that there are not evident proof of the movements of the Earth if we don’t use instruments of observation. Then, ancient and medieval astronomers were not able to measure time precisely, so they could not compare the data and, at the same time, they were not able to see and calculate the movements of the stars (that is the proof of the movements of the Earth) because they did not have the right instruments to do that. Finally, as said before, the Christian religion adopted this theory that could perfectly explain its theocentric world view and placed humankind in a privileged position in the Universe (men are the Creatures of God). Those who tried to underline and overcome the problems of the geocentrism were judged heretics and because of that, also heliocentrism could not accepted at first.

At the end of its long life, the geocentric model became so complicated that the King of Castile, Alfonso X (1221-1284) said ironically that if God had asked his advice while creating the Universe, he would have suggested a simpler design of that. During the XVIth century, this theory started collapsing: it was time to change, it was the time of heliocentrism.

Heliocentric model

In astronomy, heliocentrism is the theory according to which the Sun is at the centre of the Universe. The word comes from the Greek (helios = sun and kentron = centre). It was proposed for the first time in XVIth century by the Polish astronomer and mathematician, Nicolaus Copernicus, and then it was better elaborated and expanded by Kepler.

But the heliocentric model was not a creation of Copernicus. In fact, Aristarchus, a Greek astronomer and mathematician, had already theorised a model similar to this one. We have nowadays only little information about his theory, thanks to a passage taken from “Concerning the face which appears in the orb of the moon” written by Plutarch. One of the characters talking in there, the philosopher Cleanthes, openly criticised Aristarchus and his model saying that he should be charged with impiety for “having moved the Hearth”. But at that time, Aristarchus attracted very little attention and the heliocentric model was abandoned till Copernicus revised and rielaborated it a few centuries after.

In 1543 the publication of the “De revolutionibus orbium coelestium” written by Copernicus challenged the geocentric model threatening his future existence. This model asserted that the Earth and the other planets revolve around the Sun. The circular movements of the planets were maintained (Kepler will assert that planets have an elliptical orbit only later). At first, this new model was not accepted even if it gave a simpler explanation to the retrograde motions of the planets and of the variation of their distance from the Earth because it posed big problems for both natural science and Scripture. So, for many years, the geocentric system was still held.

The invention of the telescope in 1609 made by Galileo Galilei, who also made many others discoveries like the satellites of Jupiter, or the presence of mountains and craters on the surface of the Moon, threatened again the principles of geocentrism. In December 1610, thanks to the telescope, Galilei found out that Venus showed all the phases, as the Moon. But this observation was completely incompatible with the Ptolemaic system, while it was a proof and a direct consequence of the heliocentric system proposed by Copernicus. In fact, according to Ptolemy, Venus’ deferent and epicycle were inside the sphere of the Sun; that means that if Venus was between the Earth and the Sun, its phase should have always been crescent or all dark. Instead, if Venus was beyond the Sun, its phase should have always been gibbous or full. But Galilei saw Venus, at first full, and then crescent. This was the proof that Venus’ epicycle could not be entirely inside or outside the orbit of the Sun.

Then, Galileo discovered that our galaxy, the Milky Way is made of millions of stars that are more distant to the Earth than the planets. As a result, Ptolemaics abandoned some of their principles and started believing in a new model: the Tychonic system. According to this theory, the Earth was still the centre of the universe and the Sun revolved around it, but all the other planets revolved around the Sun. But the time of geocentrism was at that point finished.

In 1619, based on heliocentric model and on the fact that planets move on elliptical paths, Johannes Kepler constructed his three laws and on the fact that planets move on elliptical paths. These laws are:

• The orbit of every planet is an ellipse and the sun is in one of the focuses
• A planet covers equal areas in equal period of time
• The square of the orbital period of a planet is proportional to the cube of its distance from the Sun

Using these laws, Kepler was the first astronomer to predict a transit of Venus (for the year 1631). A transit of Venus happens when Venus passes between the Sun and the Earth, obscuring a small part of the Sun. During a transit, Venus is seen from the Earth as a small black disk moving across the Sun. In 1687, Isaac Newton asserted his law of universal gravitation, according to which, the Earth and all the other planets are kept together by a force called gravitation. After this discovery, scientists were finally able to construct a plausible heliocentric model of the solar system and to demonstrate it empirically.

But the biggest problem that the heliocentric model had to face in order to be accepted, was the reactions of the religious authorities. Galileo defended heliocentrism saying that it was not contrary to the Bible. He said that it’s important while reading the Scripture not to take every single word literally, because it is a book of poetry and songs and not a book of history and this was also the attitude suggested by St. Augustine. He wrote “Dialogue concerning the two chief world systems”, his best work, published in 1632, in which he compared the two models, the geocentric and the heliocentric.