PHYSICS

In medieval science, as for the Greeks, physics included the study of "all things that change" or, to use Aristotelian terminology, of all things in the world of generation and corruption. In the Islamic world the study of physics (Tabī'īyāt), more than that of any other science, followed in its fundamental lines the teachings of Aristotle. Most of the problems posed by Muslim philosophers and scientists in this field were set within the framework of the doctrines of form and matter, of power and act, of the four causes and of teleology. Aristotle was not, of course, followed in every detail, especially on the question of motion. Many Muslim authors, following the example of Giovanni Filopono, were severely critical of Aristotle and formulated various new concepts, such as that of impetus, which played an important role in the changes that would occur later in the whole structure of physics in West.
There were also anti-histotelic philosophers, such as Rhazes, whose approach to the study of nature differed substantially from that of the Stagirite. Since such critics, however, usually adopted the hermetic and alchemical perspective, we can not classify their doctrines as physics in the sense in which the term is understood in peripatetic or modern science. There were also the illuminationists, who, like Plotinus, built a physics based on the symbolism of light; even they, strictly speaking, have much in common with physicists, but rather with the "theosophists" and with the Gnostics, whose perspectives generally shared.
Many of the "new" ideas concerning time, space, the nature of matter, light, and other fundamental elements of medieval physics came not from the philosophers, who were above all related to the ideas of their Greek predecessors, but rather from theologians, who they usually opposed the Peripatetics. In the writings of theologians such as Abū'l-Barakāt al-Baghdādī, of Fakhr al-Dīn al-Rāzī and MuHammad al-Bāqillānī, who may be considered the "philosopher of Nature" of the dominant Asharite school of Sunni theology, there were doctrines of considerable interest. Theologians turned away from the path of the Peripatetics and became the founders of a distinct worldview. Although they were linked, as theologians, to those problems connected with faith, they were not limited to the premises of Peripatetic philosophy and were therefore among the most severe critics of Aristotelian physics, much of which they rejected in favor of a different conception of time, of space and causality.
The study of physics among both philosophers and theologians was based on reasoning, and did not generally depend on direct observation. Unlike the later centuries, therefore, in the medieval period it was not the rationalists, but the Gnostics and the alchemists, who appealed to the direct observation of Nature. And yet, for the last group, the external and physical aspects of things did not serve as data for rational analysis, but rather as an opportunity for insight and "reminiscence"; the phenomena of Nature were symbols for them, not simply facts.
There was also a third group that observed and performed experiments, and in this way tried to analyze the meaning of the sensitive aspects of Nature. In this group there were several important scholars of optics, such as Qutb al-Dīn al-Shīrāzī, and the most famous among all the Muslim physicists, Alhazen, and also al-Bīrūnī, who determined the specific gravity of some minerals, and Abū 'l-Fatá' Abd al-Raámān al-Khāzīnī, who also dealt with the measurement of density and gravity. This kind of physics, which resembles the works of Archimedes - at least in the approach, if not always in the techniques and results -, is very interesting from the point of view of modern science, whose one-sided approach to Nature is based on an some similar way. But from the point of view of Islamic civilization such studies, as well as those with automata and various types of machines, occupy a secondary and peripheral position in the total scheme of knowledge. They should always be considered in this way, therefore, if medieval Islamic civilization is to be seen in its own perspective. Transforming the periphery into the center and the center in the periphery would be tantamount to destroying the fundamental relationships on which the harmony of the sciences of the medieval world was based. Studies such as the Optics of Alhazen, which from the modern point of view of the "progressive development of science" may seem of the utmost importance, have never been at the center of Islamic intellectual life, which has focused its interest on the immutable aspects more than on the changeable ones of cosmic manifestation. These studies are certainly very interesting for Islamic science, but they should never be considered synonymous with it.
Alhazen is undoubtedly the greatest scholar of optics between Ptolemy and Witelo. He was a distinguished mathematician and astronomer and also a philosopher, as well as being a physicist whose results have led some modern authors to consider him the greatest of medieval physics scholars.
Alhazen gave significant contributions to the study of motion, in which he discovered the principle of inertia, celestial physics and the science of statics, but above all he transformed the study of optics into a new science. Before him the Muslim scientists knew Euclid's optics, with the commentary of Theon, the works of Heron and Archimedes, the studies on the curved mirrors of Antenio and the remarkable studies on the refraction of Ptolemy. Euclid's optics, in fact, was known in the West through al-Kindī's commentary in De Aspectibus. Even Muslim doctors like Hunain ibn Ishāq and al-Rāzī studied the eye independently, but in general the Greek sources were more or less followed.
Obviously, Alhazen also depended on such sources, from Euclid and from Ptolemy, from the Meteorology of Aristotle and from the Conics of Apollonius, but he transformed the basis of the study of optics and made it a well-ordered and definite discipline. He combined elaborate mathematical treatment with well-designed physical models and accurate experimentation. Like Archimedes, he was both a theoretical and an experimental physicist. He made experiments to determine the rectilinear motion of light, the properties of shadows, the use of lenses, the camera obscura, which he mathematically studied for the first time, and many other essential optical phenomena. He also owned a lathe, with which he built lenses and curved mirrors for his experiments.
In the catoptric, in which the Greeks had already made important discoveries, Alhazen's notable contribution was in the study of spherical and parabolic mirrors. He studied spherical aberration and realized that in a parabolic mirror all the rays are concentrated in one point, so that it is the best type of ustorious mirror. The problem of Alhazen in Optics is in fact connected to the reflection on a spherical surface: from two points on the plane of a circle draw lines that intersect at a point on the circumference and which form equal angles with the normal at that point. This leads to a fourth degree equation, which he solved with the intersection of a hyperbole and a circle.
In the field of refraction his contributions are more eminent. He applied the rectangle of velocities to the surface of refraction several centuries before Newton, and believed in the principle of "minimum time". He made careful experiments by immersing a graduated cylinder in water to measure the angle of refraction. Although familiar with the function of the breast, Alhazen preferred to work with cords; otherwise he would have discovered Snell's law, which he discovered for small angles, where the angle itself can be replaced roughly at the breast. He also studied refraction through glass cylinders and spheres, and tried to determine the magnifying effect of plane-convex lenses.
The third field of optics in which Alhazen made significant discoveries was that of atmospheric phenomena. Here he determined the extent of atmospheric refraction by measuring the distance of a fixed star from the pole to the time of its rising and to the zenith with the help of an armilla. The phenomena of dawn and twilight and the apparent change in the size of the Sun and the Moon on the horizon also aroused great interest in him, and he explained them after doing a very thorough analysis. He established that twilight ends when the Sun is 19 ° below the horizon. He also showed great interest in rainbows and, while not applying the refraction to them, he explained the rainbow on the basis of the principle of reflection more fully than Ptolemy.
Finally, among his contributions one must mention the study of the physiology of the eye and the problem of vision. Like his contemporaries Avicenna and al-Bīrūnī, Alhazen believed that in the process of vision light goes from the object to the eye. He also analyzed the function of the eye as a lens and tried to unveil the mystery of vision by combining his knowledge of physics and medicine. His study of the physiology and diseases of the eye belongs to both the history of Islamic medicine and that of optics itself.
After Alhazen in the Muslim world there was a decline in the study of optics, so much so that in the 6th / 12th century even a great scientist like Nasīr al-Dīn al-Tūsī did not know his contributions. Only in the 7th / 13th centuries, almost certainly following the influence of Suhrawardī's philosophy of illumination, the study of optics became popular again and in fact a new branch of science called the science of the rainbow arose in Persia. Qutb al-Dīn al-Shīrāzī, who was also a commentator on Suhrawardī, gave the first correct qualitative explanation of the rainbow, stating that it is caused by both reflection and refraction. His disciple Kamāl al-Dīn al-Fārsī wrote a commentary on Alhazen's masterpiece in optics, the Optics (Kitāb al-manāüir), and he brought the study of optics to his last brilliant period in the Muslim world. Meanwhile Alhazen's writings were becoming well known in the West, and particularly his Optics had a profound effect on every scholar of this discipline. His magnum opus, Opticae Thesaurus, in Latin, was printed in the X / XVI century and its influence is visible in Kepler's optical studies.
A contemporary of Alhazen, but originally from the eastern part of the Islamic world, in eastern Persia, al-Bīrūnī was perhaps the greatest compiler and scholar in this fruitful period of Islamic history, and had a knowledge of the geography, chronology and comparative religions that remained unsurpassed in the Islamic world.
He was also the most eminent astronomer and mathematician of his time: his Elements of astrology remained for centuries a textbook in the teaching of the Quadrivium, while his main astronomical work, the Qānūn al-Mas'ūdī, is undoubtedly the broader text of Islamic astronomy. Some of his other astronomical works contain parameters of Babylonian astronomy that do not appear in some still existing Greek works.
Al-Bīrūnī also made a thorough study of philosophy and physics. Although most of his philosophical works have been lost, there is little doubt that he has opposed many points at the Peripatetic school. In his letters to Avicenna, who fortunately survived, al-Bīrūnī discusses and criticizes, with his usual lucidity, some of the fundamental principles of Peripatetic physics that were dominant in the teaching of most of the schools of the time. He demonstrates considerable autonomy with respect to Aristotelian philosophy, and is severely critical of various points in peripatetic physics, such as the question of time and space, which he attacks not only by appealing to reason but also through the use of observation .
Al-Bīrūnī was also very interested in the question of the possible motion of the Earth around the Sun, and he also wrote a book about it, which was lost. As an astronomer, he realized that this question was not a problem of astronomy but of physics. He then directed the attention of physics scholars to the problem, and he himself studied the physical implications of the heliocentric system. At the end of his life, after many years of neutrality on this question, he finally decided in favor of the geocentric system, not for astronomical reasons, but because the physics of heliocentrism seemed impossible to him.
A series of notable physicists followed Alhazen and al-Bīrūnī and continued their studies especially in mechanics, hydrostatics and similar branches of physics. He also continued to criticize the theory of the motion of the projections of Aristotle along lines established by Avicenna, which led to the important studies of Avempace and other later Muslim philosophers and scientists, who exerted a great influence on Latin medieval mechanics. In this field, Muslim scientists developed the theory of "inclination", and laid the foundations of the theory of impetus and the concept of moment, which were further elaborated by later medieval scientists in the West. Moreover, Avempace's attempt to quantify the motion of the projectiles by considering the velocity proportional to the difference between strength and resistance rather than their relationship is very important in light of the later attempt by Bradwardine and the Mertonian school to describe the motion quantitatively.
Among the Hindu physical Muslims, one of the most important is Abū 'l-Faøf' Abd al-Rahmān al-Khāzinī, originally a Greek slave who flourished in Merv at the beginning of the 6th / 12th century, and who continued the study of mechanics and of hydrostatics in the tradition of al-Bīrūnī and earlier scientists. He also wrote various works of astronomy and physics, including the Book of the Balance of Wisdom, which is perhaps the most important Muslim work on mechanics and hydrostatics, and especially on the study of barycenters. Muslim scientists had from the beginning familiarity with the writing of Heron on the rise of heavy things, which itself reflects some influence of Archimedes. And although there is still no evidence of an Arabic translation of the Pseudo-Aristotelian Mechanics or the Equilibrium of Archimedes' Plans, among the Muslim physicists there is the influence on the static works of both the works and of both schools. The Liber Karatonis of Thābit ibn Qurrah very soon demonstrates the influence of these Greek schools, and it is very interesting that in this work Thābit ibn Qurrah tries to derive the law of the lever from dynamical rules following the pseudo-Aristotelian tradition , with an emphasis on dynamics and barycenters that was in contrast with Archimedes's approach.
The interest in mechanics and especially in the laws of simple machines is also found in the writings of the Banū Mūsā and in some of the apocryphal treatises attributed to Avicenna, while the study of hydrostatics was cultivated with great success by al-Bīrūnī and also by 'Umar Khayyām. Al-Khāzinī marks further development in this school. He combined his interest in hydrostatics with that for mechanics and focused particularly on the concept of center of gravity in its application to the balance. It was followed in his efforts a century later by Abū'l-'Izz al-Jazarī, whose book of knowledge of ingenious geometrical devices is the definitive work of mechanics in the Islamic world. He was followed in turn by Qayöar al-Hanafī, who was particularly expert on the mechanics of the water wheel. It was he who built the famous celestial globe preserved today at the National Museum of Naples.
Muslims, as they made the study of the rainbow a separate science, thus created a separate science of the balance, in which al-Khāzinī was the undisputed master. His Book of the Balance of Wisdom is the main work in this science, in which he discusses the opinions of earlier scholars, including al-Rāzī, Khayyām and al-Bīrūnī. It is particularly interesting that al-Khāzinī describes an instrument which, according to him, al-Bīrūnī used in his famous determinations of the specific weights of various substances, since al-Bīrūnī himself never revealed the method by which it came to its results. .
Al-Khāzanī provides a detailed exposition of the balance theory, the centers of gravity and the general way of applying the balance, in order to determine the specific weight of bodies composed of one or two substances. The choice we present below from the Book of the Balance of Wisdom - whose title itself is a reminiscence of the cosmic balance of al-Jabirian alchemy, but is applied here specifically to physical problems - demonstrates the sophistication that the use of the balance reached among physicists Muslims.
The modern reader might wonder about men such as Alhazen, al-Bīrūnī or al-Khāzinī, what their reactions would be to modern science. Would they consider this type of science the continuation and improvement of what they started or - as modern historians usually express - an example of the "progress of ideas"? The difficulty in answering the question in modern terms is that today historical time has assumed a quantitative meaning, while the qualitative nature of history itself has been almost forgotten. In fact, even a physicist like Alhazen lived in a completely different spiritual and psychological environment than the modern optician. In the world in which he lived, the phenomena of Nature were not yet completely separated from their archetypes: the light still reminded man of the divine intellect, even if he did quantitative experiments with it. One may also ask whether Alhazen, if he lived in our century, would have become a modern physicist. The answer is that, because in time there is something "definite" and "absolute" - that is, the fifth / eleventh century is qualitatively different from ours -, historical time is not the reversible time of classical physics, and Alhazen of the fifth / eleventh century could not be meta-physically the same being, with the same powers and faculties, if it were suddenly placed in the twentieth century.
If, however, the idea of ​​bringing Alhazen or al-Bīrūnī into the twentieth century could be realized, the most likely reaction of these men to modern science would be a surprise reaction to the position that quantitative science has come to occupy today. . Alhazen and al-Bīrūnī were able to practice a type of science that could be defined as "progressive", while continuing to remain within a "non-progressive" world view, because for them all the scientia was subordinate to knowledge. Their quantitative science was only an interpretation of a segment of Nature, not the interpretation of its totality. The matrix of their vision of the world remained immutable, even when they pursued their study of the world of becoming and of change. The surprise that medieval Muslim nature scientists would try, if they were placed before modern science, would not stem from the recognition of the "progress" of the ideas they had begun, but from seeing the complete reversal of relationships. They would see that the center of their perspective was made peripheral and that the periphery became central; would be amazed to learn that "progressive" science, which in the Islamic world has always remained secondary, has now become almost everything in the West, while the unchanging and "non-progressive" science or wisdom that was then primary has now reduced to almost nothing.

[Excerpts from: Seyyed Hossein Nasr, Science and civilization in Islam, Irfan Edizioni - courtesy of the publisher]