Reference is pointing out that, contrary to al-Biruni's astronomy or astrology works, on which he wrote separate treatises, there does not exist a single book devoted exclusively on physics, but it is necessary to read all the books to evaluate his physical researches. And then, after such a research, we find what al-Biruni thought on heat and light.

Aristotle considered heat to be a fundamental quality of the element fire and inherent in all things. There are two types of heat, by which the bodies can be heated: internal or external. Starting from the Aristotle’s works, Al-Biruni came to the conclusion that “heat is nothing but the rays of the Sun detached from the body of the Sun towards the Earth”. And then, “the heat exists in the rays, it is inherent in them”. As observed, the natural conclusion would be that air is heated by the Sun, but al-Biruni tells that “the warmth of the air is the result of the friction and violent contact between the sphere, moving rapidly, and his body”. This is an Aristotelian manner of thinking. In any case, al-Biruni had the merit of understanding the connection between motion and heat, the same we find in the Kinetic Theory of heat.

And heat and rays were the subjects of several letters of a correspondence between al-Biruni and Ibn Sina, Avicenna, and there we find that the heat is generated by the motion and cold by the rest, and for this reason, the Earth is hot at the Equator and cold at the Poles. Another important discussion between the two scientist was on the propagation of heat and rays of Sun. Al-Biruni’s opinion was that that light and heat are immaterial, and that the heat exists in the rays and it is inherent in them. How is therefore the propagation of heat? After this al-Biruni’s question, Avicenna answered that the heat was not propagating by itself, but the rays of the Sun are propagating, and the heat is carried by them, like a man in a boat, which is not moving, but his boat is moving. A very interesting discussion between two outstanding persons.

This problem of the propagation of heat leads al-Biruni to study the problem of the nature and propagation of light. He stated that “there is a different opinion regarding the motion of the rays. Some say, this motion is timeless, since the rays are not bodies. Others say, this motion proceeds in very short time: that, however, there is nothing more rapid in existence, by which you might measure the degree of its rapidity, e.g. the motion of the sound in the air is not so fast as the motion of the rays, therefore the former has been compared with the latter and thereby its time (the degree of its rapidity) has been determined”. According to al-Biruni's experience, this is the first reference to the problem of  measuring the speed of light.

Source: Muslimheritage

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