These notes cover the development of the current scientific concepts of space and time through history, emphasizing the newest developments and ideas. The presentation will be non-mathematical: the concepts will be introduced and explained, but no real calculations will be performed. The various concepts will be introduced in a historical order (whenever possible), this provides a measure of understanding as to how the ideas on which the modern theory of space and time is based were developed. In a real sense this has been an adventure for humanity, very similar to what a child undergoes from the moment he or she first looks at the world to the point he or she understands some of its rules. Part of this adventure will be told here.
Every single culture has had a theory of the formation of the universe and the laws that rule it. Such a system is called a cosmology (from the Greek, kosmos: world, and logia from legein: to speak). The first coherent non-religious cosmology was developed during ancient Greece, and much attention will be paid to it after a brief overview of Egyptian and Babyonian comologies . The system of the world devised by the Greeks described correctly all phenomena known at the time, and was able to predict most astronomical phenomena with great accuracy. Its most refined version, the Ptolemaic system, survived for more than one thousand years.
These promising developments came to a stop during the Middle Ages, but took off with a vengeance during the Renaissance; the next landmark in this saga. During this time Copernicus developed his system of the world, where the center of the Universe was the Sun and not the Earth. In the same era Galileo defined and developed the science of mechanics with all its basic postulates; he was also the creator of the idea of relativity, later used by Einstein to construct his Special and General theories.
The next great player was Isaac Newton, who provided a framework for understanding all the phenomena known at the time. In fact most of our daily experience is perfectly well described by Newton's mathematical formulae.
The cosmology based on the ideas of Galileo and Newton reigned supreme up until the end of the 19th century: by this time it became clear that Newton's laws were unable to describe correctly electric and magnetic phenomena. It is here that Einstein enters the field, he showed that the Newtonian approach does not describe correctly situations in which bodies move at speeds close to that of light ( in particular it does not describe light accurately). Einstein also provided the generalization of Newton's equations to the realm of such high speeds: the Special Theory of Relativity. Perhaps more importantly, he also demonstrated that certain properties of space and time taken for granted are, in fact, incorrect. We will see, for example, that the concept of two events occurring at the same time in different places is not absolute, but depends on the state of motion of the observer.
Not content with this momentous achievements, Einstein argued that the Special Theory of Relativity itself was inapplicable under certain conditions, for example, near very heavy bodies. He then provided the generalization which encompasses these situations as well: the General Theory of Relativity. This is perhaps the most amazing development in theoretical physics in 300 years: without any experimental motivation, Einstein single handedly developed this modern theory of gravitation and used it to predict some of the most surprising phenomena observed to date. of the most surprising phenomena observed to date. These include the bending of light near heavy bodies and the existence of black holes, massive objects whose gravitational force is so strong it traps all objects, including light.
These notes provide an overview of this saga. From the Greeks and their measuring of the Earth, to Einstein and his description of the universe. But before plunging into this, it is natural to ask how do scientific theories are born, and why are they discarded. Why is it that we believe Einstein is right and Aristotle is wrong? Why is it that we claim that our current understating of the universe is deeper than the one achieved by the early Greeks? The answer to these questions lies in the way in which scientists evaluate the information derived from observations and experiments, and is the subject of the next section.