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Since antiquity, human beings have sought to understand the workings of nature: why unsupported objects drop to the ground, why different materials have different properties, the character of the universe such as the form of the Earth and the behavior of celestial objects such as the Sun and the Moon, and so forth. Typically the behavior and nature of the world was explained by invoking the actions of gods. Eventually explanations were proposed based on philosophical speculation. Rarely verified by systematic experimental testing, many of them were wrong, but this is part of the dialectical nature of scientific enquiry, and even modern theories of quantum mechanics and relativity are merely considered "theories that have not been broken yet".
The growth of physics has brought not only fundamental changes in ideas about the material world, mathematics and philosophy, but also, through technology, a transformation of society. Physics is considered both a body of knowledge and the practice that makes and transmits it. The Scientific Revolution, beginning about year 1543, is a convenient boundary between ancient thought and classical physics. The emergence of physics as a science distinct from natural philosophy began with the scientific revolution of the 16th and 17th centuries, and continued through the dawn of modern physics in the early 20th century. The year 1900 marks the beginnings of a more modern physics. Today, the science shows no sign of completion, as more issues are raised, with questions rising from the age of the universe, to the nature of the vacuum, to the ultimate nature of the properties of subatomic particles. Partial theories are currently the best that physics has to offer, at the present time. The list of unsolved problems in physics is large.
Early cultures
Babylonian contributions
Babylonian astronomy was the basis for much of what was done in Greece, in India, in Sassanian Iran, in Byzantium, in Syria, in Islam, in Central Asia, and in Western Europe.[1]
The first evidence of recognition that astronomical phenomena are periodic and of the application of mathematics to their prediction is Babylonian. Tablets dating back to the Old Babylonian period document the application of mathematics to the variation in the length of daylight over a solar year. Centuries of Babylonian observations of celestial phenomena are recorded in the series of cuneiform tablets known as the Enūma Anu Enlil. The oldest significant astronomical text that we possess is Tablet 63 of the Enūma Anu Enlil, the Venus tablet of Ammi-saduqa, which lists the first and last visible risings of Venus over a period of about 21 years and is the earliest evidence that the phenomena of a planet were recognized as periodic. The MUL.APIN, contains catalogues of stars and constellations as well as schemes for predicting heliacal risings and the settings of the planets, lengths of daylight measured by a water-clock, gnomon, shadows, and intercalations. The Babylonian GU text arranges stars in 'strings' that lie along declination circles and thus measure right-ascensions or time-intervals, and also employs the stars of the zenith, which are also separated by given right-ascensional differences.[2]
Egyptian contributions
An early Egyptian interest in astronomy is demonstrated by the development of a stellar-based calendar than can be traced back as far as 4236 BC.[3]
Greek and Hellenistic contributions
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Western physics began with eminent Greek pre-Socratic philosophers such as Thales, Anaximander, possibly Pythagoras, Heraclitus, Anaxagoras, Empedocles and Philolaus, many of whom were involved in various schools. For example, Anaximander and Thales belonged to the Milesian school.
Plato, briefly and Aristotle at length, continued these studies of nature in their works, the earliest surviving complete treatises dealing with natural philosophy. Democritus, a contemporary, was of the school of Atomists who attempted to characterize the nature of matter.
Due to the absence of advanced experimental equipment such as telescopes and accurate time-keeping devices, experimental testing of physical hypotheses was impossible or impractical. There were exceptions and there are anachronisms. Greek thinkers like Archimedes proposed calculating the volume of objects like spheres and cones by dividing them into very thin disks and adding up the volume of each disk, using methods resembling integral calculus. It was also Archimedes who derived many correct quantitative descriptions of mechanics and also hydrostatics when, so the story goes, he noticed that his own body displaced a volume of water while he was getting into a bath one day. Another remarkable example was that of Eratosthenes, who deduced that the Earth was a sphere, and accurately calculated its circumference using the shadows of vertical sticks to measure the angle between two widely separated points on the Earth's surface.
Modern knowledge of many early ideas in physics, and the extent to which they were experimentally tested, is sketchy. Almost all direct record of these ideas was lost when the Library of Alexandria was destroyed, around 400 AD. Perhaps the most remarkable idea we know of from this era was the deduction by Aristarchus of Samos that the Earth was a planet that traveled around the Sun once a year, and rotated on its axis once a day (accounting for the seasons and the cycle of day and night), and that the stars were other, very distant suns which also had their own accompanying planets (and possibly, lifeforms upon those planets).
The discovery of the Antikythera mechanism points to a detailed understanding of movements of these astronomical objects, as well as a use of gear-trains that pre-dates any other known civilization's use of gears, except that of ancient China.
An early version of the steam engine, Hero's aeolipile was only a curiosity which did not solve the problem of transforming its rotational energy into a more usable form, not even by gears. The Archimedes screw is still in use today, to lift water from rivers onto irrigated farmland. The simple machines were unremarked, with the exception (at least) of Archimedes' elegant proof of the law of the lever. Ramps were in use several millennia before Archimedes, to build the Pyramids.
This period of inquiry into the nature of the world was eventually stifled by a tendency to accept the ideas of eminent philosophers, rather than to question and test those ideas. Pythagoras himself is said to have tried to suppress knowledge of the existence of irrational numbers, discovered by his own school, because they did not fit his number mysticism. For one thousand years following the destruction of the Library of Alexandria, Ptolemy's (not to be confused with the Egyptian Ptolemies) model of an Earth-centred universe in which the planets are assumed to each move in a small circle, called an epicycle, which in turn moves along a larger circle called a deferent, was accepted as absolute truth.
Indian contributions
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In Lothal (c. 2400 BC), the ancient port city of the Harappan civilization, shell objects served as compasses to measure the angles of the 8–12 fold divisions of the horizon and sky in multiples of 40–360 degrees, and the positions of stars. [4] In the late Vedic era (c. 9th–6th century BC), the astronomer Yajnavalkya, in his Shatapatha Brahmana, referred to an early concept of heliocentrism with the Earth being round and the Sun being the "centre of spheres". He measured the distances of the Moon and the Sun from the Earth as 108 times the diameters of these heavenly bodies, which were close to the modern values of 110.6 for the Moon and 107.6 for the Sun.[5][unreliable source?]
Indians in the Vedic era classified the material world into five basic elements: earth, fire, air, water and ether/space. [6] From the 6th century BC, they formulated systematic atomic theories, beginning with Kanada and Pakudha Katyayana. Indian atomists believed that an atom could be one of up to 9 elements, with each element having up to 24 properties. They developed detailed theories of how atoms could combine, react, vibrate, move and perform other actions, as well as elaborate theories of how atoms can form binary molecules that combine further to form larger molecules, and how particles first combine in pairs, and then group into trios of pairs, which are the smallest visible units of matter.[6][not in citation given] This parallels with the structure of modern atomic theory, in which pairs or triplets of supposedly fundamental quarks combine to create most typical forms of matter. They had also suggested the possibility of splitting an atom, which as we know today, is the source of atomic energy.[citation needed]
The principle of relativity (not to be confused with Einstein's theory of relativity) was available in an embryonic form since the 6th century BC in the ancient Indian philosophical concept of "sapekshavad", literally "theory of relativity" in Sanskrit.
The Samkhya and Vaisheshika schools developed theories on light from the 6th–5th century BC. According to the Samkhya school, light is one of the five fundamental "subtle" elements out of which emerge the gross elements, which were taken to be continuous. The Vaisheshika school defined motion in terms of the non-instantaneous movement of the physical atoms. Light rays were taken to be a stream of high velocity fire atoms, which can exhibit different characteristics depending on the speed and the arrangements of these particles. The Buddhists Dignāga (5th century) and Dharmakirti (7th century) developed a theory of light being composed of energy particles, similar to the modern concept of photons.
Veteran Australian indologist A. L. Basham concluded that "they were brilliant imaginative explanations of the physical structure of the world, and in a large measure, agreed with the discoveries of modern physics."
In 499, the mathematician-astronomer Aryabhata propounded a detailed model of the heliocentric solar system of gravitation, where the planets rotate on their axes causing day & night and follow elliptical orbits around the Sun causing year, and where the planets and the Moon do not have their own light but reflect the light of the Sun. Aryabhata also correctly explained the causes of the solar and lunar eclipses and predicted their times, gave the radii of planetary orbits around the Sun, and accurately measured the lengths of the day, sidereal year, and the Earth's diameter and circumference. Brahmagupta, in his Brahma Sputa Siddhanta in 628, recognized gravity as a force of attraction and understood the law of gravitation.
A particularly important Indian contribution was the Hindu-Arabic numerals. Modern physics can hardly be imagined without a system of arithmetic in which simple calculation is easy enough to make large calculations even possible. The modern positional numeral system (the Hindu-Arabic numeral system) and the number zero were first developed in India, along with the trigonometric functions of sine and cosine. These mathematical developments, along with the Indian developments in physics, were adopted by the Islamic Caliphate, from where they spread to Europe and other parts of the world.
In most of the Indian languages, Physics is known as Bhautikasastra.
Chinese contributions
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A theory very similar to Newton's first law of motion is found in the Mo Ching of the 3rd or 4th century BC:[7]
The cessation of motion is due to the opposing force....If there is no opposing force...the motion will never stop. This is as true as that an ox is not a horse.[8]
Contemporary documentation of the 3rd century AD makes reference to the South Pointing Chariot, a mechanical compass which made early use of the differential gear.[9]
Physics in the Middle Ages
Islamic world
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- Further information: List of Muslim scientists
Like the later Scientific revolution in the West, Islamic science was built on a foundation laid in antiquity, in this case, the intellectual patrimony of the Greeks, Byzantines, Persians and Indians they conquered. The Arab and Persian scholars of the Islamic Golden Age made advances by building on previous work in astronomy, mathematics, and physics while developing new fields like alchemy.
The most important scientific development during the Middle Ages was the pioneering development of the experimental scientific method by Ibn al-Haytham (commonly Latinized Alhazen, ca. 965–1040), as recorded in his Book of Optics.[10]
Alhazen, who is regarded as the father of optics and the pioneer of the scientific method, developed a broad theory that explained vision, using geometry and anatomy, which stated that each point on an illuminated area or object radiates light rays in every direction, but that only one ray from each point, which strikes the eye perpendicularly, can be seen. The other rays strike at different angles and are not seen. He built a camera obscura and used the example of the pinhole camera, which produces an inverted image, to support his argument. This contradicted Ptolemy's emission theory of vision that objects are seen by rays of light emanating from the eyes. Alhazen held light rays to be streams of minute particles that travelled at a finite speed. He improved Ptolemy's theory of the refraction of light, and went on to discover the laws of refraction.
He also carried out the first experiments on the dispersion of light into its constituent colors. His major work Book of Optics was translated into Latin in the Middle Ages, as well as his book dealing with the colors of sunset. He dealt at length with the theory of various physical phenomena like shadows, eclipses, and the rainbow. He also attempted to explain binocular vision and the moon illusion. Through these extensive researches on optics, he is considered the father of modern optics.
Alhazen also correctly argued that we see objects because the sun's rays of light, which he believed to be streams of tiny particles traveling in straight lines, are reflected from objects into our eyes. He understood that light must travel at a large but finite velocity, and that refraction is caused by the velocity being different in different substances. He also studied spherical and parabolic mirrors, and understood how refraction by a lens will allow images to be focused and magnification to take place. He understood mathematically why a spherical mirror produces aberration.
In the mechanics field of physics, the eldest Banū Mūsā brother, Muhammad ibn Musa, in his Astral Motion and The Force of Attraction, discovered that there was a force of attraction between heavenly bodies in the 9th century,[11] foreshadowing Newton's law of universal gravitation.[12]
In the early 11th century, Ibn al-Haytham discussed the theory of attraction between masses, and it seems that he was aware of the magnitude of acceleration due to gravity. Ibn al-Haytham also discovered Galileo Galilei's law of inertia, known as Newton's first law of motion, when he stated that a body moves perpetually unless an external force stops it or changes its direction of motion.[13] Ibn al-Haytham's contemporary, Avicenna, discovered the concept of momentum, when he referred to impetus as being proportional to weight times velocity, a precursor to the concept of momentum in Newton's second law of motion.[14] He is thus considered the father of the fundamental concept of momentum in physics.[15] His theory of motion was also consistent with the concept of inertia in Newton's first law of motion.[14]
In the early 12th century, Ibn Bajjah (Avempace) was the first to state that there is always a reaction force for every force exerted, a precursor to Gottfried Leibniz's idea of force which underlies
Classical
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