History of Physics
The History of Physics is intertwined with the History of Astronomy. In fact, Astronomy is a subject area belonging to the broader topic of Physics. Astronomy is a subset of Physics.
Historically science has its roots in peoples' efforts to understand and explain the world and the universe around them of which they are a part. While many observations were made about phenomena here on earth, the heavens were also observed. People wanted to know what was going on and why. Their interest was born of concern and fear as well as curiosity. They wanted to feel some degree of control of their lives or at least be able to explain what was going on and why.
Our collective knowledge about our environment, the earth and the sky, came about slowly through observation and interpretation of those observations. Knowledge was accumulated slowly in different societies and cultures. Sometimes the knowledge found was lost and only rediscovered much later. The history of western civilization from several thousand B. C. to the present is very much a story of discovery, disagreement, loss, and rediscovery.
Early history of man involves very little ability to investigate more than could be observed with the senses. This resulted in people making up stories to explain phenomena for which they had no real understanding. This is not unlike stories made up to help little children deal with their every day experiences. Often adults won't try explain the physics principles behind a thunder storm to a frightened little child, even if they understood it themselves. Instead they make up a cute story to provide the child with something to explain a scary situation in terms of something less scary. As an example, in many Christian homes in western civilization the explanation often given to little children afraid of thunder storms was that the angels were bowling. The noise of the thunder was the noise of the bowling balls and the lightning was their using light to find the ball so they wouldn't lose it.
This explanation has nothing to do with what is going on in a thunder storm or what lightning or thunder is. On the other hand it gives the child comforting information, which they can then use to face the world. This same phenomenon can be seen in ancient civilizations. People attributed phenomena to the presence or actions of gods. A close look at ancient civilizations like the Greeks reveals a colorful history filled with such stories about gods. All civilizations have their beliefs and mythologies (creation stories) explaining their origins and how the earth and sky came about.
On the other hand, among the people of these civilizations there were some who wanted to know more. They didn't accept the myths on faith, but chose to investigate further. These people wanted to find the truth so they could understand the world and the universe better. We find such people among the members of many societies of which we have knowledge. Collectively, today, we might call such people scientists, though in their own times they may not have had that label.
"Historians have a tendency always to go further back. They justify this because of the belief that one cannot understand what happens at a certain time without understanding the past." (Daniel Barret - from his lecture notes, 1995.)
The study of mathematics and the sciences, particularly astronomy and physics, often begins with the Ancient Greeks. There were other civilizations which predate or coexisted around the eastern Mediterranean that had certain knowledge of these subjects, but in a short review of events such as this page, only some of the most significant people and events are mentioned. The most famous of the Greek philosophers Socrates, Plato, and Aristotle are often quoted or paraphrased, because it is these three that are seen as having a most significant impact on the development of western civilization. What is a curious question is "What influence did other civilizations and their knowledge, such as the Egyptians, have on forming the thinking of these great philosophers and Greek knowledge and society as a whole?" We'll leave that for you, the reader, to pursue, if you are curious. We'll begin with the Greeks and the view that came down from Aristotle. His influence was so great that it had a major impact on western civilization's philosophy, law, religion (Christianity), and science, all the way to the present.
The ancients' view was that the earth was the center of the universe. Until the beginning of the renaissance only a few in western civilization ever thought that the sun might be the center of planetary motion. It was never a popular view, though, because Aristotle's explanation of phenomena both on earth and in the heavens dominated western thinking until the renaissance. They were aware of five known planets and believed that they moved in complicated paths. The need to explain their motion in terms of complicated paths was the result of the apparent complicated motions of planets as they are observed moving across the sky. This observed motion is called apparent motion, which means the motion as seen by an observer here on earth. The planets do not appear to move at a steady rate across the sky from night to night, as one who does not watch the sky might think. In fact there are times when a planet like Mars appears to move backwards with respect to its usual forward motion. This motion is called retrograde motion and required some very creative imaginations to explain this motion based on a belief that the earth was the center of everything.
Aristotle viewed the process of learning as one of observation and thinking. He would study the ideal situation, but he would not conduct experimentation. He believed that truth could be found through good reasoning powers and experimentation would not enhance any study of the world. Experimentation was not something he supported in his ideas about how to determine the answers to questions. The insinuation of doubt and suspicion of the scientific method (the use of experimentation as well as observation) even in today's society no doubt has its roots in Aristotelian logic, though some people are less skilled in logical reasoning than others and may feel intimidated by the scientific method, because they do not really understand it for the process that it is. It wasn't until the beginning of the renaissance that we see a change in the direction of thought as experimentation and observation become equally important. Galileo Galilei in the 14th gave us two very important tools that brought about change in the way the earth and sky were studied and conclusions made. He believed in experimentation, including the theoretical experiment which he, himself, used to investigate falling bodies. He also used mathematics to describe and define phenomena, something which hadn't really been done before.
Socrates (470-399 B.C.) He is a famous Greek philosopher whose ideas are the basis for western civilization's philosophy. He is also well known for his teaching style which is based on a dialogue where the teacher and pupil both reason through an argument to the truth. This is known today as the Socratic method.
Plato (427-347 B.C.) He is a student of Socrates. His philosophy is a belief in an idealized world. He saw the real world as decaying and changing. He believed that true knowledge could not be found from such a corruptible world. Plato outlined the creation of the world as the actions of a demiurge (a supernatural being) working with less than perfect material creating the world according to a rational plan. This being fashioned the world as perfect as possible within the limitations of what it had to work with. This perfection involved using the circle, which he considered the most perfect shape, to describe the motion of the heavenly bodies.
Eudoxus of Cynidus (400-347 B.C.) He was an associate of Plato. He created a very complex astronomy in terms of his description of the movement of heavenly bodies. He described the motion by means of concentric spheres. For example the moon was explained by three concentric spheres. The outer most sphere moved it from east to west every 24 hours. The second sphere moves the moon through one complete revolution every 223 synodic months. The third sphere accounts for the moon's motion through the zodiac. The system becomes more and more complex as other heavenly bodies' motions are described in terms of concentric spheres. [These types of mental imaginings come close to describing a possible explanation for the apparent motion of objects in the sky. They do not, unfortunately have any connection to the truth, that is, what is really going on as we observe the motion of objects in the sky.]
Aristotle (384-322 B.C.) He was a student of Plato. He saw the sphere as the perfect shape. He adopted the ideas of Eudoxus about concentric spheres. He even went further and said that the system described by Eudoxus involves physically real spheres. He related this idea to the model he had created for the physical world of which he said the following.
Celestial Objects above the moon were part of a fifth, perfect element called the aether. From the moon on down everything was made of the four elements consisting of earth, fire, water, and air. These elements had their particular motions that was their nature to move in. These are listed below.
The center of the earth (the center of the universe) was where all things that moved down moved towards. The earth was spherical because of all of the things that moved downwards towards its center. Which ever element dominated an object's make up (composition in terms of the four elements) determined which way an object would move. The heavens were considered perfect, such that anything made up of aetheral matter moved in a circle that was above and distinct from the earth.
Ptolemy (90-168 A.D.) He defined a very complex movement of heavenly bodies that complimented Aristotle's physics that said matter must move either up, down, or in a circle according to its nature. He added more spheres to an already complicated system. His model was more accurate than that of Eudoxus. In fact though his model was not based upon what was really happening in terms of the movement of the planets, it did as good a job as a tool for making predictions as to apparent motion in the sky as the early renaissance "astronomers" could operating with newly rediscovered information. [This means that as a "calculating machine" of sorts, his model was useful for predicting positions of objects in the sky, but did not in any way explain the actual motions of the planets and moons in our solar system or why they moved as they did.]
It can be said at this point that the goal of explaining apparent motion of the planets etc. involved a way of thinking that focused on preserving the appearances, that which was seen, and not on the truth based on experiment and observation that would focus on what events were occurring to produce the appearances or what was seen. It should be noted here that there was a small group of Greeks called the Ionians who interpreted what they saw in the heavens as the sun being at the center of the universe. Their ideas were not held by the general population or the more influential people such as Aristotle. Their ideas would go unnoticed for close to 2,000 years until the heliocentric universe was "rediscovered".
Copernicus (1473-1543 A.D.) In 1543 year his heliocentric theory was published. His model said that the sun was at the center of things and not the earth. He said that the five known planets as well as the earth were all traveling around the sun, making sun the center of all of the action.
At the time, the known planets from the sun outwards were:
While he did make a serious bold step in the scheme of things, producing a theory of heliocentricity, he did not abandon the use of epicycles. An epicycle is a small circular path whose center moved along the circumference of a larger circle whose center was the earth. He kept the idea of epicycles because he believed that the paths of the planets were circles and the only model that would explain the motion seen in the sky using circles requires that the planets follow epicycles rather than single circles around the earth. [It was not yet learned that the true shape of the planetary orbits were ellipses and epicycles were not in anyway a true description of how planets moved about the sun.]
Tycho Brahe (1546-1601 A.D.) He was man who at a young age had a strong interest in astronomy and some connections at court. He was upset that current astronomical tables of information were noticeably off in their measurements. The Danish King, Fredrick II, gave him the small island of Hven off of the coast of Denmark. He erected an observatory (some sources say two were erected) on this island. Along with the help of assistants, but without the use of telescopes (they were not invented yet), he carried out the most exact astronomical observations ever done up to that point in time. He used a great mural quadrant, a device that works similar to a giant protractor, to determine the height above the horizon of objects at the time they cross the meridian. In 1577 he witnessed a comet which because of his accuracy showed it to be six times further from earth than the moon. This didn't bode well for the popular Aristotelian model of the universe which said that the heavens were unchanging. It also was bad for the solid sphere model because the comets path took it right through those spheres. Before this time, comets were just thought of as being "shooting stars" moving above the earth but closer to the earth's surface than the moon. He looked for stellar parallax which would indicate that the earth is moving, but with the unaided eye he could not detect any. Thus caused him to doubt the earth moved and disagreed with Copernicus. For over twenty years he did gather volumes of data, data which would eventually fall into the hands of Johannes Kepler. Tycho himself believed that Saturn, Jupiter, and Mars revolve counter clockwise around the sun. Mercury and Venus did too, but on smaller circles. The sun revolves about the earth once a day while the sphere carrying the stars turns once a day as well. Tycho supported an earth centered universe.
Johannes Kepler (1571-1630) Kepler asked question like "why were there six planets?" and "why do those further from the sun move more slowly?" Kepler studied and taught astronomy. While teaching he came to the conclusion that Copernicus was correct, but there was no need for epicycles. Instead he reasoned that the ratios of the planetary orbital radii were related to certain geometric shapes. He first noted that the ratio of Jupiter's to Saturn's radii was similar to a ratio of an inner circle to an outer circle with a triangle inscribed within it, except that actual geometric shapes that appear to work for the ratio of planetary orbits are more complex. He set out and determined the geometric shapes which would fit inside an inner - outer circle ratio of each pair of planetary radii. Below is a list of the geometric shapes that he determined would fit between the circles representing the orbits of the six known planets.
Mercury
> octahedron
Venus
> icosahedron
Earth
> dodecahedron
Mars
> tetrahedron
Jupiter
> cube
Saturn
He communicated this information both to Tycho Brahe and Galileo Galilei. He was convinced that this geometric pattern explained why only six planets existed and why they had the orbital radii that they did. Later he joined Tycho Brahe. Kepler acquired Tycho Brahe's data when Tycho died in 1601. Kepler studied this data for a good twenty years. It was during this time that he developed his three laws of planetary motion. These three laws are:
1. The planets travel in elliptical paths. (Their orbits are ellipses with the sun at one of the two foci)
2. The planets sweep out equal amounts of area in equal amounts of time. This law says that a planet such as the earth speeds up and slows down during one revolution about the sun. Since the sun is off center, being at one of the two focus points of the elliptical path it slows down as it moves away from the sun and speeds up as it moves closer to the sun.
3. The relationship between the period (the time for one revolution about the sun) and the average radius of a planet is related by the mathematical expression that says the period squared divided by the average radius cubed is equal to a numerical constant. If a person knows that constant it is easy to calculate the radius of an orbit from the period of the planet or the period of the planet from the average radius of the an orbit.
Galileo Galilei (1564-1642) Galileo was a brilliant man. He gave us the idea of doing thought experiments as well as reinforced the concept of doing physical experiments. He also gave us the idea of using mathematics to describe relationships among variables and describe phenomena.
A very big investigation on his part was the concept of motion. Aristotle, who used as his approach observation followed by logical thought to arrive at "truths", reasoned that the natural state of an object was at rest because he saw all moving objects slow down and come to rest. He either didn't perceive or couldn't conceive of the reason an object slowed down. He apparently was not aware of the presence of the force we call friction. Slowing down to him was a natural state of things. He wasn't aware of forces at work. Galileo, on the other hand, believed that a person should test ideas with observations based upon further experimentation wherever possible and see whether the results could still be explained in terms of existing theories. Theories have to be continually tested as new data through new experimentation is gathered and analyzed. Galileo reasoned that the natural state of an object is either at rest or moving with constant speed, for as long as no unbalanced forces are acting on the object. Aristotle didn't realize that on earth objects always tend to come to rest because of the unbalanced force of friction acting on the object. Galileo understood that an object at rest would continue to remain at rest until an unbalanced force acted on it. Galileo was also able to reason that if friction and other forces acting on an object were balanced (canceled out, if you prefer), an object not at rest would continue to move along with constant speed because there would be no reason for it to ever come to rest or even change its speed until an unbalanced force acted on it.
Aristotle also reasoned that heavier objects fall faster than lighter ones although today we know that that only appears to happen whenever friction is present, because friction has a greater effect on small light weight objects than on large heavy objects. Galileo reasoned that all objects fall at the same rate when there are no unbalanced forces like friction acting on them. He wasn't able to create an environment where he could reduce friction to essentially zero such as we do today in vacuum chambers, but he was able to imagine it and work through a thought experiment that led to his correct conclusion. Galileo studied the motion of objects under the influence of the force of gravity and came to understand the concept of acceleration.
Galileo also heard about the development of the telescope by the Flemish optician Hans Lipperhey. In 1609 he made one of his own. Though he didn't invent the telescope, he did make significant improvements to it. He ground his own lenses and had eight to nine power telescopes when others had only six power telescopes back in 1609. At the end of 1610 he had a twenty power scope. By today's standards these is not impressive at all, but at the beginning of this technology this was great stuff. Galileo studied the surface of the moon making some sketches of what he saw. He studied the four large moons of Jupiter. He found stars that were not visible with the naked eye. He found that unlike planets and moons which appeared larger and with more detail when observed through more powerful telescopes, stars did not appear to look larger, but did appear as brighter points of light. He published the "Starry Messenger" in which he reported his findings.
Galileo did not make known his support of Copernicus' heliocentric model until 1613. The discovery of the four moons of Jupiter appears to be the piece of the puzzle that really convinced him of the heliocentric view of the heavens as being correct. He dismissed the geocentric (earth centered) view. Of course this got him into trouble with the Roman Catholic Church which had dominated western civilization since the time of the Roman empire. The church had secured and preserved thousands and thousands of documents not just in the Vatican, but in many monasteries scattered across western Europe, etc., even as far away as Ireland. Along with the Renaissance came the reformation and the Church was being attacked by a number of groups who were trying to change Christianity and/or abolish it altogether. They were very skeptical of Galileo's ideas because they ran contrary to what had been believed true about the universe for some 2,000 years. Their cautiousness is understandable from the point of view of that they had preserved western civilization and the faith from the time of the fall of the roman empire, through the dark ages, to the beginning of the renaissance, when Galileo came out with his research results. Its a complicated story, but in the end he was placed under house arrest on a nice estate with maid service because he couldn't prove beyond a doubt that his evidence was proof positive of a heliocentric universe. He continued to write and had material snuck out to Holland for publishing.
Isaac Newton (1642-1727) Newton was born into a poor farming family. His interest lay elsewhere and he went to Cambridge to become a preacher. It was here that he studied mathematics and got into considering planetary orbits and the related physics of motion. Newton developed his famous three laws of motion which were defined and described in his large publication the "Philosophiae Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy) (1687), often shortened to Principia Mathematica or simply "the Principia.''
The story about his developing these three laws of motion says that he did it when he left the university to return to the farm so as to avoid the plague. Here is where the story of the falling apple comes from. While we won't really ever know whether he was actually struck by an apple, we do know that what got him thinking about gravity were questions such as "If an apple and the moon are both affected by gravity, then why does only the apple fall to earth and not the moon?". He took and built upon Galileo's thinking about forces and motion and investigated the concept of force much further developing his three laws of motion. In addition to his three laws of motion he developed the equation linking the force of gravity to both the mass of the two objects between which there was a force of attraction and the distance which separated them. This is referred to as Newton's Law of Universal Gravitation. His investigation of motion lead him to develop Calculus from the point of the derivative. A German scientist by the name of Leibniz (1646-1716 A.D.) also developed Calculus from the point of view of the integral.
Newton's Three Laws of Motion:
1. Newton's First Law: An object that is at rest will remain at rest and an object that is moving in a straight line with constant velocity will continue to do so, if the net force (the sum of all forces acting on the object) is equal to zero. Newton generalized Galileo's study of motion to fit motion in any direction. His first law is often referred to as the law of inertia. Inertia is the tendency of an object to resist change, as in this case, a change in its motion.
2. Newton's Second Law: The force exerted on an object is a function of both the object's mass and the rate of acceleration it experiences as a result of the force. This means that force is both proportional to mass and to acceleration when one of the two remains constant, but mass and acceleration are inversely proportional to one another when force remains constant.
The equation for Newton's Second Law
F = m a
F is Force, m is mass, and a is acceleration.
3. Newton's Third Law: Whenever one object exerts a force on a second object, the second object exerts a force on the first object. Sometimes this law is described as saying "to every action there is an equal and opposite reaction". It must always be remembered that the action and the reaction force are acting on different objects. As you think about this look at your hand when you are pushing against a heavy object. That the shape of your hand is affected is evidence that a force is acting on it and it is not just exerting a force on something else. You can feel the force as well and if you are pushing hard enough, it is pushing back on you hard enough to make your hand hurt. An ice skater could move across the ice by pushing against something, and the something will push back on the skater. For example, a stationary skater in the middle of a low friction smooth frozen pond holding a handful of baseballs could move off the ice by throwing one ball at a time until they have reached the edge of the pond The as the each ball is thrown (pushed) forward with a force it pushes backwards with an equal but opposite force. This too is how rocket engines work the hot exhaust gas is pushed out the rear of the engine while at the same time the hot gas is pushing the back on the rocket. As a final example, consider when you walk or drive a car. You or the car tire are pushed forward by the earth at the same time your foot or the car (tire) is pushing back on the earth.
Newton's Law of Universal Gravitation: Newton was able to express his law in terms that applied to the motion of planets orbiting the sun. His law agreed with Kepler's third law which provided proof that he had he and Kepler were on the right track and in agreement about the observable known universe. His law defined the force of gravity in terms of the mass of the two objects exerting gravitational force on each other and the distance which separates them. His law says that the Force of gravity between two objects varies directly with the masses of the two objects and varies inversely with the distance separating them squared.
The Equation for Newton's Law of Universal Gravitation
Fg a m1 m2 / d2. (a proportionality)
Fg = G m1 m2 / d2. (an equality)
Where G is a numerical constant changing the proportionality into an equality. Unfortunately Newton did not have the means (the equipment) to measure very small gravitational forces in a laboratory experimental setting so he wasn't able to determine the value for G. It wasn't until around 100 years later that a person by the name of Henry Cavendish was able to experimentally determine the value for G and calculations using Newton's Law of Universal Gravitation could be done.
Modern Physics (The 20th century, 1900-2000) The physics coming down to us from the past which focusses very much on motion and the nature of the universe was further defined and perfected by Isaac Newton was dominant until the beginning of the 20th century. This physics is sometimes called Newtonian physics or more often generally referred to as classical physics. This physics is adequate for most everyday phenomena involving motion where relatively large massive objects moving with relatively slow velocities are involved.
By the early 1900's though a very famous person by the name of Albert Einstein came along and shook up the world by saying the universe is not a big mechanical gadget like a lot of people who understood newton were saying. Instead it is a much more complicated and at the same time curiously interesting and different than what Newton's view gave us. The universe is very different when you consider that small high speed particles do not behave as would be predicted using Newton's Laws. Instead they behave according to a new set of principles that require one to open one's mind to exciting new principles and century of research into the unknown of the very large and the very small. Einstein kicked off the 20th century with his theory of relativity and we left the last century with emphasis on quantum theory.
This is as far as this summary goes for now. Understand that the information provided here is just the proverbial first step into the 1,000 mile journey. If you choose to take it, you'll enjoy it.
The History of Physics is intertwined with the History of Astronomy. In fact, Astronomy is a subject area belonging to the broader topic of Physics. Astronomy is a subset of Physics.
Historically science has its roots in peoples' efforts to understand and explain the world and the universe around them of which they are a part. While many observations were made about phenomena here on earth, the heavens were also observed. People wanted to know what was going on and why. Their interest was born of concern and fear as well as curiosity. They wanted to feel some degree of control of their lives or at least be able to explain what was going on and why.
Our collective knowledge about our environment, the earth and the sky, came about slowly through observation and interpretation of those observations. Knowledge was accumulated slowly in different societies and cultures. Sometimes the knowledge found was lost and only rediscovered much later. The history of western civilization from several thousand B. C. to the present is very much a story of discovery, disagreement, loss, and rediscovery.
Early history of man involves very little ability to investigate more than could be observed with the senses. This resulted in people making up stories to explain phenomena for which they had no real understanding. This is not unlike stories made up to help little children deal with their every day experiences. Often adults won't try explain the physics principles behind a thunder storm to a frightened little child, even if they understood it themselves. Instead they make up a cute story to provide the child with something to explain a scary situation in terms of something less scary. As an example, in many Christian homes in western civilization the explanation often given to little children afraid of thunder storms was that the angels were bowling. The noise of the thunder was the noise of the bowling balls and the lightning was their using light to find the ball so they wouldn't lose it.
This explanation has nothing to do with what is going on in a thunder storm or what lightning or thunder is. On the other hand it gives the child comforting information, which they can then use to face the world. This same phenomenon can be seen in ancient civilizations. People attributed phenomena to the presence or actions of gods. A close look at ancient civilizations like the Greeks reveals a colorful history filled with such stories about gods. All civilizations have their beliefs and mythologies (creation stories) explaining their origins and how the earth and sky came about.
On the other hand, among the people of these civilizations there were some who wanted to know more. They didn't accept the myths on faith, but chose to investigate further. These people wanted to find the truth so they could understand the world and the universe better. We find such people among the members of many societies of which we have knowledge. Collectively, today, we might call such people scientists, though in their own times they may not have had that label.
"Historians have a tendency always to go further back. They justify this because of the belief that one cannot understand what happens at a certain time without understanding the past." (Daniel Barret - from his lecture notes, 1995.)
The study of mathematics and the sciences, particularly astronomy and physics, often begins with the Ancient Greeks. There were other civilizations which predate or coexisted around the eastern Mediterranean that had certain knowledge of these subjects, but in a short review of events such as this page, only some of the most significant people and events are mentioned. The most famous of the Greek philosophers Socrates, Plato, and Aristotle are often quoted or paraphrased, because it is these three that are seen as having a most significant impact on the development of western civilization. What is a curious question is "What influence did other civilizations and their knowledge, such as the Egyptians, have on forming the thinking of these great philosophers and Greek knowledge and society as a whole?" We'll leave that for you, the reader, to pursue, if you are curious. We'll begin with the Greeks and the view that came down from Aristotle. His influence was so great that it had a major impact on western civilization's philosophy, law, religion (Christianity), and science, all the way to the present.
The ancients' view was that the earth was the center of the universe. Until the beginning of the renaissance only a few in western civilization ever thought that the sun might be the center of planetary motion. It was never a popular view, though, because Aristotle's explanation of phenomena both on earth and in the heavens dominated western thinking until the renaissance. They were aware of five known planets and believed that they moved in complicated paths. The need to explain their motion in terms of complicated paths was the result of the apparent complicated motions of planets as they are observed moving across the sky. This observed motion is called apparent motion, which means the motion as seen by an observer here on earth. The planets do not appear to move at a steady rate across the sky from night to night, as one who does not watch the sky might think. In fact there are times when a planet like Mars appears to move backwards with respect to its usual forward motion. This motion is called retrograde motion and required some very creative imaginations to explain this motion based on a belief that the earth was the center of everything.
Aristotle viewed the process of learning as one of observation and thinking. He would study the ideal situation, but he would not conduct experimentation. He believed that truth could be found through good reasoning powers and experimentation would not enhance any study of the world. Experimentation was not something he supported in his ideas about how to determine the answers to questions. The insinuation of doubt and suspicion of the scientific method (the use of experimentation as well as observation) even in today's society no doubt has its roots in Aristotelian logic, though some people are less skilled in logical reasoning than others and may feel intimidated by the scientific method, because they do not really understand it for the process that it is. It wasn't until the beginning of the renaissance that we see a change in the direction of thought as experimentation and observation become equally important. Galileo Galilei in the 14th gave us two very important tools that brought about change in the way the earth and sky were studied and conclusions made. He believed in experimentation, including the theoretical experiment which he, himself, used to investigate falling bodies. He also used mathematics to describe and define phenomena, something which hadn't really been done before.
Socrates (470-399 B.C.) He is a famous Greek philosopher whose ideas are the basis for western civilization's philosophy. He is also well known for his teaching style which is based on a dialogue where the teacher and pupil both reason through an argument to the truth. This is known today as the Socratic method.
Plato (427-347 B.C.) He is a student of Socrates. His philosophy is a belief in an idealized world. He saw the real world as decaying and changing. He believed that true knowledge could not be found from such a corruptible world. Plato outlined the creation of the world as the actions of a demiurge (a supernatural being) working with less than perfect material creating the world according to a rational plan. This being fashioned the world as perfect as possible within the limitations of what it had to work with. This perfection involved using the circle, which he considered the most perfect shape, to describe the motion of the heavenly bodies.
Eudoxus of Cynidus (400-347 B.C.) He was an associate of Plato. He created a very complex astronomy in terms of his description of the movement of heavenly bodies. He described the motion by means of concentric spheres. For example the moon was explained by three concentric spheres. The outer most sphere moved it from east to west every 24 hours. The second sphere moves the moon through one complete revolution every 223 synodic months. The third sphere accounts for the moon's motion through the zodiac. The system becomes more and more complex as other heavenly bodies' motions are described in terms of concentric spheres. [These types of mental imaginings come close to describing a possible explanation for the apparent motion of objects in the sky. They do not, unfortunately have any connection to the truth, that is, what is really going on as we observe the motion of objects in the sky.]
Aristotle (384-322 B.C.) He was a student of Plato. He saw the sphere as the perfect shape. He adopted the ideas of Eudoxus about concentric spheres. He even went further and said that the system described by Eudoxus involves physically real spheres. He related this idea to the model he had created for the physical world of which he said the following.
Celestial Objects above the moon were part of a fifth, perfect element called the aether. From the moon on down everything was made of the four elements consisting of earth, fire, water, and air. These elements had their particular motions that was their nature to move in. These are listed below.
- Fire moves up
- Air moves up
- Water moves down
- Earth moves down
The center of the earth (the center of the universe) was where all things that moved down moved towards. The earth was spherical because of all of the things that moved downwards towards its center. Which ever element dominated an object's make up (composition in terms of the four elements) determined which way an object would move. The heavens were considered perfect, such that anything made up of aetheral matter moved in a circle that was above and distinct from the earth.
Ptolemy (90-168 A.D.) He defined a very complex movement of heavenly bodies that complimented Aristotle's physics that said matter must move either up, down, or in a circle according to its nature. He added more spheres to an already complicated system. His model was more accurate than that of Eudoxus. In fact though his model was not based upon what was really happening in terms of the movement of the planets, it did as good a job as a tool for making predictions as to apparent motion in the sky as the early renaissance "astronomers" could operating with newly rediscovered information. [This means that as a "calculating machine" of sorts, his model was useful for predicting positions of objects in the sky, but did not in any way explain the actual motions of the planets and moons in our solar system or why they moved as they did.]
It can be said at this point that the goal of explaining apparent motion of the planets etc. involved a way of thinking that focused on preserving the appearances, that which was seen, and not on the truth based on experiment and observation that would focus on what events were occurring to produce the appearances or what was seen. It should be noted here that there was a small group of Greeks called the Ionians who interpreted what they saw in the heavens as the sun being at the center of the universe. Their ideas were not held by the general population or the more influential people such as Aristotle. Their ideas would go unnoticed for close to 2,000 years until the heliocentric universe was "rediscovered".
Copernicus (1473-1543 A.D.) In 1543 year his heliocentric theory was published. His model said that the sun was at the center of things and not the earth. He said that the five known planets as well as the earth were all traveling around the sun, making sun the center of all of the action.
- 1 Mercury
- 2 Venus
- 3 Earth
- 4 Mars
- 5 Jupiter
- 6 Saturn
While he did make a serious bold step in the scheme of things, producing a theory of heliocentricity, he did not abandon the use of epicycles. An epicycle is a small circular path whose center moved along the circumference of a larger circle whose center was the earth. He kept the idea of epicycles because he believed that the paths of the planets were circles and the only model that would explain the motion seen in the sky using circles requires that the planets follow epicycles rather than single circles around the earth. [It was not yet learned that the true shape of the planetary orbits were ellipses and epicycles were not in anyway a true description of how planets moved about the sun.]
Tycho Brahe (1546-1601 A.D.) He was man who at a young age had a strong interest in astronomy and some connections at court. He was upset that current astronomical tables of information were noticeably off in their measurements. The Danish King, Fredrick II, gave him the small island of Hven off of the coast of Denmark. He erected an observatory (some sources say two were erected) on this island. Along with the help of assistants, but without the use of telescopes (they were not invented yet), he carried out the most exact astronomical observations ever done up to that point in time. He used a great mural quadrant, a device that works similar to a giant protractor, to determine the height above the horizon of objects at the time they cross the meridian. In 1577 he witnessed a comet which because of his accuracy showed it to be six times further from earth than the moon. This didn't bode well for the popular Aristotelian model of the universe which said that the heavens were unchanging. It also was bad for the solid sphere model because the comets path took it right through those spheres. Before this time, comets were just thought of as being "shooting stars" moving above the earth but closer to the earth's surface than the moon. He looked for stellar parallax which would indicate that the earth is moving, but with the unaided eye he could not detect any. Thus caused him to doubt the earth moved and disagreed with Copernicus. For over twenty years he did gather volumes of data, data which would eventually fall into the hands of Johannes Kepler. Tycho himself believed that Saturn, Jupiter, and Mars revolve counter clockwise around the sun. Mercury and Venus did too, but on smaller circles. The sun revolves about the earth once a day while the sphere carrying the stars turns once a day as well. Tycho supported an earth centered universe.
Johannes Kepler (1571-1630) Kepler asked question like "why were there six planets?" and "why do those further from the sun move more slowly?" Kepler studied and taught astronomy. While teaching he came to the conclusion that Copernicus was correct, but there was no need for epicycles. Instead he reasoned that the ratios of the planetary orbital radii were related to certain geometric shapes. He first noted that the ratio of Jupiter's to Saturn's radii was similar to a ratio of an inner circle to an outer circle with a triangle inscribed within it, except that actual geometric shapes that appear to work for the ratio of planetary orbits are more complex. He set out and determined the geometric shapes which would fit inside an inner - outer circle ratio of each pair of planetary radii. Below is a list of the geometric shapes that he determined would fit between the circles representing the orbits of the six known planets.
Mercury
> octahedron
Venus
> icosahedron
Earth
> dodecahedron
Mars
> tetrahedron
Jupiter
> cube
Saturn
He communicated this information both to Tycho Brahe and Galileo Galilei. He was convinced that this geometric pattern explained why only six planets existed and why they had the orbital radii that they did. Later he joined Tycho Brahe. Kepler acquired Tycho Brahe's data when Tycho died in 1601. Kepler studied this data for a good twenty years. It was during this time that he developed his three laws of planetary motion. These three laws are:
1. The planets travel in elliptical paths. (Their orbits are ellipses with the sun at one of the two foci)
2. The planets sweep out equal amounts of area in equal amounts of time. This law says that a planet such as the earth speeds up and slows down during one revolution about the sun. Since the sun is off center, being at one of the two focus points of the elliptical path it slows down as it moves away from the sun and speeds up as it moves closer to the sun.
3. The relationship between the period (the time for one revolution about the sun) and the average radius of a planet is related by the mathematical expression that says the period squared divided by the average radius cubed is equal to a numerical constant. If a person knows that constant it is easy to calculate the radius of an orbit from the period of the planet or the period of the planet from the average radius of the an orbit.
Galileo Galilei (1564-1642) Galileo was a brilliant man. He gave us the idea of doing thought experiments as well as reinforced the concept of doing physical experiments. He also gave us the idea of using mathematics to describe relationships among variables and describe phenomena.
A very big investigation on his part was the concept of motion. Aristotle, who used as his approach observation followed by logical thought to arrive at "truths", reasoned that the natural state of an object was at rest because he saw all moving objects slow down and come to rest. He either didn't perceive or couldn't conceive of the reason an object slowed down. He apparently was not aware of the presence of the force we call friction. Slowing down to him was a natural state of things. He wasn't aware of forces at work. Galileo, on the other hand, believed that a person should test ideas with observations based upon further experimentation wherever possible and see whether the results could still be explained in terms of existing theories. Theories have to be continually tested as new data through new experimentation is gathered and analyzed. Galileo reasoned that the natural state of an object is either at rest or moving with constant speed, for as long as no unbalanced forces are acting on the object. Aristotle didn't realize that on earth objects always tend to come to rest because of the unbalanced force of friction acting on the object. Galileo understood that an object at rest would continue to remain at rest until an unbalanced force acted on it. Galileo was also able to reason that if friction and other forces acting on an object were balanced (canceled out, if you prefer), an object not at rest would continue to move along with constant speed because there would be no reason for it to ever come to rest or even change its speed until an unbalanced force acted on it.
Aristotle also reasoned that heavier objects fall faster than lighter ones although today we know that that only appears to happen whenever friction is present, because friction has a greater effect on small light weight objects than on large heavy objects. Galileo reasoned that all objects fall at the same rate when there are no unbalanced forces like friction acting on them. He wasn't able to create an environment where he could reduce friction to essentially zero such as we do today in vacuum chambers, but he was able to imagine it and work through a thought experiment that led to his correct conclusion. Galileo studied the motion of objects under the influence of the force of gravity and came to understand the concept of acceleration.
Galileo also heard about the development of the telescope by the Flemish optician Hans Lipperhey. In 1609 he made one of his own. Though he didn't invent the telescope, he did make significant improvements to it. He ground his own lenses and had eight to nine power telescopes when others had only six power telescopes back in 1609. At the end of 1610 he had a twenty power scope. By today's standards these is not impressive at all, but at the beginning of this technology this was great stuff. Galileo studied the surface of the moon making some sketches of what he saw. He studied the four large moons of Jupiter. He found stars that were not visible with the naked eye. He found that unlike planets and moons which appeared larger and with more detail when observed through more powerful telescopes, stars did not appear to look larger, but did appear as brighter points of light. He published the "Starry Messenger" in which he reported his findings.
Galileo did not make known his support of Copernicus' heliocentric model until 1613. The discovery of the four moons of Jupiter appears to be the piece of the puzzle that really convinced him of the heliocentric view of the heavens as being correct. He dismissed the geocentric (earth centered) view. Of course this got him into trouble with the Roman Catholic Church which had dominated western civilization since the time of the Roman empire. The church had secured and preserved thousands and thousands of documents not just in the Vatican, but in many monasteries scattered across western Europe, etc., even as far away as Ireland. Along with the Renaissance came the reformation and the Church was being attacked by a number of groups who were trying to change Christianity and/or abolish it altogether. They were very skeptical of Galileo's ideas because they ran contrary to what had been believed true about the universe for some 2,000 years. Their cautiousness is understandable from the point of view of that they had preserved western civilization and the faith from the time of the fall of the roman empire, through the dark ages, to the beginning of the renaissance, when Galileo came out with his research results. Its a complicated story, but in the end he was placed under house arrest on a nice estate with maid service because he couldn't prove beyond a doubt that his evidence was proof positive of a heliocentric universe. He continued to write and had material snuck out to Holland for publishing.
Isaac Newton (1642-1727) Newton was born into a poor farming family. His interest lay elsewhere and he went to Cambridge to become a preacher. It was here that he studied mathematics and got into considering planetary orbits and the related physics of motion. Newton developed his famous three laws of motion which were defined and described in his large publication the "Philosophiae Naturalis Principia Mathematica" (Mathematical Principles of Natural Philosophy) (1687), often shortened to Principia Mathematica or simply "the Principia.''
The story about his developing these three laws of motion says that he did it when he left the university to return to the farm so as to avoid the plague. Here is where the story of the falling apple comes from. While we won't really ever know whether he was actually struck by an apple, we do know that what got him thinking about gravity were questions such as "If an apple and the moon are both affected by gravity, then why does only the apple fall to earth and not the moon?". He took and built upon Galileo's thinking about forces and motion and investigated the concept of force much further developing his three laws of motion. In addition to his three laws of motion he developed the equation linking the force of gravity to both the mass of the two objects between which there was a force of attraction and the distance which separated them. This is referred to as Newton's Law of Universal Gravitation. His investigation of motion lead him to develop Calculus from the point of the derivative. A German scientist by the name of Leibniz (1646-1716 A.D.) also developed Calculus from the point of view of the integral.
Newton's Three Laws of Motion:
1. Newton's First Law: An object that is at rest will remain at rest and an object that is moving in a straight line with constant velocity will continue to do so, if the net force (the sum of all forces acting on the object) is equal to zero. Newton generalized Galileo's study of motion to fit motion in any direction. His first law is often referred to as the law of inertia. Inertia is the tendency of an object to resist change, as in this case, a change in its motion.
2. Newton's Second Law: The force exerted on an object is a function of both the object's mass and the rate of acceleration it experiences as a result of the force. This means that force is both proportional to mass and to acceleration when one of the two remains constant, but mass and acceleration are inversely proportional to one another when force remains constant.
F = m a
3. Newton's Third Law: Whenever one object exerts a force on a second object, the second object exerts a force on the first object. Sometimes this law is described as saying "to every action there is an equal and opposite reaction". It must always be remembered that the action and the reaction force are acting on different objects. As you think about this look at your hand when you are pushing against a heavy object. That the shape of your hand is affected is evidence that a force is acting on it and it is not just exerting a force on something else. You can feel the force as well and if you are pushing hard enough, it is pushing back on you hard enough to make your hand hurt. An ice skater could move across the ice by pushing against something, and the something will push back on the skater. For example, a stationary skater in the middle of a low friction smooth frozen pond holding a handful of baseballs could move off the ice by throwing one ball at a time until they have reached the edge of the pond The as the each ball is thrown (pushed) forward with a force it pushes backwards with an equal but opposite force. This too is how rocket engines work the hot exhaust gas is pushed out the rear of the engine while at the same time the hot gas is pushing the back on the rocket. As a final example, consider when you walk or drive a car. You or the car tire are pushed forward by the earth at the same time your foot or the car (tire) is pushing back on the earth.
Newton's Law of Universal Gravitation: Newton was able to express his law in terms that applied to the motion of planets orbiting the sun. His law agreed with Kepler's third law which provided proof that he had he and Kepler were on the right track and in agreement about the observable known universe. His law defined the force of gravity in terms of the mass of the two objects exerting gravitational force on each other and the distance which separates them. His law says that the Force of gravity between two objects varies directly with the masses of the two objects and varies inversely with the distance separating them squared.
Fg a m1 m2 / d2. (a proportionality)
Fg = G m1 m2 / d2. (an equality)
Modern Physics (The 20th century, 1900-2000) The physics coming down to us from the past which focusses very much on motion and the nature of the universe was further defined and perfected by Isaac Newton was dominant until the beginning of the 20th century. This physics is sometimes called Newtonian physics or more often generally referred to as classical physics. This physics is adequate for most everyday phenomena involving motion where relatively large massive objects moving with relatively slow velocities are involved.
By the early 1900's though a very famous person by the name of Albert Einstein came along and shook up the world by saying the universe is not a big mechanical gadget like a lot of people who understood newton were saying. Instead it is a much more complicated and at the same time curiously interesting and different than what Newton's view gave us. The universe is very different when you consider that small high speed particles do not behave as would be predicted using Newton's Laws. Instead they behave according to a new set of principles that require one to open one's mind to exciting new principles and century of research into the unknown of the very large and the very small. Einstein kicked off the 20th century with his theory of relativity and we left the last century with emphasis on quantum theory.
This is as far as this summary goes for now. Understand that the information provided here is just the proverbial first step into the 1,000 mile journey. If you choose to take it, you'll enjoy it.
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