Welcome, everyone, to the study of physics. In this course, you will be learning about the contributions of a few famous physicists as well as many more people whose work in mathematics, physics, and engineering did not bring them eternal fame. Of course, most of these people lived and died long ago. There will be times, however, when the work of modern physicists presents itself. Occasionally ,we may even get an inkling about the physics of the next millennium.
Be warned however: understanding physics isn’t as hard as you think. How else to explain the way scientists have connected to physics to virtually every discipline? Long gone are the days (circa 1700 a.d.) when a recluse such as Newton could know everything that was known about physics, and reshape one paradigm after another with a stroke of his quill pen.

So, what is physics exactly? Let’s start from the beginning of time. Actually, In the beginning there was nothing, or at least nothing much to write about. As time went on, things got more complex (or perhaps complexity increased and produced more things?). Eventually hairless and clawless human beings showed up and really added to the complexity of life. In our continual effort to stay alive in hostile environments, we created confusion by trying to explain everything. After doing this life-improvement work for a few hundred millennia, it got to be habit forming.

Pretty soon folks like Plato, Aristotle, Copernicus, Galileo, Kepler, Newton, Maxwell, Einstein, Bohr, Heisenberg, Hawking, and Chu were explaining the inner and outer workings of the Universe. At first, they were describing reality in a way that seemed irrelevant to our immediate well being. Every so often, however, one of these scholarly principles turned out to have a practical side--often when it was least expected.

For Example
Copernicus's Earth centered model of the cosmos led, 200 years later, to Newton's laws of motion. Application of these three laws led to the construction of heavy machinery, the industrial revolution, and the space program. About two hundred years after Newton, Maxwell's laws of electricity and magnetism led to the invention of radio and television, and stimulated a very young physicist named Einstein to ponder the meaning of matter and energy.

In the 1920's Nils Bohr's theory of atomic energy levels, and Werner Heinsberg’s probability calculations ushered in the quantum revolution, and paved the way for the invention of transistors and high speed digital computers. Nowadays, physicists ponder the nature of apparently ghostlike particles called neutrinos, and space and time bending objects like black holes. In these cutting edge investigations, physicists are expectantly looking for than the tiniest flaw in our current theories, or the missing bit of data that proves an existing model correct. Its not at all obvious how these investigations might lead to practical innovation, but if history is any guide, they certainly will.

A Brief History of Physics

Recall that human beings have been trying to explain how and why things happen for quite some time. Before the advent of written language, we have little evidence of what those explanations were. It is the ancient Greeks who get considerable credit for starting the ball rolling (if you pardon the pun)--Partially because they were great writers, as well as great thinkers, Most of us are familiar with the names of Socrates, Plato, and Aristotle. While both Socrates and Plato shaped the questions that were asked and answers that could be given (the discourse of physics), it is Aristotle (384 - 322 BC) who gets most of the credit for asking and answering the questions that would form the basis for our understanding of the "natural" world.

Aristotle created a working model of the cosmos, placing the Earth at the center, and the sun, moon, planets, and stars on concentric spheres that revolved about the Earth. He also formed a "complete" theory of motion by separating all substances in four elementals: earth, air, fire, and water. Later a fifth element was proposed, the quintessence, or the "Aether" which filled all otherwise empty space. According to Aristotle, objects sought their natural place—period. Other than seeking their natural place, objects moved if they were in “violent motion,” meaning that they were pushed or pulled somehow.

The only detail was determining the natural place for the elementals. Well, water belonged in the ocean (and presumably in lakes), so water fell from the sky, ran downhill in rivers, and ended up in the ocean. Air (or any gas) belonged in the sky, so vapors trapped in the Earth, or released in a fire rose upward. Earth-like stuff (a rock for example) belonged in the Earth, so stones, hail, people, and puppy dogs fell downward. Fire is a tough one, but invariably went where it belonged.

Now imagine that you are student of Aristotle, in the Lyceum in Athens (the school he founded), about 330 BC. You are bright and confident, or you wouldn't be there! Could you argue with the old man? Find a hole in his theory? What about the "gray" areas? A feather, for example. Why does it flutter downward, instead of plummeting like a stone? Why does an arrow fly horizontally through the air--is it seeking its natural place in a target somewhere? How does sound or light get from one place to another? Remember, however, that you don't know a thing about gravity or air molecules, or that the Earth orbits the sun and spins on its axis, or that the air doesn't go on forever (that empty space lies 50 kilometers overhead). Be warned, however, Aristotle is smarter than he looks, and he looks pretty smart (from statues anyway). His theories of elementals and violent motion were pretty hard to argue with given the information available.

Now imagine that it is one thousand five hundred years later. Aristotle's theories are still in place. In fact, if you live in Europe, his theories are more than "natural laws," they are part of the Church's sacred doctrine. Aristotle is not around to argue with, and anyone protesting too loudly that that Earth orbits the sun, or that objects (such as planets) move through empty space without being pushed about by angels or their equivalent, risks inciting the wrath of church officials who consider their power absolute, and your opinion irrelevant. It was dangerous to point out, for example, that the planets were not where they were supposed to be at the times predicted by the best Earth centered models, or that Jupiter has moons that orbit Jupiter and not the Earth.

At this point, you may be wishing that you lived in the second millennium AD. Well, lucky us! But perhaps we take some of the next 16 weeks or so to honor the courage and conviction of those to argued against the brilliant, but incorrect Aristotelian principles.

Shifting paradigms:

Skipping to 1543, Nicholas Copernicus wrote a book describing the Heliocentric (sun centered) model of the cosmos, with the Earth and all the known planets orbiting the sun, and only the moon orbiting the Earth. As a monk, with good relations in a permissive wing of the Catholic church, he was able to publish his work--with a couple of safeguards in place of course. For one, he wrote the book in Latin, so that only the scholarly community of Europe could read it. For another, he claimed that the book only described a calculational model--a way for astrologers, farmers, and navigators to predict and interpret the motions of the heavens. By so doing, he was able to die of natural causes soon after publication, rather than suffer any form of corporal punishment. A far less lucky natural philosopher was the Italian monk Geodorno Bruno, who was burned at the stake soon after for teaching the Copernican system (in spite of warnings to keep his belief's to himself). A lively plaza in Rome now surrounds Bruno's statue. In the late seventeenth century, Galileo used the new telescope to defend Copernicus, and developed many of the physical concepts needed to relate the idea of an orbiting, spinning earth to the motion of an arrow or an ice skater.

The Ancient Greeks produced many enduring principles. While Aristotle's first four elementals have bit the dust, so to speak, Archimedes principles of geometry, leverage, buoyancy , and mechanics are part of our current catalog of knowledge. In fact, you will study some of these later in the course to solve some very modern problems. Using geometry, Eratosthenes not only used geometrical reasoning to prove that the Earth was spherical, but also measured its circumference in 235 BC. A few years earlier, Aristarchus accurately determined the ratio of the Earth's diameter to the Moon's diameter. If Columbus had better understood Eratosthenes estimation method, he might not have tried to sail the larger actual distance westward to India

Even earlier, Democritus, (about 400 BC) postulated that all matter was composed of small elementals, called atoms. Few people believed him at the time. But how to prove the existence of something you cannot see? In those parts of the world largely freed from ancient dogma, physicists got right to work on it, finally succeeding after the start of the 20^th century.

What makes a good theory?
These results and the methods that precede them, have withstood the test of time, which in science generally means “the unending scrutiny of scientists and mathematicians.” Nowadays, a scientific theory has to be both verifiable, and falsifiable. In other words, a "good" theory must be testable. If I propose, for example, that helium balloons rise in the air, not because helium gas is less dense than the air molecules outside the balloon, but rather because of invisible-undetectable gremlins that are attracted to helium balloons and invariably push them upward, then you, as a member of the literate scientific community, have every right to scoff. There is no way to test my theory.

By the same token, every good theory is out there only so long as experimental evidence supports it. Newton's theory of gravity was perhaps the "best" theory in all of physics until Einstein showed that it is incompatible with the bending of light. Now, Einstein's general theory of relativity is the preeminent theory of gravity--having survived every devious test we can throw its way.

What about the 5^th element?
What has replaced the Aristotelian idea of elementals seeking their natural place? Our Standard model now includes four different forces and associated “fields” that act at a distance, pulling baseballs to the ground, and electrons to the atom, protons and neutrons together within the nucleus, and governing the decay of the nucleons into more fundamental particles such as quarks and neutrinos. The except number, mass, and other properties of these fundamental particles are also key aspects of the Standard Model.
But gravity is still causing trouble. Most physics students have already heard that the Universe is expanding. This is old news, relativity speaking. An expanding universe can be explained with Einstein’s theory of gravity. The news of the day, however, is that the universe is expanding faster and faster—that it is accelerating. This possibility was not predicted by Einstein or anyone else! After lots of careful thought, however, theorists have come up with the “dark energy” model, which merges Einstein’s theory with the existence of a “quintessence” or dark energy that fills all of space and drives the expansion. Famously, Einstein himself adopted and then rejected the idea of a “cosmological constant” or quintessence-like Aether that kept the universe from collapsing in on itself. Aristotle of 225 BC would never have recognized his theory, but today’s young Aristotle’s are taking the idea at face value and testing its compatibility with increasingly accurate and unexpected data.

So remember, the Paradigm of physics is summarized in a Standard Model that includes all the verified particles and interactions between them, as well as the nature of space and time itself. However, these particles and interactions are subject to daily scrutiny. They hold as long as the data from experiments support them. We already know, for example, that parts of our main theories are not compatible with one another. What happens when massive objects shrink to the sizes of atoms, for example? New models are in the works, mainly to explain just such discrepancies, such as “string” and “brane” theory, which replace the very idea of particles in three or four dimensions with vibrations and interactions in multiple dimensions. We won’t be studying these theories here, but as you continue learning and reading, put new models in context as the best fit to available data—nothing more and definitely nothing less.