physics notes

Physics is all around us. It is in the electric light you turn on in the morning; the car you drive to work; your wristwatch, cell phone, CD player, radio, and that big plasma TV set you got for Christmas. It makes the stars shine every night and the sun shine every day, and it makes a baseball soar into the stands for a home run.

Physics is the science of matter, energy, space, and time. It explains ordinary matter as combinations of a dozen fundamental particles (quarks and leptons), interacting through four fundamental forces. It describes the many forms of energy—such as kinetic energy, electrical energy, and mass—and the way energy can change from one form to another. It describes a malleable space-time and the way objects move through space and time.

There are many fields of physics, for example: mechanics, electricity, heat, sound, light, condensed matter, atomic physics, nuclear physics, and elementary particle physics. Physics is the foundation of all the physical sciences—such as chemistry, material science, and geology—and is important for many other fields of human endeavor: biology, medicine, computing, ice hockey, television…the list goes on and on.

A physicist is not some geek in a long white coat, working on some weird experiment. Physicists look and act like you or me. They work for research laboratories, universities, private companies, and government agencies. They teach, do research, and develop new technologies. They do experiments on mountaintops, in mines, and in earth orbit. They go to movies and play softball. Physicists are good at solving problems—all kinds of problems, from esoteric to mundane. How does a mirror reflect light? What holds an atom together? How fast does a rocket have to go to escape from earth? How can a worldwide team share data in real time? (Solving this last problem led physicists to invent the World Wide Web.)

Mechanics is an important field of physics. Developed by Sir Isaac Newton in the 17th century, the laws of mechanics and the law of gravity successfully explained the orbits of the moon around the earth and the planets around the sun. They are valid over a large range of distances: from much less than the height of an apple tree to much more than the distance from the earth to the moon or the sun. Newton’s laws are used to design cars, clocks, airplanes, earth satellites, bridges, buildings—just about everything, it seems, except electronics.

Electricity is another example of physics, one that you may experience as a spark when you touch a doorknob on a dry winter day. The electrical attraction of protons and electrons is the basis for chemistry. Magnetism is another force of nature, familiar to us from refrigerator magnets and compasses. In the 19th century, James Clerk Maxwell combined electricity and magnetism. He showed that light is an electromagnetic wave that travels through empty space. (Waves had always required a medium, for example, water is the medium for ocean waves.) Other electromagnetic waves besides light also travel through empty space; hence radio signals can reach us from a Mars explorer.

Maxwell’s theory also showed that electromagnetic waves travel with the same speed (the speed of light), even if the person who sees it is moving. This is in conflict with Isaac Newton’s principle of relativity, which said a train’s headlight beam would have one speed as seen by the engineer and a different speed as seen by a person watching the train go by. Newton and Maxwell could not both be right about this matter, and in 1905, Albert Einstein resolved the conflict by allowing space and time to change, depending on motion. His special theory of relativity predicted that an object passing by would look shorter and a passing clock would run slower. These changes are too small to notice unless the object is moving very fast—Newton’s laws work just fine at the speeds of ordinary moving objects. But space really does shrink and time really does expand for particles moving at speeds near the speed of light (300,000 kilometers per second).

Another remarkable consequence of special relativity is the famous equation E=mc2, which says that mass is just another form of energy. This equivalence of mass and energy is the source of the energy that comes to earth as sunlight. In the intense heat at the core of the sun, four hydrogen nuclei fuse into one helium nucleus and the mass difference is converted into radiant energy, which emerges as sunlight. E=mc2 is also responsible for the release of energy from fission of uranium in a nuclear reactor, and this energy is used around the world to make large amounts of electric power.

Einstein went on to replace Newton’s theory of gravity with his general theory of relativity, which says that space and time are changed not only by speed, but also by the presence of matter. Imagine space-time as a large sheet of rubber, and set a bowling ball on the sheet; it will be dimpled near the ball. A tennis ball rolled slowly near the bowling ball will curve around it and may settle into an orbit, just as the earth orbits the sun. Today, the general theory of relativity is well-tested and is used to accurately determine the location of your car if you have a GPS (Global Positioning System) device.

Newton’s laws also break down on the tiny distance scales of atoms and molecules, and must be replaced by the theory of quantum mechanics. For example, quantum mechanics describes how electrons can only travel around the nucleus of an atom in orbits with certain specific energies. When an electron jumps from one of these orbits to another, the atom will absorb or emit energy in discrete bundles of electromagnetic radiation. Because the energies of different states of an atom are known with high precision, we can create highly accurate devices such as atomic clocks and lasers.

Quantum mechanics is also necessary to understand how electrons flow through solids. Materials that normally do not conduct electric current can be made to conduct when “doped” with atoms of a particular element. This is how we make transistors, microscopic electrical on-off switches, which are the basis of your cell phone, your iPod, your PC, and all the modern electronics that has transformed our lives and our economy.