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Private Notes
What distinguishes science from the many other ways humanity has devised to understand the cosmos? What makes knowledge "scientific"? Why is scientific literacy so important for citizens in the modern world?
Science is a search for answers, and thus needs well-conceived questions. How are these questions formed? At what do they aim? What is "the scientific method"? Is science purely systematic, or do accident and serendipity play a role?
Scientists believe that our senses don't lie. Although this was not obvious to the ancients, the Roman scholar Pliny the Elder catalogued thousands of "facts." Ptolemy's famous geocentric model of the solar system was an early application of the scientific method.
Pivotal figures in early-modern science, Nicolas Copernicus, Tycho Brahe, and Johannes Kepler, made significant contributions to astronomy. Galileo Galilei, the great Italian physicist and astronomer, was also a pioneer of experimental methods.
Isaac Newton built on the works of Kepler and Galileo by showing that motion everywhere obeys a single set of mathematical laws. During a rural sojourn in 1665–66, he formed many of his major contributions, including calculus, some basic laws of optics, the three laws of motion, and the law of gravity.
Did a falling apple really inspire Newton to deduce the mathematical description of the universal force known as gravity? What do Newton's universal laws of motion and gravity reveal about the world? What are their implications for the study of natural phenomena?
Energy is the ability to do work—i.e., to exert a force over a distance. What are the various forms in which energy comes? How have scientists defined and studied them?
Energy constantly changes forms all around us. Study of such transformations has led to countless useful devices. Learn why, to many scientists, the first law of thermodynamics tells us something profound about the symmetry of nature.
What does the second law of thermodynamics mean? What is the difference between heat and temperature? How does heat flow? What does the second law imply about the limits on an engine's ability to convert heat energy into useful work?
In its most general form, the second law of thermodynamics states that the degree of disorder, or entropy, of any system tends to increase over time. Among the deep and far-reaching questions raised by this concept is the origin of highly ordered local systems, such as life.
Magnetism is one of the forces that can be studied in light of Newton's laws of motion. Because compasses are magnetic, magnetism was of great importance in the age of ocean exploration and commerce. Static electricity, by contrast, was little more than a fascinating curiosity.
Most modern uses of electricity rely on electrons that move. Why was Alessandro Volta's battery a turning point in electrical science? What are the components of an electrical circuit?
H. C. Oersted found that electricity can produce magnetic fields, leading to the electromagnet, the telegraph, and the electric motor. Michael Faraday showed that moving magnets induce electricity—the principle behind most electric-power generation. James Clerk Maxwell described the links between electricity and magnetism in four elegant equations.
Maxwell's equations predicted the existence of electromagnetic waves. He predicted that invisible wavelengths would be found; Hertz discovered radio waves in 1889. How do scientists divide the electromagnetic spectrum?
The discovery and application of electromagnetic radiation has transformed science and technology in ways that you'll find familiar, but also in ways that may surprise you.
Pondering a paradox that arose from Maxwell's equations, Albert Einstein stated and explored the principle of relativity, both special and general. Fatefully, Einstein also discovered that mass must be a form of energy.
While the concept of the atom, the basic building block of all matter, was first proposed at least 2,500 years ago, its existence was not verified until the 20th century. John Dalton presented the first modern statement of the atomic theory. Learn how the discovery of radioactivity and a mathematical demonstration by Einstein provided the compelling evidence.
Learn why Rutherford's concept of the atom was physically impossible, and what Niels Bohr proposed as an alternative. Bohr's model helped to explain many of the properties of light-matter interactions. Lasers make special use of the "quantum" interactions between light and matter.
In 1900, Max Planck theorized that energy comes in discrete bundles called "quanta." Einstein's research later reinforced this idea. At the atomic scale, according to Werner Heisenberg's famous uncertainty principle, every measurement changes its object. Thus quantum-scale events can only be described in terms of probabilities, and electrons display the characteristics of both particles and waves.
Long before Bohr, chemists realized that there are many kinds of atoms—the chemical elements. Elements cannot be broken down into other substances by any ordinary physical or chemical means. In 1869, Dmitri Mendeleev used observed similarities to draw up a periodic table of 63 chemical elements. Subsequent discoveries have lengthened the table but not altered its basic form.
Learn why atoms bond to one another, and what makes some types of atoms particularly unstable and reactive. Learn what distinguishes covalent from ionic and metallic bonding. The most versatile of all covalently bonded elements is carbon, the element of life.
Carbon's unparalleled ability to form covalent bonds makes it the major focus of modern chemical research. More than 90 percent of known compounds are organic; that is, they contain carbon. Polymers, the chemical building blocks of plastics, form an important class of organic molecules.
The states of matter—solid, liquid, gas, and plasma—manifest the submicroscopic organization of atoms and molecules. How do scientists define these four states?
Change is a hallmark of the material world. Wood burns, glue hardens, eggs cook, dead organisms decay, carbon graphite under high pressure becomes a diamond. Physical transformations reflect changes in the arrangement of atoms and their chemical bonds. What distinguishes a phase transformation from a chemical reaction? What are types of chemical reactions, and how do they occur?
Materials are useful because of distinct physical properties, including strength, hardness, and a variety of optical, thermal, magnetic, and electrical properties. These properties result from the kinds of atoms and their arrangements in three dimensions, and the way they are bonded.
If conductors and insulators were the only materials we had, the world of electronics would be quite limited. Computers and other marvels of modern electronics rely on the microchip, or integrated circuit, which is a single semiconductor device. Learn what semiconductors are, and how they work.
The discovery of radioactivity, and the subsequent exploration of the atomic nucleus, led to nuclear physics and nuclear chemistry. About one atom in a million is radioactive. Such atoms can decay through alpha, beta, or gamma radiation, all of which are dangerous because they can disrupt chemical bonds
Prodigious amounts of energy can be released when atoms are split (fission) or when two nuclei, usually hydrogen, are forced together (fusion). Fission reactions can be controlled in reactors or unleashed by bombs. Attempts are now underway to control fusion reactions, which would provide sustained energy.
Nearly all the information that we have about distant stars comes from electromagnetic radiation traveling at 186,000 miles per second. Astronomers collect, analyze, and interpret this data to understand the spatial distribution, dynamic state, and past and future of the universe.
Our Sun is an ordinary or "main sequence" star, 4.5 billion years old. It has several billion more years of hydrogen-burning life left, during which the contractive force of gravity will strive against the expansive force of nuclear fusion. How do stars like the Sun die, and what is left behind?
In 1924, Edwin Hubble discovered that galaxies are immense collections of gravitationally bound stars. Astronomers have since catalogued thousands of galaxies. Hubble also found a close relationship between a galaxy's distance and its "red shift," a change in light wavelengths caused by rapid movement away from us. As telescopes have improved, the estimated number of galaxies has grown to tens of billions.
The Big Bang theory proposes that the universe came into existence at one moment in time and has expanded rapidly. The Big Bang was not an explosion but an expansion—of space itself, with all its matter and energy. What observations support this theory? What surprising conclusions do astronomers draw from galactic red shifts?
The search for a "theory of everything," a set of equations that describes all matter and forces in the universe, is one of the great frontiers in physics today. What will determine whether or not we make progress in this search? What are the four fundamental forces and particles in the universe, and why do some scientists think that, at some level, they are all the same?
According to Pierre Simon Laplace's widely accepted nebular hypothesis, a star forms when gravity draws interstellar dust and hydrogen gas into an increasingly dense, small cloud that flattens into a rotating disc with most of its mass pulled to the center. If solar systems form from such discs, then there must be many in our own galaxy. The Hubble Space Telescope has produced dramatic images of star-forming regions in nearby space.
In this lecture we journey through the solar system. We voyage from Mercury, alternately seared by the Sun and frozen in darkness, to Jupiter, whose four largest moons are distinct planetlike worlds of their own, and then beyond Uranus to the beautiful blue planet Neptune.
We complete our review of the solar system and look at the fascinating research field of extrasolar planetary systems. More than a dozen planets the size of Jupiter or larger have been detected, and more are being found every month. The Earth shares many characteristics with other planets of the solar system but is unique because it has so much liquid water—the essential medium for life.
The Earth's topography seems permanent, but a close look reveals signs of constant change. What first led James Hutton to propose the key geological doctrine of uniformitarianism, which holds that great changes occur incrementally over eons?
The plate tectonics theory produced one of the great scientific revolutions of the 20th century. Before the mid-1960s, Earth studies were localized and fragmented into subdisciplines. We examine the separate lines of observational evidence that led to this grand theory, and the wealth of specific and testable predictions that flow from it.
The mechanism of plate tectonics depends on the rigidity of rocks. The lithosphere, which includes the crust and the upper mantle, floats on the relatively soft, hot asthenosphere. The Earth's surface is divided into about a dozen lithospheric plates, with earthquakes and volcanoes clustered at their boundaries. How do geologists explain the presence of volcanism in mid-plate "hot spots"?
All elements and compounds take part in geochemical cycles, which are described by identifying all the principal reservoirs, as well as the processes by which materials move from one reservoir to another. Three major Earth cycles are the water cycle, the atmospheric cycle, and the rock cycle.
Our atmosphere is an envelope of gases. Weather is the state of the atmosphere at a given time and place; climate is a long-term average of weather for a given region. What variables define the state of the atmosphere? What does paleoclimatology tell us about climate change?
The rock cycle is epic both in terms of time and scale. What are the three major types of rock recognized by geologists? How does each form? Learn some of the amazing stories that rocks tell.
Biology is the study of living systems. What characteristics do all living organisms share? What share of the estimated 50 million species has been identified? How does the Linnaean system for classifying species work?
Metabolism is the cell's process of obtaining energy from its surroundings and converting that energy into molecules. Kingdoms of organisms adopt different strategies for supporting metabolic activity—in other words, for staying alive.
All living organisms are exceptionally complex chemical systems, yet these systems are built from relatively simple parts. Life's varied chemical substances are constructed from a few molecular building blocks, which share a few essential characteristics.
What are proteins? What do they do that makes them the chemical workhorses of life? What are amino acids, and what do they have to do with proteins?
All living things are composed of cells, the fundamental unit of life. All cells arise from previous cells. How can cells be compared to chemical factories?
Classical genetics, founded in the 19th century by Gregor Mendel, is the study of how biological information is passed from parents to offspring at the level of organisms and their traits. Mendel's work was ignored and unappreciated during his lifetime, but it formed a basis for genetic discoveries in the 20th century.
Mendel's laws of genetics were purely descriptive. Cellular genetics, the study of the transfer of biological information at the level of cells, set the stage for research in molecular mechanisms of genetics. The double-helix structure of DNA was first described in 1952 by James Watson and Francis Crick.
No scientific discovery of the 20th century has had a greater impact than the deciphering of the genetic code. The Human Genome Project will map for the genes on each of the 23 pairs of human chromosomes, and determine the sequence of all three billion letters of the human genetic message.
Our growing understanding of genes raises troubling ethical questions. While each person's interests, abilities, and behavior arise from a complex interplay of environment and genetic attributes, a number of genetic diseases reveal that genes play an important role as well. What would it take to establish definitive links between heredity and personal traits?
Humans, never content simply to observe nature, have begun to read and edit the genetic code. The questions that swirl around genetic engineering exemplify the opportunities and concerns associated with these new abilities.
Genetic research in humans is driven primarily by efforts to cure inherited diseases. Yet as we learn more about "editing" genes, we may learn to design entirely new organisms. Then the central question of genetics will not be "What is the language of life?" but rather "What limits must we place on using the language of life?"
If all cells come from other cells, where did the first cell come from? What can science tell us here, and what are the competing scientific hypotheses?
Biological evolution is the central unifying theme in the life sciences. What is the evidence that guides us in understanding life's history on our planet? What is molecular phylogeny now revealing about this history?
Evolution is an observational fact, though there are competing theories about how it occurs. The primary source of evidence for the evolution of life comes from the fossil record.
When Charles Darwin first formed his theory of natural selection, he was troubled by the lack of a known physical mechanism for change. What do we know today that fills that gap?
Species always occur as part of an ecosystem—an interdependent community of species and its physical environment. The law of unintended consequences states that any change in one part of a complex system may affect other parts of the system, often in unpredictable ways. How can we improve our understanding of our impact on ecosystems?
Modern technology and population growth have led to many concerns about their effects on the environment and global climate. Local problems are fairly straightforward, but as problems become less localized, both diagnoses and solutions grow more elusive. This lecture reviews three such problems: the ozone hole, acid rain, and the greenhouse effect.
Recently a number of science watchers have claimed that science is approaching its end—that all there is of significance to be learned about the natural world will soon be known. Are they right?
