General knowledge, oriented

The SI rests on seven base units: meter, kilogram, second, ampere, kelvin, mole and candela, and every other SI unit is built from them, such as the newton (kg m/s^2) and the joule (N m). Since May 20, 2019, all seven have been fixed by exact constants of nature rather than objects: the meter through the speed of light, exactly 299,792,458 meters per second, and the kilogram through the Planck constant, which retired the platinum-iridium cylinder kept near Paris. A mole is a count, about 6.022 x 10^23 entities, not a mass.

Many everyday scales are logarithmic, so each step multiplies rather than adds. Adding 10 decibels makes a sound ten times as intense, yet it seems only about twice as loud; one whole step of earthquake magnitude is about 32 times the energy; and each pH unit is a tenfold change in acidity.

The classic trap is a category error. A light-year is a distance, about 9.46 trillion kilometers, not a time. A food Calorie is a kilocalorie. A degree Celsius and a kelvin are the same size, and only the zero point differs, so 0 degrees C is 273.15 K. For example, if a headline calls a magnitude 7 quake twice as strong as a magnitude 6, check the scale: one step is about 32 times the energy.

Rule: before you compare two numbers, ask what the unit measures and whether its scale adds or multiplies.

A few laws and models organize most of science. Newton's laws say a force changes motion rather than sustaining it (F = ma), and gravity weakens with the square of distance, so doubling the distance leaves a quarter of the pull. Energy is conserved, but the combined entropy of a system and its surroundings never falls, which is why heat flows from hot to cold on its own. In biology, DNA is transcribed into RNA and translated into protein, natural selection needs inherited variation plus unequal reproduction, and every cell comes from an existing cell. On Earth, plates creeping over softer rock below cause earthquakes, volcanoes and mountain ranges, and the greenhouse effect is natural: gases that absorb infrared keep the surface near 15 C on average.

Several famous ideas are commonly garbled, and the popular version is wrong in a specific way. Heavier objects do not fall faster: apart from air resistance, heavy and light objects fall at the same rate. The Big Bang was not an explosion at one point in space; space itself expanded everywhere, so there is no center. As usually stated, the uncertainty principle limits how sharply a state can be prepared, not just the quality of instruments. And there is no Nobel Prize in mathematics.

Say someone claims a vaccine works by giving you the disease. A vaccine shows the immune system an antigen, a killed or weakened microbe or only a piece of one, so memory cells can respond faster and more strongly to a real exposure later.

Rule: learn each law as the specific thing it predicts or forbids, not as a slogan.

History here is a chain of turning points, and the useful skill is knowing what each one changed. Farming began around 12,000 years ago in the Fertile Crescent, and writing began with Sumerian cuneiform around 3100 BC, to keep records of goods and payments. Rome's Republic gave way to an empire under Augustus; the western empire ended in 476, while the eastern empire survived until the Ottomans took Constantinople in 1453. Printing spread Luther's writings after 1517, Copernicus, Galileo, Kepler and Newton remade astronomy, and industrialization in England after about 1750 was led by cotton textiles. The modern world runs through the American, French and Russian revolutions, two world wars, decolonization in Asia and Africa between 1945 and 1960, the Cold War and the opening of the Berlin Wall on November 9, 1989.

A common mistake is to compress causes and credit. Julius Caesar was never emperor; his heir Augustus was the first. Gutenberg is credited with uniform metal type, but he did not invent printing: paper and block printing came from China. The 1863 Emancipation Proclamation freed people only in areas in rebellion, and the 13th Amendment ended U.S. slavery in December 1865. And the Great Depression had several causes, not only the 1929 crash.

For example, if someone asks when Rome fell, ask which Rome: the West in 476 or the East in 1453.

Rule: tie every date to what it changed, and distrust any story with a single cause.

Institutions are easiest to understand by asking who holds which power and who can check it. Separation of powers splits making law, carrying it out and judging disputes among branches that check one another. In a parliamentary system the executive governs only with the legislature's support, while a president is elected separately and cannot be removed by a no-confidence vote. Federal systems divide power between national and constituent governments; unitary states keep final authority at the center. Courts can strike down laws that violate a constitution: the US Supreme Court first did so in Marbury v. Madison (1803).

Internationally, the bodies are easy to confuse. The Security Council has five permanent members with a veto, and UN members agree to carry out its decisions; the General Assembly gives every state one vote, but its resolutions are recommendations. The International Court of Justice settles disputes between states, while the International Criminal Court prosecutes individuals and is not part of the UN. The International Monetary Fund (IMF) steadies countries in financial crisis, while the World Bank funds development.

Say a news story reports that the UN ordered something. Check which body acted: members agree to carry out Security Council decisions, while a General Assembly resolution is a recommendation.

Rule: for any institution, ask who chooses it, what it can decide, and who can overrule it.

Art history reads best as a sequence of reactions. Renaissance painters added linear perspective and chiaroscuro, modeling form with light and dark; Baroque art, from around 1600, turned to drama and theatrical light; Neoclassicism revived Greek and Roman models; Romanticism put feeling and nature ahead of formal rules; Impressionism, from the 1860s, recorded fleeting light with short, visible strokes; and Post-Impressionism and Cubism pushed further past the optical record. Music has its own eras: Baroque (about 1600-1750) with Bach, Classical with Haydn and Mozart, Romantic with Chopin and Wagner, and Modern, which stretches or abandons tonality. In literature, epic plunges into the middle of events, Greek tragedy grew from the worship of Dionysus, and Shakespeare's First Folio (1623) sorted his plays into comedies, histories and tragedies.

Common confusions: classical music broadly covers several eras, while the Classical era is only one. Cezanne is classed with the Post-Impressionists, not the Impressionists. Aristotle required only unity of action; the three unities were codified by later critics. And the main idea of Gothic architecture is gathering a roof's weight onto piers through rib vaults and flying buttresses, not the pointed arch as such.

For example, to tell Greek columns apart, look at the top: a plain rounded capital is Doric, large paired scrolls are Ionic, and acanthus leaves are Corinthian.

Rule: place a work by what it reacts against, then confirm the label with one concrete marker.

English is a layered language, and most word puzzles dissolve once you know the layers. It belongs to the Indo-European family, which links it to Latin, Greek, Persian and Sanskrit. Latin arrived in waves: church words with Christianity, Latin-based French after the Norman Conquest, and scholarly borrowings in the Renaissance. Greek supplies the building blocks of science and medicine, such as -itis and -logy. The Normans explain why the animal keeps a Germanic name and the meat a French one: cow and beef, swine and pork, sheep and mutton. Between about 1350 and 1700 the Great Vowel Shift moved the long vowels while printing fixed the spelling, which is why English letters and sounds parted ways.

Scripts differ in what a sign stands for. An alphabet writes consonants and vowels, an abjad writes consonants and leaves vowels optional, an abugida gives each consonant a built-in vowel, a syllabary has a sign per syllable, and Chinese characters stand for whole morphemes. Hieroglyphs were cracked when Champollion showed in 1822 that their signs also spell sounds.

The trap is the tidy origin story. Sirloin was never knighted; it comes from French surloin, upon the loin. Smog was coined by a Londoner in 1905, not in Los Angeles. Say you hear a clever story about how a word began: trace the word's earliest recorded form before you repeat the story.

Rule: trust the documented history of a word over the best story about it.

Myths and religions are best learned as systems rather than lists of names. The twelve Olympians each have a domain and symbols, such as Zeus's thunderbolt and Poseidon's trident, and the Romans matched their gods to Greek ones by role: Zeus to Jupiter, Ares to Mars, Aphrodite to Venus. Norse myth centers on Odin, Thor and Loki and ends in Ragnarok; Egyptian myth on Ra, Osiris and the weighing of the heart against a feather. For the major religions, learn each one's core: Judaism's Torah, Talmud and festivals; Christianity's Catholic, Orthodox and Protestant branches, formed by splits in 1054 and from 1517; Islam's five pillars and the Sunni-Shia difference over Muhammad's successor; Buddhism's Four Noble Truths; and the Hindu triad of Brahma, Vishnu and Shiva.

Common confusions: the Roman mapping pairs roles, not words, so Apollo kept his Greek name and Janus has no Greek twin. Achilles' vulnerable heel is a later tale, not in Homer. A myth, a legend and a folktale are different kinds of story, and myth does not simply mean false.

For example, flood stories appear in Mesopotamia (Utnapishtim), the Bible (Noah) and Greece (Deucalion); such parallels show a recurring theme, not necessarily one shared source.

Rule: learn each tradition's own structure first, then compare across traditions with care.

Much of world geography follows from a few physical facts. Earth's axis tilts about 23.4 degrees, which sets the tropics and causes the seasons. Moon phases come from how much of the Moon's sunlit half faces us, and tides from the pull of the Moon and, about half as strongly, the Sun. Deserts cluster near 30 degrees north and south, where dry air sinks. Plates meet at divergent, convergent and transform boundaries, which explains ridges, trenches, volcanoes and ranges such as the Himalayas, raised when India collided with Asia.

The human map is conventional. The usual count is seven continents, though the Europe-Asia line is disputed; time zones bend around borders; the International Date Line zigzags and has been moved; and the prime meridian at Greenwich was chosen in 1884. Capitals are chosen, not ranked by size: Canberra, not Sydney, is Australia's capital.

The trap is mistaking a convention for a law of nature, or the reverse. The far side of the Moon is not a dark side: it gets as much sunlight as the near side. For example, if someone says summer comes because Earth is nearer the Sun, point out that Earth is closest to the Sun during the northern winter.

Rule: separate what physics fixes from what people agreed on, and check each claim against the right one.