Karen Hill

For all of history, matter was matter and energy was energy. The two were separate, unrelated concepts. Then Albert Einstein established the relationship between matter and energy by creating the most famous equation in the history of humankind, E = mc2. (The second most famous is the Pythagorean Theorem for a right triangle, A2 = … Read more

A neutron is a subatomic particle located in the nucleus of an atom with the mass of a proton but no electrical charge. The discovery of neutrons has been hailed as a major landmark of twentieth-century science. First, this discovery completed our understanding of the structure of atoms. Second, because they have no electrical charge, … Read more

At the dawn of the twentieth century, scientists thought that all control signals in the human body were sent electrically along nerve fibers. Then William Bayliss and Ernst Starling discovered that chemical messengers (called hormones) as well as electric signals trigger body organs to function. This startling discovery started a whole new field of medical … Read more

What could be more basic to understanding planet Earth than to know what lies between the surface and Earth’s center, or between the surface and outer space? Yet the twentieth century dawned with science having virtually no concept of what the atmosphere was like more than two miles above the earth’s surface. Teisserenc de Bort … Read more

Marie Curie’s discovery of two naturally radioactive elements, polonium and radium, made headline news, but her real discovery was that atoms were not small solid balls and that there must be even smaller particles inside them. This discovery opened the door to all atomic and subatomic research and even to the splitting of the atom. … Read more

Mitochondria are tiny energy producers in every cell. One of many tiny structures floating in the cell’s cytoplasm (fluid) that are collectively called organelles, mitochondria are considered the most important of all cell parts, besides the nucleus. Amazingly, mitochondria have their own separate DNA. You depend on them. They depend on you. And yet they … Read more

Albert Claude was the first scientist to develop procedures for isolating and studying individual structures within a cell. He is the one who mapped the inner organization and activity of a cell and its many components. He is rightly called the founder of modern cell biology. Although he never graduated from high school, Claude pioneered … Read more

Albert Einstein is one of only three or four scientists in history who have changed the fundamental ways in which humans view the universe. Einstein’s theory of relativity changed humankind’s core assumptions concerning the nature of the universe and of Earth’s and of humans’ place in it. The twentieth century’s developments in technology, science, and … Read more

We label foods by their vitamin content. We spend billions of dollars every year buying vitamin supplements. Vitamins are essential to life and health. Yet any awareness of vitamins, even the very notion of vitamins, is only 100 years old. It had not occurred to anyone to search for trace elements in food that human … Read more

Nothing is more basic than knowing your age, or the age of your house, or of a tree in your yard. For science, the same is true for Earth and for the rocks that make up Earth’s crust. Scientists had been estimating Earth’s age for thousands of years. However, these were little more than guesses. … Read more

The word “antibiotic” comes from the Greek words meaning “against life.” Early folk medicine relied on some natural compounds that cured certain diseases, the ground bark of a tree, certain cheese molds, certain fungi. Doctors knew that these natural compounds worked, but had no idea of how or why they worked. Paul Ehrlich conducted the … Read more

In the first 20 years of the twentieth century, physics buzzed with the incredible discovery of the subatomic world. Long before microscopes were powerful enough to allow researchers to see an atom, scientists used mathematics to probe into the subatomic world of electrons, protons, and alpha and beta particles. Albert Einstein, Werner Heisenberg, Max Planck, … Read more

Marie Curie opened the century by proving that there was a subatomic world. Einstein, Dirac, Heisenberg, Born, Rutherford, and others provided the new theoretical descriptions of this subatomic world. But proving what lurked within an atom’s shell, and what governed its behavior, lingered as the great physics challenges of the early twentieth century. It was … Read more

A black hole is a collapsed star that is so dense, and whose gravitational pull is so great, that not even light can escape it. Such stars would look like black holes in a black universe. Many consider black holes to be the ultimate wonder of the universe, the strangest of all stellar objects. Black … Read more

Insulin is a hormone produced by the pancreas that allows the body to pull sugar from blood and burn it to produce energy. Frederick Banting discovered a way to remove and use the pancreatic “juice” of animals to save the lives of diabetic humans. The discovery of the hormone insulin has saved millions of human … Read more

Like cracking the genetic code, like the creation of the atomic bomb, the discovery of how the brain’s system of neurons communicates is one of the fundamental science developments of the twentieth century. Nerves signal sensations to the brain; the brain flashes back commands to muscles and organs through nerves. But how? Otto Loewi’s discovery … Read more

Humans have always wondered how we came to be on this planet. Virtually every culture and religion has created myths to explain the creation of humans. In the early twentieth century, most scientists believed that the first humans appeared in Asia or Eastern Europe. Then Dart discovered the Taung skull and provided the first solid … Read more

Far smaller than cells and bacteria, viruses are the smallest life forms on Earth, so small they can only reproduce inside some host cell and do it by taking over control of that cell. Viruses are so small they easily slip through virtually any filter or trap. Their discovery answered many medical questions at the … Read more

Atoms had never been seen before. Defined as the smallest particles possible and the basic building blocks of all matter, they were invisibly small, in the late nineteenth century still more theoretical than real. How could someone claim to have found something smaller? How could particles get any smaller? J. J. Thomson discovered the electron … Read more

If you have ever had an X-ray as part of a medical checkup, you owe thanks to Wilhelm Roentgen. Medical X-rays have been one of the most powerful, useful, and life-saving diagnostic tools ever developed. X-rays were the first noninvasive technique developed to allow doctors to see inside the body. X-rays led to the more … Read more

The modern worlds of chemistry and physics depend on knowing and studying the universe of atoms. But no one could actually see an atom until the invention of the electron microscope in 1938. Centuries before that, atoms were well known and were an important part of chemistry and physics research. It was John Dalton who … Read more

Edwin Hubble’s twin discoveries (that there are many galaxies in the universe, not just the Milky Way, and that all of those galaxies are traveling outward, expanding the universe) rank as the most important astronomical discoveries of the twentieth century. These discoveries radically changed science’s view of the cosmos and of our place in it. … Read more

What Is It? A medication used during surgery that causes loss of awareness of pain in patients. Who Discovered It? Humphry Davy Anesthesia created safe surgery and made many medical and dental operations practical and plausible. The trauma suffered by patients from the pain of an operation was often so dangerous that it kept doctors … Read more

In the eighteenth century scientists still believed that Earth’s surface had remained unchanged until cataclysmic events (the great flood of Noah’s ark fame was the most often sited example) radically and suddenly changed the face of our planet. They tried to understand the planet’s surface structures by searching for those few explosive events. Attempts to … Read more

Scientists once believed that heat was an invisible, weightless liquid called caloric. Things that were hot were stuffed with caloric. Caloric flowed from hot to cold. They also believed that fire (combustion) came from another invisible substance called phlogiston, a vital essence of combustible substances. As a substance burned, it lost phlogiston to air. The … Read more

Werner Heisenberg is famed worldwide for discovering the Uncertainty Principle, which states that it is impossible to determine both the position and momentum (motion) of an elementary particle at the same time since the effort to determine either would change the other in unpredictable ways. This pivotal theorem marked a fundamental turning point in science. … Read more

In the late 1800s discovering the true the speed of light had only minor importance because astronomers were the only ones who used this number. Distances across space are measured in light-years, how far light travels in one year’s time. Since their measurements were only approximations anyway, they could accept a 5 percent (or even … Read more

Blood was blood, or so the world thought. Then Austrian physician Karl Landsteiner discovered that there were four types of blood. Some could be safely mixed and some could not. That discovery has saved millions of lives. The day that Karl Landsteiner’s results were published, blood transfusions became a safe and risk-free part of surgery. … Read more

Before 1820, the only known magnetism was the naturally occurring magnetism of iron magnets and of lodestones, small, weak direction finders. Yet the modern world of electric motors and electric generating power plants is muscled by powerful electromagnets. So is every hair dryer, mixer, and washing machine. Our industry, homes, and lives depend on electric … Read more

Gregor Mendel conducted the first serious study of heredity. His findings, his methods, and his discoveries laid the foundation for the field of genetics and the study of genes and heredity. The discoveries of genes, chromosomes, DNA, and the decoding of the human genome (completed in 2003) are all direct descendents of Mendel’s work. The … Read more

When most people think of the chemical elements, they picture Mendeleyev’s Periodic Chart of the Elements. This organizational table has served as the one accepted organizing system for the elements that make up our planet for 125 years. It is so important that it is taught to every student in beginning chemistry classes. It led … Read more

Charles Thomson radically changed science’s view of deep oceans and of the requirements for life in the oceans. There existed no light in the ocean depths, yet he discovered abundant and varied life. He proved that life can exist without light. He even proved that plants can thrive in the lightless depths (though it took … Read more

If atoms are the basic building block of each element, then molecules are the basic building blocks of each substance on Earth. Scientists were stalled by their inability to accurately imagine, let alone detect, particles as small as an atom or a molecule. Many had theorized that some tiny particle (that they called an atom) … Read more

Humphry Davy discovered that the chemical bonds between individual atoms in a molecule are electrical in nature. We now know that chemical bonds are created by the sharing or transfer of electrically charged particles, electrons, between atoms. In 1800, the idea that chemistry somehow involved electricity was a radical discovery. Davy’s discovery started the modern … Read more

Infrared and ultraviolet radiation are key parts of our scientific development over the past 200 years. Yet until 1800 it never occurred to anyone that radiation could exist outside the narrow band that human eyes detect. The discovery of infrared and ultraviolet light expanded science’s view beyond the visible light to the whole radiation spectrum, … Read more

Have you had smallpox? Polio? Typhoid? Probably not. However, such infectious diseases used to plague humankind. The word plague comes from one of these killer diseases, the bubonic plague. Throughout the fourteenth and fifteenth centuries, the plague killed nearly half of the population of Europe. Smallpox killed over 100,000 people a year for a century … Read more

Chromosomes carry genes that hold the blueprints for building, operating, and maintaining the cells of your body. Genetics and heredity research could not advance until these physical structures inside the nucleus of each cell had been discovered and studied. Our basic understanding of biology also depends, in part, on our knowledge of how cells divide, … Read more

Photosynthesis is the process that drives plant production all across Earth. It is also the process that produces most of the oxygen that exists in our atmosphere for us to breathe. Plants and the process of photosynthesis are key elements in the critical (for humans and other mammals) planetary oxygen cycle. When Jan Ingenhousz discovered … Read more

Joseph Priestley’s discovery of oxygen sparked a chemical revolution. He was the first person to isolate a single gaseous element in the mixture of gasses we call “air.” Before Priestley’s discovery, scientific study had focused on metals. By discovering that air wasn’t a uniform thing, Priestley created a new interest in the study of gasses … Read more

Electricity is one of our greatest energy resources and one of the few natural energy sources. Benjamin Franklin’s electricity experiments were the first scientific ventures into the nature and use of electricity and uncovered its true nature. They set the stage for much of the scientific and engineering development in the nineteenth century and for … Read more

The discovery that stars are clumped into galaxies represents the first advance in efforts to describe the actual shape of the universe and the distribution of stars in it. Thomas Wright’s theory of galaxies was the first astronomical work to place our sun not in the center of the universe, but in a tightly packed … Read more

The Atlantic Ocean’s Gulf Stream is the most important of our world’s ocean currents. It is a major heat engine, carrying massive amounts of warm water north to warm Europe. It has directed the patterns of ocean exploration and commerce and may be a major determinant of the onset of ice ages. Finally, it is … Read more

Antoine Lavoisier was the first chemist to believe in measurement during and after experiments. All chemists before had focused on observation and description of the reactions during an experiment. By carefully measuring the weight of each substance, Lavoisier discovered that matter is neither created nor destroyed during a chemical reaction. It may change from one … Read more

Until the eighteenth century, nature was viewed as a wild profusion of life. Carl Linnaeus discovered order and organization in that seeming randomness. His system for naming, grouping, and conceptually organizing plants and animals provided insights into botany, biology, ecosystems, and biological structure that scientists still rely on almost 300 years later. For his discovery, … Read more

Darwin’s theory of evolution and its concept of survival of the fittest is the most fundamental and important discovery of modern biology and ecology. Darwin’s discoveries are 150 years old and are still the foundation of our understanding of the history and evolution of plant and animal life. Darwin’s discovery answered countless mysteries for anthropology … Read more

The Doppler Effect is one of the most powerful and important concepts ever discovered for astronomy. This discovery allowed scientists to measure the speed and direction of stars and galaxies many millions of light years away. It unlocked mysteries of distant galaxies and stars and led to the discovery of dark matter and of the … Read more

Yogurt and other dairy products soured and curdled in just a few days. Meat rotted after a short time. Cow’s and goat’s milk had always been drunk as fresh milk. The consumer had to be near the animal since milk soured and spoiled in a day or two. Then Louis Pasteur discovered that microscopic organisms … Read more

Twenty new elements (beginning with the discovery of cesium in 1860) were discovered using one chemical analysis technique. That same technique allows astronomers to determine the chemical composition of stars millions of light years away. It also allowed physicists to understand our sun’s atomic fires that produce heat and light. That same technique allows other … Read more

Energy is never lost. It can change from one form to another, but the total amount of energy never changes. That principle has allowed scientists and engineers to create the power systems that run your lights and house and fuel your car. It’s called conservation of energy and is one of the most important discoveries … Read more

We now know that mechanical work, electricity, momentum, heat, magnetic force, etc., can be converted from one to another. There is always a loss in the process, but it can be done. That knowledge has been a tremendous help for the development of our industries and technologies. Only 200 years ago, the thought had not … Read more

It was a revolutionary idea: Earth’s climate had not always been the same. Every scientist for thousands of years had assumed that Earth’s climate had remained unchanging for all time. Then Louis Agassiz discovered proof that all Europe had once been covered by crushing glaciers. Earth’s climate had not always been as it was now. … Read more

Most people and scientists assumed that the world and its mix of plants and animals had always been as it was when these scientists lived. The discovery of dinosaur fossils destroyed that belief. This discovery represented the first proof that entire groups of ancient, and now extinct, animals once roamed Earth. It was the first … Read more

The concept Robert Boyle discovered (now called Boyle’s Law) laid the foundation for all quantitative study and chemical analysis of gasses. It was the first quantitative formula to describe the behavior of gasses. Boyle’s Law is so basic to understanding chemistry that it is taught to every student in beginning chemistry classes. A genius experimenter, … Read more

Throughout most of the nineteenth century, people thought that electricity, magnetism, and light were three separate, unrelated things. Research proceeded from that assumption. Then Maxwell discovered that they are all the same, forms of electromagnetic radiation. It was a startlingly grand discovery, often called the greatest discovery in physics in the nineteenth century. Maxwell did … Read more

The human circulatory system represents the virtual definition of life. No system is more critical to our existence. Yet only 400 years ago, no one understood our circulatory system. Many seriously thought that the thumping inside the chest was the voice of the conscience trying to be heard. Most thought that blood was created in … Read more

Galileo discovered that other planets have moons and thus extended human understanding beyond our own planet. His careful work with the telescopes he built launched modern astronomy. His discoveries were the first astronomical discoveries using the telescope. Galileo proved that Earth is not unique among planets of the universe. He turned specks of light in … Read more

It is a simple, seemingly obvious notion: air has weight; the atmosphere presses down on us with a real force. However, humans don’t feel that weight. You aren’t aware of it because it has always been part of your world. The same was true for early scientists, who never thought to consider the weight of … Read more

Even after Copernicus simplified and corrected the structure of the solar system by discovering that the sun, not the earth, lay at the center of it, he (like all astronomers before him) assumed that the planets orbited the sun in perfect circles. As a result, errors continued to exist in the predicted position of the … Read more

It seems a simple and obvious discovery. Heavier objects don’t fall faster. Why does it qualify as one of the great discoveries? Because it ended the practice of science based on the ancient Greek theories of Aristotle and Ptolemy and launched modern science. Galileo’s discovery brought physics into the Renaissance and the modern age. It … Read more

The human anatomy references used by doctors through the year A.D. 1500 were actually based mostly on animal studies, more myth and error than truth. Andreas Vesalius was the first to insist on dissections, on exact physiological experiment and direct observation, scientific methods, to create his anatomy guides. His were the first reliable, accurate books … Read more

Copernicus measured and observed the planets and stars. He gathered, compiled, and compared the observations of dozens of other astronomers. In so doing Copernicus challenged a 2,000-year-old belief that the earth sat motionless at the center of the universe and that planets, sun, and stars rotated around it. His work represents the beginning point for … Read more

Newton’s three laws of motion form the very the foundation of physics and engineering. They are the underlying theorems that our physical sciences are built upon, just as Euclid’s basic theorems form the foundation of our modern geometry. For the creation of these laws, combined with his discovery of gravity and his creation of calculus, … Read more

By the early seventeenth century, many forces had been identified: friction, gravity, air resistance, electrical, forces people exerted, etc. Newton’s mathematical concept of gravity was the first step in joining these seemingly different forces into a single, unified concept. An apple fell; people had weight; the moon orbited Earth, all for the same reason. Newton’s … Read more

The only way we can learn about the ancient past is to examine fossil remains of now extinct plants and animals and try to re-create that long-gone life and environment. Scientists can only do this if they correctly interpret the fossil remains that are dug from ancient rock layers. That process began with Nicholas Steno. … Read more

Our understanding of the universe depends on two foundations, our ability to measure the distances to faraway stars, and our ability to measure the chemical composition of stars. The discovery that allowed scientists to determine the composition of stars is described in the 1859 entry on spectrographs. The distance to the sun has always been … Read more

Just as Galileo used his telescope to open the human horizon to the planets and stars of space, so van Leeuwenhoek used his microscope to open human awareness to the microscopic world that was invisibly small and that no one had even dreamed existed. He discovered protozoa, bacteria, blood cells, sperm, and capillaries. His work … Read more

The cell is the basic unit of anatomy. Countless millions of cells build living plants and animals. The functions of a body can be studied by studying individual cells. Just as the discovery of the molecule and atom allowed scientists to better understand chemical substances, Hooke’s discovery of the cell has allowed biologists to better … Read more

The concepts of buoyancy (water pushes up on an object with a force equal to the weight of water that the object displaces) and of levers (a force pushing down on one side of a lever creates a lifting force on the other side that is proportional to the lengths of the two sides of … Read more

The classical view of dinosaurs through the first half of the twentieth century was that they were plodding, cold-blooded monsters. Dinosaurs were also were depicted as sluggish, dull-gray, and so dumb that they weren’t capable of decent parenting. That was what expert paleontologists believed. Robert Bakker shattered those beliefs. Robert Bakker was the first to … Read more

One of the great questions for science and humanity is: Are we the only solar system with planets, and the only one with planets that could support life? The discovery of planets around other stars makes it likely that other planets exist capable of supporting life. Of great importance to astronomers, the discovery of other … Read more

A great debate began after Edwin Hubble discovered that the universe is expanding: Is that expansion slowing so that it will eventually stop and the universe will begin to collapse? Saul Perlmutter discovered that the expansion of the universe is actually accelerating, shattering all existing scientific models of the motion of the universe. The universe … Read more

Austrian monk Gregor Mendel discovered the concept of heredity in 1865, launching the field of genetics. In 1953 Francis Crick and James Watson discovered the double helix shape of the DNA molecule that carried all genetic instructions. The problem was that there were billions of genetic instructions carried on the complete human genetic code, or … Read more