Humanities › History & Culture Most Influential Scientists of the 20th Century Share Flipboard Email Print History & Culture The 20th Century People & Events Fads & Fashions Early 20th Century The 20s The 30s The 40s The 50s The 60s The 80s The 90s American History African American History African History Ancient History and Culture Asian History European History Genealogy Inventions Latin American History Medieval & Renaissance History Military History Women's History View More By Jennifer Rosenberg History Expert B.A., History, University of California at Davis Jennifer Rosenberg is a historian and writer who specializes in 20th-century history. our editorial process Jennifer Rosenberg Updated January 05, 2019 Scientists look at the world and ask, "Why?" Albert Einstein came up with most of his theories just by thinking. Other scientists, like Marie Curie, used a lab. Sigmund Freud listened to other people talk. No matter what tools these scientists used, they each discovered something new about the world we live in and about ourselves in the process. 01 of 10 Albert Einstein Bettmann Archive/Getty Images Albert Einstein (1879-1955) may have revolutionized scientific thought, but what made the public adore him was his down-to-earth sense of humor. Known for making short quips, Einstein was the people's scientist. Despite being one of the most brilliant men of the 20th century, Einstein appeared approachable, partly because he always had uncombed hair, disheveled clothing, and a lack of socks. During his entire life, Einstein worked diligently to understand the world around him and in so doing, developed the Theory of Relativity, which opened the door for the creation of the atomic bomb. 02 of 10 Marie Curie Corbis/Getty Images Marie Curie (1867-1934) worked closely with her scientist husband, Pierre Curie (1859-1906), and together they discovered two new elements: polonium and radium. Unfortunately, their work together was cut short when Pierre died suddenly in 1906. (Pierre had been trampled by a horse and carriage while trying to cross a street.) After Pierre's death, Marie Curie continued to research radioactivity (a term she coined), and her work eventually earned her a second Nobel Prize. Marie Curie was the first person to be awarded two Nobel Prizes. Marie Curie's work led to the use of X-rays in medicine and laid the foundation for the new discipline of atomic physics. 03 of 10 Sigmund Freud Bettmann Archive/Getty Images Sigmund Freud (1856-1939) was a controversial figure. People either loved his theories or hated them. Even his disciples got into disagreements. Freud believed that every person has an unconscious that can be discovered through a process called "psychoanalysis." In psychoanalysis, a patient would relax, perhaps on a couch, and use free association to talk about whatever they wanted. Freud believed that these monologues could reveal the inner workings of the patient's mind. Freud also postulated that slips of the tongue (now known as "Freudian slips") and dreams were also a way to understand the unconscious mind. Although many of Freud's theories are no longer in regular use, he established a new way of thinking about ourselves. 04 of 10 Max Planck Bettmann Archive/Getty Images Max Planck (1858-1947) didn't mean to but he completely revolutionized physics. His work was so important that his research is considered the pivotal point where "classical physics" ended, and modern physics began. It all started with what seemed an innocuous discovery — energy, which appears to be emitted in wavelengths, is discharged in small packets (quanta). This new theory of energy, called quantum theory, played a role in many of the most important scientific discoveries of the 20th century. 05 of 10 Niels Bohr Bettmann Archive/Getty Images Niels Bohr (1885-1962), a Danish physicist, was only 37 when he won the Nobel Prize in Physics in 1922 for his progress in understanding the structure of atoms (specifically his theory that electrons lived outside the nucleus in orbits of energy). Bohr continued his important research as the director of the Institute for Theoretical Physics at the University of Copenhagen for the rest of his life, except during World War II. During WWII, when the Nazis invaded Denmark, Bohr and his family escaped to Sweden on a fishing boat. Bohr then spent the rest of the war in England and the United States, helping the Allies create an atomic bomb. (Interestingly, Niels Bohr's son, Aage Bohr, also won the Nobel Prize in Physics in 1975.) 06 of 10 Jonas Salk Three Lions/Getty Images Jonas Salk (1914-1995) became a hero overnight when it was announced that he had invented a vaccine for polio. Before Salk created the vaccine, polio was a devastating viral disease that had become an epidemic. Each year, thousands of children and adults either died from the disease or were left paralyzed. (U.S. President Franklin D. Roosevelt is one of the most famous polio victims.) By the early 1950s, polio epidemics had been increasing in severity and polio had become one of the most feared childhood diseases. When the positive results from an extensive test trial of the new vaccine were announced on April 12, 1955, exactly ten years after Roosevelt's death, people celebrated around the world. Jonas Salk became a beloved scientist. 07 of 10 Ivan Pavlov Hulton Archive/Getty Images Ivan Pavlov (1849-1936) studied drooling dogs. While that may seem like an odd thing to research, Pavlov made some fascinating and important observations by studying when, how, and why dogs drooled when introduced to varied, controlled stimuli. During this research, Pavlov discovered "conditioned reflexes." Conditioned reflexes explain why a dog would automatically drool when hearing a bell (if usually the dog's food was accompanied by a bell being rung) or why your tummy might rumble when the lunch bell rings. Simply, our bodies can be conditioned by our surroundings. Pavlov's findings had far-reaching effects in psychology. 08 of 10 Enrico Fermi Keystone/Getty Images Enrico Fermi (1901-1954) first became interested in physics when he was 14 years old. His brother had just died unexpectedly, and while looking for an escape from reality, Fermi happened upon two physics books from 1840 and read them from cover to cover, fixing some of the mathematical errors as he read. Apparently, he didn't even realize the books were in Latin. Fermi went on to experiment with neutrons, which led to the splitting of the atom. Fermi is also responsible for discovering how to create a nuclear chain reaction, which led directly the creation of the atomic bomb. 09 of 10 Robert Goddard Bettmann Archive/Getty Images Robert Goddard (1882-1945), considered by many to be the father of modern rocketry, was the very first to successfully launch a liquid-fueled rocket. This first rocket, named "Nell," was launched on March 16, 1926, in Auburn, Massachusetts and rose 41 feet into the air. Goddard was just 17 years old when he decided he wanted to build rockets. He was climbing a cherry tree on October 19, 1899 (a day he forever after called "Anniversary Day") when he looked up and thought how wonderful it would be to send a device to Mars. From that point on, Goddard built rockets. Unfortunately, Goddard was not appreciated in his lifetime and was even ridiculed for his belief that a rocket could one day be sent to the moon. 10 of 10 Francis Crick and James Watson Bettmann Archive/Getty Images Francis Crick (1916-2004) and James Watson (b. 1928) together discovered the double helix structure of DNA, the "blueprint of life." Surprisingly, when news of their discovery was first published, in "Nature" on April 25, 1953, Watson was just 25 years old and Crick, although older than Watson by little more than a decade, was still a doctoral student. After their discovery was made public and the two men became famous, they went their separate ways, rarely speaking to each other. This may have been in part because of personality conflicts. Although many considered Crick to be talkative and brash, Watson made the very first line of his famous book, "The Double Helix" (1968): "I have never seen Francis Crick in a modest mood." Ouch!