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The 10 Most Influential Scientists of the 20th Century

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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 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.

1. Albert Einstein

A picture of the wax figure of scientist Albert Einstein at Madame Tussaud's.
(Mario Tama / 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.

2. Marie Curie

Portrait of Marie Curie.
Picture courtesy Smithsonian Institution Archives.
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.

3. Sigmund Freud

Sigmund Freud, founder of psychoanalysis, smoking cigar.
Picture courtesy of Google-hosted LIFE Photo Archive.
Sigmund Freud (1856-1939) was a controversial figure. People either loved his theories or hated them. Even his own 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.

4. Max Planck

Picture of Max Planck.
Picture courtesy of the Smithsonian Institution.
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 actually 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.

5. Niels Bohr

Picture of scientist Niels Bohr.
Picture courtesy of the Library of Congress.
Niels Bohr (1885-1962), a Danish physicist, was only 37 when he won the Nobel Prize for 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 for Physics in 1975.)

6. Jonas Salk

Picture of Jonas Salk.
Picture courtesy of the Archives Service Center, University of Pittsburgh.
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 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 was announced on April 12, 1955, exactly ten years after Roosevelt's death, people celebrated around the world. Jonas Salk became a beloved scientist.

7. Ivan Pavlov

A drawing of a cute dog eating food.
(Picture by Chet Phillips / 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.

8. Enrico Fermi

Enrico Fermi
Enrico Fermi (1901-1954) first became interested in physics when he was 14 years old. His brother had just died unexpectedly and while looking from 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.

9. Robert Goddard

A picture of Dr. Robert Hutchings Goddard, the father of American rocketry.
Picture courtesy of NASA.
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 at 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. Francis Crick and James Watson

Computer generated picture of a three dimensional DNA double helix.
(Photo by Chad Baker / 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!
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