Almost half a million of the people alive
today were alive 100 years ago. And they’ve seen a lot: the roaring 20s,
the swinging 60s, the Lost Decades, the wars, the art, and the viral cat videos. Our understanding of the world has also changed
a ton, just in their lifetimes. Around 100 years ago, humans didn’t know
much about genetics and quantum mechanics, or why certain foods and fungi can keep us
healthy. But 20th century scientists have made some
important discoveries about humans, the Earth, and our entire universe. Here are just 10 of the things we didn’t
know a century ago. Number One: We didn’t know that there are
uncharged particles in atoms, which we call neutrons. By 1920, we knew that every atom had both
negatively-charged electrons, and positively-charged protons, which sit in the nucleus and make
up most of its mass. But there was a problem: for example, it didn’t
make sense that a helium atom only had two protons because it was as heavy as four. And there wasn’t any proof that some kind
of neutral particle existed, one that added mass but not charge. At least, not until the 1930s. Researchers were experimenting with alpha
particles, basically helium nuclei, and firing them at the element beryllium, which released
a bunch of different particles. The physicist James Chadwick noticed that
these mystery particles had no charge, and they were massive enough to bump out protons
from other atoms. By 1932, he was convinced that these were
different from electrons and protons, and were the particles the scientific world had
been looking for: neutrons. Number Two: We didn’t know that DNA is in
the shape of a double helix. By the early 1950s, researchers working on
DNA were pretty sure it held a lot of genetic information, and was passed down from parents
to their children. But they couldn’t really figure out how
it worked without knowing its structure. Two scientists named Rosalind Franklin and
Maurice Wilkins were experimenting with a process called x-ray diffraction, where they
basically shined x-rays onto DNA. These x-rays bounced off the atoms and showed
up as patterns on a sheet of film, which revealed a helical structure. In 1953, these images, along with lots of
other research about the chemistry of DNA, ended up in the hands of James Watson and
Francis Crick. They pieced it all together to make the double
helix model. Knowing this structure inspired lots of new
biology research, to figure out how DNA’s chemical code can make something as complicated
as us. Number Three: We didn’t know that there’s
a vitamin C... and others. For a long time, we’ve known that eating,
or not eating, certain foods can affect our health. Like, doctors in the mid-1700s realized that
eating citrus fruits could prevent scurvy, a disease where people would develop ulcers,
anemia, and exhaustion. But they didn’t know what these fruits had
in common: vitamin C. Around a century ago, scientists began studying,
isolating, and naming all the vitamins we know today. For example, the biochemist Albert Szent-Györgyi
spent the 1920s researching how our cells use food to make energy. And he came across a molecule in the adrenal
gland, the hormone-releasers right on top of our kidneys, that sparked his interest: It had 6 carbons, and acted like a sugar and
an acid, so he called it hexuronic acid. In the 1930s, he and other researchers ran
some experiments on guinea pigs. Without this compound in their diet, these furry friends
developed scurvy… sound familiar? So they realized that hexuronic acid was,
in fact, the vitamin C that scientists had been trying to isolate for years. Number Four: We didn’t know how to kill
germs and fight off infections. The idea of killing harmful microbes has been
around for a long time. Some ancient cultures treated infected wounds
with chunks of moldy bread. But they didn’t know why those treatments worked. The first medicine that could fight off microbial
infections, called an antibiotic, was discovered by a scientist named Alexander Fleming in
1928. After coming back from vacation, Fleming was
looking at petri dishes with bacteria that could cause nasty infections and sore throats
in humans. And he noticed something strange: there was
a little bit of mold at the edge of one dish, with no bacterial colonies growing nearby. Fleming figured out that that this Penicillium
notatum mold produced a compound, which he called penicillin, that could kill different
kinds of harmful bacteria. For years, many researchers worked to purify
penicillin from different molds, test it on humans, and eventually mass-produce the first
antibiotic. This so-called “wonder drug” has saved lots of people from life-threatening
infections. Number Five: We didn’t know that the universe
was bigger than the Milky Way. At the start of the 20th century, astronomers
knew the universe was filled with things like planets, stars, gases, and dust. But they didn’t fully understand how big
it was. Or, why certain clouds of gas and dust, or nebulae, were spiral-shaped, and
appeared to be so far away. This spiraled into the so-called “Great
Debate” of astronomy in the 1920s: Some astronomers thought everything in the
universe was contained in the Milky Way galaxy, including these spiral nebulae. While others thought the universe was much
bigger, and that these nebulae could be entire galaxies on their own. To try and resolve this argument, Edwin Hubble
used the largest telescope in the world at the time, the Hooker telescope, to take pictures
of the spiral-shaped Andromeda nebula in 1924. He found stars upon stars, including some
Cepheid variable stars, which vary in brightness and can be used to figure out how far away
something is. By his calculations, Andromeda was hundreds
of thousands of light years away, and almost certainly a galaxy. So our Milky Way is actually
just one of many, many galaxies in the universe. Number Six: We didn’t know that hydrogen
is the most abundant element in the universe. Before 1925, we assumed that the rest of the
universe was made of the same kinds of elements in the same proportions as the Earth. However, the astronomer Cecilia Payne-Gaposchkin
turned to the sun and observed something very different. Her PhD research was focused on understanding
the light emitted and absorbed from stars, which can be used to figure out what elements
they’re made of. She realized that the Sun was mostly made
up of hydrogen and helium, and it was different forms of these elements that caused the variations
in the light we observed. Her 1925 thesis concluded that hydrogen was
about a million times more abundant in stars than we thought, and thus is the most abundant
element in the universe. Number Seven: We didn’t know that the Earth’s
crust was made of tectonic plates. At the start of the 20th century, most geologists
believed that the Earth’s crust was pretty much… set in stone. They figured the Earth was once a hot molten
blob, and the surface shrank and wrinkled as it cooled to form things like mountain
ranges and the oceans. Like, imagine a grape turning into a raisin. But, in 1915, a scientist named Alfred Wegener
came up with the theory of continental drift: that a long time ago the continents all made
up a single landmass called pangaea and slowly shifted apart. Evidence started piling up, like we found
similar fossils from the same time periods on completely different continents. But we still couldn’t really explain how
the continents moved. That came in the late 1960s, when geologists
connected this theory to ideas about large underwater ridges, which oozed molten rock
and gradually pushed the existing seafloor away. With some more research, they concluded that
the Earth’s crust was broken up into giant, slow-moving chunks that they called tectonic
plates. Number Eight: We didn’t know that modern
humans probably came from ancestors in Africa that migrated across the world. Throughout the 18th and 19th centuries, nobody
really agreed where modern humans came from. And it’s even still debated today. Some scientists believe that different races
of modern humans evolved separately, but from a common ancestor that migrated across the
world. While others think a population of modern
humans started on one continent and eventually migrated around the world. Starting in the late 1980s, Charles Darwin’s
Out of Africa idea became one of the most widely-accepted theories, because researchers
started looking for genetic evidence to trace human ancestries. A team of scientists compared human mitochondrial
DNA, which is passed on from mothers to their children, and found it was very similar in
humans all across the world. And more researchers keep finding evidence
that modern humans are probably descended from one original group in Africa, that grew
different as they multiplied and migrated across the Earth over thousands of years. Number Nine: We didn’t know that CFCs destroy
the Earth’s ozone layer. The ideas of air pollution, burning too many
fossil fuels, and global warming have been around for a while. But we didn’t realize how much some man-made
compounds could affect our atmosphere. Chlorofluorocarbons, or CFCs, are organic
chemicals made of some combination of carbon, hydrogen, chlorine, or fluorine. Throughout the 20th century, CFCs were being
produced as coolants in refrigerators and air conditioners, and in aerosol sprays. But some chemists realized that CFCs were
floating up into the stratosphere and being broken apart by the strong UV light from the
sun. The CFCs released chlorine that reacted with
ozone molecules, and were slowly depleting the ozone layer around the Earth that normally
protects us from harmful UV radiation. At first, the researchers’ ideas received
a lot of criticism, but in 1975, they won the Nobel Prize in Chemistry for their work. And by 1989, countries around the world began
to sign an environmental treaty called the Montreal Protocol, and promised to stop using
CFCs in order to protect our planet. Number 10 - We didn’t know just how right
Einstein was. Now, Einstein wasn’t right about everything,
but his General Theory of Relativity has really stood the test of time. The basic idea is that the universe is like
a huge fabric of space and time, and heavy masses can bend and warp it. When it was first published 101 years ago,
there wasn’t much evidence to support his ideas. But, over time, more and more tests kept proving
Einstein right. For example, astronomers noticed that Mercury’s
orbit wasn’t really acting the way they had predicted, but Einstein used his own equations
to re-calculate the orbit and was way more accurate. Then, there’s gravitational lensing, his
prediction that light should be bent by the presence of heavy objects. And Einstein was right again. When we look
into the night sky, we see light getting bent around galaxies, making circular streaks in
our telescopes. The cherry on top was the discovery of gravitational
waves. Over a hundred years ago, Einstein predicted
that there should be small ripples in the fabric of space and time, like when a boat
leaves ripples in a lake. And just a few months ago, the LIGO Scientific
Collaboration observed them for the first time. These gravitational waves could allow scientists
to peer farther out, and further back in the universe, and uncover even more of its secrets. We’ve made so many discoveries in the past
century that have changed how we understand the world, the universe, and our place in
it. And scientists are far from done exploring. So who knows what we might learn next? Thanks for watching this episode of SciShow,
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