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  • One of the funny things about owning a brain

  • is that you have no control over the things that it gathers and holds onto,

  • the facts and the stories. And as you get older, it only gets worse.

  • Things stick around for years sometimes

  • before you understand why you're interested in them,

  • before you understand their import to you.

  • Here's three of mine.

  • When Richard Feynman was a young boy in Queens,

  • he went for a walk with his dad and his wagon

  • and a ball. And he noticed that when he pulled the wagon,

  • the ball went to the back of the wagon.

  • And he asked his dad, "Why does the ball go to the back of the wagon?"

  • And his dad said, "That's inertia."

  • He said, "What's inertia?" And his dad said, "Ah.

  • Inertia is the name that scientists give

  • to the phenomenon of the ball going to the back of the wagon.

  • But in truth, nobody really knows."

  • Feynman went on to earn degrees

  • at MIT, Princeton, he solved the Challenger disaster,

  • he ended up winning the Nobel Prize in Physics

  • for his Feynman diagrams describing the movement of subatomic particles.

  • And he credits that conversation with his father

  • as giving him a sense

  • that the simplest questions could carry you out to the edge of human knowledge,

  • and that that's where he wanted to play.

  • And play he did.

  • Now Eratosthenes was the third librarian at the great Library at Alexandria,

  • and he made many contributions to science.

  • But the one he is most remembered for

  • began in a letter that he received as the librarian,

  • from the town of Swenet, which was south of Alexandria.

  • The letter included this fact that stuck in Eratosthenes' mind,

  • and the fact was that the writer said at noon

  • on the solstice, when he looked down this deep well,

  • he could see his reflection at the bottom, and he could also see that his head

  • was blocking the sun.

  • Now, I should tell you -- the idea that Christopher Columbus discovered that the world is spherical

  • is total bull. It's not true at all.

  • In fact, everyone who was educated understood that the world was spherical

  • since Aristotle's time, and Aristotle had proved it

  • with a simple observation.

  • He noticed that every time you saw the Earth's shadow on the Moon

  • it was circular,

  • and the only shape that constantly creates a circular shadow

  • is a sphere, Q.E.D. the Earth is round.

  • But nobody knew how big it was

  • until Eratosthenes got this letter with this fact.

  • So he understood that the sun was directly above the city of Swenet,

  • because looking down a well, it was a straight line

  • all the way down the well, right past the guy's head up to the sun.

  • Eratosthenes knew another fact.

  • He knew that a stick stuck in the ground in Alexandria

  • at the same time and the same day, at noon,

  • the sun's zenith, on the solstice,

  • the sun cast a shadow that showed that it was 7.2 degrees off-axis.

  • Now, if you know the circumference of a circle,

  • and you have two points on it,

  • all you need to know is the distance between those two points,

  • and you can extrapolate the circumference.

  • Three hundred and sixty degrees divided by 7.2 equals 50.

  • I know it's a little bit of a round number, and it makes me suspicious of this story too,

  • but it's a good story, so we'll continue with it.

  • He needed to know the distance between Swenet and Alexandria,

  • which is good because Eratosthenes was good at geography.

  • In fact, he invented the word geography.

  • The road between Swenet and Alexandria

  • was a road of commerce,

  • and commerce needed to know how long it took to get there.

  • It needed to know the exact distance, so he knew very precisely

  • that the distance between the two cities was 500 miles.

  • Multiply that times 50, you get 25,000,

  • which is within one percent of the actual diameter of the Earth.

  • He did this 2,200 years ago.

  • Now, we live in an age where

  • multi-billion-dollar pieces of machinery are looking for the Higgs boson.

  • We're discovering particles that may travel faster than the speed of light,

  • and all of these discoveries are made possible

  • by technology that's been developed in the last few decades.

  • But for most of human history,

  • we had to discover these things using our eyes and our ears and our minds.

  • Armand Fizeau was an experimental physicist in Paris.

  • His speciality was actually refining and confirming other people's results,

  • and this might sound like a bit of an also-ran,

  • but in fact this is the soul of science,

  • because there is no such thing as a fact that cannot be independently corroborated.

  • And he was familiar with Galileo's experiments

  • in trying to determine whether or not light had a speed.

  • So, Galileo had worked out this really wonderful experiment

  • where he and his assistant had a lamp,

  • each one of them was holding a lamp. And Galileo would open his lamp, and his assistant would open his lamp.

  • And they got the timing down really good.

  • They just knew their timing. And then they stood at two hilltops,

  • two miles distant, and they did the same thing, on the assumption

  • from Galileo that if light had a discernable speed,

  • he'd notice a delay in the light coming back from his assistant's lamp.

  • But light was too fast for Galileo.

  • He was off by several orders of magnitude when he assumed

  • that light was roughly 10 times as fast as the speed of sound.

  • Fizeau was aware of this experiment. He lived in Paris,

  • and he set up two experimental stations,

  • roughly five and a half miles distant,

  • in Paris. And he solved this problem of Galileo's,

  • and he did it with a really relatively trivial piece of equipment.

  • He did it with one of these.

  • I'm going to put away the clicker for a second

  • because I want to engage your brains in this.

  • So this is a toothed wheel. It's got a bunch of notches

  • and it's got a bunch of teeth.

  • This was Fizeau's solution to sending discrete pulses of light.

  • He put a beam behind one of these notches.

  • If I point a beam through this notch at a mirror,

  • five miles away, that beam is bouncing off the mirror

  • and coming back to me through this notch.

  • But something interesting happens as he spins the wheel faster.

  • He notices that it seems like a door is starting to close

  • on the light beam that's coming back to his eye.

  • Why is that?

  • It's because the pulse of light, it's not coming

  • back through the same notch. It's actually hitting a tooth.

  • And he spins the wheel fast enough

  • and he fully occludes the light. And then,

  • based on the distance between the two stations

  • and the speed of his wheel and the number of notches in the wheel,

  • he calculates the speed of light to within two percent of its actual value.

  • And he does this in 1849.

  • This is what really gets me going about science.

  • Whenever I'm having trouble understanding a concept, I go back and I research the people that discovered that concept.

  • I look at the story of how they came to understand it.

  • And what happens when you look at what the discoverers were thinking about

  • when they made their discoveries, is you understand

  • that they are not so different from us.

  • We are all bags of meat and water. We all start with the same tools.

  • I love the idea that different branches of science are called fields of study.

  • Most people think of science as a closed, black box,

  • when in fact it is an open field.

  • And we are all explorers.

  • The people that made these discoveries just thought a little bit harder

  • about what they were looking at, and they were a little bit more curious.

  • And their curiosity changed the way people thought about the world,

  • and thus it changed the world.

  • They changed the world, and so can you.

  • Thank you.

  • (Applause)

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B1 TED-Ed galileo alexandria wagon lamp wheel

【TED-Ed】How simple ideas lead to scientific discoveries

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    VoiceTube posted on 2012/12/30
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