#2. The Universal Constant of Gravitation Is a Load of Balls
Gravity is the weakest of the four fundamental forces. You normally need an entire planet to notice it. Some teachers demonstrate the weakness of the force by holding up two objects and saying, "There is gravitational attraction between these, but you can't see it." Henry Cavendish said, "You're just not looking hard enough."
Horrebows Basis Astronomiæ via Wikimedia Commons
"And Roemer is looking the wrong way."
Cavendish discovered hydrogen, weighed the entire planet, and pretty much single-handedly discovered most of the next century's progress in electromagnetism, but he only bothered to tell people about the first two. We found out about the other stuff when other scientists examined his notes a century after his death and discovered that he'd kept quiet about pretty much solving the universe because he didn't want to bother anyone.
His apparatus was first built by the Rev. John Michell, who sadly died before being able to undertake the experiment, and it passed on to Cavendish. He ended up upgrading most of it in order to weigh the world in his shed, which is not a euphemism for using an outhouse.
"Forget the weight of the Earth. Last night's Chili's now feels more like the atmosphere of Venus."
The apparatus was a horizontal rod suspended from the center by a twisting string. Large lead spheres at the end of the rod were attracted to smaller spheres suspended about 20 centimeters away from the ends. By measuring the twist of the wire, the force between the spheres could be measured. By measuring the oscillation of the wire, the tension constant of the wire could be found. Between them, these gave a relation between the weight of the spheres and the weight of the planet. And the smaller spheres weighed less than 2 kilograms. He caught the universal force of gravitation with less lead than some people need to bag a goose. (Of course he had those big balls too, but that goes without saying when someone takes on the weight of the entire world they're standing on).
"Forget progress, I'm winning fights with unarmed birds!"
The experiment was ridiculously sensitive, and observations would take hours. He measured the swaying rods inside a closed box using small holes and telescopes, which meant he must have looked like the first Internet user, going out to peer into a box for hours while working on his balls.
"But remember, we tell stock photo that I'm 'typing,' OK?"
He determined the average density of the Earth, around five and a half times that of water, and his final answer was only 1 percent off. He measured the planet with hardware that looked like a playground toy. He took on the world with weights most people wouldn't even use as a warm-up. The results would be used to find the mass of the planet and the first accurate value for G, the universal constant of gravitation, one of the startup constants for our entire reality. He performed this experiment before we built the first steam-powered railway.
#1. Wave Nature of Light
The double slit experiment sounds like ladies inventing a kind of sex that doesn't need men, but had even more amazing effects on the world: It proved the wave nature of light. One of the most ridiculous conclusions of light's wave nature is that two waves can cancel each other out if conditions are right: Two lights can make a dark. It was ridiculous right up until Thomas Young demonstrated it at the start of the 19th century.
Young shone sunlight through one slit to get a source, then directed that through two more slits, but got a huge number of lines on the screen, which sounds like a glitch in reality's graphic engine, but is exactly what happened. The light waves from the two slits have to travel different distances to reach a point on the board. The waves add up or cancel each other out depending on how far they have to travel (and also whether they arrive lined up with each other or out of phase). The result is that instead of two bands of light, you get a central bright line and alternating bands of light and dark, like reality's bar code. Young's original patterns consisted of several colors of bands due to the multiple wavelengths in sunlight. With lasers, we can now get crisp patterns.
KITT is built into all of reality, not just a car.
It's one of the most striking results in physics, which was useful for Young, because at the time he was going up against the theories of Isaac Newton, who said that light moved in straight lines as particles. But the great thing about science is that it doesn't matter who said something, only what they said, and whether it can be tested. Science is built to boil out as much petty human bullshit as possible so that we can get on with achieving things. Quantum mechanics would later turn this fundamental conflict into a Sesame Street episode, explaining that Newton and Young were both right, but that they'd have to work together if they wanted to solve even more exciting problems. Little things like the structure of reality and the beginning of time.
You know, those things we're working on right now.
Behold more science with positronium annihilation gamma-ray lasers in 6 Man-Made Materials You Won't Believe Exist and intergalactic hypervelocity stars with 5 Awesome Stars from Beyond the Other Stars.