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Physics investigates why the universe behaves the way that it does, and today, Hank tells us about the three physics experiments that he thinks were the most awesome at helping us understand how the universe works.

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References/Licenses
http://www.cavendishscience.org/phys/tyoung/tyoung.htm
http://physics-animations.com/Physics/English/top10.htm
http://myweb.usf.edu/~mhight/goldfoil.html
http://galileo.phys.virginia.edu/classes/252/more_atoms.html
http://en.wikipedia.org/wiki/File:Cavendish_Experiment.png
http://en.wikipedia.org/wiki/File:Cavendish_Henry_signature.jpg
http://en.wikipedia.org/wiki/File:Cavendish_Torsion_Balance_Diagram.svg
http://en.wikipedia.org/wiki/File:Thomas_Young_(scientist).jpg
http://en.wikipedia.org/wiki/File:Young_Diffraction.png
http://en.wikipedia.org/wiki/File:Plum_pudding_atom.svg
http://en.wikipedia.org/wiki/File:Rutherford_gold_foil_experiment_result...
http://en.wikipedia.org/wiki/File:Rutherford_atomic_planetary_model.svg
http://www.nist.gov/pml/data/th-arlamp/images/shutterstock_36406936_Harp...
http://en.wikipedia.org/wiki/File:Light_dispersion_conceptual_waves.gif
Physics is the study of stuff and how it moves along with everything that comes along with stuff moving: like energy and force and space and time. Basically it's how we investigate why the universe behaves the way it does. Big questions, yes, many of them, and a lot of those questions are still unanswered. But when physicists hit upon an answer in an experiment, you know it's gonna be big. If I had a, you know, say, five minutes to rattle off three of the most awesome experiments in physics ever, they would probably sound like this.

(Intro)

Number one: Henry Cavendish measure gravity. In 1797, English super-genius and wallflower, Henry Cavendish, performed his "torsion ball" experiment, which, now that I've said it, ew...

But it's not about balls, it's about gravity. He became the first to demonstrate in a laboratory setting what Isaac Newton had been telling the world all along -- that even small masses exert gravitational force on one another.

The instrument he used to do this was complicated, but the principle behind it was pretty simple. A wooden rod suspended from a wire with a small lead ball on either end -- about three quarters of a kilogram each. Then, near each small sphere, he mounted a larger one hundred and fifty-eight kilogram ball. Now, gradually, but measurably, the gravitational attraction between the bodies caused the small balls to move toward the big ones. In addition to demonstrating gravitation between objects, the results allowed Cavendish to actually measure it for the first time by determining how much the rod moved and how much forced was exerted on the wire as it twisted. From this, he was able to extrapolate all kinds of things, perhaps most importantly though, is what's now known as "the gravitational constant" -- the figure that's actually used to calculate the gravitational force between any two objects.

Number two: Thomas Young makes waves. Less than twenty years after Cavendish, another gifted Brit, Thomas Young, revolutionized how we think about light with an experiment that today is repeated in classrooms the world over.

Back then, physicists were puzzling over how light moved: as a wave, like sound does, or as a beam of particles, like Newton argued. Young thought Newton's Particle Theory didn't explain much of light's behavior, like how it refracts through a prism or the atmosphere, so he wanted to put the Particle Theory to the test.

He covered a window with paper, and then poked a small hole in it. He then bounced the beam of light that came through that hole onto a mirror, and then held a thin card up to an edgewise, which diverted the beam into two. And on the wall, the beams projected alternating bands of light that looked a lot like the pattern made when waves of water meet, with bright bands where the crests of two light waves crossed over, and dark bands where the crest from one beam met the trough of another.

When Young shared his findings, physicists all over the world were dropping their monocles into their gin classes because contradicting Newton was pretty much like mooning the royal academy. The results showed that light, of course, did have wave-like properties, launching a scientific debate that would last nearly two hundred years, in which Young's experiment would be endlessly repeated, refined, and built upon.

In the end, though, it turned out that both Young and Newton were wrong, and also they were right. Today, Quantum Theory posits that all particles exhibit the properties of both particles and waves, a condition known as "Wave-Particle Duality", and that, in physics, is what we call a win-win.

And, experiment number three: Ernest Rutherford discovers that atoms aren't made of pudding. One of modern physics most prolific and curious experiments was Ernest Rutherford. His investigations lead to the discovery like the half-life of elements and the existence of different kinds of radiation; namely positively charged alpha particles and negative beta particles.

But despite all of his intrepid research, by the early 1900s, we still didn't know what atoms looked like. At the time, electrons had just been discovered, and the prevailing theory was that they just floated around within a big positively charged cloud that held the atom together. This was known as the "plum pudding" model -- I'm not kidding -- and while it sounded tasty, it didn't answer enough questions about the behavior of atoms to satisfy Rutherford.

So in 1909, he and colleagues Hans Geiger and Ernest Marsden tested the theory. They aimed a beam of alpha particles at an infinitesimally thin foil of gold, a metal soft enough to compress into a layer just a few atoms thick. If the plum pudding model was correct and atoms were just big, spacious clouds of particles, then alpha particles would pass right through the foil. And interestingly, lots of them did, but many of them did not; they bounced off at sharp angles, sometimes flying right back at the source of the beam. Rutherford concluded that atoms must have a small, compact core: a nucleus, with a net positive charge that deflected the alpha particles, and rather than floating among the positively charged material, the electrons must be moving around it at a distance.

This lead directly to the atomic model that we know today, and its principles form the foundations of nuclear physics.

Thank you for watching this episode of SciShow. I'm sure that we missed some extremely important physics experiments and you are welcome to talk about those in the comments, and maybe we'll do another one of these sometime. And if you want to keep getting smarter with us here at SciShow, you can go to youtube.com/scishow and subscribe.