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Little Jack Horner
Sat in a corner,
Eating his Christmas pie.
He stuck in his thumb
And pulled out a plum,
And said, “What a good boy am I!”
(English Nursery Rhyme variously styled from Henry Carey, 1725)
There was never a plum in Jack’s dessert. Nor was he eating a pie, really. “Christmas Pie” was a pudding, a baked slurry embedded with raisins. The mischievous Little Jack failed to mention the customary topping of brandy sauce, too. That could explain his chipper mood and confusion of details.
Now, what does Little Jack have to do with you? Well, Jack Horner, himself, isn’t so helpful, but his plum pudding shook the very foundations of science and brought on the technologies your generation finds inseparable – computers, cell phones, geopositioning, digital photography, and the like.
It goes like this …
Everything about us is composed of matter (mass). Your computer, your desk, your associate, the bricks in the street, the glass in the window panes, and the rain falling on the umbrellas. All substance is composed of matter. As early as the Greek Golden Age, intellectuals speculated on what constituted matter. Democritus (ca. 460-370 BC) is generally credited with coining the term atom (ατομος), meaning “indivisible,” but Democritus and twenty-three centuries of scientists after him were clueless as to what that might mean. (It turns out not even to be true. The atom, as we now know, is not an indivisible entity.)
So, no one had even a hint about the real nature of the atom: not until 1911. That’s when the epiphany occurred and this year, 2011, we celebrate the 100th anniversary of that discovery and the abandonment of the plum pudding.
Ernest Rutherford (1871-1937) was a Brit born in New Zealand. By the age of 40, he had already received the Nobel Prize for his work in radioactivity. He was curious about the nature of the atom and devised a straightforward experiment to determine the constituency of matter.
Fifteen years earlier (1896), J.J. Thompson (1856-1940) discovered the electron. So, at the time of Rutherford’s experiment, most physicists thought the atom a kind of “plum pudding.” There were negative charges (electrons) spread about like raisins in a pudding of opposing positive charge. Rutherford set up an experiment to measure whether the constituency of the atom was like a plum pudding. It was even called the Plum Pudding model of the atom.
The experiment consisted of alpha particles (helium nuclei obtained from a radioactive source) shot though an ever-so-thin piece of gold
foil (hundreds of times thinner, even, than than the aluminum foil in your kitchen.) The notion was that most of the alpha particles would go virtually straight through, making a “circle” of impact immediately behind the gold foil. It would be much like a pistol firing thousands of bullets into a bulls eye printed on a piece of paper. The bullets go straight through the paper and the impact holes are variously distributed about the bulls eye. Not knowing exactly what to expect, though, the prescient Rutherford constructed a circular detector all about the gold foil so he could detect the electrons at whatever angle they came off.
Well, lo and behold, most of those alpha particles did go straight through, as expected, but some of them came bouncing back at highly oblique angles. Now, if the raisins in the pudding were really electrons (think BB’s) and you hit an electron with an alpha particle (think cannon ball), you don’t expect the cannon ball to come flying back in your direction. Alpha particles are 7,300 times more massive than electrons. The cannon ball and BB comparison are scarcely an exaggeration; the bullets and the paper target are apropos.
How did Rutherford explain the cannon ball bouncing back from the BB or, rather, the alpha particle being deflected by the electrons? To start with, he abandoned the plum pudding model. The alpha particles were not deflected by the electrons.
Rutherford’s explanation? The atom had to have almost all of its mass concentrated at the center. In terms of mass, the electron, then, was an insignificant player. That was an astounding result and totally contrary to what anyone thought. (The proton would not be discovered until several years later and the neutron not until 21-years later.) Of course, today we know that 99.9% of the entire mass of the atom is concentrated in the nucleus. But, in 1911, no one had any notion the nature of the atom. Rutherford correctly reasoned from his experiment.
Rutherford’s experiment and critical analysis were the true beginning of the modern day model of the atom. His experiment is referenced in every physics class in the world.
Little Jack would have awarded him the Christmas pie.