In part seven we watched an alpha particle escape a nucleus and fly away with enormous speed. Tonight I want to go the other direction – back into that nucleus to try to discover it secrets.
This is a path taken before. We’ll be following in the footsteps of one of my all-time science heroes, New Zealand-born Ernest Rutherford.
Rutherford is one of those characters you feel that you know after reading descriptions of him. He was boisterous, loud, like an animated bowling ball, but never mean, arrogant, pompous, or a blow-hard. He was always humble yet perspicacious (it’s OK, I had to look it up, too), assiduous about always giving credit to others where it was due, a great supporter of female scientists when practically no one else was. He was supposed to have lacked math skills, but his intuition about how things worked was uncanny. He seemed, far beyond his contemporaries, to know which questions to ask.
My favorite Rutherford anecdote is this picture:
Notice the sign in Rutherford’s laboratory. It says, “Talk Softly Please.” At first you might think Rutherford is a tyrant, forcing his subordinates into quiet work. Not so. The sign was put there by Rutherford’s team, specifically because of him. The work done in the laboratory was sensitive, and Rutherford’s loud, booming voice could actually affect the results. The team put up the sign to remind Rutherford to please curb his enthusiasm. You see him in this picture, under the sign made for him, gleefully chewing on a cigar and considering some deep mystery of the universe. When I grow up, Iwant to be Ernest Rutherford.
Rutherford named both alpha and beta radiation, and he was the first to recognize the connection between alpha particles and our helium atoms. But, far from just naming alphas, Rutherford made them dance, using these atomic bullets to enter the holy of holy.
In 1911 Rutherford asked a student to do an experiment that couldn’t possibly work. Or could it? Ernst Marsden was to fire alpha particles at a gold foil sheet. Alphas were known to be very small, very fast, and positively charged. At the time of this experiment, atoms (such as the atoms making up gold) were thought to be like plum pudding. The pudding was a sort of diffuse positively charged – something – and the plums were the electrons discovered by Rutherford’s mentor JJ Thomson in 1897. Theory said that alphas should pass right through the gold foil, as in fact every experiment done up to that time had show.
But Rutherford asked for a different experiment. Instead of watching for alphas passing through the foil, Rutherford asked Marsden to watch for alphas that bounced back. This was clearly impossible – a thin, diffuse atom couldn’t possibly stop something as fast as an alpha, let alone bounce it back the other way. And yet when Marsden looked . . . yes, there they were. Not often, but often enough to be utterly baffling, alpha particles impacted the gold foil and bounced back.
Rutherford wrote about how shocked he was. “It was about as credible,” he said, “as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you.” But I’ve always had my doubts about Rutherford’s surprise. He never seems to admit it, but I suspect that Rutherford suspected Marsden might find something, after all. If not, why look?
At any rate, whether Rutherford suspected anything or not before the experiment, there is no doubt that after the results were in Rutherford saw what they meant. The bouncing alphas showed Rutherford that the atom was not a plum pudding, but instead was almost entirely empty space. Almost all the mass and charge of the atom was concentrated in a tiny spot in the center. That spot came to be called the nucleus. My favorite comparison is that if the atom were the size of a baseball diamond, the nucleus would be a ladybug in the grass just behind the pitcher’s mound. And yet virtually all the mass that makes up the atom is found within that bug. The rest is virtually empty. We are, all of us, mostly empty space.
Anyway, this experiment shows again why quantum tunneling is so surprising. In effect, the experiment is just the opposite of quantum tunneling. Put yourself down there, at the level of an atom. An alpha is “fired” at a nucleus. It approaches, closer and closer, and gradually the positive electric charge of that nucleus becomes so large that the alpha actually bounces back, like a baseball off a trampoline. It wasn’t moving fast enough to get inside the nucleus, into that holy of holies, into that deep secret. But why?
There’s a scene in the movie Ghost that, by messing up, shows why. Of course, the rest of the movie is pristinely scientifically accurate, but in this one place they get it wrong. Near the end, the bad guy is impaled by falling glass, broken by a swinging pendulum. But the pendulum swings higher on its second swing than on its first! Anyone who’s ever done the bowling ball to the nose demonstration
knows that this can’t happen (otherwise we’d get a lot of broken noses!) For the same reason, an alpha shot out of a nucleus can’t get back in.
Remember what we learned about tunneling. The alpha particle is like the rivet that didn’t kill you. It doesn’t start right next to the nucleus, but some distance away, and as a result has a lot less energy. Therefore, just like the bowling ball or the pendulum in Ghost, it can’t get up the hill it never came down. It can’t get to the nucleus, but instead is bounced away. In the process, though, it reveals that there’s something down there, something tiny, massive, and deeply mysterious.
Still, Ernest Rutherford was able to build a universe from this discovery, a universe in which the mass and the positive charge of atoms lay deep within, while the negatively-charged electrons zipped about on the outside like planets around a massive Sun. There was only one problem with this picture. It couldn’t possiblywork.