When last we left our new helium atom, it had just emerged as an alpha particle from a radioactive nucleus. But I want to go back one step, for the process of alpha decay itself is one of the more amazing events in the universe.
Let’s think about this radioactive nucleus. Suppose it is thorium, with 90 protons and 142 neutrons. Picture it in a slightly different way, though, as a main nucleus with 88 protons and 140 neutrons and a single alpha particle, 2 and 2 respectively, bouncing around this nucleus. The most powerful force in the universe, appropriately named the strong force, holds this nucleus together. Because this attractive force is stronger than the repulsion arising from the positive electric charge of the main nucleus and the alpha particle, it should be impossible for the alpha to escape.
And it nearly is. With a half-life of 13 billion years, a thorium atom is just about as close to stable as a radioactive atom can get. Yet it is still possible, barely possible, that the alpha will, in fact, escape.
There’s another mystery here, as well. We know that alpha particles do escape from thorium, in the process turning into helium. And we know that those alphas shoot out with incredible speed, right? Well, not really so much. Although early researchers were flabbergasted that a single atomic reaction could produce so much energy as that carried by a speeding alpha, later on scientists realized that the alpha was actually moving surprisingly slowly.
Here’s an analogy. Suppose you are walking under a very tall bridge, which just happens to be under construction. As you stroll along, something compels you to look up. The moment you lift your head, you see a rivet only inches from your nose. Naturally, you are immediately concerned, for there’s only one place this rivet could have come from. The bridge is so far above your head, and therefore the rivet must have fallen so far, that you are in serious jeopardy.
And yet, when the rivet reaches your nose, it lands gently before falling off to land harmlessly at your feet. Somehow the rivet must have appeared just above you, without falling the great distance between the bridge and your nose.
This doesn’t happen in the macroscopic world, of course. But on the scale of the atom it is a routine occurance, and alpha decay is a perfect example.
The thorium atom is like the high bridge in our analogy. The alpha particle is the rivet. If the alpha appeared at the edge of the nucleus, where the repulsive force of all those protons is enormous, the alpha would rocket away with enormous speed. But that’s not what we find. Instead, the alpha seems to appear far down the “slope” of the nucleus, without ever having been in the space between.
This process is called quantum tunneling, and it happens because an alpha particle isn’t always a particle. Sometimes, it’s a wave.
“A wave of what?” you might ask, and that’s a fair question. The answer is both strange and wonderful. The wave is a wave of probability.
The wave shows you where the alpha particle is likely to be found at any moment. The wave is very large inside the nucleus, but a very small amount of the wave leaks out of the nucleus. As such, there is a tiny (very tiny) chance that the alpha particle will find itself not inside the nucleus, but instead on the outer fringes. When this happens, the alpha (released from the attraction of the strong force) feels the large repulsive force of the nearby nucleus and goes flying away.
Why, though, can the alpha not appear in the “forbidden zone” higher up on the slope? Therein lies the mysterious magic of quantum physics. Think for a moment what happens when the alpha tunnels through the barrier and goes flying away. You can analyze the movement as the result of repulsion. But you can also think about energy. Where did the energy come from? From the nucleus itself!
Find the mass of the thorium nucleus before it decays, then find the mass of the resulting nucleus (it will be a radium nucleus) plus the alpha particle. The products will have lower mass. Where did that mass go? It became energy! If the alpha were to appear “too close” to the nucleus, the speed (and therefore the kinetic energy) of the alpha would be too high. The energy would have appeared from nowhere, and nature will not allow this. The alpha won’t appear in this forbidden zone because to do so would violate the conservation of energy. Crazy!
There’s even more. Because the amount of energy available is fixed by the masses of the products, the energy (and the speed) of the alpha is incredibly consistent. There’s simply no way for the speed of the alpha to vary, because there’s noplace else for that energy to go. There’s another process, called beta decay, which is quite different, and that difference led to another amazing discovery. But that’s another story for another time. Now it’s time to go back into this mysterious nucleus to discover the secrets that lie inside. And helium will be our key once again.