A great science story is like a great Seinfeld episode. By the end, seemingly unrelated things all fit together and make the whole thing work as a whole.
Pick up a pinch of salt and drop it into a candle flame, or the burner on a gas stove. You’ll see a flash of yellow as the the salt heats up in the flame. That flash is one of the most profoundly mysterious events in the universe. Why should salt burn yellow? For hundreds of years, no one knew.
Now flash to 1914. Ernest Rutherford has just made his most important scientific contribution, in a lifetime of breathtaking contributions. He’s just discovered, as he said “what the atom looks like.” Rutherford blew apart the old plum pudding model of his mentor JJ Thomson, and showed that the atom is more like a miniature solar system, with a massive nucleus in the center surrounded by bits of fluff called electrons.
There was only one problem. The solar system model couldn’t work. In fact, it would cause every atom in the universe to collapse on itself in less than a second.
Here’s why: electrons have an electric charge. When things with an electric charge accelerate, they give off radiation. That’s what makes radio work – vibrating electrons at the transmitter send radio signals into space. The radio signals vibrate electrons in your metal antenna, and those vibrating electrons make the signal that turns into the sound you hear.
In a solar system atom, the electron would have to be spinning around the nucleus. Otherwise the negative electron would fall right into the positive nucleus like a bird that stops flying. But if the electrons are spinning, they’re also accelerating. If they’re accelerating, they’re giving off radiation, and if they’re giving off radiation, they’re losing energy. A quick calculation showed that if the electrons were accelerating like this, they’d lose so much radiation they’d fall into the nucleus in less than a second.
Since that hasn’t yet happened, despite plenty of seconds to do so, something must be screwy somewhere.
A student of Rutherford’s named Niels Bohr went to work on the screwiness. Bohr knew he had two mysteries. First, why do things like salt give off certain colors when they’re heated in a flame? Second, why don’t electrons collapse into the nucleus? Could Bohr make like a Seinfeld writer and bring it all together?
Bohr had a few tools. Albert Einstein had nine years earlier discovered that light is lumpy. The color of the lump tells you the energy of the lump. Blue lumps have more energy than yellow lumps, and yellow lumps more energy than red. Bohr also knew that the energy of an electron depended on how far it was from the nucleus, just as the energy of a brick depends on how far above your head it is when it’s dropped.
So Bohr took a guess. What if electrons could only “orbit” the nucleus at certain, regular energies? If “A” is the base energy, then the electron could orbit at 1 times A, 2 times A, and so on. (Actually Bohr based the allowed orbits on angular momentum, not energy. But the heart of the idea is still there.) Bohr tried this for the simplest atom, hydrogen, and when he did, out popped the lumps of light that came from heated-up hydrogen atoms! It all came together.
The important thing to remember is that this was screwy physics. No one knew any reason why an electron orbiting a nucleus at only certain, specific distances should not give off radiation. No one knew why, when an electron went from one of these allowed orbits down to a lower one that it should give off radiation in one lump (other than Einstein’s equally screwy idea that, well, it just did). This was physics with no basis in anything anyone had ever seen in the real world. But, for hydrogen at least, it worked.
Sadly for Bohr, nature wasn’t so simple when it came to other atoms like sodium. It took a long while before scientists could work out the rules of how electrons jump in complicated atoms like sodium. But when they did, sure enough, the yellow color of heated salt came jumping out, just as the colors of heated hydrogen came jumping out for Bohr.
Bohr’s discovery not only explained why atoms didn’t collapse. It also helped solve an ancient mystery – a mystery you can recreate yourself with just a pinch of salt and an open flame. Where does the yellow come from?