Lower than the Angels

Chapter 4, Dark Matter

They had taken a break for lunch. While they ate, Kore had asked Tengri to tell her what he was thinking, but he demurred, saying he wanted to let his thoughts gel a bit. Now they were back in the little library.

Kore said, "Okay, times up. Tell me what you think about dark matter."

Tengri nodded his assent and began. "Before the discovery of dark matter, it was thought that our periodic table of the elements was complete. Starting with atomic hydrogen, one proton and one electron, the periodic table of elements shows how the common hydrogen atom can be made to give up energy by combining its parts into lower energy configurations, other atoms, from helium to iron. Careful analysis showed that this could take place in any sufficiently dense concentration of hydrogen. 

"The obvious opportunity for this was in our sun, and other stars, where atomic hydrogen is condensed gravitationally into a plasma of protons and electrons. Some very bright thinkers worked out how, in this environment, hydrogen protons can get bound together into various lower energy configurations to make other elements. The process isn't automatic. All protons have the quality of positive electronic charge, which makes them repel each other.

Kore said, "I learned a little about this as a kid. But then, you know, I sort of accidentally turned myself into a snake, and couldn't go to school anymore. So I missed my chance to study atomic physics."

Tengri grinned and said, "Turning yourself into a snake requires quite a talent for physics. The process of making atoms and molecules may not seem as strange to you as it did to the scientists who were trying to figure it out. But they eventually realized that it took extra energy, like intense solar gravity, to push protons close enough together that they could stick to each other. In fact the nuclear binding energy by itself was not strong enough to hold two protons together. It took a couple of neutrons to do the trick."

Kore said, "I'm kind of remembering some of this from somewhere. But where did the neutrons come from?"

"Those scientists had to answer that same question. As far as they could tell from normal chemistry, protons and neutrons had the same mass, and electrons had next to no mass at all.

"But again, careful analysis showed something a little different. A neutron has very slightly more mass than a proton. The difference is very close to the mass of an electron. So they concluded that an electron, with a negative charge, and a proton, with a positive charge, have to combine to make a neutron with no charge. That seemed to make sense.

"But the masses of a proton plus an electron still don't quite add up to the mass of a neutron. They decided that the difference must come from the energy that held the neutron and proton together, and they called this 'nuclear binding energy' and everyone was happy. For a while. It got more complicated later with things called quarks, but you probably get the general idea."

Kore said, "Close enough, I guess."

So Tengri went on. "At the time, hydrogen was thought to be the most abundant form of matter in the universe. All the other known elements had to make up the rest. Chemists and physicists didn't have a clue there could be anything else anywhere. But some cosmologists began to realize that the physics of galaxies didn't make sense unless there was some extra mass somewhere holding them together. They couldn't see it, even with radio telescopes that showed them the bands of energy lower than visible light that known atoms could emit. So they called the missing mass dark matter. Now we know that dark matter is simply another form of hydrogen."

Kore said, "Right, because it's the stuff that leaks out of sun bottles. We use it to make drones and all sorts of other things go up in the air. Now everyone knows we make that from hydrogen. But why do you say that it's simply another form of hydrogen? I don't think anyone ever explained that to me."

Tengri said, "It was a very interesting situation. Chemists had known for a long time that when atoms combine into molecules they generally shift into a lower energy state. Usually that means the chemical reaction gives off heat. 

"That's what's meant by a stable reaction. If the combined energy is lower than when the atoms were separate, then energy has to be added to separate them again. Often there's no handy place for that energy to come from. So the lower energy combination stays that way. 

"Unless some extra energy does come along, like a fast-moving atomic particle from someplace, usually an electron. That can give up a photon that will knock the molecule apart. The action has to leave the separated atoms with more energy than they had when they were in the molecule.

"When a chemical reaction gives off hydrogen, for example, usually the hydrogen given off will be moving fast, or hot, which again means the same thing. In other words, it's in a higher energy state than it was in the molecule. It starts to cool off when it releases some of that extra energy, usually to combine with another atom to make another molecule that's not quite as hot."