>>>This is a Science Sunday post, delayed for a day due to unforseen circumstances. I found myself a little short of time Sunday, and decided to delay for a day rather than hastily whipping up something crappy.<<<

Probably, at some point in your life you’ve seen someone take a long stick with a flame on the end, hold it up to a balloon, and make the balloon explode in a very satisfying way. In theory you could do it with methane, but for some reason it’s almost always done with simple hydrogen. The secret to getting a really good explosion is to use not pure hydrogen, but a hydrogen:oxygen mix of exactly 2:1, because two hydrogen molecules and one oxygen molecule gives you two water molecules, and there’s no other way to react those two chemicals. The total energy of the explosion will actually be reduced (it involves using less hydrogen, the “limiting reagent” since oxygen is abundant in normal air) but the explosion will look bigger because having all the oxygen you need right there in the balloon makes it happen so much faster.

Molecular hydrogen also… what else can I say about molecular hydrogen? Sure, hydrogen is the key to nuclear fusion, but there the chemical properties don’t matter. Wikipedia tells me that the hydrogen bomb actually uses lithium hydride (specifically, lithium deuteride) as opposed to some form of molecular hydrogen. Oh, we might use molecular hydrogen for fuel cells some day, but that’s a ways in the future, and may not turn out to be the idea way to run a car anyway.

Here’s the thing, though. In spite of the hydrogen molecule being relatively humble in its practical applications, it ended up playing a pivotal role in our understanding of the universe we live in. The thing is that it’s so damn simple, making it really easy to do experiments with. It is to chemistry and physics what fruit flies are to genetics. Hydrogen was an early player in bringing chemistry into being, when someone noticed that the hydrogen-oxygen trick worked best with 2:1 ratios. Not so obvious, arguably kinda dubious, inference that turned out to be correct: gases are made of lots of little particles, and water is made of two hydrogen particles and one oxygen particle.

The thing I find really exciting about hydrogen, though, its its role uniting physics and chemistry. The idea of a unified physics has a strange history. It was dreamt of for a long time, but also went nowhere for a long time. In his /Pragmatism/ lectures delivered in 1904, William James was able to say that our scientific laws “have grown so numerous that there is no counting them.” A little later, in his 1925 /The Mind and its Place in Nature/, C. D. Broad considered the possibility of explaining everything in the world in terms of physics, and said:

Nothing that we know about Hydrogen by itself or in its combinations with anything but Oxygen would give us the least reason to expect that it would combine with Oxygen at all. And most of the chemical and physical properties of water have no known connexion, either quantitative or qualitative, with those of Oxygen and Hydrogen. Here we have a clear instance of a case where, so far as we can tell, the properties of a whole composed of two constituents could not have been predicted from a knowledge of the properties of these constituents taken separately, or from this combined with a knowledge of the properties of other wholes which contain these constituents.

Broad did admit that in theory the idea of physically explaining this relationship might work out, somehow, and sought to rest his arguments against physicalism on other grounds. Still, you could tell he was skeptical.

Then, just two years after /The Mind and its Place in Nature/, a pair of scientists named Walter Heitler and Fritz London published a paper on the hydrogen atom. It used something called the Schrödinger Equation, an equation governing particles as conceived in quantum mechanics. In QM, particles are conceived of as functions that actually go over all space, though the magnitude of the function will only have a non-negligible value over a small atomic sized region. The Schrödinger Equation places limits on what kind of functions a particle can be. It turns out that if you apply the Schrödinger Equation to a pair of protons and a pair of electrons (which is what makes up molecular hydrogen), a good way to solve the Schrödinger Equation is to make each of the functions for the two electrons go around both protons, thus binding those protons together into a molecule–a chemical bond, if you will.

Though the math for other situations would be harder, we suddenly had a way for chemistry to make sense. Newtonian mechanics supported solar systems just fine, but didn’t have any explanation for the complicated configurations of atoms scientists had spent the past century learning about without really understanding. Quantum mechanics, it was realized, could explain those chemical bond things we had been discovering. Today, the idea of chemistry reducing to physics has become a commonplace, stated without people really thinking about what they’re saying. And we owe it all to the hydrogen molecule.