Ch 6 - Atoms

In 1961, Richard Feynman said during one of his eponymous lectures:

"If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another."

Why did Feynman put so much emphasis on atoms?  Isn't this just one of those things that everyone already knows, like the existence of gravity?  There are people who deny the earth is round.  There are people who deny that man walked on the moon.  But you'd be hard-pressed to find a sane person alive today who will deny the existence of gravity or atoms.

So it comes as a surprise to many people to learn that the existence of atoms was an open question for a very long time.  Although not controversial today, atoms were bitterly contested as recently as the early 20th century.  The dispute was so fierce that it arguably drove at least one scientists, Ludwig Boltzmann, to suicide.

This is one of the reasons that Feynman emphasized it.  In 1961, the existence of atoms was a relatively recent and hard-won scientific breakthrough.  It is useful to study how this controversy was resolved, not just as another example of how the scientific sausage is made, but also because knowing about atoms is one of those foundational facts that can give you a lot of leverage in day-to-day life.

The principal objection to the existence of atoms is that you can't see them, or at least couldn't in the past.  Today we have images of atoms (or at least purported-to-be images of atoms -- more on this later) but in the late 19th and early 20th century, when this question was being most hotly contested, imaging an atom was far beyond the technological limits of the day.  Interestingly, the question was settled long before the first image of an atom was obtained.

The atomic hypothesis was first advanced by a Greek philosopher named Democritus around 400 BCE.  Despite having essentially no evidence for their existence, Democritus's theory was astonishingly close to the modern understanding.  He believed atoms were indivisible (the Greek word "atoma" means "indivisible") as well as invisible and unchanging, and that there were different kinds of atoms with different properties.  All of these things turn out to be (mostly) true.  But without evidence to support it, Democritus's theory languished in the philosophical backwaters for over 2000 years.  The theory that dominated that period of time was Aristotle's, which held that everything was made of four elements: earth, air, water, and fire.  (There was also a fifth element, aether, which existed only in the heavens.)

Aristotle's theory seems quaint by modern standards, but it is actually better supported by the evidence available in his day than Democritus's theory.  The biggest factor in favor of Aristotle is that the physics of the heavens seems manifestly and radically different than the physics of earth.  On earth, things fall.  In the heavens, they don't (except for meteorites).  On earth, things are dirty and messy.  In the heavens, they are clean and pure.  On earth, objects don't move unless something comes along and pushes them.  In the heavens, objects are in constant unending motion with no apparent causal agent.  All of this (except meteorites) is easily accounted for by Aristotle's theory: things in the heavens are made of different stuff, which acts according to different laws, than things on earth.

And that was where the matter (pun very much intended!) stood for 2000 years, until the mid-1600s when a German alchemist named Johann Joachim Becker pointed out that there were some phenomena here on earth that could not be straightforwardly accounted for by Aristotle's theory.  For example, producing fire required air.  No air, no fire.  Furthermore, producing fire changed the property of the air used to produce it.  If you burned something in an enclosed container, the air would become "depleted" and could not sustain any further fire.  Moreover, this "depleted" air would not sustain animal life, but was beneficial for plants.  Also, metals kept in "depleted" air did not corrode.

All of these observations were *problems* in the sense of the scientific process, i.e. things that the then-best explanation could not account for.  I cannot over-emphasize the extent to which all scientific progress begins with identifying a *problem* in this sense.  The reason I harp on this is that one of the diagnostic features of anti-scientific crackpots and charlatans is pointing out a problem and claiming that its existence demonstrates some kind of deficiency in the scientific process.  It doesn't.  Problems are a *feature*, not a bug.  They are the seeds of progress.  (Another diagnostic feature of such charlatans is that they will often follow up with an offer to provide you access to some secret knowledge available only to a select few.)

Becher's proposed solution to this problem was that there was an additional element that Aristotle had overlooked, which Becher named "phlogiston".  Phlogiston, according to Becher's hypothesis, was similar to air in that it was invisible and incorporeal.  Air and plants could both absorb phlogiston.  When a plant burned, it released its absorbed phlogiston into the surrounding air, but air had only a limited capacity for absorbing phlogiston.  Once this saturation limit had been reached, no further burning could take place.  Phlogiston was poisonous to animals.  And metals were not elements but compounds consisting of earth and phlogiston.  In the presence of "de-phlogisticated air", the phlogiston in metals would leak out, leaving pure earth (i.e. rust and other corroded metals) behind.

The phlogiston hypothesis was not a bad guess, but it had problems of its own.  For example, corroded metals are *heavier* than their un-corroded precursors, which is hard to account for if something "leaks out" of the metal in the corrosion process.  But plants get heavier when they *grow* (obviously!) which, according to the theory, involves *absorbing* phlogiston from the surrounding air.  These two facts are very difficult to reconcile with each other under the phlogiston theory.

Despite these difficulties, phlogiston theory was considered an improvement over Aristotle and was the dominant view for about the next 100 years.  In the mid-1700s a flurry of new discoveries were made, mostly by rich white guys riding the wave of prosperity brought about by the invention of the steam engine and the resulting industrial revolution.  In 1755 Joseph Black discovered what he called "fixed air", which today we know as carbon dioxide.  In 1766 Henry Cavndish reported "inflammable air", what we now call hydrogen.  In 1772 Joseph Priestly discovered "dephlogisticated air" -- oxygen.  The properties of these materials, and methods of reliably producing them, were written down and shared among the wealthy elites, one of which was a Frenchman named Antoine Lavoisier.

Lavoisier is considered the father of modern chemistry.  Entire books have been written just about him and his work, which I will now try to distill into a few sentences: Lavoisier showed (among many other things) that water is made of hydrogen and oxygen.  This definitively debunked Aristotle's theory that water was an element which could not be decomposed into constituent parts.  He demonstrated the principle of conservation of mass: in a chemical reaction, the mass of the output products was always the same as the mass of the inputs.  And he showed that in any chemical reaction, the constituents combined in ratios of small integers.

(Exercise: why did I go out of my way to emphasize the word "small" in the previous sentence?  What does "small" even mean here?  Does 100 count as "small"?)

Let's push the pause button on the history lesson for a moment and think about the implications of Lavoisier's discovery.  In retrospect we can say that this is strong evidence for the existence of atoms.  The *reason* that chemicals combine in (small) integer ratios is that matter is made of atoms, and specific chemicals are made of specific (integer) numbers of atoms.  Water is two hydrogen atoms and one oxygen atom, for example.  But is this *proof* that atoms exist?  No, it is not.  Although atoms are one possible explanation for Lavoisier's law of integer ratios (henceforth LLIR) they are not the only possible explanation.  Another possibility -- one of many -- is that LLIR is simply a fundamental and irreducible law of nature, kind of like gravity.  Another possibility is that it's just a coincidence that happens to hold for certain chemical reactions, but that there may be other reactions yet to be studied that violate this law.  Maybe LLIR holds in test tubes but not in (say) living things, or not in reactions that happen in stars.

Throughout the 19th century, evidence for the existence of atoms continued to pile up.  The biggest pile came in the form of a new field of study pioneered by Ludwig Boltzmann (among others) called *statistical mechanics*, which examines mathematically the behavior of systems comprised of a large number of particles.  In other words, if you *assume* that matter is made of atoms, and model the behavior of the resulting system, do the results match observations?  And the answer is a resounding "yes".  Statistical mechanics was able to predict many of the thermodynamic properties of materials, like what happens when you heat them up.

Still, the skeptics were unconvinced.  The counter-argument was: just because the math happens to work out doesn't necessarily mean that atoms are *real*.  They might be like Newton's "gravitational field", a label for something that is described by the math but is in fact intangible.  Besides, if atoms are real, why can't we *see* them?

The controversy was finally settled in 1905 by an obscure Swiss patent clerk named Albert Einstein.  (Maybe you've already heard of him?)  Einstein studied a phenomenon called Brownian motion, where bits of dust appear to move around in a drop of water when seen through a microscope.  Einstein showed that the pattern in which the dust moves is exactly what you would expect if it were being knocked around by atoms moving according to the predictions of statical mechanics.  You might not be able to see atoms, but you could see them beating on things.

In the subsequent hundred years the evidence for the existence of atoms has, of course, only grown stronger.  In particular, we actually can "see" atoms now, after a fashion.  Of course we still cannot see them *directly* -- they are too small for that.  But we can build instruments that can measure their shape and positions and make renderings of what they would look like if we could see them.  Nevertheless, it is important to keep in the back of your mind the fact that pictures of atoms are not actual photographs.  This will be important later when we get around to talking about quantum mechanics.

For now, the idea that matter is made of atoms is about as well-established as a fact could be.  Not only that, but there is only a finite repertoire of atoms: only 92 which occur in nature, and a few dozen others that we can produce artificially, and that's it.  Those are the building blocks of the universe.  Everything we can see, touch, smell, everything from your toaster to the moon and the stars, all of it (except neutron stars and black holes) is made of those 92 varieties of atoms, and nothing else.

(Yes, there is probably dark matter too.  But we can't see, tough, or smell it.  We can't interact with it in any way.  That is what makes it hard to study.)

And you yourself are made of atoms.  This turns out to have some pretty significant consequences, which we will begin to explore in the next chapter.