By Ethan Siegel
“The atoms come into my brain, dance a dance, and then go out – there are always new atoms, but always doing the same dance, remembering what the dance was yesterday.” -Richard Feynman
Here you are, a human being, a grand Universe of atoms that have organized themselves into simple monomers, assembled together into giant macromolecules, which in turn comprise the organelles that make up your cells. And here you are, a collection of around 75 trillion specialized cells, organized in such a way as to make up you.
But at your core, you are still just atoms. A mind-bogglingly large number of atoms — some 1028 of them — but atoms nonetheless.
Those two things — you and an atom — may seem so different in scale and size that it’s hard to wrap your head around. Here’s a fun way to think about atoms: if you broke down a human being into all the atoms that make you up, there are about as many atoms that make up you (~1028) as there are “a-human’s-worth-of-atoms” to make up the entire Solar System!
All the matter in the Solar System, all summed together, contains about 1057 atoms, or 1029 human-beings-worth of atoms. So an atom, compared to you, is as tiny as you are in comparison to the entire Solar System, combined.
But that’s just for perspective. The 1028 atoms that are existing-as-you-right-now each have their own story stretching back to the very birth of the Universe. Each one has its own story, and so today I bring you the story of just one atom in the Universe.
There was a time in the distant past — some 13.7 billion years ago — when there were no atoms. Yes, the energy was all there, but it was far too hot and too dense to have even a single atom. Imagine all the matter in the entire Universe, some 1091 particles, in a volume of space about equal to that of a single, giant star.
The whole Universe, compressed into a volume of space that one large star takes up.
Yes, back then it was too hot to have any atoms at all. But the Universe didn’t stay that way for long: it may have been incredibly hot and dense, but it was expanding and cooling incredibly rapidly back then. After less than a second, the quarks and gluons had condensed into stable protons and neutrons, the building blocks of all atomic nuclei.
The atom we’re thinking of started out as a neutron. Protons tried to fuse with it to create deuterium, but the Universe was too hot for that to happen, and each time it formed deuterium, it was blasted apart less than a nanosecond later.
After about three minutes, a few of the neutrons had decayed into protons, but this one remained, and finally the Universe had cooled enough so that nuclear fusion could proceed. The neutron quickly formed deuterium, then Helium-3, and finally found another deuteron to become a Helium-4 nucleus. Only about 8% of the atoms in the Universe became Helium-4 like this one; the other 92% were just plain old protons, also known as Hydrogen nuclei.
It took another 380,000 years for the Universe to cool enough for this to become a neutral atom, and for two electrons to join this nucleus. The Universe — despite its rapid expansion and cooling — remains 100% ionized until the temperature drops to just a few thousands of degrees, which simply takes that much time.
Over the next hundred-million years or so, this atom found itself caught up in the gravitational pull of the Universe, which began to form stars and galaxies. But the vast majority of atoms — more than 95% — weren’t a part of the first generation of stars, and neither was this one in particular.
Instead, when the first stars formed, they kicked the electrons out of the atoms that surrounded them, creating ions once again.
It was only by luck that this atom we’re following wound up in a dense molecular cloud, shielded from this radiation. After more than a billion years in this collection of neutral atoms, it finally found itself pulled in by gravitational attraction to what would become a giant star.
This atom lost its electrons and fell to the core of the star, where it lay dormant for millions of years, as hydrogen nuclei fused into other helium nuclei just like this one. When the core ran out of hydrogen fuel, helium fusion began, and our atom fused with two others to become a carbon nucleus!
While other atoms even closer to the center of the star fused further, carbon was as far as this particular atom went. When the core of the star collapsed and the star went supernova, our atom was blown out into the interstellar medium, where it resided for billions of years.
While billions of other stars went through the life-and-death cycle, this carbon atom remained in interstellar space, eventually picking up six electrons to become neutral. It found its way into a gravitational collection of neutral gas, and cooled, eventually getting sucked in to another gravitational perturbation, as star-formation happened all over again.
This time, the atom didn’t find its way into the central star of its system, but rather into the dusty disk that surrounded it. Over time, the disk separated into planetoids and planetesimals, and this atom found itself aboard one of those.
It first joined together with four hydrogen atoms, becoming methane, and went through millions of different chemical reactions over time.
After life took hold on Earth, it became a part of a bacterium’s DNA, then a part of a plant’s cell wall, and eventually became part of a complex organism that would find itself consumed by you.
The atom is currently in a red blood cell of yours, where it will remain for a total of about 120 days, until the cell is destroyed and replaced by a different one.
Although the cell — and all cells in your body — will be destroyed and replaced, you will remain the same person you are, and the atom will simply take on a different function, whether in your body or out of it. The atoms in your body are temporary, and can all be replaced — unnoticed by you — by another of the same type.
And each of the 1028 atoms in your body has a story as spectacular and unique as this one! As Feynman famously said,
“I / a Universe of atoms / an atom in the Universe.”
The story of the Universe is inside every atom in your body, each and every one. And after 13.7 billion years, 10,000,000,000,000,000,000,000,000,000 of them have come together, and that’s you. The Universe is inside of you, as surely as you’re inside the Universe.
You, a Universe of atoms, an atom in this Universe.
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