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One of the most remarkable facts about our existence was first postulated over 2000 years ago: that at some level, every part of our material reality could be reduced to a series of tiny components that still retained their important, individual characteristics that allowed them to assemble to make up all we see, know, encounter, and experience. What began as a simple thought, attributed to Democritus of Abdera, would eventually grow into the atomistic view of the Universe.

Although the literal Greek word meaning uncuttable doesnt quite apply to atoms, being that theyre made of protons, neutrons, and electrons, any attempt to divide the atom further causes it to lose its essence: the fact that its a certain, specific element on the periodic table. Thats the essential property that allows it to build up all of the complex structures that exist within our observed reality: the number of protons contained within its atomic nucleus.

An atom is such a small thing that if you were to count up the total number of atoms contained within a single human body, youd have to count up to somewhere around 1028: more than a million times as great as the number of stars within the entire visible Universe. And yet, just the very fact that we, ourselves, are made of atoms is perhaps the greatest miracle in the entire Universe.

Whether in an atom, molecule, or ion, the transitions of electrons from a higher energy level to a lower energy level will result in the emission of radiation at a very particular wavelength defined by the fundamental constants. If these constants changed, so would the properties of atoms throughout the Universe.

Its a simple fact that the humble atom is whats at the core of all the matter we know of within the Universe, from plain old hydrogen gas to humans, planets, stars, and more. Everything thats made up of normal matter within our Universe whether solid, liquid, or gas is made of atoms. Even plasmas, found in very high-energy conditions or in the sparse depths of intergalactic space, are simply atoms that have been stripped of one or more electrons. Atoms themselves are very simple entities, but even with such simple properties, they can assemble to make complex combinations that truly boggle the imagination.

The behavior of atoms is truly remarkable. Consider the following.

Molecules, examples of particles of matter linked up into complex configurations, attain the shapes and structures that they do owing primarily to the electromagnetic forces that exist between their constituent atoms and electrons. The variety of structures that can be created is almost limitless.

There are two keys to understanding how atoms interact.

The energy levels and electron wavefunctions that correspond to different states within a hydrogen atom, although the configurations are extremely similar for all atoms. The energy levels are quantized in multiples of Plancks constant, but the sizes of the orbitals and atoms are determined by the ground-state energy and the electrons mass. Only two electrons, one spin up and one spin down, can occupy each of these energy levels owing to the Pauli exclusion principle, while other electrons must occupy higher, more voluminous orbitals. When you drop from a higher energy level to a lower one, you must change the type of orbital youre in if youre only going to emit one photon, otherwise youll violate certain conservation laws that cannot be broken.

To an extremely good approximation, this view of matter within the Universe:

can explain almost everything in our familiar, everyday lives.

Atoms assemble with one another to make molecules: bound states of atoms that fold together in almost innumerable sets of configurations, and that can then interact with one another in a variety of ways. Link a large number of amino acids together and you get a protein, capable of carrying out a number of important biochemical functions. Add an ion onto a protein, and you get an enzyme, capable of changing the bond structure of a variety of molecules.

And if you construct a chain of nucleic acids in just the right order, and you can encode both the construction of an arbitrary number of proteins and enzymes, as well as to make copies of yourself. With the right configuration, an assembled set of atoms will compose a living organism.

Although human beings are made of cells, at a more fundamental level, were made of atoms. All told, there are close to ~10^28 atoms in a human body, mostly hydrogen by number but mostly oxygen and carbon by mass.

If all of human knowledge were someday wiped out in some grand apocalypse, but there were still intelligent survivors who remained, simply passing on the knowledge of atoms to them would go an incredibly long way toward helping them not only make sense the world around them, but to begin down the path of reconstructing the laws of physics and the full suite of the behavior of matter.

The knowledge of atoms would lead, very swiftly, to a reconstruction of the periodic table. The knowledge that there were interesting things in the microscopic world would lead to the discovery of cells, of organelles, and then of molecules and their atomic constituents. Chemical reactions between molecules and the associated changes in configurations would lead to the discovery of both how to store energy as well as how to liberate it, both biologically as well as inorganically.

What took human civilization hundreds of thousands of years to achieve could be re-discovered in a single human lifetime, and would bring fascinating hints of more to come when properties like radioactivity or the interaction possibilities between light and matter were discovered as well.

The periodic table of the elements is sorted as it is (in row-like periods and column-like groups) because of the number of free/occupied valence electrons, which is the number one factor in determining each atoms chemical properties. Atoms can link up to form molecules in tremendous varieties, but its the electron structure of each one that primarily determines what configurations are possible, likely, and energetically favorable.

But the atom is also a sufficient key to take us beyond this Dalton-esque view of the world. Discovering that atoms could have different masses from one another but could still retain their elemental properties would lead not only to the discovery of isotopes, but would help investigators discover that atomic nuclei were composed of two different types of particles: protons (with positive charges) as well as (uncharged) neutrons.

This is more profound than almost anyone realizes, at first pass. Within the atomic nucleus, there are:

and that the full nucleus is orbited by electrons: particles that have the equal-and-opposite charge that a proton has, and that have a smaller mass than the mass difference between the proton and the neutron inside the nucleus.

Where, if you take a free proton, it will be stable.

And if you take a free electron, it, too, will be stable.

And then, if you take a free neutron, it wont be stable, but will decay into a proton, an electron, and (perhaps) a third, neutral particle.

Schematic illustration of nuclear beta decay in a massive atomic nucleus. Beta decay is a decay that proceeds through the weak interactions, converting a neutron into a proton, electron, and an anti-electron neutrino. Before the neutrino was known or detected, it appeared that both energy and momentum were not conserved in beta decays; it was Wolfgang Paulis proposal that a new, tiny, neutral particle existed.

That small realization, all of a sudden, would teach you a tremendous amount about the fundamental nature of reality.

First, it would immediately tell you that there must be some additional force that exists between protons and/or neutrons than the electromagnetic force. The existence of deuterium, for example (an isotope of hydrogen with 1 proton and 1 neutron) tells us that some sort of attractive force between protons and neutrons exists, and that it cannot be explained by either electromagnetism (since neutrons are neutral) or gravity (because the gravitational force is too weak to explain this binding). Some sort of nuclear binding force must be present.

This force must, at least over some small distance range, be able to overcome the electrostatic repulsion between protons within the same atomic nucleus: in other words, it must be a stronger nuclear force than even the (quite strong in its own right) repulsive force between two protons. Because there are no stable atomic nuclei made solely out of two (or more) protons, the neutron must play a role in the stability of the nucleus.

In other words, just from discovering that atomic nuclei contain both protons and neutrons, the existence of the strong nuclear force or something very much like it becomes a necessity.

Individual protons and neutrons may be colorless entities, but the quarks within them are colored. Gluons can not only be exchanged between the individual gluons within a proton or neutron, but in combinations between protons and neutrons, leading to nuclear binding. However, every single exchange must obey the full suite of quantum rules.

In addition, once one either:

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the implication is immediate for the existence of a fourth fundamental interaction in addition to gravity, electromagnetism, and the strong nuclear force: what we call the weak nuclear force.

Somehow, some sort of interaction must occur that allows one to take multiple protons, fuse them together, and then have it transform into a state that is less massive than the original two protons, where one proton gets converted into at least a neutron and a positron (an anti-electron), and where both energy and momentum are still conserved. The ability to convert one type of particle into another thats different than the sum of its parts or than the creation of equal amounts of matter-and-antimatter is something that none of the other three interactions can accommodate. Simply by studying atoms, the existence of the weak nuclear force can be deduced.

The most straightforward and lowest-energy version of the proton-proton chain, which produces helium-4 from initial hydrogen fuel. Note that only the fusion of deuterium and a proton produces helium from hydrogen; all other reactions either produce hydrogen or make helium from other isotopes of helium.

In order to have a Universe with many types of atoms, we needed our reality to exhibit a certain set of properties.

The existence of a Universe rich with a variety of atoms, but dominated by hydrogen, demands all of these factors.

The anatomy of a very massive star throughout its life, culminating in a Type II Supernova when the core runs out of nuclear fuel. The final stage of fusion is typically silicon-burning, producing iron and iron-like elements in the core for only a brief while before a supernova ensues. Many of the elements found throughout the Universe, including iron, silicon, sulfur, cobalt, nickel and more, are primarily created inside the cores of massive stars such as this one.

If an intelligent being from another Universe were to encounter us and our reality for the very first time, perhaps the very first thing wed want to make them aware of was this fact: that were made of atoms. That within everything thats composed of matter in this Universe are tiny, little entities atoms that still retain the essential characteristic properties that belong only to that specific species of atom. That you can vary the weight of the nuclei inside these atoms and still get the same type of atom, but if you vary their charge, youll get an entirely different atom. And that these atoms are all orbited by the number of negatively charged electrons required to precisely balance the positive charge within the nucleus.

By looking at how these atoms behave and interact, we can understand almost every molecular and macroscopic phenomenon that emerges from them. By looking at the internal components of these atoms and how they assemble themselves, we can learn about the fundamental particles, forces, and interactions that are the very basis of our reality. If there were only one piece of information to pass on to a surviving group of humans in a post-apocalyptic world, there might be no piece of information as valuable as the mere fact that were all made of atoms. In some sense, its the most miraculous property of all pertaining to our Universe.

Original post:

Why atoms are the Universe's greatest miracle - Big Think