How a single atom contains the entire quantum Universe

If you wanted to uncover the secrets of the Universe for yourself, all you’d have to do is interrogate the Universe in a way that compelled nature itself to provide the answers to your deepest questions in a fashion that was comprehensible to you. When any two quanta of energy interact — irrespective of their properties, including whether they’re particles or antiparticles, massive or massless, fermions or bosons, etc. — the result of that interaction has the potential to inform you about the underlying laws and rules that the system has to obey. If you can know:

• all the possible outcomes of any interaction,

• including what their relative probabilities were,

• and can validate your theoretical model by matching it up with the experimental/observational data you collect,

then and only then can you claim to have some understanding of what was going on. Being quantitative in precisely this fashion, asking not only “what happens” but also “by how much” and “how often,” is what makes physics the robust science that it is.

Quite surprisingly, everything that we know about the Universe can, in some way, be traced back to the most humble of all the entities we know of: an atom. An atom remains the smallest unit of matter we know of that still retains the unique characteristics and properties that apply to the macroscopic world, including what we observe as physical and chemical properties of matter. And yet, an atom is a fundamentally quantum entity inherently, with its own energy levels, allowable and forbidden transitions, and conservation laws. Moreover, even the humble atom couples to all four of the known fundamental forces, and also, importantly, possesses a non-zero mass. In a very real way, all of physics is on display, even inside a single atom. Here’s what the humble atom, all on its own, can tell us about the Universe.

Here on Earth, there are approximately ~90 elements that occur naturally: left over from the cosmic processes that created them, or created downstream from the quantum decays that lead to their production. An element is fundamentally an atom, with an atomic nucleus made of protons and (possibly) neutrons and orbited by a number of electrons that’s equal to the number of protons. Each element has its own unique set of properties, including:

• hardness (in its solid phase),

• color,

• melting and boiling points,

• density (how much mass exists within a given volume of that element),

• conductivity (how easily its electrons are transported, or flow, when a voltage is applied),

• electronegativity (how strongly its atomic nucleus holds onto electrons when covalently bound to other atoms),

• ionization energy (how much energy is required to kick an electron off of that atom),

among many others. What’s remarkable about atoms is that there’s only one property that defines what type of atom you have (and hence, what these properties are): the number of protons in its atomic nucleus.

Given the diversity of atoms out there and the quantum rules that govern the electrons — all of which are identical particles to one another — that orbit the nucleus, it’s not hyperbole at all to make the claim that everything under the Sun is truly made, in some form or other, of atoms.

Each and every atom, whose species is defined by the number of protons inside its nucleus, will form a unique set of bonds with other atoms, enabling a practically unlimited set of possibilities for assembly: the types of molecules, ions, salts, and larger structures (such as crystals) that it can form. Primarily through the electromagnetic interaction, the subatomic particles that compose atoms will exert forces on one another, leading — given enough time — to the macroscopic structures we observe not only on Earth, but everywhere throughout the Universe.

At their very core, however, atoms all have the property of being massive in common with one another. The greater the number of protons and neutrons that are present in the atomic nucleus, the more massive your atom is going to be. Even though these are quantum entities, with an individual atom spanning no more than a single ångström in diameter, there’s no limit to the range of the gravitational force, nor is there any sort of “negative mass” that would cancel out an atom’s gravitational force on some scale.

Any object with energy — including the rest energy that gives particles their masses — will curve the fabric of spacetime according to Einstein’s theory of General Relativity. No matter how small or large the mass, or how small or great the distance scales are that we work with, the curvature of space induced by any number of atoms, whether 1057 (the approximate number of atoms in a star like our Sun), 1028 (the number of atoms composing a human being), or just one (such as a monatomic helium atom), will occur exactly as the rules of General Relativity predict.

Atoms themselves are also made up of multiple different types of electrically charged particles.

• Protons have a positive electric charge inherent to them.

• Neutrons are electrically neutral overall.

• And electrons have an equal-and-opposite charge to the proton.

All of the protons and neutrons that appear in an atom are bound together in an atomic nucleus just around a femtometer (10-15 m) in diameter (or a few femtometers for large atoms), while the electrons orbit in a cloud that’s some 100,000 times larger in size (about 10-10 m). Each electron occupies its own unique energy level and exists in a unique quantum state: no two electrons can occupy the same one within an atom. And furthermore, electrons can only transition between those discrete energy states; no other transitions are permissible.

However, these specific restrictions only apply to individual, isolated, unbound atoms, which isn’t the only set of conditions that apply to atoms throughout the Universe.

When an atom comes into the vicinity of another atom (or group of atoms), those various atoms can interact. At a quantum level, the wavefunctions of those multiple atoms can overlap, allowing atoms to bind together into molecules, ions, lattices, and salts, with these bound structures possessing their own unique shapes and configurations as far as their electron clouds are concerned. Correspondingly, these bound states also take on their own unique sets of energy levels, which absorb and emit photons (particles of light) only over a particular set of wavelengths.

You should care about these electron transitions within an atom or group of atoms a great deal, as they are what gives everything in the world the shape, feel, and variety that we get to experience. The electron transitions of every bound structure are unique: particular to the atom or the configuration of a group of multiple atoms that exists within that structure. When you detect a set of spectral lines from an atom or molecule — regardless of whether they’re emission or absorption lines — they immediately reveal what type of atom or molecule you’re looking at. The internal transitions that are allowed for the electrons within that bound system gives a unique set of energy levels, and the transitions of those electrons reveal unambiguously what type and configuration of atom (or collection of atoms) you’re investigating.

From anywhere in the Universe, atoms and molecules obey these same rules: the laws of classical and quantum electrodynamics, which govern every charged particle in the Universe. Even inside the atomic nucleus itself, which is internally composed of (charged) quarks and (uncharged) gluons, the electromagnetic forces between these charged particles is tremendously important. This internal structure explains why the magnetic moment of a proton is almost three times the magnitude of the electron’s magnetic moment (but of opposite sign), while the neutron has a magnetic moment that’s almost twice as large as the electron’s, but the same sign.

While the electric force has a very long range — the same, infinite range as gravitation, in fact — the fact that atomic matter is electrically neutral as a whole plays a tremendously important role in understanding how the Universe we experience behaves. The electromagnetic force is fantastically large, as two protons will repel each other with a force that’s approximately 1036 times larger than their gravitational attraction!

But because there are so many atoms making up the macroscopic objects we’re used to, and atoms themselves are electrically neutral overall, we only notice electromagnetic effects when either:

• something has a net charge, like a charged-up electroscope,

• when charges flow from one location to another, like during a lightning strike,

• or when charges get separated, creating an electric potential (or voltage), such as in a battery.

One of the simplest and most fun examples of this comes from rubbing a blown-up balloon on your shirt, and then attempting to stick the balloon either to your hair or to the wall. This works only because the transfer or redistribution of a small number of electrons can cause the effects of a net electric charge to completely overcome the force of gravity; these van der Waals forces are intermolecular forces, and even objects that remain neutral overall can exert electromagnetic forces that — over short distances — can themselves overcome the power of gravity.

At both a classical and quantum level, an atom encodes a tremendous amount of information about the electromagnetic interactions in the Universe, while “classical” (non-quantum) General Relativity is completely sufficient to describe the gravitational behavior of every atomic and subatomic interaction we’ve ever observed and measured. If we venture even further inside the atom, however, to the interior of the protons and neutrons inside the atomic nucleus, we can begin to discover the nature and properties of the remaining fundamental forces: the strong and weak nuclear forces.

As you venture down to ~femtometer (~10-15 m) scales, you’ll first start to notice the effects of the strong nuclear force. It first shows up between the different nucleons: the protons and neutrons that make up each nucleus. Overall, there’s an electric force that either repels (since two protons both have like electric charges) or is zero (since neutrons have no net charge) between the different nucleons. But at very short distances, there’s an even stronger force than the electromagnetic force: the strong nuclear force, which occurs between quarks through the exchange of gluons. Bound structures of quark-antiquark pairs — known as mesons — can be exchanged between different protons and neutrons, binding them together into a nucleus and, if the configuration is right, overcoming the repulsive electromagnetic force. (This aspect of how mesons are exchanged to bind protons and neutrons together is known as the residual strong force.)

Deep inside these atomic nuclei, however, there’s a different manifestation of the strong force: the individual quarks inside are continuously exchanging gluons. In addition to the gravitational (mass) charges and the electromagnetic (electrical) charges that matter possesses, there’s also a type of charge specific to the quarks and gluons: a color charge. Instead of being always positive and attractive (like gravity) or negative and positive where like charges repel and opposites attract (like electromagnetism), there are three independent colors — red, green, and blue — and three anti-colors. The only allowable combination is “colorless,” where all three colors (or anticolors) combined, or a net colorless color-anticolor combination are permitted.

The exchange of gluons, particularly when quarks get farther apart (and the force gets stronger), is what holds these individual protons and neutrons together. The higher the energy that you smash something into these subatomic particles, the more quarks (and antiquarks) and gluons you can effectively see: it’s like the inside of the proton is filled with a sea of particles, and the harder you smash into them, the “stickier” they behave. As we go to the deepest, most energetic depths we’ve ever probed, we see no limit to the density of these subatomic particles inside every atomic nucleus, and no limit to the species of quark that can be found within them, including the heavy, exotic quarks: strange, charm, bottom, and even top quarks and antiquarks.

However not every atom is capable of lasting forever in the configuration in which it is created; not every atom is stable. Many atoms, if they are unstable, will undergo some type of radioactive decay, meaning that eventually they will spit a particle (or a set of particles) out, fundamentally changing the type of atom that they are. The most common type of radioactive decay is alpha decay, where an unstable atom spits out a helium nucleus with two protons and two neutrons, which relies on the strong force, bumping it down two elements on the periodic table. But the second most common type is beta decay, where an atom spits out an electron and an anti-electron neutrino, and one of the neutrons in the nucleus transforms into a proton in the process, bumping it up one element on the periodic table instead.

This latter type of decay, beta decay, requires yet another novel force: the weak nuclear force. This force relies on a wholly new type of charge: weak charge, which itself is a combination of weak hypercharge and weak isospin. The weak charge has proven tremendously difficult to measure, since the weak force is millions of times smaller than either the strong force or the electromagnetic force until you get down to extraordinarily small distance scales, like 0.1% the diameter of a proton. With the right atom, one that’s unstable against beta decay, the weak interaction can be both seen and measured, meaning that all four of the fundamental forces can be probed simply by looking at an atom on various scales: from outside of it to its deepest interiors.

This realization carries along with it another remarkably profound implication: that if there’s any particle in the Universe, even one we have yet to discover, that interacts through any of these four fundamental forces, it will also exhibit interactions with atoms. In fact, the way that we’ve detected a great many particles, including all of the Standard Model particles, even including the different types of neutrinos and antineutrinos, is through their interactions with the particles found within the humble atom. Even though it’s the very thing that makes us up, it’s also, in a fundamental way, our greatest window into the true nature of matter.

This remarkable story, of the Universe that exists and can be discovered inside an atom, isn’t just the story of how how humanity discovered what makes up the Universe on the smallest scales of all, it’s also the story of all of us, and what makes us up ourselves. Back in 2023, I co-wrote a story with my particle physicist collaborator, Laura Manenti, to tell the story of the Universe that exists inside an atom in a way that was both educational for and understandable by children of all ages, and here in 2026, it’s now available:

• in English,

• Italian,

• and Arabic,

with more languages on the way in the future. After all, atoms make up everything: even me and you!

All matter is made up of the same building blocks: the same building blocks that composes the atoms that we’re familiar with. The deeper inside those building blocks we choose to look, the better and better we come to understand the very nature of the Universe itself. These various quanta bind together in an almost limitless fashion to create the Universe we observe and everything in it. By measuring and probing that matter, we can learn about the underlying rules that every particle and antiparticle within it obeys.

This has a profound implication to it: it’s only by interrogating the Universe that we have that we can learn about it and how it works. That’s the key to science: if you want to know anything about the composition and rules of the cosmos itself, you must probe it in a way that compels it to tell you about itself. That’s how we learn what nature is truly all about

As long as the science and technology we’re capable of constructing is capable of investigating it further, it would be a pity to give up on the search simply because a new, paradigm-shattering discovery isn’t guaranteed. The only guarantee we can be certain of is this: if we fail to look more deeply, we won’t find anything at all. That’s why it’s vital to stay curious, to keep looking, and to invest in the next generation of apparatuses that can take us beyond the current frontier. Despite all we’ve learned about the Universe, there’s always more to explore.

This article was first published in August of 2023. It was updated in June of 2026.

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