A micrometer, one millionth of a meter, is the standard unit for things like bacteria and red blood cells. Below it lies a cascade of progressively tinier scales, each governed by different physics and requiring radically different tools to observe. The nanometer (a thousandth of a micrometer) is the realm of molecules and viruses; the picometer captures the spacing between atoms in a crystal; the femtometer measures the radius of an atomic nucleus; and below that, the attometer begins to describe the internal structure of protons and neutrons. The journey does not stop there. Theoretical physics proposes a hard floor on smallness itself, a length so tiny it may be physically meaningless to speak of anything shorter.
Nanometers and the World of the Very Small
Step one thousand times smaller than a micrometer and you reach the nanometer, written as 10⁻⁹ meters. This is where the familiar rules of bulk materials start to break down. A gold bar is inert and yellow, but gold nanoparticles just a few nanometers across can appear red or purple and become chemically reactive. The change happens because at this scale, a significant fraction of the material’s atoms sit on its surface rather than buried inside, and electrons become confined in ways that alter the material’s optical and electronic behavior.
Viruses occupy the nanoscale. A typical influenza virus is roughly 80 to 120 nanometers across, and the smallest known viruses (parvoviruses) are around 20 nanometers in diameter. DNA, the molecule carrying genetic instructions, is about 2.5 nanometers wide. Researchers in nanoscience exploit these dimensions for everything from drug delivery particles to carbon-based nanostructures like carbon dots, which exhibit quantum confinement effects and unusually high electrical conductivity for their size.1PubMed. Carbon-based nanostructured materials incorporating carbon dots for supercapacitors: a review
Seeing individual nanometer-scale objects is itself a challenge. Conventional optical microscopes hit a resolution wall because visible light has wavelengths of roughly 400 to 700 nanometers, and the diffraction limit restricts resolution to about half the wavelength of the light used.2PubMed Central. Optical microscopy beyond the diffraction limit That means a standard light microscope cannot distinguish two objects separated by less than about 200 nanometers. To image anything smaller, scientists rely on electron microscopes, scanning probe instruments, or specialized super-resolution optical techniques that sidestep the diffraction barrier.
Picometers and the Gaps Between Atoms
One thousand times smaller than a nanometer sits the picometer, or 10⁻¹² meters. At this level, you are measuring the distances between individual atoms inside a solid material. A hydrogen atom, the smallest element, has a radius of roughly 25 picometers. Most atoms range from about 30 to 300 picometers in radius, depending on the element and how many electrons it has.
Chemists and materials scientists care deeply about picometer-scale precision because the exact spacing between atoms determines a material’s strength, conductivity, optical properties, and behavior under stress. In a silicon crystal, the distance between neighboring silicon atoms is about 235 picometers. Researchers studying semiconductor interfaces have used aberration-corrected electron microscopes to resolve the dumbbell-like arrangement of silicon and germanium atoms separated by roughly 136 picometers, achieving point resolutions below one ångström (which is 100 picometers).3PubMed Central. Interfacial atomic structure analysis at sub-angstrom resolution using aberration-corrected STEM
The ångström, at 0.1 nanometers or 100 picometers, deserves a quick mention because it appears constantly in chemistry and crystallography even though it is not an official SI unit. When a biologist says a protein bond is “1.5 ångströms,” they mean 150 picometers. You will see both units in the literature depending on the field and the era the paper was written in.
Scanning tunneling microscopes operate comfortably in picometer territory. These instruments can image individual atoms on a surface and even push them around one at a time, enabling researchers to build structures atom by atom.4Advanced Functional Materials. Atomic‐Scale Manipulation and In Situ Characterization with Scanning Tunneling Microscopy The ability to position single atoms with picometer-level control has opened up entire subfields in quantum computing and molecular electronics.
Femtometers and the Size of a Nucleus
Shrink another thousand-fold from picometers and you enter the femtometer, 10⁻¹⁵ meters. This is nuclear territory. The nucleus of an atom, where nearly all its mass is concentrated, occupies a volume measured in femtometers. A single proton has a charge radius of about 0.84 femtometers based on muonic hydrogen measurements, or around 0.88 femtometers from electron scattering experiments.5Atoms. Proton Charge Radius from Electron Scattering That discrepancy, known as the proton radius puzzle, occupied physicists for years and remains an active area of research.
To put the femtometer in perspective, an atom’s electron cloud (the part that determines its chemical behavior and its size as seen by neighboring atoms) is typically about 100,000 times larger than its nucleus. If you scaled a hydrogen atom up so that the nucleus was a marble sitting at the center of a football field, the electron cloud would extend to the edge of the stadium. Almost all of an atom is empty space.
Heavier nuclei are bigger but still absurdly small. A uranium-238 nucleus, one of the largest stable nuclei, has a radius of about 6 femtometers. Even uranium’s bulky nucleus, crammed with 238 protons and neutrons, is tens of thousands of times smaller than the atom it sits inside.
Measuring anything at the femtometer scale requires high-energy particle physics. You cannot bounce light off a nucleus to image it the way you would photograph a cell; the wavelength of visible light is roughly a trillion times too long. Instead, physicists slam electrons or other particles into nuclei at enormous speeds. The way those projectiles scatter reveals the size and shape of the target. This is the principle behind the deep-inelastic scattering experiments at the Stanford Linear Accelerator in the late 1960s, which probed the internal structure of protons and neutrons and confirmed that they contain point-like constituents, eventually understood as quarks.6Progress in Particle and Nuclear Physics. The theory of deeply inelastic scattering
Attometers and Inside the Proton
Below the femtometer lies the attometer, 10⁻¹⁸ meters. At this scale, you are looking inside protons and neutrons at the quarks and gluons that make them up. Individual quarks, as far as current experiments can tell, have no measurable size at all. Every attempt to probe their structure has returned results consistent with them being true point particles, at least down to scales of about 10⁻¹⁸ meters. That does not necessarily mean quarks are infinitely small; it means our most powerful accelerators have not yet found any substructure.
The attometer is also where the strong force, the force that binds quarks together inside protons and neutrons, operates at full ferocity. The strong force has a peculiar property: it grows stronger as quarks move farther apart, which is why isolated quarks have never been observed. The energy required to pull two quarks apart is so enormous that the energy itself generates new quark pairs before separation can occur. This confinement means the attometer scale is, in a sense, permanently cloaked. You can probe it with high-energy collisions, but you can never extract the pieces and lay them on a table.
The prefix “atto” also shows up in the measurement of ultra-fast events. Attosecond pulses of light, lasting roughly 10⁻¹⁸ seconds, are now routinely generated in laboratories and used to watch electrons rearrange themselves inside atoms in real time. Researchers have used these pulses to observe coherent electron motion in ionized xenon atoms, measuring oscillation periods as short as about 3 femtoseconds.7PubMed Central. Table-top all-attosecond transient absorption spectroscopy The 2023 Nobel Prize in Physics recognized the development of attosecond spectroscopy as a tool for studying matter at these fleeting timescales.
Zeptometers, Yoctometers, and the Newest Prefixes
The SI system continues below the attometer with the zeptometer (10⁻²¹ meters) and the yoctometer (10⁻²⁴ meters). In 2022, the international metrology community added two more prefixes: ronto (10⁻²⁷) and quecto (10⁻³⁰). These were introduced partly to keep up with data science, where extremely large numbers were outstripping the available prefix system, but they apply equally to the small end of the scale.
Whether these tiniest prefixes correspond to anything physically real is a separate question. No known particle or structure has a measured size in the zeptometer range. The electron, like the quark, appears to be a point particle down to the best resolution current experiments can achieve, which is on the order of 10⁻¹⁸ meters. The neutrino, even harder to pin down because it barely interacts with matter, is similarly structureless. These prefixes exist as mathematical tools for expressing extremely small quantities, but the physical world may not offer any object that naturally sits at 10⁻²⁷ meters.
The Planck Length and the Edge of Meaning
At about 1.6 × 10⁻³⁵ meters sits the Planck length, a value derived from three fundamental constants of nature: the speed of light, the gravitational constant, and the quantum of action. It is not just a very small number. Multiple approaches to quantum gravity suggest that it represents the smallest distance that has physical meaning, a kind of resolution limit built into reality itself.8Symmetry. Planck Length Emerging as the Invariant Quantum Minimum Effective Length Determined by the Heisenberg Uncertainty Principle in Manifestly Covariant Quantum Gravity Theory
The reasoning, in broad strokes, goes like this. To probe smaller and smaller distances, you need higher and higher energies. But at the energy scale corresponding to the Planck length, the energy concentrated in a tiny region becomes so extreme that it would collapse into a black hole. At that point, the act of measurement destroys the thing you are trying to measure, and the concept of “distance” ceases to be well-defined. This is not a technological limitation that better equipment could overcome; it appears to be a feature of how spacetime itself works.
The Planck length is almost inconceivably small. If you could somehow magnify an atom to the size of the observable universe, the Planck length would still be smaller than a single atom at the original scale. No experiment has directly probed anything remotely close to this distance. Even the Large Hadron Collider, the most powerful particle accelerator ever built, probes structures down to roughly 10⁻¹⁹ meters, which is still a staggering sixteen orders of magnitude larger than the Planck length.
Vacuum Fluctuations at the Smallest Scales
Even “empty” space is not truly empty at these extremes. Quantum field theory predicts that the vacuum, what we think of as nothing, is actually a seething mess of temporary fluctuations. Particle-antiparticle pairs blink into and out of existence on timescales so short that they cannot be directly observed individually. These vacuum fluctuations are a consequence of the Heisenberg uncertainty principle, which guarantees that the energy of a quantum field can never be pinned down to exactly zero, even in its lowest-energy ground state.9arXiv. Imaging the vacuum fluctuations of a quantum field
These fluctuations have measurable consequences. The Casimir effect, where two uncharged metal plates placed very close together in a vacuum experience a tiny attractive force, is one well-established example. The Lamb shift, a small change in the energy levels of hydrogen that was measured in the 1940s, is another. Both are real, experimentally confirmed effects that arise because empty space at the smallest scales is anything but still.
At or near the Planck length, these quantum fluctuations are expected to be so violent that the smooth fabric of spacetime itself would look foamy and turbulent, a concept sometimes called “quantum foam.” No one has observed this directly, and it may require an entirely new theory of quantum gravity to describe it properly. Spacetime, at the very bottom of the size ladder, may not resemble anything we would recognize as geometry at all.
Why You Cannot Just Keep Dividing
A natural impulse when thinking about scales is to assume you can always zoom in further, that between any two points there is always a midpoint, and so on forever. Classical geometry works this way. A line segment is infinitely divisible. But nature may not cooperate.
Several candidate theories of quantum gravity, including certain versions of string theory and loop quantum gravity, predict that space comes in discrete chunks at the Planck scale. If that is true, asking “what is smaller than the Planck length?” is a bit like asking “what is south of the South Pole?” The question is grammatically correct but physically empty. This is still theoretical, and the honest answer is that nobody knows for certain whether space is continuous or granular. But the mathematical and physical arguments pointing toward some kind of minimum length are compelling enough that most theoretical physicists take the idea seriously.
The practical upshot is that the hierarchy from micro down to Planck is not just a matter of slapping on smaller prefixes. Each step down brings you into a domain governed by fundamentally different physics. Micrometers are the world of cells and classical optics. Nanometers introduce quantum effects in materials. Picometers are where chemistry’s atomic bonds live. Femtometers are governed by the strong nuclear force. Attometers probe quark confinement. And the Planck length is where spacetime itself becomes the object of study, a frontier where our best theories break down and new physics is needed.
Everyday Comparisons That Put the Scale in Context
Numbers like 10⁻¹⁵ and 10⁻³⁵ are so far removed from human experience that they are almost meaningless without comparison. One way to build intuition is to note how many times you have to multiply by a thousand to move between scales. From a micrometer to a nanometer is one step of a thousand. From a nanometer to a picometer is another. Each step of a thousand feels modest when you say it quickly but represents a revolution in the kinds of objects you can find and the physics that governs them.
Another useful comparison involves time. Light, the fastest thing in the universe, takes about 3.3 femtoseconds to cross a single micrometer. To cross the diameter of a proton, light needs roughly 3 × 10⁻²⁴ seconds. To cross a Planck length, light would take about 5.4 × 10⁻⁴⁴ seconds, a duration so short that there is no known physical process that operates on that timescale.
Perhaps the most striking comparison is the gap between the smallest things we can measure and the theoretical floor. The difference in scale between the proton (about 10⁻¹⁵ meters) and the Planck length (about 10⁻³⁵ meters) is twenty orders of magnitude. That is the same ratio as the difference in size between a human being and the entire observable universe. The territory below the femtometer, in other words, is not a thin sliver at the bottom of reality. It is a vast, mostly unexplored expanse, as rich in its range of scales as everything above it that we can see and touch.