# The faintest sound you can hear moves your eardrum less than an atom

https://mariuscomper.uk/pana-la-atom/en/

Start from a candle seen far away and go down into your own body, order of magnitude after order of magnitude, to a single atom. At every step you find something of yours at work at that size: a nerve, a cell, a molecule, your eardrum.

Keep going down. Everything is drawn to scale, and the bar in the corner shows how long it is.

4 kilometres

## A candle, far away

We start far away. On a clear night in the countryside, with your eyes used to the dark and your pupils wide open, you could catch the flame of a candle about four kilometres off.

It is a calculation that starts from a laboratory experiment. In 1942 Selig Hecht and his colleagues estimated that an eye fully adapted to the dark notices a faint flash when it absorbs between 5 and 14 photons. Starting from that threshold, the page on the eye's limit works out that air leaves the flame visible to about 4.3 km on a clear rural night, and to 2.9 km through light city haze.

[Nine Photons in the Dark: The Quantum Limit of Human Vision](https://mariuscomper.uk/noua-fotoni/en/)

2 metres

## You, and a thread longer than you

Now it is you: 1.70 metres, the height of the reference adult in the cell censuses.

Beside you lies the DNA from a single cell, unwound: 2.08 metres, longer than you are. Almost every cell with a nucleus carries it packed tight, and laid end to end the threads in your whole body would run to about 6.2 billion kilometres, some 21 round trips to the Sun. We will meet it again, much further down.

[All the DNA in you would reach the Sun and back. How many times?](https://mariuscomper.uk/cat-adn-ai-in-tine/en/)

1 metre

## A caress travels slower than you walk

From the forearm to the spinal cord there are about 60 centimetres of nerve. Two pieces of news about the same touch set off along it.

The fibre that feels a caress carries its impulses at about 0.8 metres a second, slower than walking. The fast touch fibres, measured in a separate study on the hand, averaged about 59 metres a second. In the drawing both impulses leave at once; watch the slow one.

[The speed of a caress: stroke an arm and hear the nerve fibre for tenderness](https://mariuscomper.uk/the-speed-of-a-caress/)

20 centimetres

## The first face you saw

Thirty centimetres: about the distance between a newborn and the face of whoever is holding it.

At that distance the narrowest stripe a newborn can tell apart is about 2.7 mm wide. An eyebrow, yes; an eyelash, no. By three months, the stripes a baby tells apart at the same distance are down to 0.65 mm, four times finer. The first face you knew had no fine detail, and most studies find adult sharpness arrives only at around six or seven.

[The first face](https://mariuscomper.uk/the-first-face/)

6 centimetres

## Two needles on the skin

Touch the skin with two points at once. On the tip of your index finger, the threshold is about 2 millimetres (2 to 3 in Weinstein's 1968 measurements). On your upper back the two needles can be about 40 millimetres apart, four centimetres, and still feel like one.

These are classic averages from the compass test, and the page warns that the test is sensitive to how hard you press. The gap is still enormous: the threshold is more than ten times smaller on the finger than on the back.

[Forty Millimetres on the Back, Two on the Fingertip](https://mariuscomper.uk/patruzeci-de-milimetri/en/)

Also: [Thirteen Nanometres: The Biophysics of Fingerprints and the Nanoscale Limit of Touch](https://mariuscomper.uk/treisprezece-nanometri/en/)

1 centimetre

## Three and a half millimetres of sound

When a sound does not come from straight ahead, it reaches one ear slightly before the other. For a sound close to straight ahead, the best listeners can tell apart a difference in arrival of just 10 microseconds.

Translated into distance, the sound has travelled about 3.4 millimetres further through the air to the far ear. That is enough for your brainstem to tell which side it came from.

[Ten Microseconds Between the Ears: How the Brain Computes Sound Location](https://mariuscomper.uk/zece-microsecunde/en/)

20 micrometres

## A cell wider than its road

We zoom into a blood vessel, down to the narrowest. A red blood cell at rest is about 7.8 micrometres across. Many capillaries are only 5 to 6 micrometres wide inside.

So red cells deform to squeeze through many capillaries, and there the blood almost stops: fractions of a millimetre a second, long enough for the oxygen to get out.

[Half a Millimetre per Second: Why Blood Nearly Stops in the Capillaries](https://mariuscomper.uk/jumatate-de-milimetru/en/)

6 micrometres

## The film on your eye

On your cornea lies a film of tears that you spread again with every blink. In one recent measurement its watery layer was 3,282 nanometres thick, about three micrometres.

On top, like a lid, sits a layer of oil just 52.7 nanometres thick: 62 times thinner. Everything you see passes through both.

[Three Micrometres of Water Between Air and Sight](https://mariuscomper.uk/trei-micrometri/en/)

1.5 micrometres

## Your finger, at thirteen nanometres

Here we meet the fingertip again. Pressed down, it tells two needles apart only from about 2 millimetres. But when it slides across a surface, the ridges of your fingerprint turn texture into vibration.

In 2013 Lisa Skedung and her colleagues had people run a finger across films with parallel grooves. They told apart grooves with a relief of just 13 nanometres: about 40 times less than the wavelength of green light. In the drawing the grooves are to scale: an almost straight line.

[Thirteen Nanometres: The Biophysics of Fingerprints and the Nanoscale Limit of Touch](https://mariuscomper.uk/treisprezece-nanometri/en/)

600 nanometres

## Where light cannot follow

We have reached the wavelength of light. Hecht ran his experiment with blue-green light of about 510 nanometres; in the drawing, a single wave already spans half the screen.

A little further down, a light microscope can no longer resolve detail: its limit is about 2,500 ångströms, a quarter of a micrometre. To see past it, George Palade used a beam of electrons.

[A Romanian doctor timed a protein's journey through the cell and won a Nobel Prize](https://mariuscomper.uk/george-palade/en/)

Also: [Nine Photons in the Dark: The Quantum Limit of Human Vision](https://mariuscomper.uk/noua-fotoni/en/)

60 nanometres

## Palade's grains

In nearly all the cells he studied, Palade saw small, dense grains, many stuck to membranes. Today we call them ribosomes and know that proteins are made in them; their role was proved later.

Most of the grains in his 1955 paper measured 10 to 15 nanometres. The atomic model of the human ribosome, published in 2015, puts it at 26 to 30. The sizes were determined differently, but no source the page cites explains the gap.

[A Romanian doctor timed a protein's journey through the cell and won a Nobel Prize](https://mariuscomper.uk/george-palade/en/)

15 nanometres

## A ten-nanometre turbine

Real motors spin in the mitochondria of your cells. ATP synthase, the rotary turbine that recharges the cell's batteries, has a head about 10 nanometres across.

Together these turbines remake, every day, an amount of ATP comparable to your body weight. The pages about them give slightly different figures for how fast they spin, so here we stay with the size.

[Seventy Kilograms a Day: The 21,000 RPM Turbine Inside Your Cells](https://mariuscomper.uk/saptezeci-de-kilograme/en/)

Also: [Sixty Kilograms a Day: The Bioenergetics of ATP Recycling](https://mariuscomper.uk/saizeci-de-kilograme/en/)

2.5 nanometres

## The thread, again

The 2.08-metre thread that stood beside you up above is built from steps this small. Each letter of the genome adds 0.34 nanometres to its length, as X-rays measured.

Two sets of over three billion letters, each a third of a nanometre. That is how an invisible thread ends up longer than you.

[All the DNA in you would reach the Sun and back. How many times?](https://mariuscomper.uk/cat-adn-ai-in-tine/en/)

250 picometres

## Less than an atom

Here is a hydrogen atom: about 106 picometres, a tenth of a nanometre. Atoms have no sharp edge; the figure is a convention, but the order of magnitude does not change.

And this is where your hearing reaches. At the faintest sound you can hear, your eardrum moves, by calculation, about 10 picometres: ten times less than the atom. It is not a direct measurement; it is what follows from the sound pressure at threshold. The bar in the drawing swings across that distance.

[Ten Picometres in the Eardrum](https://mariuscomper.uk/zece-picometri/en/)

## Try it, with a paperclip

Unbend a paperclip into a U, with its two tips three or four millimetres apart. Touch your fingertip lightly: you will usually feel two points. Now touch your forearm: there you usually feel only one.

Spread the tips, a centimetre, then three, then four, and look for the gap at which your forearm starts to feel two; the page gives 38 millimetres for the forearm. Do not press hard: the test is sensitive to force, and the thresholds on the page are averages from studies, not yours.

[Forty Millimetres on the Back, Two on the Fingertip](https://mariuscomper.uk/patruzeci-de-milimetri/en/)

## Silence

You have gone down more than thirteen orders of magnitude, from four kilometres to a tenth of a nanometre, and at every stop you found part of yourself at work: an eye that counts photons, a finger that feels thirteen nanometres, an eardrum that moves less than an atom.

Further in, the cochlea amplifies the motion: at the same threshold, a hair cell's bundle of stereocilia is displaced by about 0.3 nanometres, some three times the diameter of an atom. Between the eardrum and the auditory nerve, your ear turns almost nothing into something.

Tonight, when the house goes quiet, listen for the faintest sound you can still make out. For that sound, your eardrum is moving less than the width of an atom.

## Further

Every step above is a whole page. Here they are, with the other measurements of your body.

- [Ten Picometres in the Eardrum](https://mariuscomper.uk/zece-picometri/en/)If sound waves struck inner ear fluid directly, 99.89% of acoustic energy would be reflected.
- [Your Ear Has Its Own Amplifier](https://mariuscomper.uk/patruzeci-de-decibeli/en/)The inner ear is not a passive sensor, but a piezoelectric mechanical motor: 12,000 outer hair cells pump force at over 20,000 cycles per second using the motor protein Prestin, adding 40 decibels to faint sounds (a 100-fold boost in amplitude and 10,000-fold in energy).
- [Why You Don't Deafen Yourself Talking](https://mariuscomper.uk/douazeci-de-decibeli/en/)When we speak or chew, internal vibrations reach the inner ear through tissue and bone.
- [Ten Microseconds Between the Ears: How the Brain Computes Sound Location](https://mariuscomper.uk/zece-microsecunde/en/)The human brainstem detects sound arrival time differences down to 10 microseconds between the ears — 100 times finer than a single action potential.
- [Nine Photons in the Dark: The Quantum Limit of Human Vision](https://mariuscomper.uk/noua-fotoni/en/)In darkness, Hecht's estimated threshold is 5–14 absorbed photons; 9 is representative.
- [Tens of Picoamperes in the Dark](https://mariuscomper.uk/patruzeci-de-picoamperi/en/)In darkness, rods sustain a dark current on the order of tens of picoamperes, while direct human recordings measured flash-evoked photocurrents up to about 20 pA.
- [The Blind Spot in the Middle of Your Vision](https://mariuscomper.uk/punctul-orb/en/)A fist at arm's length covers ten degrees of sky — the amateur astronomer's rule.
- [Pupil squared](https://mariuscomper.uk/pupila-la-patrat/en/)When the pupil moves from 2 to 8 millimetres, its diameter grows fourfold while the circular opening grows sixteenfold.
- [Forty-Three Dioptres: Why We See Completely Blurred Underwater and Where the Eye's True Lens Really Is](https://mariuscomper.uk/patruzeci-si-trei-de-dioptrii/en/)Over 70% of human eye focusing power (+43 of 60 dioptres) occurs at the air-cornea interface.
- [Three Micrometres of Water Between Air and Sight](https://mariuscomper.uk/trei-micrometri/en/)The tear film is about 3,282 nanometres in its muco-aqueous region and 52.7 nanometres in its surface lipid layer — a 62.3-fold difference.
- [Seven Milliseconds Behind the Head: The Biophysics of the Vestibulo-Ocular Reflex and Why Your Eyes Beat Any Gimbal](https://mariuscomper.uk/sapte-milisecunde/en/)While the visual cortex operates with a 150–200 millisecond delay and blurs completely when shaking a phone above 1 Hz, the 3-neuron inner ear vestibulo-ocular reflex responds in just 7–10 milliseconds, allowing you to read with razor-sharp clarity while running.
- [Thirteen Nanometres: The Biophysics of Fingerprints and the Nanoscale Limit of Touch](https://mariuscomper.uk/treisprezece-nanometri/en/)A static press resolves two separate points only at 1.6 millimetres, but sliding a finger detects nanoscale grooves of just 13 nanometres.
- [Forty Millimetres on the Back, Two on the Fingertip](https://mariuscomper.uk/patruzeci-de-milimetri/en/)On the fingertip we resolve two points at 1.6 millimetres, but on the upper back two points fuse into one even at 42 millimetres.
- [Nine Degrees Under Your Fingertip: Why Metal Feels Colder Than Wood](https://mariuscomper.uk/noua-grade-sub-deget/en/)Aluminium and wood can share one temperature, yet skin initially meets them at roughly 20.6 °C and 29.2 °C.
- [Forty-Three Degrees on the Tongue: Why Chili Pepper Activates the Fire Receptor at Body Temperature](https://mariuscomper.uk/patruzeci-si-trei-de-grade/en/)The thermal pain receptor TRPV1 responds probabilistically to heat, with an operational range around 42–43 °C.
- [The Two Speeds of Pain](https://mariuscomper.uk/doua-viteze/en/)When you stub your toe, the reflex arrives in 80 ms, while the dull ache takes 1.60 s.
- [Eighty Milliseconds: Why Consciousness Lives in the Past](https://mariuscomper.uk/optzeci-de-milisecunde/en/)To synchronize instantaneous light with sluggish sound and asynchronous nerve impulses, the brain delays conscious perception by 80 milliseconds.
- [Two Hundred Milliseconds: Why You Can't Tickle Yourself](https://mariuscomper.uk/de-ce-nu-te-poti-gadila-singur/en/)In an experiment with 16 participants, touch produced by one's own movement was rated as less ticklish; at 200- and 300-millisecond delays, ratings did not differ significantly from external stimulation.
- [One Hundred Milliseconds of Too Much Precision](https://mariuscomper.uk/o-suta-de-milisecunde/en/)Touch made by your own hand is less surprising because the nervous system predicts its consequences.
- [Ten Oscillations a Second: The Hand's Physiological Tremor](https://mariuscomper.uk/zece-oscilatii/en/)The acceleration of a healthy adult's outstretched hand often carries energy between 8 and 12 Hz.
- [One and a Half Watts: The Hydraulic Mechanics of the Human Heart](https://mariuscomper.uk/un-watt-si-jumatate/en/)The human heart pumps 7,200 litres of blood daily across 100,000 continuous pressure cycles at a mechanical power output of just 1.33 watts — less than a nightlight LED.
- [Seven Metres per Second: Why the Pulse at Your Wrist Is Not the Blood Flowing](https://mariuscomper.uk/sapte-metri-pe-secunda/en/)The pulse at your wrist arrives in 80 milliseconds, yet the pumped blood takes over two seconds.
- [Half a Millimetre per Second: Why Blood Nearly Stops in the Capillaries](https://mariuscomper.uk/jumatate-de-milimetru/en/)At a resting cardiac output of 5 L/min, Q = A × v gives about 185 mm/s in the aorta and 0.19–0.24 mm/s across a 3,500–4,500 cm² capillary bed.
- [Ninety Millimetres of Mercury: The Calf's Second Heart and the Pressure Drop in the Ankle](https://mariuscomper.uk/nouazeci-de-milimetri/en/)In a standing adult, the hydrostatic column raises ankle pressure to 90 millimetres of mercury.
- [Twenty-Five Percent in Cold Water: The Mammalian Dive Reflex and the Vagal Brake on the Human Heart](https://mariuscomper.uk/inima-in-apa-rece/en/)When your face touches 10–15 °C water during breath-holding, the trigeminal nerve fires the mammalian dive reflex: resting heart rate plummets by 25%, while limb blood flow is cut by 80% to conserve oxygen.
- [One Hundred and Fifty Millilitres: Anatomical Dead Space and the Alveolar Ventilation Paradox](https://mariuscomper.uk/o-suta-cincizeci-de-mililitri/en/)In every breath, 150 millilitres of air remains trapped in the trachea and bronchi without reaching the alveoli.
- [Two Millinewtons per Metre: The Surface Tension of Lungs and the 480-Million-Bubble Paradox](https://mariuscomper.uk/doi-milinewtoni/en/)Across 480 million alveoli, 130 square metres of wet membrane should collapse under Laplace's law.
- [The Two-Hour Nostril: The Hidden Asymmetry of Breathing](https://mariuscomper.uk/o-nara-la-doua-ore/en/)At any given moment, ~80% of resting airflow passes through just one nostril.
- [Four Grams in the Entire Bloodstream: Why the Body Lives on a Single Teaspoon of Glucose](https://mariuscomper.uk/patru-grame/en/)Across all 5 litres of human blood, only 4.5 grams of glucose circulate at any moment — a single teaspoon of sugar that the brain oxidizes in 54 minutes.
- [Sixty Kilograms a Day: The Bioenergetics of ATP Recycling](https://mariuscomper.uk/saizeci-de-kilograme/en/)At a daily burn of 2,500 kcal, your body synthesizes and breaks down 57.3 kg of ATP via F₀F₁ rotary nanomotors at 9,000 RPM, despite holding only 50 g in stock.
- [Seventy Kilograms a Day: The 21,000 RPM Turbine Inside Your Cells](https://mariuscomper.uk/saptezeci-de-kilograme/en/)Your body stores just 50 grams of ATP, yet synthesizes 70 kilograms daily — spinning over 10¹⁷ rotary nanomachines at 21,000 RPM with near 100% thermodynamic efficiency.
- [One Million to One: The Stomach's Impossible Acid Gradient](https://mariuscomper.uk/un-milion-la-unu/en/)The H⁺/K⁺-ATPase creates a more-than-million-to-one gradient between parietal-cell cytosol and the secretory canaliculus.
- [Fifty Grams in the Skull](https://mariuscomper.uk/cincizeci-de-grame/en/)The human brain weighs 1,400 grams in air, yet the skull base bears only 50 grams thanks to Archimedean buoyancy.
- [One Volt per Centimetre: The Skin's Epithelial Battery and the Electric Field That Heals Wounds](https://mariuscomper.uk/un-volt-pe-centimetru/en/)Intact human skin acts as a living bioelectric battery maintaining a 20 to 50 millivolt transepithelial potential.
- [Seven Hundred Newtons on the Molar: The Biomechanics of the Bite and Enamel's 100-Atmosphere Strength](https://mariuscomper.uk/sapte-sute-de-newtoni/en/)The jaw's Class III lever delivers 700–900 N onto molars, creating contact pressures exceeding 1,500 atmospheres on body's hardest biomaterial: tooth enamel.
- [Twenty Minutes for a Bubble: The Biophysics of Knuckle Cracking and the Cavitation Refractory Period](https://mariuscomper.uk/douazeci-de-minute/en/)MRI links the crack to rapid cavity inception under negative pressure in <310 ms; the refractory period is about 20 minutes, while the precise acoustic mechanism remains debated.
- [All the DNA in you would reach the Sun and back. How many times?](https://mariuscomper.uk/cat-adn-ai-in-tine/en/)About 21 times: 6.2 billion kilometres of DNA in 3 trillion nucleated cells.
- [A Romanian doctor timed a protein's journey through the cell and won a Nobel Prize](https://mariuscomper.uk/george-palade/en/)In 1953 George Palade described the grains on the cell's membranes, the ribosomes, and with Lucien Caro and James Jamieson he measured how many minutes new proteins take from the ribosome to the granules.
- [The speed of a caress: stroke an arm and hear the nerve fibre for tenderness](https://mariuscomper.uk/the-speed-of-a-caress/)The skin of your arm has a slow nerve fibre that answers to gentle stroking.
- [The first face](https://mariuscomper.uk/the-first-face/)A newborn can see you, but not the way you think.
- [The first voice you ever heard](https://mariuscomper.uk/the-first-voice/)For months before birth, you could hear.
- [What your dog sees of you](https://mariuscomper.uk/what-your-dog-sees/)Dogs don't see in black and white, but they don't see what you see either.
- [Most of your atoms are 13.8 billion years old: where each part of your body was made](https://mariuscomper.uk/made-of-stars/)Six in ten of your atoms are hydrogen from the Big Bang.

## How this was made

This essay draws on 43 pages on mariuscomper.uk. From each I extracted the quotations, figures and caveats, and the text here is written only from them; the titles and summaries on the shelf are the pages' own.

The key figures were also checked in primary sources: Hecht, Shlaer and Pirenne (1942) for the visual threshold, measured with 510-nanometre light; Skedung and colleagues (2013, Scientific Reports) for the 13 nanometres; Klumpp and Eady (1956) for the 10 microseconds; Hudspeth (1989) for the 0.3 nanometres of the stereocilia; for the eardrum, the plane-wave calculation at 0 dB SPL and 1 kHz gives about 8–11 picometres, and Wilska (1935) measured displacements of the order of a tenth of a nanometre directly, near 300 Hz.

The drawing is to scale: at every point of the scroll, the bar in the corner shows how long its length is. The objects are schematic, but their sizes are those in the text; the 760-nanometre spacing of the grooves under the finger comes from Skedung’s paper. The candle's distance is a calculation, not an observation; the two-point thresholds are classic averages from a test that researchers now consider imprecise.

Published 26 September 2026.

[mariuscomper.uk](https://mariuscomper.uk/) · Marius Comper