Smallest Thing: a journey down the scale of size

What is the smallest thing in the universe? As far as anyone has measured, the smallest things are fundamental particles such as quarks and electrons: no experiment has found any size for them, so if they have one it is smaller than about 10⁻¹⁸ m. Below that sits the Planck length, about 1.6 × 10⁻³⁵ m, where today's physics stops working. The Smallest Thing course takes you all the way down, in 23 short sections.

It opens with a 3D zoom from a water molecule into its oxygen atom, then the nucleus, then a single proton, with the size at every level.

What the course covers

Counting zeros

Tiny sizes need so many zeros that scientists count them instead. A millimetre is 10⁻³ m, a micrometre 10⁻⁶ m and a nanometre 10⁻⁹ m. A typical cell is about 10 µm, or 10⁻⁵ m, and a small atom about 10⁻¹⁰ m, so 10⁻⁵ ÷ 10⁻¹⁰ = 10⁵, about 100,000 atoms fit across a cell.

Seeing small things

Your eyes can make out about 0.1 mm. A light microscope reaches about 200 nm, because light cannot show details much smaller than about half its wavelength. Electron microscopes go further, and particle accelerators probe inside protons by smashing them together.

Inside the atom and beyond

An atom is almost entirely empty space, with nearly all its mass in a tiny nucleus. Protons and neutrons are about a femtometre across and are made of quarks. Quarks and electrons have no measured size, and at the Planck length, gravity and quantum physics should both matter at once, which no tested theory yet handles.

Try a question from the course

Every time you divide by ten, the power of ten goes down by one. So 0.001 m is 10⁻³ m, because it is 1 divided by ten three times. Going the other way, 10⁻⁶ m is 0.000001 m: a decimal point followed by five zeros, then a 1.

Now write a new size as a power of ten, then press Mark to check your answer in the course.

Who this course is for

• GCSE students who need standard form, prefixes and the sizes of cells, atoms and nuclei.
• Curious learners of any age who want a clear answer to how small things go.
• Teachers looking for a visual way to show the scale of the very small.

What you need to know first

Just multiplying and dividing by ten. The course builds powers of ten and scientific notation from scratch before using them.

How long it takes

There are 23 sections and most take a few minutes. Learners without a plan can open two sections of a course a day, in order, so the whole course takes 12 days. A plan removes the daily limit and lets you go straight to any level, such as inside the nucleus.

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