🔬 Inside the proton
Never a lone quark
A proton has three quarks inside it, and you can never get one of them out on its own.
Every atom has protons in its nucleus.
Each proton is a tiny bag with three things inside.
None of the three can ever be taken out on its own.
The setup
What’s inside a proton
Atoms are made of protons, neutrons and electrons. As far as anyone can tell, the electron is fundamental, which means it isn’t made of anything smaller. A proton is made of smaller things: it’s a bag of three quarks jiggling about, held together by the strong force, the strongest force in nature.
Your turn: grab any quark and pull. The band will pull it straight back. Then switch the proton to a neutron and see what happens to the charge.
The proton · u u d
+1
+2/3 +2/3 −1/3 = +1
A proton has two up quarks and one down quark. Their three charges (+⅔, +⅔, −⅓) add up to exactly +1, which is the charge of the proton.
The trap
Can you pull one quark out?
Suppose you reach into a proton, grab one quark and try to pull it out on its own. Before you try it below, pick what you think will happen.
Pull one quark away from the other two as hard as you can. You’ll…
Think of a strange elastic band that keeps pulling back just as hard however far you stretch it, storing more and more energy. If you pull hard enough, it snaps into two bands, each with two ends, so you never get one loose end. A quark behaves almost exactly like this. Try it below.
The harder you pull, the harder the string pulls back.
If you pull hard enough, something surprising happens.
The why
Pull a quark and watch the string snap
Drag the loose quark away from its clump. The strong-force string between them stretches and stores more and more energy, and the pull stays just as strong however far you go. Keep going until the string snaps, and see what the energy turns into.
Energy stored in the string
0: barely stretched
Each time, the energy you’ve put in becomes a new quark and antiquark, and they pair off into two clumps. You end up with more particles and still no lone quark. Physicists call this confinement, and it’s why nobody has ever seen a quark on its own.
Other forces get weaker as things move apart.
The strong force between quarks doesn’t.
The why
The strong force is the odd one out
The forces you know about get weaker with distance. Move two magnets apart and the pull between them drops away fast, and gravity weakens with distance too. The strong force behaves the other way: pull the quarks apart and its pull doesn’t fade. It keeps pulling back just as hard however far you go. Slide the two apart to compare them.
Distance between the two things
Close up, ordinary forces are strong, but they fade towards nothing as things move apart, while the strong force keeps growing. That’s why quarks can never escape.
Now zoom back out.
Everything you’ve ever touched…
…is made of just three little pieces, arranged in different ways.
Explore
Three pieces make almost everything
You, this screen, the sea and the stars are nearly all made of just three fundamental particles: the up quark, the down quark and the electron. “Fundamental” means that, as far as anyone can tell, they’re not made of anything smaller. Tap each one.
u
Up quark
charge +2/3
FundamentalIt’s one of the two kinds of quark in everyday matter. Two ups and a down make a proton, and nobody has ever found an up quark on its own.
The heavier quarks (charm, strange, top and bottom) and the electron’s heavier cousins do exist, but they’re rare and quickly turn into the light ones. Everyday matter needs only the three. This whole picture is called the Standard Model, and it’s our best-tested description of what everything’s made of.
Quarks are real, and physicists can measure them and count them.
But nobody has ever seen one alone.
Better machines wouldn’t change that, because it’s how quarks behave. If you pull hard enough on a quark, the energy makes a new quark and antiquark, so you never end up holding one on its own.
One more thing
So if someone tells you a proton is a tiny solid ball, you can explain that it’s three quarks in a bag of strong force, and that nearly everything you’ve ever touched is made of up quarks, down quarks and electrons, arranged in countless different ways. Then show them the snap: when you pull a quark, a new pair appears and the quark never comes free.