Assemble a hadron a piece at a time. Choose three quarks, or a quark with an antiquark, and the simulator adds their fractional charges in front of you — landing every time on a whole number of e, and naming the particle you have just built if anyone has ever found one.
What Is the Quark Simulator?
The quark simulator is a free interactive physics lab that runs in your browser — nothing to install and no sign-up. Build protons, neutrons and mesons from up, down and strange quarks and watch the fractional charges add to a whole number. It reports Electric charge, Baryon number, Sum of quark masses, Matched particle and quark mass as a share of the real particle as you drag the sliders.
| Quantity reported | Example value |
|---|---|
| Electric charge Q | +1 e |
| Baryon number B | 1 |
| Sum of quark masses | 9.02 MeV/c² |
| Matched particle | Proton |
| Quark mass as a share of the real particle | 0.96% |
How to Use the Quark Simulator
Two sets of buttons drive the whole lab. The three across the top decide what you are assembling — three quarks, a quark paired with an antiquark, or three antiquarks — and the arrow pairs beneath them step each slot through the six flavours in mass order: up, down, strange, charm, bottom, top. Nothing is typed and nothing is dragged, so it is quick to walk the entire table rather than check one combination and stop. In meson mode the third slot goes inert and says so, because there is no third seat to fill.
Watch the charge readout while you click. It is accumulated in whole thirds of e and divided by three exactly once, at the moment it is printed, so it cannot drift into a ragged decimal the way a running total of 0.666 would. Step slot 3 from down to up and the charge goes from +1 to +2 without passing through anything in between. Underneath sits the mass sum in MeV/c2 — the energy-equivalent unit the E = mc2 calculator works in — and for the proton and the neutron the lab also reports that sum as a share of the particle's measured mass. It is barely one per cent.
The baryon-number readout is what separates the three modes at a glance: 1 for three quarks, 0 for a meson, and −1 once you flip to antiquarks. It is not decoration. Baryon number is conserved in every interaction ever measured, which is the reason a proton has nowhere to decay to and ordinary matter lasts.
The habit this lab is built to break is the expectation that a fractional charge should be reachable. Try to find one. Every route through the buttons lands on an integer, because the strong force only binds combinations whose thirds happen to cancel — the confinement rule that keeps a lone quark permanently out of reach. What looks like an accident is the selection rule doing its job.
Frequently asked questions
Why does the charge always come out as a whole number?
Because every quark carries a charge that is two thirds or minus one third of e, and the only combinations nature binds are ones whose thirds happen to add to a multiple of three. Three quarks give a multiple of three; a quark paired with an antiquark cancels down to one as well. The simulator adds those thirds as integers and divides by three once, at the end, so the answer it prints is exactly an integer rather than a decimal that rounds to look like one.
What is the difference between the baryon and meson modes?
Baryon mode gives you three quarks and a baryon number of 1; meson mode gives you one quark and one antiquark, and the baryon number falls to 0 because the antiquark counts as minus one third against the quark's plus one third. That single number is what separates the two families. Protons and neutrons are baryons, which is why baryon number is conserved and ordinary matter does not simply evaporate. Pions and kaons are mesons, and they decay freely.
Why do the quark masses not add up to the proton mass?
Because most of a proton's mass is not its quarks at all. The simulator prints the sum for you: two up quarks and a down quark come to about 9 MeV/c2, against a measured proton mass of 938 MeV/c2, so the quarks account for roughly one per cent. The remainder is the energy stored in the gluon field binding them, converted to mass by E = mc squared. Mass here is mostly bound-up energy, not stuff.
Can I build a particle that does not exist?
Yes, and the lab will tell you so rather than inventing a name. Only combinations that have actually been observed in experiments are in its table; anything else reports as not a known hadron while still showing you a perfectly well defined charge and baryon number. Every combination containing a top quark falls into that group for a specific reason: the top decays in about 10 to the minus 25 seconds, far too fast to bind into anything.
References & formula source
- Particle Data Group (2025 Review of Particle Physics) — quark masses in the MS-bar scheme, and the observed baryon and meson tables.
- Griffiths — Introduction to Elementary Particles, Chapter 1 (Historical Introduction) and Chapter 5 (Bound States) on quark content and the eightfold way.
- Halliday, Resnick & Walker — Fundamentals of Physics, Chapter 44 (Quarks, Leptons and the Big Bang).
- CODATA / PDG — measured proton mass 938.272 MeV/c² and neutron mass 939.565 MeV/c².