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Yang–MillsA QFT.ORG PROJECT

Yang–Mills & the Mass Gap

Understand the gap. Explore the frontier.

YANG–MILLS & THE MASS GAP

Understand the gap.
Explore the frontier.

A guided path from your first gauge theory course to one of the deepest open problems in physics and mathematics.

Begin with two semesters of QFT. Build the mathematics along the way.

THE PHYSICAL SPECTRUMFIG. 01
A schematic physical energy spectrumThe vacuum has zero energy. In the gapped example, no excited states have energy between zero and Delta. This is a schematic, not a computed Yang–Mills spectrum.EEXCITEDSTATESΔTHE MASS GAP0VACUUM
Schematic · not numerical data

A positive threshold above the vacuum. That is what must be proved.

An open Millennium ProblemFour-dimensional quantum Yang–MillsExistence and a positive mass gapOfficial status · 24 Oct 2026 ↗

A CLEAR PLACE TO BEGIN

Big questions.
A manageable first step.

You know the language of fields and gauge symmetry. Start with the physical question, then discover exactly what remains to be proved.

Find your route →
01
THE QUESTION

What is the mass gap problem?

Understand the target, the physical picture, and why the word “existence” matters.

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04
THE FIRST CALCULATION

Read a spectrum from a correlator

See how Euclidean time reveals the energies of physical states.

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12 connected lessons · worked examples · exercisesThe first course →

FOLLOW THE IDEAS

At the research frontier

Open the research atlas ↗

What has been established, in which setting, and what comes next. Every result keeps its assumptions in view.

8 curated research dossiersHow we classify evidence →

SEE THE WHOLE PROBLEM

See how each result
connects to the whole.

A result can control short distances, establish an area law on a lattice, or reveal a mechanism in a compactified theory. The proof map shows how each connects to the full question.

Explore the proof map ↗
01Construct the continuum theoryRemove the regulator · retain the physics
02Control infinite volume
03Reconstruct physical states
Δ > 0Establish the physical mass gapIn the same constructed theory

An editorial map of obligations, not a progress score or a prescribed proof.

BUILD YOUR UNDERSTANDING

Explore the subject

LEARN BY CHANGING SOMETHING

Make the ideas tangible.

Explore how a correlator reveals a spectrum, what changes as the lattice spacing shrinks, how positivity reconstructs states, and when the box creates an apparent gap.

Enter the laboratories ↗
A RESOURCE THAT GROWS WITH THE FIELD

From the first question
to the next open problem.

Visit the reading room ↗