https://madrid.hostmaster.org/articles/equation_of_everything/en.html
At the highest level of abstraction, our knowledge of the physical universe can be compressed into a single symbolic expression. Written in the language of path integrals, it reads:
$$ W = \int_{k<\Lambda} [Dg][DA][D\psi][D\Phi] \, \exp \left\{ i \int d^4x \, \sqrt{-g} \, \Bigg[ \frac{m_p^2}{2} R - \tfrac{1}{4} F^a_{\mu\nu} F^{a\mu\nu} + i \bar{\psi}^i \gamma^\mu D_\mu \psi^i + \big(\bar{\psi}_L^i V_{ij} \Phi \psi_R^j + h.c.\big) - |D_\mu \Phi|^2 - V(\Phi) \Bigg] \right\}. $$
This expression, dense and compact, is the path-integral form of the Standard Model plus gravity. It unifies quantum mechanics, spacetime, matter, forces, and mass generation into a single framework. Let us dissect it part by part.
The prefactor
W = ∫[Dg][DA][Dψ][DΦ] eiS
is the generating functional of quantum field theory.
It states that to compute any process, one must sum over all possible field configurations: geometries g, gauge fields A, fermion fields ψ, and the Higgs field Φ. Each configuration contributes with a weight eiS, where S is the action.
This is the essence of quantum mechanics extended to fields: reality is the interference pattern of all possible histories.
The term
$$ \frac{m_p^2}{2} R $$
represents the Einstein–Hilbert action, where R is the Ricci scalar curvature and mp is the reduced Planck mass.
It encodes general relativity: spacetime is dynamical, curved by the presence of energy and momentum.
Although the quantum consistency of gravity is still unresolved, the inclusion of this term expresses our best effective theory of spacetime.
$$ -\tfrac{1}{4} F^a_{\mu\nu} F^{a\mu\nu} $$
This compact term encodes the dynamics of the gauge fields: gluons (strong force), W and Z bosons (weak force), and the photon (electromagnetism). The symbol Fμνa generalizes the electromagnetic field tensor to non-abelian Yang–Mills fields.
From this single structure, one can derive Maxwell’s equations in the abelian limit, as well as the full machinery of quantum chromodynamics (QCD) and the electroweak theory.
iψ̄iγμDμψi
This is the Dirac action for fermions: quarks and leptons. The index i runs over three generations.
The covariant derivative Dμ couples matter fields to gauge fields, ensuring consistency with the symmetries of the Standard Model.
This is the mathematical statement of how matter particles propagate and interact with forces.
ψ̄LiVijΦψRj + h.c.
These terms describe the Yukawa interactions: the couplings of fermions to the Higgs field Φ.
Once the Higgs acquires a vacuum expectation value, these interactions translate into fermion masses.
The coefficients Vij encode the structure of flavor mixing (e.g., the CKM matrix for quarks).
− |DμΦ|2 − V(Φ)
Here lies the Higgs field itself.
The kinetic term |DμΦ|2 couples it to gauge bosons, while the potential
V(Φ) = μ2Φ†Φ + λ(Φ†Φ)2
drives spontaneous symmetry breaking.
This breaks SU(2)L × U(1)Y → U(1)em, giving mass to the W and Z bosons while leaving the photon massless.
The discovery of the Higgs boson at CERN in 2012 confirmed this framework.
Taken together, this action expresses:
It is not the ultimate “theory of everything” — it omits dark matter, dark energy, and a full quantum theory of gravity — but it is the most complete description of reality humanity has yet achieved.
If another intelligence were to ask for our account of the laws of nature, we would present this equation.
It is not poetry, yet it carries profound beauty: a single expression encoding the dynamics of space, time, matter, and interaction.
This is our current understanding of the universe, condensed into mathematics.