User:JMvanDijk/Sandbox 16/Box3

Fundamental forces
These equations need to be refined such that the notation is defined as has been done for the previous sets of equations.
 * {| class="wikitable"

! Name ! Equations !Strong force \begin{align} \mathcal{L}_\mathrm{QCD} & = \bar{\psi}_i\left(i \gamma^\mu (D_\mu)_{ij} - m\, \delta_{ij}\right) \psi_j - \frac{1}{4}G^a_{\mu \nu} G^{\mu \nu}_a \\ & = \bar{\psi}_i (i \gamma^\mu \partial_\mu -  m )\psi_i - g G^a_\mu \bar{\psi}_i \gamma^\mu T^a_{ij} \psi_j - \frac{1}{4}G^a_{\mu \nu} G^{\mu \nu}_a \,,\\ \end{align} \,\!$$ !Electroweak interaction
 * $$\mathcal{L}_{EW} = \mathcal{L}_g + \mathcal{L}_f + \mathcal{L}_h + \mathcal{L}_y.\,\!$$
 * $$\mathcal{L}_g = -\frac{1}{4}W_a^{\mu\nu}W_{\mu\nu}^a - \frac{1}{4}B^{\mu\nu}B_{\mu\nu}\,\!$$
 * $$\mathcal{L}_f =  \overline{Q}_i iD\!\!\!\!/\; Q_i+ \overline{u}_i^c iD\!\!\!\!/\; u^c_i+ \overline{d}_i^c iD\!\!\!\!/\; d^c_i+ \overline{L}_i iD\!\!\!\!/\; L_i+ \overline{e}^c_i iD\!\!\!\!/\; e^c_i \,\!$$
 * $$\mathcal{L}_h = |D_\mu h|^2 - \lambda \left(|h|^2 - \frac{v^2}{2}\right)^2\,\!$$
 * $$\mathcal{L}_y = - y_{u\, ij} \epsilon^{ab} \,h_b^\dagger\, \overline{Q}_{ia} u_j^c - y_{d\, ij}\, h\, \overline{Q}_i d^c_j - y_{e\,ij} \,h\, \overline{L}_i e^c_j + h.c.\,\!$$

!Quantum electrodynamics
 * $$\mathcal{L}=\bar\psi(i\gamma^\mu D_\mu-m)\psi -\frac{1}{4}F_{\mu\nu}F^{\mu\nu}\;,\,\!$$
 * }

General Relativity
see: General relativity, Einstein field equations, List of equations in gravitation

where $Rμν$ is the Ricci curvature tensor, $R$ is the scalar curvature, $gμν$ is the metric tensor, $Λ$ is the cosmological constant, $G$ is Newton's gravitational constant, $c$ is the speed of light in vacuum, and $Tμν$ is the stress–energy tensor.

One can write the EFE in a more compact form by defining the Einstein tensor
 * $$G_{\mu \nu} = R_{\mu \nu} - \tfrac{1}{2} R g_{\mu \nu},$$

which is a symmetric second-rank tensor that is a function of the metric. The EFE can then be written as
 * $$G_{\mu \nu} + \Lambda g_{\mu \nu} = \frac{8 \pi G}{c^4} T_{\mu \nu}.$$

In standard units, each term on the left has units of 1/length2. With this choice of Einstein constant as 8πG/c4, then the stress-energy tensor on the right side of the equation must be written with each component in units of energy-density (i.e., energy per volume = pressure).

Using geometrized units where $G = c = 1$, this can be rewritten as
 * $$G_{\mu \nu} + \Lambda g_{\mu \nu} = 8 \pi T_{\mu \nu}\,.$$

The expression on the left represents the curvature of spacetime as determined by the metric; the expression on the right represents the matter/energy content of spacetime. The EFE can then be interpreted as a set of equations dictating how matter/energy determines the curvature of spacetime.

These equations, together with the geodesic equation, which dictates how freely-falling matter moves through space-time, form the core of the mathematical formulation of general relativity.

Sign convention
The above form of the EFE is the standard established by Misner, Thorne, and Wheeler. The authors analyzed all conventions that exist and classified according to the following three signs (S1, S2, S3):



\begin{align} g_{\mu \nu} & = [S1] \times \operatorname{diag}(-1,+1,+1,+1) \\[6pt] {R^\mu}_{\alpha \beta \gamma} & = [S2] \times \left(\Gamma^\mu_{\alpha \gamma,\beta}-\Gamma^\mu_{\alpha \beta,\gamma}+\Gamma^\mu_{\sigma \beta}\Gamma^\sigma_{\gamma \alpha}-\Gamma^\mu_{\sigma \gamma}\Gamma^\sigma_{\beta \alpha}\right) \\[6pt] G_{\mu \nu} & = [S3] \times \frac{8 \pi G}{c^4} T_{\mu \nu} \end{align} $$

The third sign above is related to the choice of convention for the Ricci tensor:
 * $$R_{\mu \nu}=[S2]\times [S3] \times {R^\alpha}_{\mu\alpha\nu} $$

With these definitions Misner, Thorne, and Wheeler classify themselves as $(+ + +)$, whereas Weinberg (1972) and Peacock (1994) are $(+ − −)$, Peebles (1980) and Efstathiou et al. (1990) are $(− + +)$, Rindler (1977), Atwater (1974), Collins Martin & Squires (1989) are $(− + −)$.

Authors including Einstein have used a different sign in their definition for the Ricci tensor which results in the sign of the constant on the right side being negative
 * $$R_{\mu \nu} - \tfrac{1}{2} R g_{\mu \nu} - \Lambda g_{\mu \nu} = -\frac{8 \pi G}{c^4} T_{\mu \nu}.$$

The sign of the (very small) cosmological term would change in both these versions, if the $(+ − − −)$ metric sign convention is used rather than the MTW $(− + + +)$ metric sign convention adopted here.