# nLab classical double copy

Contents

### Context

#### Gravity

gravity, supergravity

## Quantum theory

#### Duality in string theory

duality in string theory

general mechanisms

string-fivebrane duality

string-string dualities

M-theory

F-theory

string-QFT duality

QFT-QFT duality:

# Contents

## Idea

The classical double copy-method is the counterpart in classical field theory of the double copy-phenomenon for scattering amplitudes in perturbative quantum field theory. It relates classical solutions of the field equations of a Yang-Mills gauge theory with solutions of the Einstein equation in general relativity.

## Double copy and classical field theory

### Double copy and Kerr-Schild metric

#### Definitions

• A Kerr-Schild metric is a perturbation of a flat Minkowski metric $\eta_{\mu\nu}$ of the form

$g_{\mu\nu} = \eta_{\mu\nu} + \kappa \phi k_{\mu}k_{\nu}$

where $\kappa=\sqrt{32\pi G_{\mathrm{N}}}$ is a constant with $G_{\mathrm{N}}$ Newton's constant, $\phi$ is a scalar field and $k_\mu$ is a null covector satisfying the geodesic property, i.e.

$\eta_{\mu\nu}k^\mu k^\nu = g_{\mu\nu}k^\mu k^\nu =0, \quad (k\cdot\partial)k^\mu =0.$
• The single copy gauge field (MOW 15) of this gravitational field is defined for any gauge group $G$ by

$A_{\mu} = (c^a \mathbf{T}_a) \phi k_\mu$

where $c^a\mathbf{T}_a \in \mathfrak{g}$ is an arbitrary constant color charge, specified by a vector $c^a$ in the basis $\{\mathbf{T}_a\}$ of the Lie algebra $\mathfrak{g}$.

• Conversely, if we start from a gauge field of the form $A_{\mu} = (c^a \mathbf{T}_a) \phi k_\mu$ for any constant color charge $c^a\mathbf{T}_a \in \mathfrak{g}$ and null covector $k_\mu$ satisfying the geodesic property, we can define its double copy gravitational field by the Kerr-Schild metric $g_{\mu\nu}=\eta_{\mu\nu} + \kappa \phi k_{\mu}k_{\nu}$.

• Otherwise, if we repeat the procedure of replacing a covector $k_\mu$ with any fixed color charge $(\tilde{c}^b \tilde{\mathbf{T}}_b)\in\tilde{\mathfrak{g}}$ we can get a zeroth copy scalar field, defined by

$\Phi = (c^a \mathbf{T}_a)\otimes(\tilde{c}^b \tilde{\mathbf{T}}_b)\phi$

where the new gauge group $\tilde{G}$ can be chosen different from the previous $G$.

#### Field equations

By following (MOW 15) we have a comparison of the field equations. Assume without loss of generality that $k^0=1$. We get the following:

• The vacuum Einstein equations for the metric $g_{\mu\nu}$ are $R=0$ (where $R$ is the Ricci curvature), which reduce to

\begin{aligned} R^0_{\;0} &= \frac{1}{2}\nabla^2\phi && = 0 \\ R^i_{\;0} &= \frac{1}{2}\partial_j \left(\partial^j(\phi k^i)-\partial^i(\phi k^j)\right) && =0 \\ R^i_{\;j} &= \frac{1}{2}\partial_l \left(\partial^i(\phi k^l k^j)+\partial_j(\phi k^l k^i)-\partial^l(\phi k^i k_j)\right) && =0 \end{aligned}
• The Maxwell equations for the gauge field $A$ are $\mathrm{d} F = 0$, which reduce to

\begin{aligned} (\mathrm{d}F)^0 &= \nabla^2\phi && = 0 \\ (\mathrm{d}F)^i &= \partial_j \left(\partial^j(\phi k^i)-\partial^i(\phi k^j)\right) && = 0 \end{aligned}
• The Klein-Gordon equation for the scalar field $\Phi$ are

$\nabla^2\Phi = 0$

#### Outlook

Summarizing, we have the following table:

zeroth copysingle copydouble copy
$\;\Phi = (c^a \mathbf{T}_a)\otimes(\tilde{c}^b \tilde{\mathbf{T}}_b)\phi\;$$\;A_{\mu} = (c^a \mathbf{T}_a) \phi k_\mu\;$$\;g_{\mu\nu} = \eta_{\mu\nu} + \kappa \phi k_{\mu}k_{\nu}\;$

## Examples

Examples of classical double copy of gauge fields:

gauge theory solutiongravity solutionref.
electric monopoleSchwarzschild spacetime(MOW 15)
magnetic monopolemassless Taub-NUT spacetime(LMOW 15)
planar wavepp-wave(MOW 15)
planar shockwaveAichelburg-Sexl shockwave(BSW 20)

## Double copy and topology

From (LMOW 15) we know that in terms of charges we have the following correspondence:

gauge theory solutiongravity solution
electric chargemass
magnetic chargeNUT charge

The topological consequences were explored by (AWW 20):

• A magnetic monopole is geometrically a principal bundle of the form

$\underset{ \text{worldline} }\underbrace{\mathbb{R}^{1}} \times \underset{ \text{transverse space} }\underbrace{ (\mathbb{R}^3-\{0\}) } \;\,\simeq_{\mathrm{diff}}\;\, \underset{ \text{worldline} }\underbrace{\mathbb{R}^{1}} \times \underset{ \text{radial dir.} }\underbrace{\mathbb{R}^+} \times \underset{ \text{angular dir.} }\underbrace{S^2} \longrightarrow B U(1)$

which is trivial only on the worldline $\mathbb{R}^{1}$ of the monopole. Therefore, since we have the homotopy $\mathbb{R}^3-\{0\} \simeq S^2$, the first Chern class of the bundle will be an element $[F] \in H^2(S^2,\mathbb{Z})\cong \mathbb{Z}$. In other words we have

$[F] = \tilde{g} [\mathrm{Vol}_{S^2}]$

where $\mathrm{Vol}_{S^2}$ is the volume form of $S^2$ and $\tilde{g}\in\mathbb{Z}$ is the quantized magnetic charge.

• The massless Taub-NUT spacetime with NUT charge $N$ is a circle bundle too. In fact it is diffeomorphic to the manifold $\mathbb{R}^+\times L(1,N)$, where $\times L(1,N)$ is the $3$-dimensional Lens space with quantized first Chern class $N\in\mathbb{Z}\cong H^2(S^2,\mathbb{Z})$. In this case the $S^1$ fiber has the interpretation of time direction, which is periodic and non-trivially fibrated on the sphere $S^2$ of the angular directions.

Therefore the double copy procedure exchange the first Chern class of the magnetic monopole with the one of Taub-NUT spacetime, i.e.

$\tilde{g} \mapsto N.$

## Double copy and Wilson lines

The classical double copy of Wilson lines was introduced by (AWW 20). We can use as gravitational Wilson lines on spacetime $M$ the action functional $e^{i S_{\mathrm{kin}}}: [S^1, M]\rightarrow U(1)$ of a test particle. For any loop $\gamma\in[S^1, M]$ we can then write

$W_{\mathrm{grav}}(\gamma) = e^{i S_{\mathrm{kin}}}(\gamma) = \exp \left(i m \int_\gamma \mathrm{d}s \left(g_{\mu\nu}\frac{\mathrm{d}x^\mu}{\mathrm{d}s}\frac{\mathrm{d}x^\nu}{\mathrm{d}s}\right)^{\frac{1}{2}} \right) \;\in\, U(1)$

If we assume that the metric is of the form $g_{\mu\nu} = \eta_{\mu\nu} + \kappa h_{\mu\nu}$, we can expand $W_{\mathrm{grav}}(\gamma)$ at first order in $\kappa$ and obtain

$W_{\mathrm{grav}}(\gamma) = \exp \left(\frac{i \kappa}{2} \int_\gamma \mathrm{d}s\, h_{\mu\nu}\frac{\mathrm{d}x^\mu}{\mathrm{d}s}\frac{\mathrm{d}x^\nu}{\mathrm{d}s} \right) \;\in\, U(1)$

where the mass $m$ is absorbed into the new parameter $s$. If now we write the holonomy of the single copy gauge field along the same path $\gamma$ we get

$W_{\mathrm{gauge}}(\gamma) = \mathcal{P}\exp \left(i g \int_\gamma \mathrm{d}s\, A_{\mu}^a \frac{\mathrm{d}x^\mu}{\mathrm{d}s}\mathbf{T}_a \right) \;\in\, G$

Thus we immediately see that the double copy rules for a Wilson line are the following:

$\mathbf{T}_a \mapsto \frac{\mathrm{d}x^\nu}{\mathrm{d}s}, \quad\; g \mapsto \frac{\kappa}{2}$

Notice that they precisely mirror the BCJ prescription of double copy for scattering amplitudes by exchanging color data with kinematic data and gauge coupling constant with its gravitational analogue.

This suggests that this formulation can be a bridge to formally connect classical double copy with double copy for scattering amplitudes.

## Double copy and S-duality

In (ABSP 19) it was proved that an electric-magnetic duality (i.e. S-duality) transformation on the single copy gauge fields corresponds to an Ehlers transformation on the double copy gravitational field. In other words the following ideal diagram commutes:

$\array{{electric \; monopole} & \overset{{\;\; double \; copy \;\;}}{\to} & {Schwarzschild \; black \; hole}\\ & \\ ^{{S-duality}}\downarrow && \downarrow^{{Ehlers \; transformation}}\\ & \\ {magnetic \; monopole}& \underset{{\;\; double \; copy \;\;}}{\to} & {NUT-charged \; spacetime}}$

Fundamental bibliography:

• Ricardo Monteiro, Donal O’Connell, Chris D. White, Black holes and the double copy (arXiv:1410.0239)

• Andrés Luna, Ricardo Monteiro, Donal O’Connell, Chris D. White, The classical double copy for Taub-NUT spacetime (arXiv:1507.01869)

• Chris D. White, The double copy: gravity from gluons (arXiv:1708.07056)

• David Berman, Erick Chacón, Andrés Luna, Chris D. White, The self-dual classical double copy, and the Eguchi-Hanson instanton (arXiv:1809.04063)

• Kwangeon Kim, Kanghoon Lee, Ricardo Monteiro, Isobel Nicholson, David Peinador Veiga, The Classical Double Copy of a Point Charge (arXiv:1912.02177)

• Nadia Bahjat-Abbas, Ricardo Stark-Muchão, Chris D. White, Monopoles, shockwaves and the classical double copy (arXiv:2001.09918)

Foundational issues:

• Chris D. White, A Twistorial Foundation for the Classical Double Copy (arXiv:2012.02479)

Some global aspects of the classical double copy were explored in the following paper:

In the following paper it is shown that a S-duality on a gauge field corresponds to an Ehlers transformation on its double copy:

The following paper is a proposal of extension of classical double copy to double field theory:

• Kanghoon Lee, Kerr-Schild Double Field Theory and Classical Double Copy (arXiv:1807.08443)