PHYSICAL REVIEW D 99, 125006 (2019) Editors' Suggestion Asymptotic structure of electromagnetism in higher spacetime dimensions Marc Henneaux1,* and C´edric Troessaert2 1Universit´e Libre de Bruxelles and International Solvay Institutes, ULB-Campus Plaine CP231, B-1050 Brussels, Belgium 2Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Am Mühlenberg 1, DE-14476 Potsdam, Germany (Received 2 April 2019; published 10 June 2019) We investigate the asymptotic structure of electromagnetism in Minkowski space in even and odd spacetime dimensions ≥ 4. We focus on d>4 since the case d ¼ 4 has been studied previously at length. We first consider spatial infinity where we provide explicit boundary conditions that admit the known physical solutions and make the formalism well defined (finite symplectic structure and charges). Contrary to the situation found in d ¼ 4 dimensions, there is no need to impose parity conditions under the antipodal map on the leading order of the fields when d>4. There is, however, the same need to modify the standard bulk symplectic form by a boundary term at infinity involving a surface degree of freedom. This step makes the Lorentz boosts act canonically. Because of the absence of parity conditions, the theory is found to be invariant under two independent algebras of angle-dependent uð1Þ transformations (d>4). We then integrate the equations of motion in order to find the behavior of the fields near null infinity. We exhibit the radiative and Coulomb branches, characterized by different decays and parities. The analysis yields generalized matching conditions between the past of I þ and the future of I −. DOI: 10.1103/PhysRevD.99.125006 I. INTRODUCTION of the curvature that may be too stringent and thus may fail to be satisfied by any solution that would allow gravita- Most of the studies of the asymptotic properties of tional waves.” This point has also been forcefully stressed gravity in the asymptotically flat context have been in the recent work [9]. performed at null infinity [1–4] (for recent useful reviews, One remarkable by-product of the studies at null infinity see [5–7]). This is quite natural, and would seem to be even was the discovery that the asymptotic symmetry group of mandatory, in order to decipher the intricate properties of gravity in the asymptotically flat context was the infinite- gravitational radiation. One conceptual difficulty with analyses at null infinity, dimensional Bondi-Metzner-Sachs (BMS) group. Initially however, is that the existence of a null infinity with the received with some skepticism because the physical sig- smoothness properties usually assumed in the asymptotic nificance of this infinite-dimensional enlargement of the treatments is a difficult dynamical question: given reason- Poincar´e group was not clear, the emergence of the BMS able initial data on a Cauchy hypersurface, will their group was understood recently to be related to profound Cauchy development give rise to a null infinity with the infrared properties of gravity having to do with soft – requested properties? Strong doubts that this would be the graviton theorems and memory effects [10 16] (see [17] – case have been expressed in [8], which we quote verbatim: for an exposition of this recent work and [18 20] for earlier “...it remains questionable whether there exists any non- investigations). The conclusion of the huge amount of trivial solution of the field equations that satisfies the activity that flourished since then is that the BMS group is a Penrose requirements [of asymptotic simplicity]. Indeed, gift rather than an embarrassment. An even further enlarge- “ ” his regularity assumptions translate into fall-off conditions ment of the Poincar´e group including super-rotations have even been argued to be useful [21–23]. The BMS transformations are diffeomorphisms leaving *On leave from Coll`ege de France, 11 place Marcelin the boundary conditions at null infinity invariant. They Berthelot, 75005 Paris, France. are exact symmetries of the theory. That is, they leave the action exactly invariant up to a surface term, without Published by the American Physical Society under the terms of having to make approximations. As exact symmetries the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to of the theory, they should be visible in any description, the author(s) and the published article’s title, journal citation, and, in particular, in slicings of spacetime adapted to and DOI. Funded by SCOAP3. spatial infinity. In such slicings, they would appear as 2470-0010=2019=99(12)=125006(21) 125006-1 Published by the American Physical Society MARC HENNEAUX and CEDRIC´ TROESSAERT PHYS. REV. D 99, 125006 (2019) diffeomorphisms leaving the boundary conditions at this problem [30], we put forward alternative boundary spatial infinity invariant. For this to be the case, however, conditions that were also BMS invariant, but which yielded the boundary conditions at spatial infinity should be a singular behavior (∼ ln r) for some components of the equivalent, or at least compatible in a sense that we shall Weyl tensor as one went to null infinity. In spite of this make more precise below, with the boundary conditions singular behavior at null infinity, nothing spectacular at null infinity. This brings us back to the dynamical occurred at spatial infinity and the BMS symmetry was question on null infinity mentioned above. untouched.] Earlier investigations of the asymptotic symmetries at Similar features arise in the discussion of the asymptotic spatial infinity showed no sign of the BMS group. One behavior of the electromagnetic field, where the null either found the Poincar´e group with no enlargement infinity analysis [32–34] seemed to be at variance with [24,25]; the smaller homogeneous Lorentz group [26]; the spatial infinity analysis [35]. The tension was solved in or an even larger extension, the Spi group [27,28], but in no [36], again by providing appropriate boundary conditions case the BMS group uncovered at null infinity. One logical at spatial infinity. The null infinity matching conditions of possibility for this discrepancy would be that the set of electromagnetism were also shown there to be a conse- boundary conditions at spatial infinity is incompatible with quence of the boundary conditions at spatial infinity. the set of boundary conditions at null infinity, so that there Extension of the asymptotic analysis to higher dimen- exists transformations preserving one set but not the other. sions raises interesting issues, which have led to a some- (Invariance of the action cannot be the issue—provided what unclear situation at null infinity where some studies the action is well defined—since we are dealing with yield infinite-dimensional asymptotic symmetries as in four diffeomorphisms.) spacetime dimensions, while some others do not [37–46]. If true, this situation would be very disappointing and The question is further complicated in odd spacetime physically unsatisfactory. Motivated by the desire to under- dimensions because half-integer fractional powers of r−1 stand better these earlier puzzling results, we have mix with integer powers, leading to problems with the reexamined the asymptotic structure of gravity at spatial conformal definition of null infinity [47–49], and the infinity [29,30]. We have provided in [29] boundary frequent necessity to split the analysis according to whether conditions at spatial infinity that eliminate the previous the spacetime dimension is odd or even since only in the tensions between spatial infinity and null infinity analyses, latter case does one avoid nonanalytic functions at null in the sense that (i) these boundary conditions are invariant infinity. under the BMS group, which acts nontrivially on the fields This provides strong motivations for investigating the and has generically nonvanishing conserved charges, and asymptotic structure of the electromagnetic and gravita- (ii) integration of the symmetry generators from spatial to tional fields at spatial infinity in higher dimensions, where null infinity enables one to show that it is the same BMS the falloff of the fields is more uniform (no fractional group that acts both at spatial infinity and at null infinity, powers of r). This is done here for electromagnetism. We expressed in different parametrizations that can be explic- show that the methods developed in our previous work [36] itly related [31]. generalize straightforwardly to higher dimensions, with no Furthermore, the matching conditions imposed at null new conceptual difficulty. The discussion proceeds along infinity on the leading order of the gravitational field [17] similar lines independently of the spacetime dimension. are automatic consequences of the asymptotic behavior of One finds in particular the same need to modify the the Cauchy data at spatial infinity. Although not equivalent standard bulk symplectic structure by a surface term, as (they are stronger), the boundary conditions at null infinity shown necessary also by different methods in d ¼ 4 are compatible with those at spatial infinity, in the sense spacetime dimensions [50,51]. that they can be shown to obey the conditions that are One remarkable feature, however, is that a second angle- implied at null infinity by the behavior at spatial infinity. It dependent uð1Þ asymptotic symmetry emerges. This sec- is of interest to point out in this respect that while the ond uð1Þ is eliminated in 4 spacetime dimensions because leading order of the Cauchy development of the gravita- of parity conditions that must be imposed to get rid of tional field coincides with the generally assumed leading divergences in the symplectic structure and of divergences order at null infinity, the subsequent terms in the expansion in some components of the fields as one goes to null infinity ln r differ in general, since subleading terms of the type rk [36], but these parity conditions turn out to be unnecessary (k ≥ 1) will develop from generic initial data.