My primary interests are quantum field theory (and string theory), mostly focused on properties and applications of scattering amplitudes involving point-particle approximation of non-pointlike objects. I have been looking into classical physics of spin hiding in scattering amplitudes since my PhD. A particle with spin greater than a threshold value (spin-2 for gravitational interactions) is a good example of a non-pointlike object approximated by a point-particle, as its description as an elementary particle cannot be valid to arbitrarily high energies.

The era of "gravitational eyes" has unfolded before us as gravitational wave observatories have started to detect gravitational waves coming from binary mergers of stellar bodies. One of the preparations needed for detecting the signals is to have expectations of signals to be detected, and such educated guesses are based on theoretical computations of stellar binary dynamics. Techniques for computing scattering amplitudes provide a tool to compute such dynamics of stellar binaries in the inspiral phase, where stellar bodies are distant from each other so that they can be considered as point particles. Including spins into the discussion is rather subtle, and this was the problem that I have explored during my PhD.

During my PhD I have used massive spinor-helicity variables as the main tool for describing dynamics of spinning particles. A note I have written on massive spinor-helicity variables can be found on the Notes section from this webpage. An approach I have adopted together with my collaborators is to incorporate scattering amplitude tools into effective field theory techniques of general relativity community. The use of effective action in general relativity community is rather different from that of high energy community in that sources of gravitation are considered as classical objects in the former, while all objects are intrinsically quantum in the latter. The final framework we have developed lies somewhere in between and could be called "semi-classical", in the sense that spin of the gravitating sources are quantised but allowed to have arbitrarily large values in units of Planck's constant. This framework has the advantage that it does not suffer from gauge-dependency of classical spin variables and also does not suffer from quantum fluctuation ambiguities of quantum spin variables. An interesting observation we have made is that minimal coupling in spinor-helicity language describes how Kerr-Newman black holes couple to gravitons and photons. This provides a justification for the validity of using tools of scattering amplitudes to compute binary dynamics of black holes.

My postdoctoral research has been centred on deepening my understanding of quantum field theory methods and perturbative two-body dynamics in gravity. The directions include; efficient evaluation of Feynman integrals; resolving conceptual problems for relativistic spin degrees of freedom; efficient organisation of spin for classical limit of scattering amplitudes; and perturbative description of "magnetic gravitational charge".

My current major interest is resummation of spin effects in binary dynamics, where the spinning particles are given spin-induced multipole moments generated by (a dynamical version of) the Newman-Janis shift. This topic has both theoretical and phenomenological motivations. On the theoretical side, we are studying the dynamics of "complexified" worldlines obtained from the Newman-Janis shift, which shifts the original worldline of the spinning particle by the imaginary spin direction. This will lead us to a deeper understanding of the Newman-Janis shift, which was originally introduced as a solution generating technique for stationary solutions to the Einstein-Maxwell equations. On the phenomenological side, we are studying strong spin effects in binary dynamics. Current waveform models used in LIGO-Virgo-KAGRA data analysis (SEOBNRv5 family) are known to perform worse for binaries with fast-spinning black holes, and inaccurate modelling of spin effects in waveform models is expected to limit the performance of next-generation gravitational wave observatories. A better understanding of spin effects in binary dynamics will foster development of better waveform models.

One of my minor interests is quantum effects of gravity, especially at low energy scales. Contrary to the folklore, general relativity can be quantised when treated as an effective field theory and provides unambiguous physical predictions. I have some works on quantum gravity effects in this context, which include (apparent) violations of the equivalence principle due to quantum corrections. I am also interested in holography, although I was not given the opportunity of finding good problems to work on. My master's thesis is on reconstructing the entanglement wedge dual to the given boundary subregions.

My long-term interest is understanding the point-particle approximation; when and why the approximation breaks down, and whether the breakdown can be reorganised to reconstruct the non-pointlike properties. This is why I am also interested in alternative descriptions of scattering amplitudes; some mathematical structures obscured in one description may become obvious in another. One of my interests in this direction is the problem of fully relativistic dyon-dyon quantum scattering amplitudes, where dyons are considered as (structureless) point particles; dyons are attached to nonlocal objects called Dirac strings and thus are not pointlike objects, but nevertheless the dyons themselves are point particles and Dirac strings are understood as gauge artifacts. Ultimately, I would like to understand if we are required to think beyond quantum field theory since quantum field theory (mainly) describes point particles. Some concrete questions that can branch out from this topic could be; 'is string theory inevitable?' and 'do known string theories exhaust all possible "completions" of quantum field theories?'