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. The topic can be viewed as a simplified setup for understanding nonlocal effects in quantum field theory—we expect nature to be nonlocal at the most fundamental scales when quantum gravity effects dominate the physics, and quantum field theory is the most successful language for describing nature at the smallest experimentally probed scales.

I have been looking into the 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. Moreover, the topic also has phenomenological implications—gravitational wave science.

Today we live in a truly remarkable age: we observe the skies with three-quarters of the fundamental interactions ever known to humanity—light, neutrinos, and gravitational waves. Although the youngest of our eyeglasses for observing the skies—the gravitational wave observatories—have not yet reached the resolving power comparable to that of their elder cousins, planned upgrades to the LIGO-Virgo-KAGRA network and the next generation of detectors—LISA, Einstein Telescope, and more to come—will eventually bring us the future where we observe the skies with our high-resolution gravitational eyes.

Our gravitational eyes detect gravitational waves predominantly generated from binary mergers of stellar bodies. Detecting the signals—the waveforms of gravitational waves—requires a bank of templates of expected signals. These templates are mostly generated by waveform models, which are based on our theoretical understanding of stellar binary dynamics. Scattering amplitude techniques provide tools to compute stellar binary dynamics in the inspiral phase, where stellar bodies are distant from each other so that they can be considered as point particles. Including spin effects 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 the dynamics of spinning particles. A note I wrote on massive spinor-helicity variables can be found in the Notes section from this webpage. With my collaborators, an approach I have adopted is to incorporate scattering amplitude tools into effective field theory techniques of the general relativity community. The use of effective action in the general relativity community is rather different from that of the high-energy community: 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 the spins 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 the gauge dependency of classical spin variables and also does not suffer from the 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. The framework 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 the 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 an extended object consisting of a pair of point particles due to the Newman-Janis shift: the original spinning particle is split into two, where each is shifted oppositely in the imaginary spin direction and moves on "complexified" worldlines. The topic 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 that can improve waveform models: current waveform models used in LIGO-Virgo-KAGRA data analysis (such as the SEOBNRv5 family) are known to perform worse for binaries with fast-spinning black holes. The degrading performance of current waveform models in the high-spin regime is anticipated to be the limiting factor for precision measurements in next-generation gravitational wave observatories. In fact, we have already seen the consequences: the source properties related to spin orientation could not be determined for the detection GW231123—the binary black hole merger with the largest observed constituent masses and spins to date—due to uncertainties in our waveform models in the high-spin regime. A better understanding of spin effects in binary dynamics will foster the 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 not pointlike objects since they are attached to nonlocal objects called Dirac strings, but nevertheless the dyons themselves are point particles and Dirac strings are understood as gauge artefacts. 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?'