Publications

Written by Kays Haddad.

Based on the article ‘Heavy Black Hole Effective Theory‘, P. H. Damgaard,
K. Haddad, A. Helset, [arXiv.org].

Over a century after its formulation, predictions of Einstein’s theory of General Relativity, describing gravitational interactions, continue to be verified experimentally. One of the predicted phenomena is gravitational waves (GWs), gravitational radiation emitted when gravitationally interacting bodies orbit one another, and whose propagation in space results in the contraction and expansion of space itself. Their observation for the first time in 2015 by the LIGO and VIRGO collaborations, from the collision of two black holes, has attracted much renewed attention from the scattering amplitudes community.

General Relativity is a theory of classical physics; that is to say it describes processes that take place on large distance scales, where quantum mechanical effects are negligible. With this in mind, it is natural to wonder what a classical phenomenon such as the emission of GWs has to do with scattering amplitudes, which describe the interactions of quantum mechanical particles. The conventional wisdom in quantum field theory is that only the terms in a scattering amplitude that contain the fewest powers of a theory’s coupling constant (the so-called tree-level amplitudes) encode classical effects. While it is true that tree-level amplitudes contain classical effects – the classical Newtonian gravitational force and the Coulomb electromagnetic force can both derived from tree-level amplitudes – it was shown in the mid 90s that terms with higher powers of the coupling constant (sub-leading, or loop-level amplitudes) can also contain classical corrections to long-distance physics! In particular, one can use scattering amplitudes to correct the interaction potential between two bodies subject to, say, Newton’s law of gravitation.

This hints at a connection between the amplitudes community and those interested in classical gravitational phenomena such as GWs. By calculating gravitational amplitudes that are more and more sub-leading, one can extract ever higher order corrections to the classical gravitational interaction potential between two bodies. With recent advances in the computational efficiency of scattering amplitudes, it can even be easier to obtain these corrections using quantum field theory than using Einstein’s General Relativity! Furthermore, there is a sense in which it is appropriate to identify quantum mechanical particles with black holes: similarly to these particles, black holes are entirely describable in terms of their mass and how fast they spin around themselves. With the interpretation of scattering amplitudes describing the interactions of black holes, the corrections to the gravitational interaction potential can be processed to improve the theoretical predictions of what signal a gravitational wave coming from colliding black holes should produce in experiments.

While the scattering amplitudes approach appears to have much applicability to classical physics, the computation of classical effects from scattering amplitudes remains an inefficient task. When an amplitude is computed, the vast majority of terms are only relevant to quantum mechanical systems. Someone who is interested in classical physics then needs to identify and isolate which terms in the amplitude represent the sought after classical corrections. There are reliable techniques for doing this, but it is natural to wonder if one can avoid having to compute the quantum portion of the amplitude in the first place. Heavy Black Hole Effective Theory (HBET) was formulated with this question in mind.

HQET schematic. — Figure 1: A bound state of one heavy quark (red) and one light quark (blue), with interactions mediated via the strong force (wavy line). The total momentum p is decomposed into a large part mv, where m is the mass of the heavy quark and v is its velocity, and a small residual momentum |k| << m that parameterises the energy and momentum carried by the light quark and by the particles mediating the strong force.

HBET is inspired by a well-established effective quantum field theory called Heavy Quark Effective Theory (HQET). HQET was formulated to describe systems where a heavy quark is in a bound state with a much lighter quark, which interact via the strong force. The characteristic property of HQET is that the total momentum of the system is decomposed into a large part describing the energy and momentum of the heavy quark, and a small residual momentum which contains the energy and momentum of the light quark and the energy due to the interactions between the light and heavy quark (see Figure 1). The residual momentum is the key to connecting this momentum decomposition with the question at hand: it can be argued that the “classicality” of terms in the amplitudes depends on how many factors of the residual momentum they carry. When deriving the building blocks of a scattering amplitude – the so-called Feynman rules – from HQET, one can then keep only terms that contain the correct number of factors of the residual momentum to contribute to classical terms in the amplitude.

These same properties of HQET can be applied to gravitationally interacting systems, resulting in HBET. In the linked article, the residual momentum was reinterpreted to represent the momentum transfer between two bodies (see Figure 2), and the understanding of the residual momentum as parameterising classicality was applied to the gravitational scattering of two spin-1/2 particles. It was shown there that known classical results (and some new ones) could be obtained by using only the portions of the Feynman rules with a predetermined number of residual momentum factors. Crucially, there was no need to compute entire scattering amplitudes, or to manually separate classical from quantum terms in the amplitude at any point in the calculation.

HBET schematic — Figure 2: A system of two interacting black holes, with interactions mediated via the gravitational force (wavy line). The total momentum of each black hole p₁ and p₂ is decomposed into a large part m|v|, where m_i is the mass of black hole i and v_i is its velocity, and a small residual momentum |k| << m_i that parameterises the energy and momentum transferred in the gravitational interaction. The residual momentum flows from the black hole on the left to the one on the right.

The developments of HBET have answered the question we set out to understand: it is indeed unnecessary to compute an entire quantum scattering amplitude when one is only interested in classical physics. Nevertheless, our work is not done. HBET has so far only been formulated for spin-0 and 1/2 particles. If we are interested in describing classical black holes, it is essential to be able to describe a system with arbitrary values of spin. There are two ways to achieve this: the first is to explicitly formulate models for every spin that allow us to extract the desired Feynman rules for a given spin – an approach which is quite cumbersome. Alternatively, modern approaches to scattering amplitudes allow one to construct amplitudes using minimal building block amplitudes that can be easily generalized to arbitrary spin. Expressing the degrees of freedom of HBET in terms of these minimal amplitudes would provide a much more tractable and elegant extension to any spin. Whichever direction future studies take, one thing is certain: quantum scattering amplitudes have much to say about classical physics.

Cover image courtesy of the LIGO collaboration.

Written by Davide Polvara

Based on the article ‘Quantum anomalies in A^{(1)}_r Toda theories with defects’, S. Penati, D. Polvara, [arXiv.org]

Obtaining exact solutions in quantum ﬁeld theory (the theory studying quantum mechanical properties of subatomic particles) is one of the most important goals of modern physics and it is still far from being achieved. In most of the cases perturbation theory and standard Feynman diagrams techniques are the only tools that we can use to handle real physical processes but unfortunately they become more and more inadequate as the interaction force becomes strong. The impossibility to access to this interaction regime leaves us unable to complete understand many physical processes including for example Quantum Chromodynamics, the force keeping quarks bound together inside protons and neutrons, the same particles that compose the universe in which we live. When we accelerate particles in colliders this force becomes small and the quarks inside protons are free to move as they want.

Indeed at high energies quarks are asymptotically free and they do not behave as a single body, the proton, but rather as many components of it in such a way that we can keep track of them. On the other hand at the energy scale of the life of everyday quarks are completely hidden, ﬁrmly frozen in atomic nuclei, so that it is completely impossible to ﬁnd an isolated quark in nature. They are like connected to each other by little springs of inﬁnite force to form protons and neutrons. A complete mathematical formulation of this phenomenon, as well as other involving non-perturbative physics, is still lacking and it needs a strong intellectual eﬀort to be understood. For this reason in the last years new theoretical tools to unveil non-perturbative properties of physical systems have been studied for some classes of quantum ﬁeld theories. In doing this a fundamental role is played by symmetries. What often mathematicians and theoretical physics do is not to study a certain real phenomenon, that is generally too diﬃcult to face it directly, but rather to study a more symmetrical model and come back to the original one applying a deformation of the latter. Computations that are incredibly diﬃcult become often straightforward problems and acquire a very simple structure when they are seen under the light of the symmetries in play.

We can think for example to the orbits of the planets around the Solar System. It is in general extremely diﬃcult to evaluate these orbits considering only the forces acting on these bodies and ignoring all the rest. If instead we realise that there is a rotational symmetry in the problem (the gravitational force of the Sun on other planets indeed does not have any preferred direction) we will make our life a bit easier. The rotational symmetry manifests itself in the conservation of the angular momentum and constrains the planets to move in ellipses lying on ﬁxed planes. This is incredible! We have converted a three-dimensional problem (planets moving in the space) into a two-dimensional one (planets moving on planes). The rotational symmetry has extremely simpliﬁed our life. This fact is not an accident of the Solar System but happens always, any time there is a symmetry we can ﬁnd a charge (or equivalently a current) associated to that particular symmetry that is conserved in time (in the particular case of Solar System that charge is the angular momentum and the fact that it is conserved constraints the motion of the planets from three to two dimensions). Also in the microscopic world, at subatomic scales, symmetries when are present help us to solve problems apparently impossible. In many cases calculations that could be done only computationally can be avoid and physical observables can be found only with the help of such symmetries.

In this sense Toda theories (a particular class of quantum ﬁeld theories) are really special. They own an inﬁnite number of hidden symmetries producing an inﬁnite number of higher spin conserved charges in involution. While it is possible to study scattering processes between particles using Feynman diagrams techniques and completely ignore the presence of such conservation laws, it is deﬁnitely simpler to use these conserved charges to constrain and solve the equations of motion. In this sense we can start from some axiomatic assumptions on the structure of the S-matrix (the operator containing all the information on particle-collisions), and ﬁnd all the quantities of the theory in a quantum exact way. In this sense we say that these theories are integrable, that is exactly solvable. This fact makes these models accessible to mathematics computations whatever is the regime of the force in play, being able to shed new light on strong interaction.

In this framework the study of defects (i.e. internal boundaries of the theory) is of mandatory importance; they indeed represent a quite natural deformation of such ideal models bringing them closer to the real physics systems. Defects are walls on which particles can collide and the manner in which they collide depends on the particular kind of potential lying on these walls. Some applications of defects in physics can be found in the study of topological phases of condensed matter, entangled systems separated by domain walls, but also in string theory where boundaries and defects arise quite naturally.

An image of incoming and outgoing particles after interacting with a defect. — Figure 1: If the defect preserves its integrable nature at quantum level, particles interact elastically with it. Particles incoming to the left side of the wall will emerge from right side in the same number and with the same velocities.

In our case we start from a particular class of classical models with integrability-preserving defects, i.e. we consider walls preserving the conserved charges of the original theory. These barriers have been studied in the past and it has been found a particular potential-structure so that an inﬁnite sub-set of currents can cross them without being dissipated. We addressed the following question: “Can we promote such defects from classical objects,described in terms of wave functions, to quantum potentials preserving their integrable nature?”. In other words the question that we tried to answer is if we can scatter particles with such impurities preserving the quantum numbers and the momenta of the interacting particles. This is a non-trivial request, indeed the quantum mechanics laws allow in general to modify the nature of the colliding particles. For example if we throw a “red” particle against the left face of the defect a “blue” particle could emerge from the right face. This is an example of defect non-preserving the original symmetries of the problem (contrary an example of charges-preserving defect is shown in ﬁgure 1).

If these barriers can be promoted to be integrable quantum objects they preserve the charges of the original theory at any loop order. This property can then be used to determine an exact quantum description of the scattering between particles and defects using only axiomatic properties (that continue to be valid also in regimes which are inaccessible to standard perturbation theory).

Studying the conservation laws of these models we have discovered that, except for the energy and momentum that can be properly redeﬁned, higher spin conserved charges are spoiled by defect quantum ﬂuctuations. Loop corrections due to small perturbations of the defect potential around its classical value produce anomalies that at quantum level destroy the integrable nature of the theory. In this sense it is not possible to extend the axiomatic principles in a straightforward way, because the symmetries are destroyed by quantum eﬀects. Despite this negative result, it might be possible to restore integrability studying these defects in a more general scenario, for example adding additional degrees of freedom (particles) living on them.

The investigation of such phenomena is interesting since it represents one of the most direct connections between standard perturbation theory and a diﬀerent non-perturbative approach to quantum ﬁeld theories, highlighting possible new techniques to ﬁnd exact results for physical systems.

Category: Publications

Classical Gravitation from Quantum Scattering Amplitudes

Particles-defects interactions: do special walls in scattering theory survive the quantisation ?