arXiv subjects

L. Gavassino

Publications and source records attributed to L. Gavassino.

4 recordsLinked to original sources

arXiv2024

Universality Classes of Relativistic Fluid Dynamics: Applications

Using a formalism that was recently developed in a companion paper, we rigorously prove the equivalence, in the linear regime, of a number of apparently different relativistic hydrodynamic theories proposed in the literature. In particular, we show that Hydro+ is indistinguishable from the Israel-Stewart theory for bulk viscosity, which in turn is indistinguishable from a reacting mixture. The two-fluid model for superfluidity coincides with the Israel-Stewart theory for heat conduction in the limit of infinite conductivity, and this explains why the latter has a second sound. Also, MIS$^*$ is equivalent to the Burgers model for viscoelasticity, and this implies that the former must exhibit an elastic behavior at high frequencies. Additionally, we show that if the degrees of freedom and the conservation laws of a hydrodynamic theory have the same geometric character as those of the Israel-Stewart theory, then such theory must be indistinguishable from the Israel-Stewart theory in the linear regime. This explains why all second-order theories turn out to be identical near equilibrium. Finally, we construct the first linearized model for a relativistic supersolid that is proven to be causal, stable, and strongly hyperbolic.

nucl-th↗

arXiv2023

Bounds on transport from hydrodynamic stability

It was recently shown that the dispersion relations describing singularities of retarded two-point functions in causal quantum field theories always satisfy the fundamental inequality $\mathfrak{Im} \, ω\leq |\mathfrak{Im} \, k|$, and that several rigorous bounds on transport coefficients follow directly from such inequality. Here, we prove that the same inequality, $\mathfrak{Im} \, ω\leq |\mathfrak{Im} \, k|$, is a necessary condition for a fluid theory to be covariantly stable (i.e. stable in all frames of reference). Hence, the same bounds on transport that follow from causality in quantum field theory also emerge as stability conditions within relativistic hydrodynamics. This intimate connection between bounds from causality and bounds from stability stems from the fact that covariant stability is possible only in the presence of causality. As a quick application, we show that fluids with luminal speed of sound have vanishing viscosities.

hep-th↗

arXiv2022

Should Unstable Quantum Field Theories be Lorentz Invariant?

An unstable field theory is what we obtain when we linearise the equations of an interacting field theory near an unstable state. Theories of this kind are adopted to model the onset of spontaneous symmetry breakings, when the fields are sitting on the top of the Mexican hat, and they start to ''roll down'' to the bottom. At present, there exists no rigorous proof that unstable quantum field theories are Lorentz-invariant (in the sense of Wigner's theorem). Here, we show that they shouldn't be. In fact, unstable theories always have a limited regime of applicability, and they are valid only for a very short time. As consequence, there is a preferred simultaneity hyperplane, along which the unstable theory is everywhere applicable, while a generic observer (whose four-velocity is not orthogonal to such hyperplane) must use the full non-linear theory. In summary: the current quantization schemes are ''ok'', independently from whether they lead to a Lorentz-invariant theory.

hep-th↗

arXiv2022

Boosting unstable particles

In relativity, there is no absolute notion of simultaneity, because two clocks that are in different places can always be desynchronized by a Lorentz boost. Here, we explore the implications of this effect for the quantum theory of unstable particles. We show that, when a wavefunction is boosted, its tails travel one to the past and the other to the future. As a consequence, in the new frame of reference, the particle is in a quantum superposition "decayed + non decayed", where the property "decayed-ness" is entangled with the position. Since a particle cannot be localised in a region smaller than the Compton wavelength, there is a non-zero lower bound on this effect, which is fundamental in nature. The surprising implication is that, in a quantum world, decay probabilities can never be Lorentz-invariant. We show that this insight was the missing ingredient to reconcile the seemingly conflicting views about time dilation in relativistic quantum mechanics and quantum field theory.

physics.gen-ph↗