#quantumfieldtheory — Public Fediverse posts

#paperOfTheDay "Über die Eigenkräfte der Elementarteilchen I" from 1933.
This is another paper from the very early days of #quantumFieldTheory , concerned with the question of the seemingly infinite self-energy of the electron in its own electromagnetic field, namely: If the electron is point-like, then its classical electromagnetic field should be infinite at its location, which is clearly nonsense.
The present paper presents a more refined relativistic analysis, starting from the assumption that the locations where the electron "generates" the field and where it "feels" it are distinct by a small vector r. If r is space like (i.e. the two locations differ by a distance that is farther than the distance that light could travel in the same time interval), one recovers the familiar divergence. On the other hand, if r is inside the light cone (i.e. the electron "feels" its own field in its causal future or past), the divergence is absent even in the limit r->0. However, this computation only works for a classical electron in a classical electromagnetic field. Using the Dirac equation for the electron, new obstacles appear.
The present article is typical for the time when #quantum theory was being developed, but it was not at all clear how to interpret it, or whether it was even correct. Schrödinger coined the term "Zitterbewegung" for the intuition of the electron making infinitely fine random jumps at light speed; the present paper mentions this Zitterbewegung as an obvious reason for difficulties in the self-energy. Today, I would say that Zitterbewegung can be an intuitive picture, but the laws of classical #physics are simply not valid at so small scales.
https://link.springer.com/article/10.1007/BF01341363

Registration is open for the #academicConference "Effective Theories for Nonperturbative #physics " 24 August-4 September in Durham, UK. This conference continues a programme to connect and build a community of researchers in #effectiveFieldTheory and #quantumFieldTheory with emphasis on #nonperturbative aspects, that has been begun with a 2024 conference at the #MITP and a 2025 theory workshop at #CERN
Deadline is 24 June.
https://conference.ippp.dur.ac.uk/event/1546/

In July, there is the YETI 2026 #summerSchool for PhD students from the UK, to foster collaboration between theory and experiments in collider and particle #physics phenomenology and #quantumFieldTheory . It takes place in #Durham UK, meals and accommodation are provided free of charge.
https://conference.ippp.dur.ac.uk/event/1551/

In July, there is the YETI 2026 #summerSchool for PhD students from the UK, to foster collaboration between theory and experiments in collider and particle #physics phenomenology and #quantumFieldTheory . It takes place in #Durham UK, meals and accommodation are provided free of charge.
https://conference.ippp.dur.ac.uk/event/1551/

In July, there is the YETI 2026 #summerSchool for PhD students from the UK, to foster collaboration between theory and experiments in collider and particle #physics phenomenology and #quantumFieldTheory . It takes place in #Durham UK, meals and accommodation are provided free of charge.
https://conference.ippp.dur.ac.uk/event/1551/

In July, there is the YETI 2026 #summerSchool for PhD students from the UK, to foster collaboration between theory and experiments in collider and particle #physics phenomenology and #quantumFieldTheory . It takes place in #Durham UK, meals and accommodation are provided free of charge.
https://conference.ippp.dur.ac.uk/event/1551/

In July, there is the YETI 2026 #summerSchool for PhD students from the UK, to foster collaboration between theory and experiments in collider and particle #physics phenomenology and #quantumFieldTheory . It takes place in #Durham UK, meals and accommodation are provided free of charge.
https://conference.ippp.dur.ac.uk/event/1551/

#PaperOfTheDay is "Full phase diagram of the massive Gross-Neveu model" from 2006.
The Gross-Neveu model is a #quantumFieldTheory of fermions in two spacetime dimensions. In the absence of a bare mass term, it has a discrete chiral symmetry (a few days ago I shared a paper where that symmetry is promoted to a continuous symmetry by adding a second interaction term). The present paper, conversely, is about the case where the mass is non-zero. Then, chiral symmetry is broken.
They consider the theory from a #thermodynamics perspective, where the variables are temperature (i.e. the Planck constant in quantum field language), chemical potential (i.e. a constant offset to energy), and the mass of the particle. Using a clever ansatz, they are able to compute the full phase diagram of the Gross-Neveu field as function of the three variables. As had long been known, if the bare mass vanishes, then the model generates a dynamic mass for the fermion at low enough temperature and chemical potential. It had previously been believed that at non-vanishing bare mass, there would be a discontinuous change in the effective mass from large to small as the temperature is increased. The central result of the present paper is that this is false: Instead, in the boundary region, there is another phase where the field is spacially non-homogeneous, it forms a periodic crystal. Below, one has a heavy fermion, above the fermion is light, and the new phase is something different altogether.
https://www.sciencedirect.com/science/article/abs/pii/S0003491605002915

> One of the key insights of #quantummechanics is that absolute #nothingness, a concept already discussed by Greek philosophers, is nowhere to be found in reality. Quite to the contrary, #QuantumFieldTheory has shown that seemingly empty space is filled by #fluctuations of #light and #matter fields, leading to a continuous #popping into existence and disappearance of #photons as well as massive #particles. In the founding days of quantum mechanics, these consequences of Heisenberg's uncertainty principle were often not taken too seriously. However, modern physics is increasingly discovering how our universe is shaped by fluctuations of physical fields, which not only lead to tiny shifts of spectral lines of atoms, but moreover may cause the evaporation of black holes, and are ultimately responsible for the large-scale #structure of our #universe, formed during the inflationary period following the big bang.

Understanding #vacuum fluctuations in #space
https://phys.org/news/2020-08-vacuum-fluctuations-space.html

#physics #vacuumfluctuations