#paperoftheday — Public Fediverse posts

#paperOfTheDay is "Exact evolution equation for the effective potential" from 1993 by Christof Wetterich. This is the paper where the Wetterich equation is first introduced in its modern form.
In #quantumFieldTheory , there are "quantum fluctuations" which lead to the full theory being different from the Lagrangian or action one starts with. A famous example is light by light scattering in QED: The Lagrangian in quantum electrodynamics contains basically one type of allowed interaction, namely matter (such as electrons) emitting or absorbing one photon. This can take many different forms in practice, for example Bremsstrahlung (electron being accelerated and producing photon), or electrostatic repulsion (photons being exchanged between two electrons, accelerating them away from each other). QED does not allow for an elementary interaction between photons. But such interaction does in fact take place due to #quantum fluctuations with "virtual" intermediate electrons. A central task for theoretical #physics is to compute such effects.
The effective action contains all such quantum effects, that is, if one uses the effective action as a "classical" one (without adding further quantum corrections), one obtains the full quantum answer. Many different methods are known to compute the effective action, several of them based on the renormalization group. Compared to the previous work by Wilson, Wegner, and Polchinski, the Wetterich equation is somewhat more explicit in terms of interpretation, and it has the advantage of directly giving the quantum effective action and not some proxy quantity. It has by now become the cornerstone of functional renormalization group ( #FRG ) methods. https://www.sciencedirect.com/science/article/pii/037026939390726X?via%3Dihub

#paperOfTheDay is "Exact evolution equation for the effective potential" from 1993 by Christof Wetterich. This is the paper where the Wetterich equation is first introduced in its modern form.
In #quantumFieldTheory , there are "quantum fluctuations" which lead to the full theory being different from the Lagrangian or action one starts with. A famous example is light by light scattering in QED: The Lagrangian in quantum electrodynamics contains basically one type of allowed interaction, namely matter (such as electrons) emitting or absorbing one photon. This can take many different forms in practice, for example Bremsstrahlung (electron being accelerated and producing photon), or electrostatic repulsion (photons being exchanged between two electrons, accelerating them away from each other). QED does not allow for an elementary interaction between photons. But such interaction does in fact take place due to #quantum fluctuations with "virtual" intermediate electrons. A central task for theoretical #physics is to compute such effects.
The effective action contains all such quantum effects, that is, if one uses the effective action as a "classical" one (without adding further quantum corrections), one obtains the full quantum answer. Many different methods are known to compute the effective action, several of them based on the renormalization group. Compared to the previous work by Wilson, Wegner, and Polchinski, the Wetterich equation is somewhat more explicit in terms of interpretation, and it has the advantage of directly giving the quantum effective action and not some proxy quantity. It has by now become the cornerstone of functional renormalization group ( #FRG ) methods. https://www.sciencedirect.com/science/article/pii/037026939390726X?via%3Dihub

#paperOfTheDay "Integrating out Gluons in Flow equations" from 1996 is another early article about the functional renormalization group #frg , but this time applied to #QCD. The article is relatively long and contains many technicalities, but the main idea is the following: Like every #quantumFieldTheory , QCD contains "quantum fluctuations" on every energy scale, which can be integrated out from high to low energy with the help of a renormalization group flow equation. Unlike the scalar field theories that are often studied as toy models, QCD contains two fundamentally different types of fields: The fermions (quarks), which represent matter, and the bosons (gluons), which are particles of the strong force. Now it turns out that one can arrange the flow equations in such a way that only one type of field is (at first) integrated out, and serves as an "input" for the flow of the other. In principle, this would be exact and yield a full solution of QCD (which still today would be a breakthrough in #physics ), but in practice of course one has to use truncations and approximations. In fact, the computations presented in the paper are rather "coarse" and don't really produce new results; the point is rather to establish the method.
What is interesting is that here, the gluons are integrated out, and one obtains an effective theory for the interaction of matter. This sounds reasonable, but it is the opposite of how lattice simulations (another well-developed approach at non-perturbative QCD) work: There, the gluon field is being simulated, and the fermions are merely a correction term.
https://arxiv.org/abs/hep-ph/9604227

#paperOfTheDay "Integrating out Gluons in Flow equations" from 1996 is another early article about the functional renormalization group #frg , but this time applied to #QCD. The article is relatively long and contains many technicalities, but the main idea is the following: Like every #quantumFieldTheory , QCD contains "quantum fluctuations" on every energy scale, which can be integrated out from high to low energy with the help of a renormalization group flow equation. Unlike the scalar field theories that are often studied as toy models, QCD contains two fundamentally different types of fields: The fermions (quarks), which represent matter, and the bosons (gluons), which are particles of the strong force. Now it turns out that one can arrange the flow equations in such a way that only one type of field is (at first) integrated out, and serves as an "input" for the flow of the other. In principle, this would be exact and yield a full solution of QCD (which still today would be a breakthrough in #physics ), but in practice of course one has to use truncations and approximations. In fact, the computations presented in the paper are rather "coarse" and don't really produce new results; the point is rather to establish the method.
What is interesting is that here, the gluons are integrated out, and one obtains an effective theory for the interaction of matter. This sounds reasonable, but it is the opposite of how lattice simulations (another well-developed approach at non-perturbative QCD) work: There, the gluon field is being simulated, and the fermions are merely a correction term.
https://arxiv.org/abs/hep-ph/9604227

#paperOfTheDay "Critical Exponents from the Effective Average Action" from 1993 is one of the early works of what is now known as the functional renormalization group #frg , called at that time "exact non-perturbative evolution equation". In #quantumFieldTheory and statistical #physics , the behaviour of a system is different for different energy scales. This change is captured by the renormalization group: Changing the energy scale gives back a similar system, but with different numerical values of couplings or masses.
The functional renormalization group equation is a "flow equation" for the quantum effective action. Basically, it expresses the change of all correlation functions under change of energy scale. One can also view it as a successive solution of the path integral, where one starts with the classical (tree-level) action, and successively integrates out high-energy modes, so that , when one reaches zero energy, the full path integral has been performed and one has found the full quantum effective action.
Of course, the functional renormalization group equation can not be solved in closed form for any meaningful theory, so one is forced to introduce approximations. One can recover the usual coupling/loop expansion ( #FeynmanIntegral s), but also other types of approximation schemes are possible, for example including only 1PI correlation functions up to a certain number of legs.
The present paper is concerned with O(N) symmetric scalar fields in D=3 space dimensions. They demonstrate that with a suitable low-order approximation of the flow equations, one can indeed compute the critical exponents of this theory to a few percent accuracy.
https://arxiv.org/abs/hep-ph/9308214

#paperOfTheDay "Critical Exponents from the Effective Average Action" from 1993 is one of the early works of what is now known as the functional renormalization group #frg , called at that time "exact non-perturbative evolution equation". In #quantumFieldTheory and statistical #physics , the behaviour of a system is different for different energy scales. This change is captured by the renormalization group: Changing the energy scale gives back a similar system, but with different numerical values of couplings or masses.
The functional renormalization group equation is a "flow equation" for the quantum effective action. Basically, it expresses the change of all correlation functions under change of energy scale. One can also view it as a successive solution of the path integral, where one starts with the classical (tree-level) action, and successively integrates out high-energy modes, so that , when one reaches zero energy, the full path integral has been performed and one has found the full quantum effective action.
Of course, the functional renormalization group equation can not be solved in closed form for any meaningful theory, so one is forced to introduce approximations. One can recover the usual coupling/loop expansion ( #FeynmanIntegral s), but also other types of approximation schemes are possible, for example including only 1PI correlation functions up to a certain number of legs.
The present paper is concerned with O(N) symmetric scalar fields in D=3 space dimensions. They demonstrate that with a suitable low-order approximation of the flow equations, one can indeed compute the critical exponents of this theory to a few percent accuracy.
https://arxiv.org/abs/hep-ph/9308214

#paperOfTheDay "Derivative expansion of the exact renormalization group" from 1994 is a follow-up on yesterday's paper, by the same author. Here, he uses the (then) new functional #renormalization group #FRG equations and introduces a certain expansion in momenta, which he then studies for the scalar flip-symmetric model (i.e. the avatar of phi^4 theory). This gives rise to two non-linear differential equations, which can be solved numerically, and produce numerical values of the critical exponents rather close to the correct ones.
It seems to me that this method is surprisingly simple -- solving few differential equations instead of coupled integral equations -- and the author claims repeatedly that it can be systematically improved at will. It didn't entirely become clear to me why he does not do that. After all, computing critical exponents for phi^4 theory is one of the globally accepted benchmarks for methods in field theory and statistics, which would give much credibility to this new method. Perhaps the concrete technical challenges were too big, even if a systematic expansion is possible conceptually.
I would be interested to know if now, 30 years later, this systematic momentum expansion has been continued to higher order, or if it has been replaced by another method.
doi.org/10.1016/0370-2693(94)90767-6

#paperOfTheDay "Derivative expansion of the exact renormalization group" from 1994 is a follow-up on yesterday's paper, by the same author. Here, he uses the (then) new functional #renormalization group #FRG equations and introduces a certain expansion in momenta, which he then studies for the scalar flip-symmetric model (i.e. the avatar of phi^4 theory). This gives rise to two non-linear differential equations, which can be solved numerically, and produce numerical values of the critical exponents rather close to the correct ones.
It seems to me that this method is surprisingly simple -- solving few differential equations instead of coupled integral equations -- and the author claims repeatedly that it can be systematically improved at will. It didn't entirely become clear to me why he does not do that. After all, computing critical exponents for phi^4 theory is one of the globally accepted benchmarks for methods in field theory and statistics, which would give much credibility to this new method. Perhaps the concrete technical challenges were too big, even if a systematic expansion is possible conceptually.
I would be interested to know if now, 30 years later, this systematic momentum expansion has been continued to higher order, or if it has been replaced by another method.
doi.org/10.1016/0370-2693(94)90767-6

My #paperOfTheDay was "Galilei invariance, action-reaction principle, and center of mass theorem" from 1983.
This is an article about #generalRelativity , without anything quantum. From daily experience, we know that every object has a mass, but thinking more closely, the parameter we call mass actually appears in different ways in #physics, and it is not a priory clear how they are logically related. Einstein's famous thought experiment was about the "falling elevator", that is, if you are in a box and can't look outside, you can not distinguish whether you fall freely, or you are located far away from a planet where there is no gravitational field. This "weak equivalence principle" asserts the equivalence between "inertial mass", the parameter which determines how hard it is to accelerate something, and "passive gravitational mass", the parameter that determines how strongly a gravitational field acts on an object.
But there is a third type of mass, the "active gravitational mass", which determines how much gravitational field is generated by an object. The "strong equivalence principle" asserts that all three masses are the same.
The present article demonstrates that, as far as classical celestial mechanics is concerned, the strong equivalence principle can not been distinguished from the weak one. That is, the observed motion of celestial bodies can already be explained by the weak equivalence principle, regardless of whether the strong one holds or not.
I don't know what the current state of affairs is in that question, in particular regarding quantum theory.
https://link.springer.com/article/10.1007/BF01889417

My #paperOfTheDay was "Galilei invariance, action-reaction principle, and center of mass theorem" from 1983.
This is an article about #generalRelativity , without anything quantum. From daily experience, we know that every object has a mass, but thinking more closely, the parameter we call mass actually appears in different ways in #physics, and it is not a priory clear how they are logically related. Einstein's famous thought experiment was about the "falling elevator", that is, if you are in a box and can't look outside, you can not distinguish whether you fall freely, or you are located far away from a planet where there is no gravitational field. This "weak equivalence principle" asserts the equivalence between "inertial mass", the parameter which determines how hard it is to accelerate something, and "passive gravitational mass", the parameter that determines how strongly a gravitational field acts on an object.
But there is a third type of mass, the "active gravitational mass", which determines how much gravitational field is generated by an object. The "strong equivalence principle" asserts that all three masses are the same.
The present article demonstrates that, as far as classical celestial mechanics is concerned, the strong equivalence principle can not been distinguished from the weak one. That is, the observed motion of celestial bodies can already be explained by the weak equivalence principle, regardless of whether the strong one holds or not.
I don't know what the current state of affairs is in that question, in particular regarding quantum theory.
https://link.springer.com/article/10.1007/BF01889417

#paperOfTheDay "Theres' plenty of room in the middle: The unsung revolution of the renormalization group" from 2023 is a meta-article to commemorate 50 years of the #renormalization group in memory of one of its pioneers, Michael E. Fisher.
First of all, this article is a masterly and dense historical overview, basically every single reference in it is a breakthrough article worth reading. This being said, the article builds upon "Fisher's least cited article". There, Fisher introduces the thesis, which is elaborated in the present article, that almost all interesting #physics happens in the "middle"; concretely in the realm of collective phenomena and effective #fieldTheory governed by the renormalization group.
The present article discusses various examples, each with a provocative title, for example the BCS theory of superconductivity (famously a pure quantum effect) under the slogan that quantum mechanics is really not needed for condensed matter physics. The reasoning is: Such effects are described by effective field theories and "minimal models" with certain "asymptotic" properties (this is discussed in detail), in concrete cases these models can be derived from quantum mechanics, but this derivation does not really add anything to the practical understanding. At the same time, one can often obtain them from thermodynamic considerations alone, regardless of any more fundamental theory.
The article ends with the standard model of elementary particle physics, with the same thesis: One can view this as "just some EFT", which is valid as a very accurate model to describe observations, regardless of whether it is "fundamental"
. https://arxiv.org/abs/2306.06020v1

#paperOfTheDay "Theres' plenty of room in the middle: The unsung revolution of the renormalization group" from 2023 is a meta-article to commemorate 50 years of the #renormalization group in memory of one of its pioneers, Michael E. Fisher.
First of all, this article is a masterly and dense historical overview, basically every single reference in it is a breakthrough article worth reading. This being said, the article builds upon "Fisher's least cited article". There, Fisher introduces the thesis, which is elaborated in the present article, that almost all interesting #physics happens in the "middle"; concretely in the realm of collective phenomena and effective #fieldTheory governed by the renormalization group.
The present article discusses various examples, each with a provocative title, for example the BCS theory of superconductivity (famously a pure quantum effect) under the slogan that quantum mechanics is really not needed for condensed matter physics. The reasoning is: Such effects are described by effective field theories and "minimal models" with certain "asymptotic" properties (this is discussed in detail), in concrete cases these models can be derived from quantum mechanics, but this derivation does not really add anything to the practical understanding. At the same time, one can often obtain them from thermodynamic considerations alone, regardless of any more fundamental theory.
The article ends with the standard model of elementary particle physics, with the same thesis: One can view this as "just some EFT", which is valid as a very accurate model to describe observations, regardless of whether it is "fundamental"
. https://arxiv.org/abs/2306.06020v1

Today's #paperOfTheDay is "Why there is Nothing rather than something: A theory of the cosmological constant" from 1988. Like yesterday's paper, it deals with the intersection between quantum field theory and #generalRelativity, but the 30 years between them clearly show. Coleman's 1988 paper is an argument in the style of that time (which structurally is quite similar to much of the older #renormalon literature): Heuristic manipulations of formal objects such as the wave function of the universe, or divergent sums over all spacetime geometries. The outcome of this argument is that if #wormholes exist (caused by quantum effects at a scale that is much smaller than observations, but larger than the Planck scale), they can drive the cosmological constant to zero in an Euclidean path integral formulation of general relativity. As always with Coleman, the language is quite funny and frank about the paper's limitations: He writes "Although I find this theory in many ways very attractive, I must honestly stress its speculative character. It rests on wormhole dynamics and the Euclidean formulation of quantum gravity. This is doubly a house built on sand. [...] the Euclideon formulation of gravity is not a subject with firm foundations and clear rules of procedure; indeed, it is more like a trackless swamp". Observations like these have by now, 30 years later, led to a style of theoretical physics that is much more systematic and mathematical than in the 1980s, but also sometimes less intuitive. #dailyPaperChallenge https://doi.org/10.1016%2F0550-3213(88)90097-1

Today's #paperOfTheDay is "Why there is Nothing rather than something: A theory of the cosmological constant" from 1988. Like yesterday's paper, it deals with the intersection between quantum field theory and #generalRelativity, but the 30 years between them clearly show. Coleman's 1988 paper is an argument in the style of that time (which structurally is quite similar to much of the older #renormalon literature): Heuristic manipulations of formal objects such as the wave function of the universe, or divergent sums over all spacetime geometries. The outcome of this argument is that if #wormholes exist (caused by quantum effects at a scale that is much smaller than observations, but larger than the Planck scale), they can drive the cosmological constant to zero in an Euclidean path integral formulation of general relativity. As always with Coleman, the language is quite funny and frank about the paper's limitations: He writes "Although I find this theory in many ways very attractive, I must honestly stress its speculative character. It rests on wormhole dynamics and the Euclidean formulation of quantum gravity. This is doubly a house built on sand. [...] the Euclideon formulation of gravity is not a subject with firm foundations and clear rules of procedure; indeed, it is more like a trackless swamp". Observations like these have by now, 30 years later, led to a style of theoretical physics that is much more systematic and mathematical than in the 1980s, but also sometimes less intuitive. #dailyPaperChallenge https://doi.org/10.1016%2F0550-3213(88)90097-1

The #paperOfTheDay for my #dailyPaperChallenge is "on the theory of superconductivity" from 1950. At that time, it was not understood which physical mechanism causes superconductivity, so Ginzburg and Landau constructed a phenomenological theory: There must be some sort of superconducting charge carriers, therefore, they introduced a "density field" for those. From experimental findings, one has that it vanishes when the material becomes too hot, it is negatively affected by magnetic fields, and it can only support a finite amount of current. Using this as input, one can make an ansatz for the laws governing the charge density field, and this produces meaningful predictions for the behaviour of #superconductors. Nowadays, the BCS theory gives a more systematic description of (certain) superconductors, but Ginzburg-Landau theory has found many applications in other fields apart from superconductivity because the construction works whenever one does not know the precise laws governing microscopic constituents, but one has information about the observed large-scale properties.
https://www.sciencedirect.com/science/chapter/edited-volume/abs/pii/B978008010586450078X?via%3Dihub

The #paperOfTheDay for my #dailyPaperChallenge is "on the theory of superconductivity" from 1950. At that time, it was not understood which physical mechanism causes superconductivity, so Ginzburg and Landau constructed a phenomenological theory: There must be some sort of superconducting charge carriers, therefore, they introduced a "density field" for those. From experimental findings, one has that it vanishes when the material becomes too hot, it is negatively affected by magnetic fields, and it can only support a finite amount of current. Using this as input, one can make an ansatz for the laws governing the charge density field, and this produces meaningful predictions for the behaviour of #superconductors. Nowadays, the BCS theory gives a more systematic description of (certain) superconductors, but Ginzburg-Landau theory has found many applications in other fields apart from superconductivity because the construction works whenever one does not know the precise laws governing microscopic constituents, but one has information about the observed large-scale properties.
https://www.sciencedirect.com/science/chapter/edited-volume/abs/pii/B978008010586450078X?via%3Dihub

#paperoftheday

Amazing work showing that #genetic #disruption of the #kinase module (CDK) of Mediator complex in chimeric #antigen receptor T cell (CAR T) resulted in enhanced effector function and increased #antitumor activity (disruption of CDK module increases core Mediator #chromatin occupancy at #enhancers)

https://www.science.org/doi/10.1126/science.abn5647

#paperoftheday

Amazing work showing that #genetic #disruption of the #kinase module (CDK) of Mediator complex in chimeric #antigen receptor T cell (CAR T) resulted in enhanced effector function and increased #antitumor activity (disruption of CDK module increases core Mediator #chromatin occupancy at #enhancers)

https://www.science.org/doi/10.1126/science.abn5647

#PaperOfTheDay
"The origin of extracellular fields and currents — EEG, ECoG, LFP and spikes" by Buzsáki et al. (2012)

https://www.nature.com/articles/nrn3241

I'd been hearing/reading interesting stuff about ephaptic coupling (distant interactions between #neurons via EM fields) and wanted to know more.

Coupled to this preprint https://www.biorxiv.org/node/1403526.abstract it gave me a nice overview of neurons as EM emitting #dipoles and the main sources of EM-based effects.

#science #neuroscience #eeg #electromagnetism

#PaperOfTheDay
"The origin of extracellular fields and currents — EEG, ECoG, LFP and spikes" by Buzsáki et al. (2012)

https://www.nature.com/articles/nrn3241

I'd been hearing/reading interesting stuff about ephaptic coupling (distant interactions between #neurons via EM fields) and wanted to know more.

Coupled to this preprint https://www.biorxiv.org/node/1403526.abstract it gave me a nice overview of neurons as EM emitting #dipoles and the main sources of EM-based effects.

#science #neuroscience #eeg #electromagnetism