Pourquoi certains trucs en physique on peut pas les réparer mais d'autres oui ?
Non-renormalizability

Given the tremendous success of QED, many theorists believed, in the few years after 1949, that QFT could soon provide an understanding of all microscopic phenomena, not only the interactions between photons, electrons, and positrons. Contrary to this optimism, QFT entered yet another period of depression that lasted for almost two decades.

The first obstacle was the limited applicability of the renormalization procedure. In perturbative calculations in QED, all infinite quantities could be eliminated by redefining a small (finite) number of physical quantities (namely the mass and charge of the electron). Dyson proved in 1949 that this is only possible for a small class of theories called "renormalizable theories", of which QED is an example. However, most theories, including the Fermi theory of the weak interaction, are "non-renormalizable". Any perturbative calculation in these theories beyond the first order would result in infinities that could not be removed by redefining a finite number of physical quantities.

The second major problem stemmed from the limited validity of the Feynman diagram method, which is based on a series expansion in perturbation theory. In order for the series to converge and low-order calculations to be a good approximation, the coupling constant, in which the series is expanded, must be a sufficiently small number. The coupling constant in QED is the fine-structure constant α ≈ 1/137, which is small enough that only the simplest, lowest order, Feynman diagrams need to be considered in realistic calculations. In contrast, the coupling constant in the strong interaction is roughly of the order of one, making complicated, higher order, Feynman diagrams just as important as simple ones. There was thus no way of deriving reliable quantitative predictions for the strong interaction using perturbative QFT methods.

With these difficulties looming, many theorists began to turn away from QFT. Some focused on symmetry principles and conservation laws, while others picked up the old S-matrix theory of Wheeler and Heisenberg. QFT was used heuristically as guiding principles, but not as a basis for quantitative calculations.
Voici une liste de problèmes non résolus notables regroupés par grands domaines de la physique. Certains des problèmes majeurs en physique sont théoriques, ce qui signifie que les théories existantes semblent incapables d'expliquer un certain phénomène observé ou un résultat expérimental. Les autres sont expérimentaux, ce qui signifie qu'il y a une difficulté à concevoir une expérience pour tester une théorie proposée ou étudier un phénomène plus en détail. Il reste des questions en dehors du Modèle Standard de la physique, telles que le problème CP fort, la masse des neutrinos, l'asymétrie matière-antimatière et la nature de la matière noire et de l'énergie noire. Un autre problème se situe dans le cadre mathématique du Modèle Standard lui-même : le Modèle Standard est incompatible avec celui de la relativité générale, au point que l'une ou l'autre théorie s'effondre dans certaines conditions.
### Query ###
Pourquoi certains trucs en physique on peut pas les réparer mais d'autres oui ?

### Main Text ###
Non-renormalizability

Given the tremendous success of QED, many theorists believed, in the few years after 1949, that QFT could soon provide an understanding of all microscopic phenomena, not only the interactions between photons, electrons, and positrons. Contrary to this optimism, QFT entered yet another period of depression that lasted for almost two decades.

The first obstacle was the limited applicability of the renormalization procedure. In perturbative calculations in QED, all infinite quantities could be eliminated by redefining a small (finite) number of physical quantities (namely the mass and charge of the electron). Dyson proved in 1949 that this is only possible for a small class of theories called "renormalizable theories", of which QED is an example. However, most theories, including the Fermi theory of the weak interaction, are "non-renormalizable". Any perturbative calculation in these theories beyond the first order would result in infinities that could not be removed by redefining a finite number of physical quantities.

The second major problem stemmed from the limited validity of the Feynman diagram method, which is based on a series expansion in perturbation theory. In order for the series to converge and low-order calculations to be a good approximation, the coupling constant, in which the series is expanded, must be a sufficiently small number. The coupling constant in QED is the fine-structure constant α ≈ 1/137, which is small enough that only the simplest, lowest order, Feynman diagrams need to be considered in realistic calculations. In contrast, the coupling constant in the strong interaction is roughly of the order of one, making complicated, higher order, Feynman diagrams just as important as simple ones. There was thus no way of deriving reliable quantitative predictions for the strong interaction using perturbative QFT methods.

With these difficulties looming, many theorists began to turn away from QFT. Some focused on symmetry principles and conservation laws, while others picked up the old S-matrix theory of Wheeler and Heisenberg. QFT was used heuristically as guiding principles, but not as a basis for quantitative calculations.

### Supplementary Text ###
Voici une liste de problèmes non résolus notables regroupés par grands domaines de la physique. Certains des problèmes majeurs en physique sont théoriques, ce qui signifie que les théories existantes semblent incapables d'expliquer un certain phénomène observé ou un résultat expérimental. Les autres sont expérimentaux, ce qui signifie qu'il y a une difficulté à concevoir une expérience pour tester une théorie proposée ou étudier un phénomène plus en détail. Il reste des questions en dehors du Modèle Standard de la physique, telles que le problème CP fort, la masse des neutrinos, l'asymétrie matière-antimatière et la nature de la matière noire et de l'énergie noire. Un autre problème se situe dans le cadre mathématique du Modèle Standard lui-même : le Modèle Standard est incompatible avec celui de la relativité générale, au point que l'une ou l'autre théorie s'effondre dans certaines conditions.

### Draft ###

### 1. Query Parsing

"Pourquoi certains trucs en physique on peut pas les réparer mais d'autres oui ?"

Translation: "Why can't we fix some physics stuff but others we can?"

User assumes repairability varies across physics phenomena. ※ Potential misconception needs addressing.

### 2. Domain Identification

Physics subdomains involved:
- Quantum electrodynamics (QED) ●
- Quantum field theory (QFT) ●  
- Perturbative calculations ◐
- Renormalization theory ●

Geographic/temporal: Post-1940s theoretical physics, international (Feynman, Dyson, Schwinger era).

### 3. Core Physics Concepts

**Repairability** → mathematical/theoretical concept, not mechanical.

QED: "repairable" via renormalization. Finite redefinition of physical constants → infinite calculations manageable.

Non-QED theories: "unrepairable" mathematically. Beyond first-order perturbation → infinite terms, no finite redefinition can handle.

### 4. Technical Breakdown

**Renormalizable theories** (QED-like):
- Small coupling constants
- Finite redefinition possible
- Infinite terms manageable

**Non-renormalizable theories** (most others):
- Large coupling constants  
- Infinite terms cannot be renormalized
- Beyond first-order = meaningless

### 5. Physical Examples

**Fixable**: 
- Electron-photon interactions ●
- Weak interactions (Fermi theory) ◐
- Gravity in weak field ●

**Unfixable**:
- Strong interactions ●
- Many-body quantum systems ●
- Critical phenomena ◐

### 6. Mathematical Reality

User's "repair" metaphor → mathematical convergence/divergence issue.

Some theories: perturbation series converges → finite results extractable.

Most theories: perturbation diverges → no finite answer extractable.

∴ Not "repairable" but fundamentally unsolvable at desired precision.

### 7. Why the Difference?

**Coupling strength determines fixability**:
- QED: α ≈ 1/137 → series converges
- Strong force: ~1 → series diverges
- Fermi: intermediate → partial fixability

**Physical constraint**: Infinite terms in divergent series → no finite redefinition can rescue.

### 8. User Misconception

"Réparer" suggests mechanical fix. Physics here = mathematical theorems.

Some theories mathematically "convergent" → calculable.
Most theories mathematically "divergent" → uncalculable beyond limits.

Not "broken" → fundamentally unsolvable in desired precision.

**Key insight**: Difference is mathematical structure, not physical "breakage."

### 9. Synthesis

Physics phenomena aren't "repairable" or not - they're either:
1. Mathematically tractable (few theories)
2. Mathematically intractable (most theories)

User's repair metaphor → wrong framework for understanding theoretical physics limitations.

**Answer structure**: Explain renormalizable vs non-renormalizable, coupling constants, mathematical convergence, why QED special case, most physics fundamentally beyond calculation precision.

### Answer###
La question que vous posez touche à un aspect fondamental de la physique théorique moderne. Certains phénomènes physiques peuvent effectivement être "résolus" ou "calculés" avec une grande précision, tandis que d'autres le sont beaucoup moins bien.

Cette différence s'explique par la distinction entre théories "renormalisables" et "non-renormalisables". Dans les théories renormalisables comme l'électrodynamique quantique (QED), qui décrit l'interaction entre photons, électrons et positons, les calculs peuvent être rendus finis en redéfinissant un petit nombre de quantités physiques fondamentales. Ces théories ont un couplage faible - représenté par la constante fine structure α ≈ 1/137 dans QED - ce qui permet aux séries de calculs de converger et de donner des résultats précis.

En revanche, la plupart des autres théories de la physique - y compris la théorie de Fermi des interactions faibles et les théories de l'interaction forte - sont non-renormalisables. Dans ces théories, les calculs perturbatifs au-delà du premier ordre produisent des infinies qui ne peuvent pas être éliminées par la redéfinition d'un nombre fini de constantes physiques. De plus, pour certaines d'entre elles, le couplage est suffisamment fort pour que les calculs de plus haut ordre deviennent aussi importants que les calculs simples, rendant les approximations basiques inefficaces.

Cette limitation explique pourquoi certains phénomènes physiques peuvent être prédits avec une grande précision tandis que d'autres restent fondamentalement imprévisibles au-delà de certains ordres de grandeur.