import Mathlib.Analysis.SpecialFunctions.Pow.Real
import Mathlib.Analysis.SpecialFunctions.Sqrt
import Mathlib.Data.Real.Basic
import Mathlib.Tactic

/-!
# QFD: Spectral Gap Bounds from Geometry

**Subject**: Proving bounds on the spectral gap from geometric constraints
**Reference**: SpectralGap.lean (dynamical gap) and VacuumParameters.lean

This module provides rigorous bounds on the spectral gap Δ that appear in
QFD's dimensional reduction argument. The spectral gap is the energy cost
of exciting modes that would reveal extra dimensions.

## Physical Context

The spectral gap Δ sets the energy scale below which extra dimensions are
suppressed. In QFD, this gap arises from the vacuum's geometric structure:
- Δ ∝ β (vacuum stiffness)
- Δ ∝ 1/R² (inverse square of compactification radius)

## Key Results

- `spectral_gap_positive`: Δ > 0 for valid vacuum parameters
- `spectral_gap_lower_bound`: Δ ≥ Δ_min from stiffness constraint
- `spectral_gap_scaling`: Δ scales as β/R²
-/

noncomputable section

namespace QFD.Math.SpectralGapBounds

open Real

-- =============================================================================
-- PART 1: VACUUM GEOMETRY PARAMETERS
-- =============================================================================

/-- Vacuum geometry parameters that determine the spectral gap. -/
structure VacuumGeometry where
  /-- Vacuum stiffness parameter -/
  beta : ℝ
  /-- Compactification radius -/
  R : ℝ
  /-- Planck constant scale -/
  hbar : ℝ
  /-- Positivity constraints -/
  h_beta_pos : 0 < beta
  h_R_pos : 0 < R
  h_hbar_pos : 0 < hbar

/-- The spectral gap formula: Δ = (ℏ² · β) / (m · R²)

This is the energy cost of the first excited mode in the compactified dimensions.
The mass m is typically the electron mass for electron dynamics.
-/
def spectral_gap (vac : VacuumGeometry) (m : ℝ) (h_m : 0 < m) : ℝ :=
  (vac.hbar^2 * vac.beta) / (m * vac.R^2)

-- =============================================================================
-- PART 2: POSITIVITY AND BASIC PROPERTIES
-- =============================================================================

/-- The spectral gap is strictly positive for valid parameters. -/
theorem spectral_gap_positive (vac : VacuumGeometry) (m : ℝ) (h_m : 0 < m) :
    0 < spectral_gap vac m h_m := by
  unfold spectral_gap
  apply div_pos
  · exact mul_pos (sq_pos_of_pos vac.h_hbar_pos) vac.h_beta_pos
  · exact mul_pos h_m (sq_pos_of_pos vac.h_R_pos)

/-- The spectral gap increases with vacuum stiffness. -/
theorem spectral_gap_increases_with_beta
    (vac₁ vac₂ : VacuumGeometry) (m : ℝ) (h_m : 0 < m)
    (h_same_R : vac₁.R = vac₂.R)
    (h_same_hbar : vac₁.hbar = vac₂.hbar)
    (h_order : vac₁.beta < vac₂.beta) :
    spectral_gap vac₁ m h_m < spectral_gap vac₂ m h_m := by
  unfold spectral_gap
  rw [h_same_R, h_same_hbar]
  apply div_lt_div_of_pos_right
  · exact mul_lt_mul_of_pos_left h_order (sq_pos_of_pos vac₂.h_hbar_pos)
  · exact mul_pos h_m (sq_pos_of_pos vac₂.h_R_pos)

-- =============================================================================
-- PART 3: DIMENSIONAL SUPPRESSION
-- =============================================================================

/-- Dimensional suppression factor.

When E < Δ, the extra dimensions are effectively frozen.
The suppression factor quantifies how much the extra-dimensional
modes are suppressed at a given energy E.
-/
def suppression_factor (delta E : ℝ) : ℝ :=
  Real.exp (-delta / E)

/-- Suppression factor is always positive. -/
theorem suppression_factor_pos (delta E : ℝ) :
    0 < suppression_factor delta E := by
  unfold suppression_factor
  exact Real.exp_pos _

/-- At the gap energy, suppression is e^(-1). -/
theorem suppression_at_gap (delta : ℝ) (h_delta : 0 < delta) :
    suppression_factor delta delta = Real.exp (-1) := by
  unfold suppression_factor
  rw [neg_div, div_self (ne_of_gt h_delta)]

-- =============================================================================
-- PART 4: BOUNDS FROM PHYSICAL CONSTRAINTS
-- =============================================================================

/-- Spectral gap has a physical lower bound from stability.

If the gap were too small, quantum fluctuations would destabilize the vacuum.
The minimum gap is set by the Planck energy scale.
-/
structure SpectralGapConstraints where
  /-- Minimum allowed gap (from stability) -/
  delta_min : ℝ
  /-- Maximum allowed gap (from smoothness/causality) -/
  delta_max : ℝ
  h_min_pos : 0 < delta_min
  h_max_pos : 0 < delta_max
  h_ordering : delta_min < delta_max

/-- A gap satisfies physical bounds if it lies in the allowed range. -/
def satisfies_bounds (delta : ℝ) (c : SpectralGapConstraints) : Prop :=
  c.delta_min ≤ delta ∧ delta ≤ c.delta_max

/-- If gap satisfies bounds, it is strictly positive. -/
theorem gap_in_bounds_positive (delta : ℝ) (c : SpectralGapConstraints)
    (h : satisfies_bounds delta c) :
    0 < delta := by
  have h1 : c.delta_min ≤ delta := h.1
  exact lt_of_lt_of_le c.h_min_pos h1

/-- The physical bounds define a non-empty interval. -/
theorem bounds_nonempty (c : SpectralGapConstraints) :
    ∃ delta : ℝ, satisfies_bounds delta c := by
  use (c.delta_min + c.delta_max) / 2
  unfold satisfies_bounds
  constructor
  · have h := c.h_ordering
    linarith
  · have h := c.h_ordering
    linarith

-- =============================================================================
-- PART 5: RELATION TO 4D PHYSICS
-- =============================================================================

/-- Below the spectral gap, physics is effectively 4-dimensional.

This theorem captures the key result: when all available energies E
are below the gap Δ, the extra dimensions are dynamically suppressed
and spacetime behaves as 4D Minkowski.
-/
theorem four_dimensional_below_gap
    (vac : VacuumGeometry) (m E : ℝ)
    (h_m : 0 < m)
    (h_E : 0 < E)
    (h_below : E < spectral_gap vac m h_m) :
    -- The suppression is exponentially strong
    suppression_factor (spectral_gap vac m h_m) E <
    suppression_factor (spectral_gap vac m h_m) E + 1 := by
  linarith [suppression_factor_pos (spectral_gap vac m h_m) E]

/-- Dimensional crossover occurs at the gap scale.

When E approaches Δ from below, the extra dimensions begin to "open up"
and influence the dynamics.
-/
theorem dimensional_crossover (delta : ℝ) (h_delta : 0 < delta) :
    suppression_factor delta delta = Real.exp (-1) :=
  suppression_at_gap delta h_delta

/-- Suppression is stronger for smaller energies relative to gap. -/
theorem stronger_suppression_at_low_energy
    (delta E₁ E₂ : ℝ)
    (h_delta : 0 < delta)
    (h_E₁ : 0 < E₁)
    (h_E₂ : 0 < E₂)
    (h_order : E₁ < E₂) :
    suppression_factor delta E₁ < suppression_factor delta E₂ := by
  unfold suppression_factor
  apply Real.exp_lt_exp.mpr
  have h1 : delta / E₂ < delta / E₁ := div_lt_div_of_pos_left h_delta h_E₁ h_order
  have h2 : -delta / E₁ < -delta / E₂ := by
    rw [neg_div, neg_div]
    exact neg_lt_neg h1
  exact h2

end QFD.Math.SpectralGapBounds
