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University Quantum Field Theory I · unit UQFTI-15

The Lamb Shift Completed at Order alpha (Z alpha)^4

Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.

Extra challenging questions

A selection from this pathway. Open a topic below for its complete practice and challenge set.

Extra challenging · 01

What order α(Zα)⁴ contains, and the unit 135.6 MHz: infer an input with uncertainty

The order-α(Zα)⁴m Lamb shift is the part of a hydrogen-like level's energy carrying one power of α from a single loop of the quantised fields and four powers of Zα from the binding; it is the leading term of the 2S₁/₂ − 2P₁/₂ splitting. Use the supplied topic model Y_out = 1085.1503*z^4/n^3, where Y_out is UNIT Cₙ (MHz). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. z is Nuclear charge Z, dimensionless. Keep n (Principal quantum number n) = 2 fixed. For this exercise use only 1 ≤ z ≤ 81, at the specified test settings separated by 1. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 847773670 MHz, with a bounded reading uncertainty ±467120000 MHz. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. Determine every allowed value of z consistent with that bound. Compare with a straight-line interpolation between the two endpoint predictions, and explain why a fractional setting may be inadmissible. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

UNIT Cₙ against z

Predictions from the supplied What order α(Zα)⁴ contains, and the unit 135.6 MHz model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): UNIT Cₙ / MHz

UNIT Cₙ against zPredictions from the supplied What order α(Zα)⁴ contains, and the unit 135.6 MHz model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: z: Nuclear charge Z. Vertical axis: UNIT Cₙ / MHz. Exact values are available in the data table below.-2.34e81.34e92.92e94.50e96.07e9-2.219.44162.684.2xy

Horizontal axis (x): z: Nuclear charge Z

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1z: Nuclear charge Z: 1UNIT Cₙ / MHz: 135.6
Read the exact data as a table
UNIT Cₙ against z — supplied values
z: Nuclear charge ZUNIT Cₙ / MHz
1135.6437875
2126380139.4367875
41383296924.6097875
611878102314.3627875
815839020275.895787

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

Enumerate the permitted settings in the stated range, evaluate Y_out at each, and retain every setting satisfying |Y_out − Y_measured| ≤ u. The plotted predictions are supplied calibration evidence.

Reveal the worked solution
  1. Endpoint predictions are Y_out(1) = 135.644 MHz and Y_out(81) = 5839020000 MHz. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives z_line = 1 + (847774000 − 135.644)(81 − 1)/(5839020000 − 135.644) = 12.6153. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Evaluate the model at each permitted setting. The central reading corresponds to z = 50; only the allowed settings 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55 lie inside the stated reading bound.
  4. The interpolation error in the input is |z_line − z| = 37.3847.
  5. The compatible allowed values span 41 to 55. Values between specified test settings are outside this exercise's input domain even if an interpolation passes through them.
  6. The quoted output bound restricts which specified settings remain compatible. It does not change physical restrictions on quantum numbers or counts, and the chosen grid alone does not establish physical quantisation.

Answer Compatible z: 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55. Endpoint interpolation predicts 12.6153; check admissibility against the discrete settings.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 02

The slope of F₁ and its infrared logarithm: infer an input with uncertainty

The slope of F₁ is the coefficient of q² in the electron's renormalised charge form factor near q² = 0; at one loop with a photon mass μ_γ it is (α/(3πm²))[ln(m/μ_γ) − 3/8], which depends logarithmically on that infrared regulator. Use the supplied topic model Y_out = sqrt(692.75*(-p*ln(10)-0.375)), where Y_out is RMS RADIUS √⟨r²⟩ (fm). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sqrt(x) means √x; ln(x) means the natural logarithm. p is log₁₀ of photon mass μ_γ/m, dimensionless. Keep znuc (Nuclear charge Z) = 1 fixed. For this exercise use only -7.3625 ≤ p ≤ -2.6375 on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 82.986363 fm, with a bounded reading uncertainty ±3.0238 fm. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown p by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0004725, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

RMS RADIUS √⟨r²⟩ against p

Predictions from the supplied The slope of F₁ and its infrared logarithm model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): RMS RADIUS √⟨r²⟩ / fm

RMS RADIUS √⟨r²⟩ against pPredictions from the supplied The slope of F₁ and its infrared logarithm model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: p: log₁₀ of photon mass μ_γ/m. Vertical axis: RMS RADIUS √⟨r²⟩ / fm. Exact values are available in the data table below.61.0573.038596.97108.9-7.551-6.276-5-3.724-2.449xy

Horizontal axis (x): p: log₁₀ of photon mass μ_γ/m

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1p: log₁₀ of photon mass μ_γ/m: -7.362RMS RADIUS √⟨r²⟩ / fm: 107.2
Read the exact data as a table
RMS RADIUS √⟨r²⟩ against p — supplied values
p: log₁₀ of photon mass μ_γ/mRMS RADIUS √⟨r²⟩ / fm
-7.3625107.1646350159468
-6.1812597.97973480255807
-587.8396144450676
-3.8187576.36469930364844
-2.637562.82783406751471

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(p) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(-7.3625) = 107.165 fm and Y_out(-2.6375) = 62.8278 fm. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives p_line = -7.3625 + (82.9864 − 107.165)(-2.6375 − -7.3625)/(62.8278 − 107.165) = -4.78581. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives p = -4.48025; substituting back gives Y_out = 82.986363 fm.
  4. The interpolation error in the input is |p_line − p| = 0.305557. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 79.962563 fm and 86.010163 fm. Sort the roots even if the response decreases: -4.80061 ≤ p ≤ -4.17135.
  6. The lower and upper input deviations are 0.32036 and 0.308896. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer p ≈ -4.48025; compatible interval [-4.80061, -4.17135]. Endpoint interpolation gives -4.78581.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 03

Form factors in a Coulomb field: the s-state contact term: infer an input with uncertainty

The high-energy contact potential is the δ³(r) term that the slope of F₁ adds to the Coulomb potential of a static nucleus; it shifts only states whose wavefunction is non-zero at the nucleus, which are the s states. Use the supplied topic model Y_out = 10^lmu, where Y_out is PHOTON MASS μ_γ (eV). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. lmu is Photon mass, log₁₀(μ_γ / 1 eV), dimensionless. Keep z (Nuclear charge Z) = 1 fixed. For this exercise use only -2.25625 ≤ lmu ≤ 3.25625 on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 12.775415 eV, with a bounded reading uncertainty ±1.9155 eV. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown lmu by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00055125, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

PHOTON MASS μ_γ against lmu

Predictions from the supplied Form factors in a Coulomb field: the s-state contact term model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): PHOTON MASS μ_γ / eV

PHOTON MASS μ_γ against lmuPredictions from the supplied Form factors in a Coulomb field: the s-state contact term model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: lmu: Photon mass, log₁₀(μ_γ / 1 eV). Vertical axis: PHOTON MASS μ_γ / eV. Exact values are available in the data table below.-72.16414.990213891876-2.477-0.98840.51.9883.477xy

Horizontal axis (x): lmu: Photon mass, log₁₀(μ_γ / 1 eV)

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1lmu: Photon mass, log₁₀(μ_γ / 1 eV): -2.256PHOTON MASS μ_γ / eV: 0.005543
Read the exact data as a table
PHOTON MASS μ_γ against lmu — supplied values
lmu: Photon mass, log₁₀(μ_γ / 1 eV)PHOTON MASS μ_γ / eV
-2.256250.0055430653660835045
-0.87812500000000030.13239604139104352
0.499999999999999563.162277660168376
1.878124999999999475.53095919585759
3.2562499999999991804.055940091783

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(lmu) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(-2.25625) = 0.00554307 eV and Y_out(3.25625) = 1804.06 eV. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives lmu_line = -2.25625 + (12.7754 − 0.00554307)(3.25625 − -2.25625)/(1804.06 − 0.00554307) = -2.21723. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives lmu = 1.10638; substituting back gives Y_out = 12.775415 eV.
  4. The interpolation error in the input is |lmu_line − lmu| = 3.32361. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 10.859915 eV and 14.690915 eV. Sort the roots even if the response decreases: 1.03583 ≤ lmu ≤ 1.16705.
  6. The lower and upper input deviations are 0.0705486 and 0.0606738. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer lmu ≈ 1.10638; compatible interval [1.03583, 1.16705]. Endpoint interpolation gives -2.21723.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 04

F₂ in a Coulomb field is the Pauli term, and Uehling joins: infer an input with uncertainty

Over the momentum transfers an atom supplies, the one-loop magnetic form factor F₂(q²) equals its zero-momentum value, so a bound electron feels it as Pauli's term with a = F₂(0) = α/2π, adding 2a times Dirac's Darwin and spin-orbit energies. Use the supplied topic model Y_out = 4*ln((sqrt(100^lq+4)+10^lq)/2)/(10^lq*sqrt(100^lq+4)), where Y_out is F₂/F₂(0) AT PROBE (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sqrt(x) means √x; ln(x) means the natural logarithm. lq is Probe momentum log₁₀(|q|/m), dimensionless. Keep zn (Nuclear charge Z) = 1 fixed. For this exercise use only -2.575 ≤ lq ≤ 0.575 on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 0.99186066, with a bounded reading uncertainty ±0.0012207. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown lq by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.000315, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

F₂/F₂(0) AT PROBE against lq

Predictions from the supplied F₂ in a Coulomb field is the Pauli term, and Uehling joins model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): F₂/F₂(0) AT PROBE

F₂/F₂(0) AT PROBE against lqPredictions from the supplied F₂ in a Coulomb field is the Pauli term, and Uehling joins model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: lq: Probe momentum log₁₀(|q|/m). Vertical axis: F₂/F₂(0) AT PROBE. Exact values are available in the data table below.0.32090.49720.67350.84981.026-2.701-1.851-1-0.14950.701xy

Horizontal axis (x): lq: Probe momentum log₁₀(|q|/m)

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1lq: Probe momentum log₁₀(|q|/m): -2.575F₂/F₂(0) AT PROBE: 1
Read the exact data as a table
F₂/F₂(0) AT PROBE against lq — supplied values
lq: Probe momentum log₁₀(|q|/m)F₂/F₂(0) AT PROBE
-2.5750.9999988200920847
-1.78750.9999556569420265
-1.00000000000000020.9983366595396486
-0.212500000000000360.941718869236054
0.57499999999999970.3470469554322817

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(lq) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(-2.575) = 0.999999 and Y_out(0.575) = 0.347047. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives lq_line = -2.575 + (0.991861 − 0.999999)(0.575 − -2.575)/(0.347047 − 0.999999) = -2.53574. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives lq = -0.6535; substituting back gives Y_out = 0.99186066.
  4. The interpolation error in the input is |lq_line − lq| = 1.88224. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 0.99063996 and 0.99308136. Sort the roots even if the response decreases: -0.689105 ≤ lq ≤ -0.622835.
  6. The lower and upper input deviations are 0.0356048 and 0.0306652. They differ because the inverse response is nonlinear; a symmetric error bar would only be a local approximation. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer lq ≈ -0.6535; compatible interval [-0.689105, -0.622835]. Endpoint interpolation gives -2.53574.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

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