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University Quantum Field Theory I · unit UQFTI-11

Discrete Symmetries of Quantised Fields: C, P, T and CPT

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

Parity on Fock space: scalar and Dirac quanta: infer an input with uncertainty

Parity on Fock space is the unitary operator P that reverses each quantum's momentum, keeps its spin and multiplies its ladder operators by an intrinsic phase, the phases chosen so that every field at (t, x) goes into a field at the mirror point (t, −x). Use the supplied topic model Y_out = 180*k-th, where Y_out is ANTIPARTICLE arg η′ (°). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. th is Particle phase arg η, measured in °. Keep k (Field: 0 scalar, 1 Dirac) = 1; ell (Pair orbital L) = 0 fixed. For this exercise use only 19.125 ° ≤ th ≤ 160.875 ° 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 = 74.4075 °, with a bounded reading uncertainty ±8.2924 °. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown th by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.014175 °, 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

ANTIPARTICLE arg η′ against th

Predictions from the supplied Parity on Fock space: scalar and Dirac quanta 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): ANTIPARTICLE arg η′ / °

ANTIPARTICLE arg η′ against thPredictions from the supplied Parity on Fock space: scalar and Dirac quanta 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: th: Particle phase arg η / °. Vertical axis: ANTIPARTICLE arg η′ / °. Exact values are available in the data table below.13.4651.7390128.3166.513.4651.7390128.3166.5xy

Horizontal axis (x): th: Particle phase arg η / °

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

Reading 1th: Particle phase arg η / °: 19.13ANTIPARTICLE arg η′ / °: 160.9
Read the exact data as a table
ANTIPARTICLE arg η′ against th — supplied values
th: Particle phase arg η / °ANTIPARTICLE arg η′ / °
19.125160.875
54.5625125.4375
9090
125.437554.5625
160.87519.125

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(th) = 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(19.125) = 160.875 ° and Y_out(160.875) = 19.125 °. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives th_line = 19.125 + (74.4075 − 160.875)(160.875 − 19.125)/(19.125 − 160.875) = 105.593 °. 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 th = 105.593 °; substituting back gives Y_out = 74.4075 °.
  4. The interpolation error in the input is |th_line − th| = 0 °. This is within the requested numerical tolerance.
  5. Invert both limits Y_out = 66.1151 ° and 82.6999 °. Sort the roots even if the response decreases: 97.3001 ° ≤ th ≤ 113.885 °.
  6. The lower and upper input deviations are 8.2924 ° and 8.2924 °. They agree on this branch; this does not establish linearity for changes in other inputs or outside the interval. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer th ≈ 105.593 °; compatible interval [97.3001, 113.885] °. Endpoint interpolation gives 105.593 °.

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

Extra challenging · 02

Charge conjugation on quantised fields: infer an input with uncertainty

Charge conjugation C is the unitary operator on Fock space that turns every particle into its antiparticle with the same momentum and spin and leaves the vacuum alone, so it carries a complex field into its Hermitian conjugate, times a spinor matrix for a Dirac field, and a real field into itself times a sign η_C = ±1. Use the supplied topic model Y_out = 1+kappa*eps*nd, where Y_out is Q OF THE STATE (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. nd is Antiparticles in right packet, dimensionless. Keep kappa (Field κ: −1 scalar, +1 Dirac) = 1; eps (Statistics ε: −1 anti-, +1 commutators) = -1 fixed. For this exercise use only 0 ≤ nd ≤ 3, 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 = -1, with a bounded reading uncertainty ±0.24. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. Determine every allowed value of nd 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

Q OF THE STATE against nd

Predictions from the supplied Charge conjugation on quantised fields model, with all other inputs fixed as stated in the question. 4 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): Q OF THE STATE

Q OF THE STATE against ndPredictions from the supplied Charge conjugation on quantised fields model, with all other inputs fixed as stated in the question. 4 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: nd: Antiparticles in right packet. Vertical axis: Q OF THE STATE. Exact values are available in the data table below.-2.12-1.31-0.50.311.12-0.120.691.52.313.12xy

Horizontal axis (x): nd: Antiparticles in right packet

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

Reading 1nd: Antiparticles in right packet: 0Q OF THE STATE: 1
Read the exact data as a table
Q OF THE STATE against nd — supplied values
nd: Antiparticles in right packetQ OF THE STATE
01
10
2-1
3-2

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(0) = 1 and Y_out(3) = -2. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives nd_line = 0 + (-1 − 1)(3 − 0)/(-2 − 1) = 2. 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 nd = 2; only the allowed settings 2 lie inside the stated reading bound.
  4. The interpolation error in the input is |nd_line − nd| = 0.
  5. The compatible allowed values span 2 to 2. 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 nd: 2. Endpoint interpolation predicts 2; check admissibility against the discrete settings.

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

Extra challenging · 03

Time reversal on quantised fields: an antiunitary T: infer an input with uncertainty

Time reversal on quantum fields is the antiunitary operator T that leaves the vacuum and every energy unchanged, reverses every momentum and spin, and complex-conjugates every c-number it passes, so that T φ(t, x) T⁻¹ = φ(−t, x). Use the supplied topic model Y_out = 2*pi*197.327/p, where Y_out is WAVELENGTH 2πħc/p (fm). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. p is Momentum p, measured in MeV. Keep t (Time t) = 3 fm/c; delta (Phase δ of the coherent state) = 60 ° fixed. For this exercise use only 131.875 MeV ≤ p ≤ 368.125 MeV 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 = 4.492385 fm, with a bounded reading uncertainty ±0.16866 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 0.023625 MeV, 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

WAVELENGTH 2πħc/p against p

Predictions from the supplied Time reversal on quantised fields: an antiunitary T 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): WAVELENGTH 2πħc/p / fm

WAVELENGTH 2πħc/p against pPredictions from the supplied Time reversal on quantised fields: an antiunitary T 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: Momentum p / MeV. Vertical axis: WAVELENGTH 2πħc/p / fm. Exact values are available in the data table below.3.1274.7566.3858.0149.643122.4186.2250313.8377.6xy

Horizontal axis (x): p: Momentum p / MeV

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

Reading 1p: Momentum p / MeV: 131.9WAVELENGTH 2πħc/p / fm: 9.402
Read the exact data as a table
WAVELENGTH 2πħc/p against p — supplied values
p: Momentum p / MeVWAVELENGTH 2πħc/p / fm
131.8759.401646309837545
190.93756.493444750820694
2504.959368428439305
309.06254.011622591255252
368.1253.3679921415547063

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(131.875) = 9.40165 fm and Y_out(368.125) = 3.36799 fm. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives p_line = 131.875 + (4.49238 − 9.40165)(368.125 − 131.875)/(3.36799 − 9.40165) = 324.099 MeV. 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 = 275.988 MeV; substituting back gives Y_out = 4.492385 fm.
  4. The interpolation error in the input is |p_line − p| = 48.1115 MeV. This is larger than the requested numerical tolerance.
  5. Invert both limits Y_out = 4.323725 fm and 4.661045 fm. Sort the roots even if the response decreases: 266.001 MeV ≤ p ≤ 286.753 MeV.
  6. The lower and upper input deviations are 9.98661 MeV and 10.7657 MeV. 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 ≈ 275.988 MeV; compatible interval [266.001, 286.753] MeV. Endpoint interpolation gives 324.099 MeV.

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

Extra challenging · 04

The bilinears under C and T, and the photon under P, C and T: infer an input with uncertainty

The C and T columns of the bilinear table give the sign that each Hermitian Dirac bilinear, component by component, acquires under charge conjugation or time reversal of the quantised field; the photon field then takes whatever signs keep e ψ̄γ^μψA_μ invariant. Use the supplied topic model Y_out = min(k, 4-k), where Y_out is LORENTZ INDICES n (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; min(a,b) selects the smaller value. k is k = 0 to 4: Γ = 1, γ^μ, σ^μν, γ^μγ⁵, iγ⁵, dimensionless. Keep s (Spatial indices on the component) = 1 fixed. For this exercise use only 0 ≤ k ≤ 2, 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 = 1, with a bounded reading uncertainty ±0.16. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. Determine every allowed value of k 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

LORENTZ INDICES n against k

Predictions from the supplied The bilinears under C and T, and the photon under P, C and T model, with all other inputs fixed as stated in the question. 3 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): LORENTZ INDICES n

LORENTZ INDICES n against kPredictions from the supplied The bilinears under C and T, and the photon under P, C and T model, with all other inputs fixed as stated in the question. 3 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: k: k = 0 to 4: Γ = 1, γ^μ, σ^μν, γ^μγ⁵, iγ⁵. Vertical axis: LORENTZ INDICES n. Exact values are available in the data table below.-0.080.4611.542.08-0.080.4611.542.08xy

Horizontal axis (x): k: k = 0 to 4: Γ = 1, γ^μ, σ^μν, γ^μγ⁵, iγ⁵

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

Reading 1k: k = 0 to 4: Γ = 1, γ^μ, σ^μν, γ^μγ⁵, iγ⁵: 0LORENTZ INDICES n: 0
Read the exact data as a table
LORENTZ INDICES n against k — supplied values
k: k = 0 to 4: Γ = 1, γ^μ, σ^μν, γ^μγ⁵, iγ⁵LORENTZ INDICES n
00
11
22

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(0) = 0 and Y_out(2) = 2. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives k_line = 0 + (1 − 0)(2 − 0)/(2 − 0) = 1. 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 k = 1; only the allowed settings 1 lie inside the stated reading bound.
  4. The interpolation error in the input is |k_line − k| = 0.
  5. The compatible allowed values span 1 to 1. 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 k: 1. Endpoint interpolation predicts 1; check admissibility against the discrete settings.

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

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