University Physics IV · topic UPIV-15.08
Gravitational waves and interferometric detection
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.
Further practice
Work through a fresh case before tackling the extension.
Gravitational waves and detection: predict a finite change
A gravitational wave is a propagating strain in spacetime — a fractional change h in the proper distance between free masses, travelling at the speed of light, transverse to its direction of travel and carrying two polarisations. Use the supplied topic model Y_out = 4.2833*m^1.25*(1+cosd(i)^2)/d, where Y_out is LARGEST h DRAWN (×10⁻²¹). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sind/cosd/tand use degrees. d is Luminosity distance D, measured in Mpc. Keep m (Chirp mass M_c) = 30 M☉; i (Orbital inclination ι) = 0 ° fixed. For this exercise use only 342.5 Mpc ≤ d ≤ 657.5 Mpc 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. Calculate Y_out at d = 411.8 Mpc and 455.9 Mpc. Find the signed output change ΔY_out and the average sensitivity ΔY_out/Δd across this interval. State whether the response increases or decreases; distinguish this interval average from a derivative at one point.
LARGEST h DRAWN against d
Predictions from the supplied Gravitational waves and detection 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): LARGEST h DRAWN / ×10⁻²¹
Horizontal axis (x): d: Luminosity distance D / Mpc
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| d: Luminosity distance D / Mpc | LARGEST h DRAWN / ×10⁻²¹ |
|---|---|
| 342.5 | 1.7561009645862344 |
| 421.25 | 1.427809092868333 |
| 500 | 1.2029291607415704 |
| 578.75 | 1.0392476550683114 |
| 657.5 | 0.9147750271799016 |
These are the supplied values, not a worked answer. No fitted trend is drawn.
Show a hint
Substitute each value of d separately while keeping every other input fixed, then subtract in the same order.
Reveal the worked solution
- At d = 411.8 Mpc, substitute into Y_out = 4.2833*m^1.25*(1+cosd(i)^2)/d: Y₀ = 1.46057 ×10⁻²¹.
- At d = 455.9 Mpc, the same fixed-parameter model gives Y₁ = 1.31929 ×10⁻²¹.
- Signed change: ΔY_out = Y₁ − Y₀ = -0.141284 ×10⁻²¹; the input change is Δd = 44.1 Mpc.
- Average sensitivity is ΔY_out/Δd = -0.00320372 ×10⁻²¹ per Mpc. Its negative sign means that this response decreases over the specified interval.
- This is a secant slope between two settings. It equals a point derivative only for a locally linear relation or in a justified small-interval limit.
Answer Y₀ = 1.46057 ×10⁻²¹, Y₁ = 1.31929 ×10⁻²¹; ΔY_out = -0.141284 ×10⁻²¹; average sensitivity = -0.00320372 ×10⁻²¹ per Mpc.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.
Extra challenging questions
Work through the linked stages and explain what each result means.
Gravitational waves and detection: infer an input with uncertainty
A gravitational wave is a propagating strain in spacetime — a fractional change h in the proper distance between free masses, travelling at the speed of light, transverse to its direction of travel and carrying two polarisations. Use the supplied topic model Y_out = 4.2833*m^1.25*(1+cosd(i)^2)/d, where Y_out is LARGEST h DRAWN (×10⁻²¹). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; sind/cosd/tand use degrees. d is Luminosity distance D, measured in Mpc. Keep m (Chirp mass M_c) = 30 M☉; i (Orbital inclination ι) = 0 ° fixed. For this exercise use only 342.5 Mpc ≤ d ≤ 657.5 Mpc 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 = 1.1249688 ×10⁻²¹, with a bounded reading uncertainty ±0.031529 ×10⁻²¹. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown d by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.0315 Mpc, 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.
LARGEST h DRAWN against d
Predictions from the supplied Gravitational waves and detection 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): LARGEST h DRAWN / ×10⁻²¹
Horizontal axis (x): d: Luminosity distance D / Mpc
Axes show the supplied calibration interval; the origin may lie outside the plot.
Read the exact data as a table
| d: Luminosity distance D / Mpc | LARGEST h DRAWN / ×10⁻²¹ |
|---|---|
| 342.5 | 1.7561009645862344 |
| 421.25 | 1.427809092868333 |
| 500 | 1.2029291607415704 |
| 578.75 | 1.0392476550683114 |
| 657.5 | 0.9147750271799016 |
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(d) = 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
- Endpoint predictions are Y_out(342.5) = 1.7561 ×10⁻²¹ and Y_out(657.5) = 0.914775 ×10⁻²¹. The target lies between them. On the selected branch the response is strictly decreasing.
- Endpoint interpolation gives d_line = 342.5 + (1.12497 − 1.7561)(657.5 − 342.5)/(0.914775 − 1.7561) = 578.802 Mpc. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
- Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives d = 534.65 Mpc; substituting back gives Y_out = 1.1249688 ×10⁻²¹.
- The interpolation error in the input is |d_line − d| = 44.1516 Mpc. This is larger than the requested numerical tolerance.
- Invert both limits Y_out = 1.0934398 ×10⁻²¹ and 1.1564978 ×10⁻²¹. Sort the roots even if the response decreases: 520.074 Mpc ≤ d ≤ 550.066 Mpc.
- The lower and upper input deviations are 14.5759 Mpc and 15.4165 Mpc. 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 d ≈ 534.65 Mpc; compatible interval [520.074, 550.066] Mpc. Endpoint interpolation gives 578.802 Mpc.
Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.