Linearised gravity and gravitational waves · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Ripples in the shape of space

Science · Astronomy & Astrophysics · ages 23-24
Name ______________________   Date ____________

  1. Which idealized source has a changing mass quadrupole and emits leading gravitational radiation?

    • An isolated star remaining perfectly spherical while pulsating
    • An isolated mass distribution that remains static throughout
    • Two masses orbiting in a circular binary

  2. What is a gravitational wave in the weak-field description?

    • A radiative spacetime perturbation
    • An ordinary pressure wave in interstellar gas
    • A time-independent gravitational potential everywhere

  3. An isolated star that remains perfectly spherical emits gravitational waves solely because its radius pulsates.

    Circle one:   True   False

  4. Why does leading weak-field radiation from an isolated source lack mass-monopole and mass-dipole terms?

    • Mass-energy and momentum are conserved
    • An isolated source has neither energy nor momentum
    • A gravitational wave carries only electric charge

  5. A weak plus-polarized plane wave passes perpendicular to a ring of free test masses. What is the first-order transverse response?

    • Every separation remains fixed while the whole ring translates
    • Alternating stretch and contraction along two axes
    • Every separation grows equally without later contraction

  6. A 2 km detector arm changes in length by 2×10⁻¹⁸ m. What is its response strain ΔL/L?

    • 10⁻²¹
    • 10⁻¹⁸ m
    • 10⁻¹⁵

  7. Theo says GW150914 was a photograph showing two black holes merging. What corrects that claim?

    • A telescope photographed the entire merger directly
    • The signal contained no information about its source
    • A measured waveform matched merger models

  8. Which measurement principle lets LIGO detect a passing gravitational wave?

    • Microphones compare ordinary sound pressure in the arms
    • Returning laser beams interfere
    • Thermometers compare the heat carried along the arms
LightMySky · lightmysky.comW1-mt_z0tMZDGMHi-s1

Answer key

For grown-ups. Fold this page away before handing over the rest.

Ripples in the shape of space W1-mt_z0tMZDGMHi-s1

  1. Two masses orbiting in a circular binary · The binary's directional mass pattern rotates even if its angular speed is steady.
  2. A radiative spacetime perturbation · The radiative metric perturbation propagates at light speed and produces tidal effects.
  3. False · A perfectly spherical pulsation does not generate gravitational radiation.
  4. Mass-energy and momentum are conserved · Conservation removes the time-changing monopole and radiative mass-dipole terms; the quadrupole is the first allowed mass contribution.
  5. Alternating stretch and contraction along two axes · For this polarization, the two transverse axes change in opposite ways and then reverse.
  6. 10⁻²¹ · Convert 2 km to 2000 m, then divide: (2×10⁻¹⁸)/2000=10⁻²¹. Strain has no units.
  7. A measured waveform matched merger models · GW150914 was a gravitational-wave signal interpreted through relativistic waveform models, not a photograph.
  8. Returning laser beams interfere · Different arm responses change the returning beams' relative optical phase and interference signal.
Worksheet · LightMySky