What Gravity Still Hides: teacher key

Gravity, grade 12 edition, written for 12th grade and up. Newton gave gravity a law without saying how it works, and Einstein gave it a geometry. Follow the evidence for both and the questions still open.

Read the book. Print this page for the key.

Questions and answers

1. Why was the Principia published at Halley's expense?
   • Newton refused to be paid
   • The Royal Society was short of funds (answer)
   • Einstein asked him to
2. What has eluded physicists for more than 300 years?
   • The law of falling bodies
   • The mass of the Moon
   • An exact value of the gravitational constant (answer)
3. What caused the clocks to tick at different rates in the 2010 NIST experiment?
   • A power cut
   • A slight difference in gravity (answer)
   • A difference in temperature
4. What did the 1919 measurements arguably align with?
   • Einstein's general relativity (answer)
   • Newton's law of tides
   • Halley's comet
5. What may the prediction of a singularity signal?
   • A flaw in LIGO
   • A new star
   • The limits of relativity (answer)

Words

• precession: orbit slowly turning
• spacetime: space with time
• singularities: points relativity fails
• lensing: light bending