Measuring the Effect of Gravity on Antimatter

Recent experiments have measured the effect of gravity on antimatter with the first “direct”, i.e., freefall-style or Galilean Leaning Tower of Pisa-style measurements. In agreement with theory and indirect experiments, these experiments, performed by CERN’s ALPHA collaboration, show that antimatter, specifically antihydrogen atoms, fall downward with an acceleration within about 25% of g=9.8ms2. Strongly ruled out is the possibility of antimatter falling upwards.  Thus, the results are compatible with the weak equivalence principle. This talk will review why this topic remained a perhaps open question, document some of the current other experiments attempting to make a direct measurement, discuss some of the history of early, failed, attempts to do a direct measurement, and conclude with a description of how the measurement was actually made using a magnetic balance.