A Search and Discovery piece by Steven K. Blau (Physics Today, June 2003, page 21) discusses Jack Wisdom’s work showing that the position of an object can be changed when that object modifies its shape by extending and retracting across a Schwarzschild metric. Although this result may be new to theorists, it is already known to rocket scientists.
In a paper presented in 1990, I showed that by changing the length of a tether in a gravitational gradient—that is, in curved spacetime—one can modify the orbit, even to the extent of making the object climb away from the planet without expenditure of reaction mass. 1 In 1987, Manuel Martinez-Sanchez and Sarah Gavit had calculated similar results. 2 One can’t push on flat spacetime, but in a region of curved space—or, if you prefer, tidal forces—one can use the difference in force to push against gravity.
