An illustration of an accelerating space rocket in Tom Siegfried’s essay “Getting a grip on gravity” (SN: 10/17/15, p. 16), explained that a clock at the top of the ship would tick faster than one at the bottom thanks to gravitational time dilation. A few perplexed readers argued that acceleration aboard the rocket ship would be the same for both clocks, so they should tick at the same rate.
“Once the rocket has reached equilibrium, both clocks are accelerating at the same rate and will run at the same rate,” Bart Bresnik wrote in an e-mail.
The issue, Siegfried says, isn’t the equal acceleration of the clocks. It’s the direction of the