# Gravity, acceleration and more reader feedback

### Grasping gravity

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 acceleration and how long it takes for a light signal to travel from the bottom clock to the top clock. If the rocket were moving at a constant rate (making it an inertial frame), both clocks would keep the same time. But if the rocket is uniformly accelerating, a light beam traveling in the direction of the acceleration will take longer to reach the top clock than it would have in an inertial frame. Nevertheless the speed of light is always the same, whether in an inertial frame or in an accelerating rocket. Since the distance between the two clocks in an accelerating ship remains the same, but the light takes longer to reach the top, the top clock must tick faster to measure the same speed of light as would be measured in an inertial-frame ship.

Einstein’s theory requires gravity to be equivalent to acceleration, so the same principle applies to a tall building in a gravitational field: A clock on the building’s top floor will tick faster than a clock on the bottom floor.

### Correction

In the article “Elusive acid finally created” (SN: 10/31/15, p. 11), hydrogen fluoride was incorrectly referred to as a strong acid. While it can cause serious problems to people exposed to it, hydrogen fluoride is technically classified as a weak acid.