47 Meters Down Quotes

By the time I ran what would turn out to be my fastest 5,000, on a perfect evening in Palo Alto, California, in 2003, I’d decided I needed a new mental strategy: I would pretend I was only running 4,000 meters, and simply not worry if I had to jog the last kilometer. I wanted to run 2:45 per kilometer, and my first three kilometers were 2:45, 2:45, 2:47. The moment of truth: I knuckled down and vowed to run the fourth kilometer as hard as I could—but little by little, I drifted back from the pack I was running with.

Speed is measured as distance divided by time (miles per hour or meters per second). For the speed of light to remain constant, distance and time have to change. Let’s go back now to Galileo’s shipboard experiment, using a beam of light instead of a stone. On a boat that’s moving at a uniform speed across the water, shine a flashlight down the mast, and it will strike the deck at the base of the mast. The observer on the dock agrees with that. But from her vantage point on the dock, if she had a precision measuring tool, she would see the light travel a tiny extra distance, the distance the ship has moved in the time it took the light to reach the bottom of the mast. Brought to you by | University of Wisconsin Madison Libraries Authenticated Download Date | 5/11/19 3:47 PM 12 • GRAVITY’S CENTURY But the speed of light, which is measured in meters per second—again, distance divided by time—is a constant. So if the observer on the dock finds that light traveled an extra distance, the only way its speed can remain constant is if the light also took a longer time to travel. Time, therefore, is not immutable. The duration of time— measured as the ticks of a clock—is dif­ferent for observers who move at dif­ferent speeds. Each sees the other’s clock slow down. Even more strangely, distance is not absolute either; it appears to contract in the direction of motion.