They beam a laser at the mirrors and time the round trip. Retroreflecting devices placed on the lunar surface by the Apollo astronauts are still used by astronomers to determine the moon’s precise distance. To feel how long this is, stare at the Moon at the next opportunity and count out loud: one one thousand one. Light time finally becomes more tangible when we look at the Moon, a wistful 1.3 light seconds away at its average distance of 240,000 miles. At the Moon’s surface the laser beam spreads out to 4 miles wide and only one photon is reflected back to the telescope every few seconds. Measuring the time delay yields the Moon’s distance to within about a millimeter. Observatories beam a laser to the small array, which reflects a bit of the light back. In comparison, a blink of the eye lasts about 300 milliseconds (1/3 of a second) or 230 times longer! The Lunar Laser Ranging Experiment placed on the Moon by the Apollo 14 astronauts. During an overhead pass, light from the orbiting science lab fires up your retinas 1.3 milliseconds later. The space station orbits the Earth in outer space some 250 miles overhead. But if you happen to look up to see the tiny dark shape of a high-flying airplane trailed by the plume of its contrail, the light takes about 35,000 nanoseconds or 35 microseconds to travel the distance. In our everyday life, the light from familiar faces, roadside signs and the waiter whose attention you’re trying to get reaches our eyes in nanoseconds. Light takes about 35 microseconds to arrive from a transcontinental jet and its contrail. The average lifetime of a PsH molecule is just 0.5 nanoseconds. Since I was only about six inches away and light travels at 186,000 miles per second or 11.8 inches every billionth of a second (one nanosecond), the travel time amounted to 0.5 nanoseconds. Darn close to simultaneous by human standards but practically forever for positronium hydride, an exotic molecule made of a positron, electron and hydrogen atom. I moved closer, removed my glasses and noticed that each drop magnified a little patch of veins that thread and support the leaf.įocusing the camera lens, I wondered how long it took the drops’ light to reach my eye. How gemmy and bursting with the morning’s sunlight. ![]() My attention was focused on beaded water on a poplar leaf. A blink of an eye takes 600,000 times as much time! Credit: Bob King Moving at 186,000 miles per second, light from the leaf arrives at your eye 0.5 nanosecond later. Beads of rainwater on a poplar leaf act like lenses, focusing light and enlarging the leaf’s network of veins.
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