Light at the speed of walking

Last updated on: January 24, 2018 at 5:13 pm by Dan Peterson

Light at the speed of walking 2018-01-24T17:13:15-07:00 Dan Peterson

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Mount Timpanogos, which dominates Utah Valley (Wikimedia Commons)

An amusing thought-experiment from Chris Impey’s How It Began: A Time-Traveler’s Guide to the Universe, pp. 190-191:

Imagine the inconvenience if light were sluggish. Suppose it moved at 1 meter per second, a fast walking clip, as opposed to its measured speed, which is a blistering 300,000 kilometers per second. Everything in this hypothetical world is the same except the speed of light.

You enter a darkened house and switch on the overhead light. You patiently wait a few seconds to see the light. Then you watch as the nearby walls — and a while later the far wall — become visible. The speed of sound is still 343 meters per second, much faster than light, which leads to some interesting consequences. Face-to-face conversations tend to happen at a fixed distance of a few feet; everyone is used to lip motions slightly lagging sounds, but it gets annoying if it lags by more than a few seconds. TVs have a delay button on the remote so you can adjust the sound to match the several seconds it takes the picture to reach you across the living room.

Walking to the supermarket you wave to a neighbor across the street and half a minute later he waves back; not because he’s rude, but that’s how long it took for him to see your wave and for his wave to reach you. You spot a friend you haven’t seen for a while. He’s at the end of your aisle inspecting some salsa. By the time you reach him, he’s long moved on and is nowhere to be found. A new checkout line opens but people standing closer saw the light go on seconds before you and are all in line as you get there.

Driving is extremely dangerous. The road ahead of you contains older and older situations. People you see walking on the sidewalk 10 yards ahead might not be there any more since you’re seeing them as they were ten seconds ago. Approaching cars are already upon you as you see them. Traffic 100 yards ahead might be completely different when you get there, since your information is almost two minutes out of date. That’s during the day. Don’t even think about driving at night. All time lags double because light has to stroll from your headlights to an object ahead and then stroll back to your eye. If you drive faster than a meter per second you’ll outrace the light from your headlights, rendering them useless. In fact, just running down the street makes events play out backward, which can be quite amusing.

At the end of another long, disconcerting day, you console yourself with the sight of a beautiful sunset. Remembering, of course, that the hilly horizon is 20 miles away so the Sun set 10 hours ago and is almost ready to rise.

Posted from Salt Lake City, Utah

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