One lesson novice scuba divers learn is that determining the source of a sound underwater is more difficult than it is above water. When we hear a sound traveling through air, our brains detect the slight difference between the arrival times of vibrations as they reach our separated ears and use that slight difference to determine the direction from which a sound emanates. But what works on dry land doesn’t work well underwater because sound travels faster in the denser fluid. We’re used to sound waves traveling about 1,100 feet per second through air, but in water, depending on temperature and salinity, sound can travel between 4,600 and 5,000 feet per second. For us, that makes sound seem to hit both ears simultaneously, giving us little chance to determine its origin.
A magnifying glass works because it has varying thickness; the time light has to travel through the thinner circumference differs from the time for it to travel through the thicker middle of the lens. Here’s Richard Feynman’s explanation: “By…putting [in the paths of the photons] just the right thickness of glass to compensate for the time along each [photon’s] path, we can make all the times the same...it’s a focusing lens. By arranging things so that the times are equal, we can focus light—we can make the probability very high that light will arrive at a particular point, and very low that it will arrive anywhere else” (p. 58).
Arriving at the same time. It obsesses us. “Meet me at the coffee shop at ten,” “Get me to the church on time,” “Had I been at the corner just a second before, I might have been killed by that truck.” But isn’t it interesting that in the physical world the close timing of sound waves befuddles us whereas the close timing of photons makes things clearer, even to the point of magnification?
And maybe when we fault someone for bad timing, we need to determine whether or not we are judging by what we hear or what we see. Often, there is more than a single perspective. And that perspective, even in the physical world, might determine whether or not we really have a grasp on what we have witnessed by ears or eyes. Always check the medium through which you make your judgment.
*Feynman, Richard. QED: The Strange Theory of Light and Matter. Princeton U. Press, 1985.
A magnifying glass works because it has varying thickness; the time light has to travel through the thinner circumference differs from the time for it to travel through the thicker middle of the lens. Here’s Richard Feynman’s explanation: “By…putting [in the paths of the photons] just the right thickness of glass to compensate for the time along each [photon’s] path, we can make all the times the same...it’s a focusing lens. By arranging things so that the times are equal, we can focus light—we can make the probability very high that light will arrive at a particular point, and very low that it will arrive anywhere else” (p. 58).
Arriving at the same time. It obsesses us. “Meet me at the coffee shop at ten,” “Get me to the church on time,” “Had I been at the corner just a second before, I might have been killed by that truck.” But isn’t it interesting that in the physical world the close timing of sound waves befuddles us whereas the close timing of photons makes things clearer, even to the point of magnification?
And maybe when we fault someone for bad timing, we need to determine whether or not we are judging by what we hear or what we see. Often, there is more than a single perspective. And that perspective, even in the physical world, might determine whether or not we really have a grasp on what we have witnessed by ears or eyes. Always check the medium through which you make your judgment.
*Feynman, Richard. QED: The Strange Theory of Light and Matter. Princeton U. Press, 1985.
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