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Physics Lesson: Things Aren’t What They Seem


A snapshot in time

Think that image you see in the mirror is you in real time? If so, you’d be wrong. As our fifth graders can tell you, when it comes to physics, things aren’t what they seem. That image you see in the mirror is you some infinitesimally small fraction of a second ago.


As part of a lesson on light and distance, fifth graders traced each other’s shadows on the playground. When they came back a few minutes later, they observed changes in the length and position of their shadows, a function of the movement of the earth relative to the sun. The earth, teacher Joseph Schneider notes, spins at 822.13 mph in our neighborhood (more than 1,000 mph at the equator).


“We tried different games with our shadows,” Joseph says. “We tried to detach from them (jumping will do the trick), touching your real hand to your shadow hand (which is harder than you think), and trying to disappear completely in someone else’s shadow.”


The students learned that light travels at 186,000 miles per second and that we’re about eight “light minutes” from the sun—93 million miles. (A light year is 6 trillion miles.)


“The most critical idea conveyed in these discussions is that light travels very fast, but not infinitely fast. A great many of the stars we see are ‘ghosts’—long-dead, collapsed, or exploded husks whose light continues to reach us until we catch up, temporally speaking, with the star's death.


“As soon as the students grasp the concept that sunlight is eight minutes ‘old,’” Joseph continues, “we can scale that down to any distance—even a couple of feet. Two people conversing don’t perceive each other as they actually appear, but only as they have appeared. And considering our other senses, whose respective stimuli travel at the merest fraction of the speed of light, we process nothing in real time. Everything exists to us sometime in the past.

“We can then begin to appreciate the fallibility of sense perception (like that image in the mirror), and perhaps begin asking deeper questions about the value of what we're able to perceive relative to what we can't. The illusion of the physical world as we experience it begins to crack.”

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