Move toward the light
Dr. Maureen Hunter | TLT From the Editor January 2009
But don’t rush—it’ll reach you at 186,000 miles a second, no matter where you are.
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Last winter we went with my husband’s family to a night-time show where skiers, boarders and snowmobilers juggled and threw lighted balls back and forth to each other as they moved down the ski slope.
“Look man,” said my snowboarder nephew, pushing back his dreadlocks. “That light is traveling faster than the speed of light.”
“What?” I questioned.
“If someone coming toward you throws a ball, it comes at you faster than if he’s moving away. Velocities are additive, and that skier coming toward us threw the lighted ball in our direction. It’s all relative.”
“Not really,” I said. “The Principle of Relativity states that all laws of nature are the same for all observers. You should have learned in school that the speed of light is a universal constant—186,000 miles/second.”
“Says who?” said my nephew.
“Says experimental measurements and Maxwell’s equations,” I answered. “The speed of a light beam is constant no matter how fast its source or the observer is moving.”
“So you mean that skier moving down the hill would see the light beam from that snowmobile’s headlight zip by him at the exact speed that we see it zip by us just standing here.”
“That’s right. And you can even think of it this way. The skier’s speed relative to the snowmobile varies depending on how fast the skier is moving, but his speed relative to the snowmobile’s headlight doesn’t vary.”
“Says who?” he said again. And then added, “That’s not possible! That’s a useless metaphysical concept.”
“Says time,” I answered, “which is a useless metaphysical concept.”
“Hmh?” said my nephew.
“There is no absolute time. All moving reference frames have their own relative time,” I said.
“Really?”
“Suppose the light in the chairlift station at the top of the hill and the light in the clubhouse at the bottom of the hill are both turned on. We would say that the lights were turned on simultaneously if we were standing exactly half way between them so that the light from each reached us at the exact same time. Now let’s say that a skier moving downhill was right next to us when we calculated the lights to go on. He would be slightly downhill by the time the light arrives, so he will see the light from the clubhouse before he sees the light from the lift. There’s no way to say that two events are really simultaneous.”
“Told you that it’s all relative!”
“Another consequence of special relativity is that we, standing here on the hillside, will observe that time goes more slowly for the skier speeding past us.”
“You’re freaky,” he said, flipping back his long dreadlocks again.
“I’m not the one who’s freaky,” I said. “Think of it like this. Look at that skier throwing a lighted ball up and down as he skis down the hill. We see the lighted ball move up and down in a long parabolic zigzag moving from the top of the hill to the bottom. But he sees that lighted ball just going straight up and then straight down, only about two feet in each direction, as he throws and catches it.”
“Whoa!”
“If the lighted ball were just a light beam traveling at 186,000 miles/second, we, standing here would observe the distance that the light has to travel as longer than the skier would observe it to be. So from our perspective, time is going more slowly for the skier.”
“Cool! Time goes more slowly if you’re moving faster. Then I’m going to snowboard as fast as I can, everyday, all day long.”
“Don’t bother,” I said. “If you boarded every second of your life, you’d only gain a fraction of a micro-second, even if I stood here my whole life and watched you. No one can travel fast enough to amount to a significant difference.”
“Major bummer.”
“The only situation where anyone really has to deal with these relativistic effects is in particle accelerators, where the electrons travel close to the speed of light so the time experienced by those electrons changes relative to the scientists’ frame of reference. So they need to correct for this in all of their calculations.”
“Metaphysical man. That’s the job I want,” my nephew exclaimed.
“You’ll probably need to get your hair cut,” I said.
“Now that’s really a useless ‘mega’ physical concept,” he answered.
Maureen Hunter is technical service manager for King Industries in Norwalk, Conn. You can reach her at mhunter@kingindustries.com.