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ROBERT SIEGEL, host: Commentator Adam Frank has been pondering relativity this summer, and he's qualified to do that. He's an astrophysicist. Frank describes to us a modern-day theory involving space and time, with all due respect to Albert Einstein.

(SOUNDBITE OF A NEWSREEL)

UNIDENTIFIED MAN: And as the 20th century reaches the halfway point, the scientists' dream of unifying the basic concepts of the universe continues to occupy the genius of Professor Albert Einstein.

ADAM FRANK: Yes, it was Albert Einstein who unified space and time together into a single, coherent whole. As a physicist, I can say that was a pretty impressive feat, but as a parent - slogging across interstate whatever on the last weekend of summer - I have to ask: What's the big deal? Anyone stuck in vacation traffic with kids in tow can tell you that space and time have always been unified but not in the wiggly, abstract sense my buddy Al Einstein was talking about. No, if you're like me, you have already experienced the way the two can merge into one giant, sweaty, chewing gum wad of boredom and rest stops that is the principle of highway holiday relativity.

As the first scent of autumn rises like a ghost in the morning and the desperate desire for just one more trip to the lake burns away our good sense, it's a good time to review the basic equation of highway relativity. Time equals space equals your foot - as in your foot on the gas pedal. Now, you may think you hate physics, but I think you know exactly what I'm talking about. Let's review.

Deep into that 380-mile drive from your house to your cousin's place on Lake Arewethereyet, you pass under one of those big green interstate placards. It tells you that there are 250 miles to Middleofnowhere Falls, the last big city before you turn north to the lake. You're hungry, tired and need to go to the bathroom. This is where highway relativity and its calculations begin. The time you have left in that sweaty car equals the distance you still have to cover divided by the speed at which you are traveling.

Now, you could ask someone in the car with a calculator-enabled cell phone to divide the 250 miles you still have to cover by your current speed of 60 miles per hour. But after the entire morning in the car, everyone has already been bored into a coma. So instead, you round everything off and arrive at about four hours. Yes, it's true. You still have four interminable hours left behind the wheel. But the principle of highway relativity still has so much more to give. How much less time would it take to cover the same space if you just inched up your speed to 70 miles per hour? And hey, why be stingy? How about 75 miles per hour? Seventy-eight? Eighty even?

Einstein's theory of relativity came with no ethics attached to it, but highway relativity is not so agnostic about human affairs. Yes, more space will be covered in less time if you simply drive faster. But your family's safety and that $200 speeding ticket drag at your wheels and your conscience. And there, right there, is where the ethics of highway relativity and the physics of Einstein's relativity finally meet. Einstein taught us that time does not flow at the same rate for everyone. Highway relativity teaches us the same.

For your cousins at the lake, time is flowing as gently and freely as the breeze running off the cool water. But for you, time just hangs in the air like the hot sun beating through the windows. Oh, and look, there's road construction ahead.

SIEGEL: That's Adam Frank, an astrophysicist at the University of Rochester and author of the book "The Constant Fire."

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