# Does Time Slow Down on the Bus?

Dear Science: Is the time dilation predicted by Einstein's theory of relativity (and then confirmed after his death) active at all speeds? For instance, if I am sitting in a Sound Transit bus that is traveling at five miles per hour across the 520 bridge, does the time-dilation effect mean that clocks on the bus are traveling at a slower speed than a watch on someone who is standing still along the side of the road? Or does this happen only at higher speeds?

Anxiously awaiting your answer,

Time Traveling On Transit

Dear TTOT: Let's say you and a friend are waiting on the shore of Lake Washington, each holding a timer that works by bouncing light off the moon; each tick of the clock is measured by how long the round trip—a line up and a line back—takes to complete. You hop on the bus, and your five-mile-per-hour crawl across the lake begins. The friend's timer, sitting still on the shore, has only to wait for the light to bounce off the moon and come back. For you on the bus, the light has to take an angled bounce off the moon to catch up with your movement; this is a longer distance, so your timer starts working a little bit slower. The faster the bus goes, the longer distance the light has to travel, and the slower your timer works. For your friend's clock, the trip takes 18 minutes. For you and your moving timer, time has passed more slowly—the trip takes a teeny, tiny bit below 18 minutes. With a horrific amount of math, we can prove that this is why time begins to slow down for anything that's moving—a proton, a spaceship, Mayor Greg Nickels, whatever.

While Einstein gets all the attention for special relativity, let's give a nod to Hendrik Lorentz for dreaming up the formula used to calculate just how much time slows down. Thanks to Lorentz, we know that special relativity only becomes significant at velocities above a tenth of the speed of light—about 67 million miles per hour. At that speed, your trip across the lake would take about 81 microseconds; according to your watch, it would be a mere 80.6 microseconds. Whew.

Fun fact: You'll also become more massive. If you're 100 kilograms at rest, when zipping across the lake you'll balloon up to 100.5 kilograms, at least in apparent mass. Sneaking up to the speed of light, the fastest velocity anything can travel, takes infinite amount of energy—inefficient for even a hybrid bus. To make time go at half of the rate, you need to be almost 90 percent of the speed of light; good luck getting anything you'll fit into going that fast.

So yes, time dilation happens at all velocities—it just isn't observable at current Sound Transit speeds. After light rail is built, ask me again if you can come home younger than a twin who walks to work.

Relatively Yours,

Science

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