# How to Slow Down Time

• By Greg Kestin
• Posted 08.23.18
• NOVA

Under the right circumstances, your brain can trick you into experiencing some time intervals as longer than others. But under other circumstances, time itself really does slow down. The two phenomena—psychological and physical—are more similar than you might think. Find out more in this episode of What the Physics?!

Running Time: 03:38

## Transcript

### How to Slow Down Time

Published August 23, 2018

Greg Kestin: You're about to see a deer appear in three clearings. Watch that again.

Does it take more time for the deer to go from A to B or from B to C? Here it is a couple more times.

Most people see the deer take longer to go from B to C. But in reality, it takes the exact same amount of time to go from A to B and B to C.

This illusion slows down your perception of time—and it is just in your head—but you can actually use the same concept to understand how time, in reality, can actually slow down.

When you see the deer appear at A, then B, your brain registers how fast the deer is going, and then, subconsciously, calculates: When the deer travels three times farther, from B to C, it should take about three times as long. Or at least, it should take longer.

But this calculation is what fools your brain. It assumes the deer is moving at approximately a constant speed. And that assumption makes you expect a longer time interval from B to C. That overrides your actual observation.

So, believing the deer keeps about a constant speed changed your perception of time. Of course, that was a trick that your mind played on you. But real time dilation does exist—and exists because there is one speed in nature which is constant: the speed of light.

Light will always travel at 186,000 miles per second or, rather, one foot every nanosecond. This means you can theoretically use light to measure time by building a kind of light clock.

Imagine a light beam bouncing back and forth between two mirrors separated by six inches. One bounce up and down is one foot and therefore one nanosecond. This is where it gets strange.

Let's say you're holding your light clock on the side of some train tracks and your friend Karen has a light clock in the passing train. She's zooming sideways, so the light from her clock doesn't just go straight up and down like it does with your clock. It goes up and down, but the train is also pulling it this way, so it ends up following these diagonal lines. And those diagonal lines are longer than your vertical lines. In other words, one tick of Karen's clock covers a longer distance than one tick of your clock.

But light travels at a constant speed in both. And if light travels the same speed for a longer distance, it must take more time. So, one tick of Karen's clock is slower than one tick of your clock. Karen's clock—and Karen's breathing and Karen's heart and everything in the train—is slowed down. Time moves more slowly for anything in motion. And for things moving really fast, like, say, satellites, this time slows down by several microseconds per day.

GPS clocks have to factor in this time dilation effect, or else they wouldn't sync up with the clocks on Earth.

So, what's really cool is that the real stretching of time happens because the speed of light is constant, just like that apparent stretching of time happens because your brain thinks the speed of the deer is constant.

If you want to learn more about time dilation, check out our video on how to use time dilation to travel millions of light-years to another galaxy—all in your lifetime.

## Credits

### PRODUCTION CREDITS

Host, Producer
Greg Kestin
Filming
Greg Kestin
Research, Writing
Greg Kestin
Samia Bouzid
Editing, Animating
Samia Bouzid
Greg Kestin
Lauren Liebhaber
Editorial Input
Julia Cort
Ari Daniel
Media: Shutterstock
Special thanks
Entire NOVA team
From the producers of PBS NOVA © WGBH Educational Foundation
Funding provided by FQXi
Music provided by APM

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