Field of Science


Reasons for Short People to Gloat

So there I was, living my life, happily assuming that kooky physics notions (black holes, extra dimensions, everything made of strings) were not affecting my ordinary, three-dimensional existence. And then I read this headline: "Superaccurate Clocks Confirm Your Hair Is Aging Faster Than Your Toenails." Thanks for shattering my dreams, physics.

Really, I should be thanking Einstein and his theory of relativity. One of his ideas was that gravity affects time the same way it affects people: the closer you are to a gravitational force, the stronger it is. This is why you weigh a tiny bit less at the top of a mountain--Earth's gravity is pulling you down just a little less strongly. Time, like your body, is a little more weighed down closer to Earth. So the watch you're wearing will run a smidge slower once you come back down that mountain.

("But why?" I asked my physics-major sister. In response, she told me a bewildering story about Usain Bolt sprinting through a barn while carrying a giant pole. "I wouldn't worry about it in your day-to-day life," she reassured me.)

Einstein didn't have the equipment to test his outlandish ideas, but nowadays we do. Researchers at the National Institute of Standards and Technology (these are the people who keep the official time for the United States, among other things) used a new, hyper-accurate atomic clock to demonstrate time moving faster with an elevation increase of less than a meter.

You can also see relativity in action when one clock is in motion and the other one is at rest. (The key is any kind of difference in acceleration--gravity is a downward acceleration, but a non-downward motion works too.) With their extremely sensitive clocks, these researchers showed that even a pretty slow movement--"like the speed of you sitting on a swing, slowly swinging"--caused time to slow down.

This isn't the first time that relativity's creepy time warp has been demonstrated in real life. But in the past, most of the demonstrations have required huge differences in elevation or extreme speeds.

Your feet, then, are experiencing time at a slower pace than your head. But this difference is unbelievably small. And for practical purposes, your head is actually living--ready?--in the past.

So says neuroscientist David Eagleman. To see what he means, imagine stepping on a pebble. After the pebble touches your toe, the signal has to travel all the way up the nerves in your leg to your spinal cord, and from there to your brain. This is a fast trip, to be sure, but it's not instantaneous. It can take about a tenth of a second. Meanwhile, other information such as sights and sounds--or touches closer to your head--are arriving in the brain much more quickly.

If your brain were to process all this sensory information on a first-come, first-served basis, your world would be in total disarray. You'd see yourself stepping on the pebble before you felt it. To smooth everything out, Eagleman proposes, your brain puts a tiny delay on the moment you perceive as "now." He compares this to live TV: The show is actually on a small lag, in case, for example, someone swears and needs to be bleeped out. Since your brain has to wait for information from your farthest extremities to arrive before it assembles each moment in time, what you perceive as "now" actually happened a tiny bit in the past.

Experiments have confirmed that when people are touched on their toe and their nose simultaneously, they perceive the two events as simultaneous. That tenth of a second delay gets erased in the brain's editing. And it's not just a question of low resolution: Visually, we can perceive differences of just a few milliseconds. But when our brains assemble a moment in time, they have to wait for sounds and touches to catch up with what we've seen.

The oddest extension of this idea is that taller people have more of a delay than shorter people, since signals have to travel even farther along their gangly limbs. Short people, then, can say they live more in the present than tall folks do.

And don't forget, short people: you're also aging more slowly! Over a lifetime, that missing foot of height might add 90 billionths of a second to your life. Meanwhile, we taller people are whizzing through time and missing every moment of it.


  1. I heard about this last year on NPR, I seem to recall.

  2. Yes, the Eagleman stuff is from last year, and was covered on NPR. The fancy clocks are brand new.

  3. Oh, those people with their fancy clocks . . .


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