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Time in 298 Words

The views expressed are those of the author and are not necessarily those of Scientific American.

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Last year, in the inaugural Flame Challenge, Alan Alda and the Center for Communicating Science at Stony Brook University challenged scientists to explain what a flame is to an 11-year-old. This year, the subject was time. In particular, we were instructed to “Answer the question — ‘What is time?’ — in a way an 11-year-old will find instructive, interesting, and maybe even fun.” Written entries had a 300-word limit. This is my submission.

"We Live in a Surreal Time" by flickr user garlandcannon.

In some ways, time is a direction. Certain processes only go in one direction. You can stir chocolate syrup into your milk, but no amount of stirring will separate the syrup from the milk once they’re combined.

One fundamental law of nature is that entropy—the amount of disorder in a system—increases with time. A glass of chocolate milk, with milk and chocolate particles all jumbled together, is more disordered than a glass of milk sitting next to a spoonful of chocolate syrup. In fancy language, stirring the syrup into the milk increases the entropy of the system. The future is the direction in which entropy increases, and the past is the direction in which it decreases.

This notion of time has a big limitation: it can say that I ate my broccoli before I made my chocolate milk, but it can’t say whether it was five minutes or three years before.

From a physics point of view, determining the amount of time that passes between two events is not just a matter of finding a good stopwatch. Einstein’s theory of relativity shows that many of our instinctive notions of time are wrong. Time is not a universal measurement. Instead, the speed of an observer relative to an event affects the amount of time that seems to pass.

We humans usually aren’t traveling quickly compared to events we’re observing, so these effects are undetectable to us. But if you flew by my window at close to the speed of light (about 670 million miles per hour), you would see me make chocolate milk in slow motion.

Relativity means we can create clocks that measure time between events that occur on and near Earth, but those clocks are based on us, not on fundamental properties of the universe.

I realized after writing this post that I had scheduled it for the day most Americans switch to Daylight Saving Time. The theory of relativity fails to address the discontinuity of spacetime we experience in the one minute between 1:59 am and 3:00 am on the 2nd Sunday in March.

Evelyn Lamb About the Author: Evelyn Lamb is a postdoc at the University of Utah. She writes about mathematics and other cool stuff. Follow on Twitter @evelynjlamb.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. AllanRBrewer 1:04 pm 03/10/2013

    “But if you flew by my window at close to the speed of light would see me make chocolate milk in slow motion.”

    Wouldn’t I see you make chocolate milk in fast forward motion as I would be aging slower relative to you – if I then returned to your window, more time would have passed for you than me?

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  2. 2. Zephir_AWT 6:38 pm 03/10/2013

    /*In some ways, time is a direction*/

    This is correct insight and I’d take it even more consequentially. In dense aether model the hyperdimensional space-time foam can be described with 3D analogy of the water surface: after then the spatial dimensions are the directions parallel with this surface, whereas the time dimension is the remaining direction perpendicular to it.

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  3. 3. mmillerpsyd 11:03 pm 03/10/2013

    To thirsty 11 year old the combination of milk and chocolate into “chocolate milk” represents less entropy than it’s separate component parts. GULP!

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  4. 4. jtdwyer 7:11 am 03/11/2013

    Aside from any relativistic effects, I think that if someone flew by a window at near the speed of light, they’d only see a stationary scene for a very brief instant, before they were gone – they’d only have a very tiny fraction of a second to observe anything inside the window…

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  5. 5. jtdwyer 7:38 am 03/11/2013

    FYI – see
    Also, in certain special conditions, laminar flows in high viscosity fluids can be effectively reversed – see

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  6. 6. jtdwyer 7:44 am 03/11/2013

    Also, note that their is no reverse momentum – the direction of motion is actually always forward…

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  7. 7. Evelyn Lamb in reply to Evelyn Lamb 2:52 pm 03/11/2013

    AllanRBrewer: There are of course many practical problems with flying by a window at close to the speed of light, but the observer would think that the observed was moving more slowly than the observed thought she was. This works both ways: I would think the person flying by was flying by more slowly than she was, and she would think I was making chocolate milk more slowly than I was. But of course, she wouldn’t be next to the window long enough to see much of anything, and it would appear spatially distorted as well.
    I’m not a physicist, and my understanding of relativity is not as nuanced as a physicist’s is, of course, but this is my understanding of relativity. I’ve never gotten a chance to test it myself. ;)

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  8. 8. AllanRBrewer 7:31 am 03/12/2013

    Evelyn: Well I find that difficult to reconcile with the fact that when an observer is travelling at relativistic speeds, she will experience less time passing than a stationary observer. If a space traveller travels away from earth at speed for 20 years and then returns, she will find life on earth has progressed a lot more than 20 years, so watching the person on earth making hot chocolate from the fast moving spacecraft would look like fast forward rather than slowmo???

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  9. 9. Evelyn Lamb in reply to Evelyn Lamb 10:23 am 03/12/2013

    Like I said, I’m not a relativity expert, but I think you’re referencing the “twin paradox,” where one twin travels away from earth for a certain number of years and then back, returning to find that her twin has aged more than she has. My understanding is that the change of directions is important in resolving the twin paradox. The wikipedia page on it is a bit difficult for non-experts to read, but it does have some good information.
    It is hard to reconcile all the weird things about relativity!

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  10. 10. EyesWideOpen 8:01 pm 03/14/2013

    Evelyn, apparently reverse entrop is theoretically possible according to James Maxwell. Perhaps you can weigh in.

    “James Clerk Maxwell — known for his famous Maxwell’s Equations of Electrodynamics — then suggested that there might be a means by which a hypothetical demon might be at the door between the two sections of the box. This demon would see a hot molecule coming from the right side and open the door to allow its passage. At the same time, any cold molecule approaching the door from the left side, the demon would again allow passage. But in the case of hot molecules coming from the left, or cold ones from the right, the demon would close the door. Maxwell did not actually envision a “demon”, but instead some unknown means by which the entropy of the system could be reversed.

    “The solution to Maxwell’s Demon was that the demon needed to know if an approaching molecule was hot or cold. In this way, physicists brought into the equation, information, which then became a form of energy in the sense that information could be used to reverse entropy. Information could also be used to slow the rate at which entropy was increasing. Practically speaking that would mean that the more efficient a device is, the slower the rate at which entropy increases.”

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  11. 11. denisosu 6:34 am 03/16/2013

    I’ve always thought that perhaps Einstein’s greatest insight was about time. While philosophers for thousands of years had debated and wondered what time was, Einstein needed to define it precisely for his relativity theories. His definition (in just 5 words) is perfect: “Time is what clocks measure.”

    In retrospect it seems obvious. But would anyone else have found that definition – without which the whole theory of special relativity would be incomprehensible??

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