Walking with my son at 3:14pm the other day, I mentioned to him, "Hey, it's Pi Time". My son knows 35 digits of $\pi$ (don't ask), and knows that it's transcendental. He replied, "is it exactly $\pi$ time?"

This led to a discussion about whether there is ever a time each afternoon that is exactly $\pi$, meaning 3:14:15.926535...

This feels like some kind of Zeno's Paradox. I told him that (assuming time is continuous) it had to be $\pi$ time at some point between 3:14:00 and 3:15:00, but the length of that moment was 0. However, this discussion left him confused.

Can anyone suggest a good way to explain this to a child?

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    $\begingroup$ It's not really "Zeno's paradox"... More like taking photographs of someone walking along a river. If he is on one side of the river in the first series of photographs, and later in the series he is on the other side of the river, we deduce that somewhere along the line he must have crossed the river. 3:14:00 is a time on "one side of" $\pi$, and 3:15:00 is a time on "the other side of" $\pi$. Or try it with a graph: if the value is negative at $3$ and positive at $4$, and you have to draw the graph without lifting the pencil, somewhere along the line you went through the axis. $\endgroup$ Sep 9 '11 at 21:11
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    $\begingroup$ Of course, if time is quantized, then the answer is that it is never exactly $\pi$ time, since $\pi$ is incommeasurable with $1$ second, and whatever length a quantum of time might be, it is commeasurable with $1$ second... $\endgroup$ Sep 9 '11 at 21:12
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    $\begingroup$ Every number has an infinite decimal representation (just append 0's forever). So, is it ever exactly "1 time" or "2 time"? (I think "Yes", as per Arturo's first argument, but I just wanted to point out that $\pi$ is not special in this regard.) $\endgroup$ Sep 9 '11 at 21:18
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    $\begingroup$ @Arturo: I just looked it up: a second is 9,192,631,770 times the period of the radiation emitted by the transition between the two hyperfine levels of the ground state of the caesium 133 atom. So it looks like you're right! $\endgroup$
    – TonyK
    Sep 9 '11 at 21:40
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    $\begingroup$ And when is hammertime? $\endgroup$ Sep 9 '11 at 21:55

Pi time is not like a concrete slab on the sidewalk that you can stand on; it's not even like the crack between the slabs, which have a width. It is like the line precisely down the middle of that crack. When you're walking on the sidewalk, you cross right over it without stopping on it.

And so is every other precise time: like exactly noon or midnight.

Of course, the analogy fails a little bit because when you stop on the sidewalk, you cover a whole range of positions. As far as we can tell in everyday life, that's not true of time... not that we can stop in time, anyway...

That is how I would explain it to a non-mathematician.

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    $\begingroup$ And "pi time" fails even more because it takes the decimal expansion of $\pi$ and shoehorns the digits unchanged into a mixed-base system (twelve, sixty, sixty, ten, ten, ten, ...) $\endgroup$ Sep 9 '11 at 21:49
  • $\begingroup$ Yep, though it's all sort of a pretense unless you're standing precisely some integer multiple of 30 degrees east or west of Greenwich, and forget to observe daylight savings time. $\endgroup$ Sep 9 '11 at 23:02
  • $\begingroup$ Sorry... so what is the answer? I asked a similar question on physics.se about if any object is "exactly" 1 meter long, and the answers were somewhat circular (or trivial, e.g. "a meter is defined to be the length of a certain object, so yes, there is one object that is one meter long") $\endgroup$ Oct 14 '11 at 3:10
  • $\begingroup$ Assuming that time is continuous, the answer is "yes". But it is never pi time for long enough for you to notice or say that it is; it passes away after an instant. Pi time is not a time that you are ever practically at, but a point that you pass. Same thing for distances. $\endgroup$ Oct 14 '11 at 12:14

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