Ant On A Rubber Rope

Since Wikipedia doesn&#x27;t do a good job of simply stating <i>why</i> the ant will eventually reach the end:<p>The expansion of the rope is over its entire length. When the ant is halfway across the rope, an expansion of 1 km only adds 0.5 km to the length the ant must traverse. So the farther the ant walks, the less the expansion matters, and eventually it won&#x27;t even add the 1 cm the ant is moving per second.<p>At least that&#x27;s as best I understand it.<p>Here&#x27;s a Google spreadsheet that shows the process with a significantly faster ant:<p><a href="https://docs.google.com/spreadsheet/ccc?key=0AsmKPGVX0X-0dFF2d1dVMUg2WTZWLUxzMFU2VFpfUFE&amp;usp=sharing" rel="nofollow">https:&#x2F;&#x2F;docs.google.com&#x2F;spreadsheet&#x2F;ccc?key=0AsmKPGVX0X-0dFF...</a><p>No guarantee of being free of off-by-one type of errors, but it should give the general idea.

This is a great example of why I hate &#x27;word problems&#x27; in math. The person writing the problem is probably trying to demonstrate a nice mathematical property and engage the reader, but what they are <i>doing</i> is confusing the hell out of people and providing a frustrating experience.<p>Almost every piece of information you get from this hypothetical scenario is useless. Everything you know about ants, rubber, gravity, energy etc is suddenly a distraction from the extremely narrow set of ideas which the author allows to be relevant to the problem.<p>As someone who really really likes knowing material properties, insect physiology, and basic physics, while caring little for abstract mathematical properties, this kind of problem seems perfectly designed to tweak my nose.

One way of thinking of it is in terms of how far along the rope the ant is. If the ant is 20% along the rope, and the rope stretches, the ant is still 20% of the way along a now-bigger rope. Therefore the ant never loses ground, so to speak.<p>The effect of the stretching is to reduce the ant&#x27;s speed measured in units of &quot;proportion of the rope&quot;. E.g. if the rope is 100cm and the ant is going 1 cm&#x2F;s, then the ant is covering 1% of the rope per second. If the rope doubles in length and the ant keeps the same speed, the ant is covering 0.5% of the rope per second. So the ant is decelerating in this set of units; the total distance it travels becomes a matter of computing a limit.

The harmonic series grows incredibly slowly but has no upper bound.<p>.00001&#x2F;1 + .00001&#x2F;2 + .00001&#x2F;3 ... =&gt; infinity. The ant will eventually pass 100% of the band.<p>There are lots of counter intuitive results based on the fact the harmonic series has no bound. I remember an article where someone showed you could stack dominoes to produce infinite overhang, exploiting the same counter intuitive bit of math.

For the astronomical application of this problem, if the ants (photons) are reaching the earth after crossing the expanding universe, there must be a deep space observation (I don´t know if it´s technically achievable ) that will appear as if new galaxies appear where previously none existed (was visible)?<p>Does this make sense?<p>Edit: added the question sign.

A Rubber Band is 100,000 centimeters long, from <i>POINT A</i> to <i>POINT B</i>.<p>An ant starts at one end of the rubber band (<i>POINT A</i>), and walks toward the other end (<i>POINT B</i>), at 1 centimeter per second, for an infinite period of time.<p><i>POINT B</i> moves away from <i>POINT A</i> at 100,000 centimeters per second, for an infinite period of time.<p><pre><code> POINT B travels at: 100,000 centimeters &#x2F; second The ant travels at: 1 centimeter &#x2F; second In 100,000 seconds, the ant reaches the 100,000 centimeter mark, but POINT B is now 10,000,000,000 centimeters away. Both continue to move at the same speed. </code></pre> Additional Details:<p>If an average ant lives 90 days, then the ant only has 7,776,000 seconds to reach the end of the rubber band. In this time, the ant will travel 7,776,000 centimeters, but the rubber band will have extended to 100,000^7,776,000 (that&#x27;s one hundred thousand to the roughly seven-point-seven-millionth power) centimeters long. Based on these facts, we may safely conclude that a mortal ant will never reach the end of this hypothetical rubber band.<p>Did I miss a detail somewhere?

I saw this puzzle when I was kid, in a Martin Gardner book. Back then it was an inch worm instead of an ant. When I saw the answer, it blew my mind at first. Later I understood how it was related to the fact that the harmonic series diverges, and it became less mind blowing, but it&#x27;s still a little mind blowing—because it&#x27;s still a little mind blowing that the harmonic series diverges.<p>It became one of my favorite puzzles, and I tried it on friends and family. I certainly never thought it could have &quot;real-world&quot; applications.<p>But then it occurred to me, people were solving this puzzle every day.<p>Think of an ordinary loan. Each month, you make a payment that&#x27;s part interest and part principal. At first it&#x27;s mostly interest, then the ratio shifts over time, the final payments being nearly all principal.<p>You normally start with a loan amount (principal), an interest rate, and a loan duration (typically 180 or 360 months). From these parameters, you figure the monthly payment, a process called amortization. Part of the payment goes towards interest, part towards the principal.<p>Each month you&#x27;re required to service the loan, which means to pay (at least) the interest. That&#x27;s the cost of renting the money. Some loan arrangements allow you to pay down the principal at your own pace. If you only ever pay interest, the principal will remain unchanged, and the loan will go on forever. This is the Netflix model: they don&#x27;t care how long you keep a disc—you&#x27;re paying rent on it every month. Many people pay more than the monthly payment from time to time. The principal will be reduced by this extra amount.<p>The worm has taken out a loan. The twist is, we don&#x27;t yet know the principal, nor the total amount to be paid, which corresponds to the final rope length. Instead, we know the payment: 1 yard. The interest portion varies, but the worm consistently pays down the principal 1 inch each pay period.<p>Some of the added yard (monthly payment) appears in front of the worm (interest portion) some behind it (principal portion). Stretching the rope <i>uniformly</i> has the effect of servicing the interest. At first, most of the newly added rope appears in front of the worm, but as with the ordinary loan, the back&#x2F;front ratio increases over time.<p>That the worm will eventually reach the end of the rope is now evident. If your interest payment is taken care of, then even a small monthly pay-down of the principal will eventually pay off any size loan.

This is an interesting problem, but not that interesting, really.<p>The really interesting problem is an ant walking on the surface of an expanding sphere, where the rate of the expansion of the sphere is proportional to the radius of the sphere.<p>If the ant starts at one pole, will it ever reach the other pole?<p>Yes, I&#x27;ve underspecified the problem - we want the complete set of solutions.<p>Assume initial radius r, ant of length l, ant walking at speed w, and initial rate of expansion e (remembering that de&#x2F;dt is proportional to r).<p>Under what conditions will an ant at pole P1 reach P2? Under what conditions is impossible for the ant to reach P2?<p>Presumably there is a range of values such that the ant can traverse the sphere if it starts early enough or is big enough or if w is high enough.<p>Now make things even more interesting: Assume that there is a maximum speed M, and that the closer w or e gets to M, the harder it is to get incrementally close to M.<p>This is the expanding universe. We are the ant.<p>If we had developed high speed interstellar travel &quot;early enough&quot;, would we ever have been able to cross the universe? Or is it always too late?<p>Now that&#x27;s an interesting problem. Solution left as an exercise for the reader.

The moment the ant takes even one step, the far end will no longer be move away from the ant at 1 km&#x2F;s, so it will be able to move fast enough to catch up and cross the rope.<p>After one step, the ant will be somewhere in the middle, and both ends will be moving away from it. (Because when the rope stretches, every point on the rope moves away from every other point on the rope.) If the far end is moving one direction, and the near end is moving the other direction, and the two ends are moving away from each other at 1 km&#x2F;s, then neither of them can be moving away from the ant at 1 km&#x2F;s. The ant will then be able to catch up with either end.

The end of the rope is travelling at constant velocity but the ant is always accelerating, therefore will eventually catch up and overtake the end.

Is the starting point of the rope fixed and the end being pulled away from it or is the center of the rope fixed and the end points are being pulled away from each other?<p>Other fun questions: does the rope stretch uniformly or are there waves? Do the ants fore and rear legs get separated as the rope under it stretches? Is there any sag in the rope?

Sounds like a vari-ant of Zeno&#x27;s Dichotomy Paradox.[0]<p>[0] <a href="http://en.wikipedia.org/wiki/Zeno&#x27;s_paradoxes" rel="nofollow">http:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Zeno&#x27;s_paradoxes</a>