Reading the Comics, May 3, 2016: Lots Of Images Edition

After the heavy pace of March and April I figure to take it easy and settle to about a three-a-week schedule around here. That doesn’t mean that Comic Strip Master Command wants things to be too slow for me. And this time they gave me more comics than usual that have expiring URLs. I don’t think I’ve had this many pictures to include in a long while.

Bill Whitehead’s Free Range for the 28th presents an equation-solving nightmare. From my experience, this would be … a great pain, yes. But it wouldn’t be a career-wrecking mess. Typically a problem that’s hard to solve is hard because you have no idea what to do. Given an expression, you’re allowed to do anything that doesn’t change its truth value. And many approaches might look promising without quite resolving to something useful. The real breakthrough is working out what approach should be used. For an astrophysics problem, there are some classes of key decisions to make. One class is what to include and what to omit in the model. Another class is what to approximate — and how — versus what to treat exactly. Another class is what sorts of substitutions and transformations turn the original expression into one that reveals what you want. Those are the hard parts, and those are unlikely to have been forgotten. Applying those may be tedious, and I don’t doubt it would be anguishing to have the finished work wiped out. But it wouldn’t set one back years either. It would just hurt.

Christopher Grady’s Lunar Babboon for the 29th I classify as the “anthropomorphic numerals” joke for this essay. Boy, have we all been there.

Bill Holbrook’s On The Fastrack for the 29th of April, 2016. Spoiler: there aren’t any numbers in the second panel.

Bill Holbrook’s On The Fastrack for the 29th continues the storyline about Fi giving her STEM talk. She is right, as I see it, in attributing drama and narrative to numbers. This is most easily seen in the sorts of finance and accounting mathematics which the character does. And the inevitable answer to “numbers are boring” (or “mathematics is boring”) is surely to show how they are about people. Even abstract mathematics is about things (some) people find interesting, and that must be about the people too.

Rick Detorie’s One Big Happy for the 3rd of May, 2016. Ever notice how many shirt pockets Grandpa has? I’m not saying it’s unrealistic, just that it’s more than the average.

Rick Detorie’s One Big Happy for the 16th is a confused-mathematics joke. Grandpa tosses off a New Math joke that’s reasonably age-appropriate too, which is always nice to see in a comic strip. I don’t know how seriously to take Ruthie’s assertion that a 100% means she only got at least half of the questions correct. It could be a cartoonist grumbling about how kids these days never learn anything, the same way ever past generation of cartoonists had complained. But Ruthie is also the sort of perpetually-confused, perpetually-confusing character who would get the implications of a 100% on a test wrong. Or would state them weirdly, since yes, a 100% does imply getting at least half the test’s questions right.

Niklas Eriksson’s Carpe Diem for the 3rd of May, 2016. I’m a little unnerved there seems to be a multiplication x at the end of the square root vinculum on the third line there.

Niklas Eriksson’s Carpe Diem for the 3rd uses the traditional board full of mathematical symbols as signifier of intelligence. There’s some interesting mixes of symbols here. The c2, for example, isn’t wrong for mathematics. But it does evoke Einstein and physics. There’s the curious mix of the symbol π and the approximation 3.14. But then I’m not sure how we would get from any of this to a proposition like “whether we can survive without people”.

Bud Blake’s Tiger for the 3rd of May, 2016. How did Punkinhead get up to eleven?

Bud Blake’s Tiger for the 3rd is a cute little kids-learning-to-count thing. I suppose it doesn’t really need to be here. But Punkinhead looks so cute wearing his tie dangling down onto the floor, the way kids wear their ties these days.

Tony Murphy’s It’s All About You for the 3rd name-drops algebra. I think what the author really wanted here was arithmetic, if the goal is to figure out the right time based on four clocks. They seem to be trying to do a simple arithmetic mean of the time on the four clocks, which is fair if we make some assumptions about how clocks drift away from the correct time. Mostly those assumptions are that the clocks all started right and are equally likely to drift backwards or forwards, and do that drifting at the same rate. If some clocks are more reliable than others, then, their claimed time should get more weight than the others. And something like that must be at work here. The mean of 7:56, 8:02, 8:07, and 8:13, uncorrected, is 8:04 and thirty seconds. That’s not close enough to 8:03 “and five-eighths” unless someone’s been calculating wrong, or supposing that 8:02 is more probably right than 8:13 is.

How, Arguably, Very Slightly Less Well April 2016 Treated My Mathematics Blog

So now to my review of readership statistics. I’d expected another strong month. If I’ve learned anything it’s that posting a lot of stuff regularly encourages readers. I got to have another month with more than 1,000 readers here. In fact, there were a neat 1,500 page views, according to WordPress. This is a bit lower than March’s 1,557 page views. But remember that March had one more day than April did, and so had one more article. April had an average of fifty page views per post. March had 50.226. That’s no appreciable difference, I figure. February had 949 page views, although with only 14 articles. (And so about 68 page views per article posted, somehow.)

The number of unique visitors, as WordPress makes them out, was up though. April saw 757 visitors, a record around these parts. March only had 734, and February a relatively skimpy 538.

The measurements that seem to reflect reader engagement were ambiguous as ever. The number of likes was 345, technically up from March’s 320, and well above February’s 201. The number of comments, though, was 55, plummeting from March’s 84 and February’s 66. Part of that is I didn’t have any good controversies like the Continued Fractions post this month. But writing articles that encourage conversations, especially conversations between commenters (it can’t all be me chatting with individuals), has never been a strength of mine and I do need to ponder ways to improve that.

Proud as I am of the A To Z series, I must face the facts: none of the essays was in my top five most-read articles for April. One does sneak in at sixth place so I’ll list the top six articles instead. I’m going to suppose that the series pretty much balances out. That is, few of the articles have reason to read that one instead of another post. What are most popular are Reading the Comics posts, my trapezoids thing, and a couple of pointers to other people’s writing. Well, we can’t all be stars; someone has to be the starmaker. Most read in April:

There’s not any interesting search terms this month. Well, all right, there’s “what is an inversly [sic] propotional [sic] dice”. But I don’t know what the searcher was looking for there. I got the traditional appearance of “origin is the gateway to your entire gaming universe.” And I got asked “what makes a basketball tournament exciting?” I don’t know, but I was able to give at least a non-perfectly-ridiculous measure of how interesting one might be.

And for the always-popular listing of countries? As is usual for some reason, the United States sent me the greatest number of page views: 863. India was second at 80, and Canada third at 61. Austria was next at 45, and the United Kingdom and Germany tied for 42.

Single-reader countries were Belarus, Botswana, China, Dominican Republic, European Union, Greece, Guatemala, Hungary, Kuwait, Poland, Portugal, Qatar, Réunion, Serbia, South Korea, and Switzerland. Again, European Union. I’ve said that before. China, European Union, and Greece were there last month too. The European Union is somehow on a five-month single-reader streak. At this point I have to think whoever is doing it is doing so on purpose and for a bit of a giggle.

The month begins with 36,256 page views total, from 14,273 recorded visitors. I’ve reportedly got 579 WordPress readers, up from the 573 at the start of April, despite putting the Follow This Blog icon in a more prominent location. Well, there were some nice stretches of people following each of several days in a row and that’s something. It also lists eleven followers by e-mail, up from ten last month. Again, it’s all something.

The Poincaré Homology Sphere, and Thinking What I’ll Do Next

Yenergy was good enough to write a comment about this, but people might have missed it. “Dodecahedral construction of the Poincaré homology sphere, part II” is up. The post is an illustration trying to describe several pages of the 1979 paper Eight Faces Of The Poincaré Homology 3-Sphere by R C Kirby and M G Sharlemann.

I admit I have to read it almost the same way a non-mathematician would. My education never took me into topology deep enough to be fluent in the notation or the working assumptions behind the paper. I may work my way farther than a non-mathematician, since I’ve been exposed to some of the symbols. The grammar of the argument is familiar. And many points of it are common to fields I did study. Nevertheless, even if you just skim the text, skipping over anything that seems too hard to follow, and look at the illustrations you’ll get something from it.

Past that, I wanted to thank everyone for seeing me into the start of May. I am figuring to give up the post-a-day schedule. It’s exciting to have three thousand-word and four posts of more variable lengths each week, but I need to relax that schedule some. I am considering, based on the conversation I got into with Elke Stangl about the Yukawa Potential, whether to do a string of essays about closed orbits. That would almost surely involve many more equations than is normal around here. But it could make for a nice change of pace.

• howardat58 3:29 pm on Sunday, 1 May, 2016 Permalink | Reply

The more years spent studying maths the happier one becomes as one realises that on at least the first reading one can skip ALL the “math” and jst read the text. If that doesn’t make any sense the symbols will only make things worse!

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• Joseph Nebus 3:48 pm on Wednesday, 4 May, 2016 Permalink | Reply

That is the piece that took me longest to learn, really. I think the bad habit comes from textbooks where, so often, the real work is done in a string of equations and the text surrounding it is meaningless or at least uninsightful. (“Now we move x to the left side” … thanks, saw that, but why ‘x’ and why the left side?)

Reading the text, at least when it explains what the thinking is, gets the lay of the land. Then the details have something to cling to.

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Reading the Comics, April 27, 2016: Closing The Month (April) Out Edition

I concede this isn’t a set of mathematically-themed comics that inspires deep discussions. That’s all right. It’s got three that I can give pictures for, which is important. Also it means I can wrap up April with another essay. This gives me two months in a row of posting something every day, and I’d have bet that couldn’t happen.

Ted Shearer’s Quincy for the 1st of March, 1977, rerun the 25th of April, is not actually a “mathematics is useless in the real world” comic strip. It’s more about the uselessness of any school stuff in the face of problems like the neighborhood bully. Arithmetic just fits on the blackboard efficiently. There’s some sadness in the setting. There’s also some lovely artwork, though, and it’s worth noticing it. The lines are nice and expressive, and the greyscale wash well-placed. It’s good to look at.

Ted Shearer’s Quincy for the 1st of March, 1977, rerun the 25th of April, 2016. I just noticed ‘Miss Reid’ is probably a funny-character-name.

dro-mo for the 26th I admit I’m not sure what exactly is going on. I suppose it’s a contest to describe the most interesting geometric shape. I believe the fourth panel is meant to be a representation of the tesseract, the four-dimensional analog of the cube. This causes me to realize I don’t remember any illustrations of a five-dimensional hypercube. Wikipedia has a couple, but they’re a bit disappointing. They look like the four-dimensional cube with some more lines. Maybe it has some more flattering angles somewhere.

Bill Amend’s FoxTrot for the 26th (a rerun from the 3rd of May, 2005) poses a legitimate geometry problem. Amend likes to do this. It was one of the things that first attracted me to the comic strip, actually, that his mathematics or physics or computer science jokes were correct. “Determine the sum of the interior angles for an N-sided polygon” makes sense. The commenters at Gocomics.com are quick to say what the sum is. If there are N sides, the interior angles sum up to (N – 2) times 180 degrees. I believe the commenters misread the question. “Determine”, to me, implies explaining why the sum is given by that formula. That’s a more interesting question and I think still reasonable for a freshman in high school. I would do it by way of triangles.

David L Hoyt and Jeff Knurek’s Jumble for the 27th of April, 2016. I bet the link’s already expired by the time you read this.

David L Hoyt and Jeff Knurek’s Jumble for the 27th of April gives us another arithmetic puzzle. As often happens, you can solve the surprise-answer by looking hard at the cartoon and picking up the clues from there. And it gives us an anthropomorphic-numerals gag for this collection.

Bill Holbrook’s On The Fastrack for the 28th of April has the misanthropic Fi explain some of the glories of numbers. As she says, they can be reliable, consistent partners. If you have learned something about ‘6’, then it not only is true, it must be true, at least if we are using ‘6’ to mean the same thing. This is the sort of thing that transcends ordinary knowledge and that’s so wonderful about mathematics.

Bill Holbrook’s On The Fastrack for the 28th of April, 2016. I don’t know why the ‘y’ would be in kind-of-cursive while the ‘x’ isn’t, but you do see this sort of thing a fair bit in normal mathematics.

Fi describes ‘x’ and ‘y’ as “shifty little goobers”, which is a bit unfair. ‘x’ and ‘y’ are names we give to numbers when we don’t yet know what values they have, or when we don’t care what they have. We’ve settled on those names mostly in imitation of Réné Descartes. Trying to do without names is a mess. You can do it, but it’s rather like novels in which none of the characters has a name. The most skilled writers can carry that off. The rest of us make a horrid mess. So we give placeholder names. Before ‘x’ and ‘y’ mathematicians would use names like ‘the thing’ (well, ‘re’) or ‘the heap’. Anything that the quantity we talk about might measure. It’s done better that way.

A Leap Day 2016 Mathematics A To Z: The Roundup

And with the conclusion of the alphabet I move now into posting about each of the counting numbers. … No, wait, that’s already being done. But I should gather together the A To Z posts in order that it’s easier to find them later on.

I mean to put together some thoughts about this A To Z. I haven’t had time yet. I can say that it’s been a lot of fun to write, even if after the first two weeks I was never as far ahead of deadline as I hoped to be. I do expect to run another one of these, although I don’t know when that will be. After I’ve had some chance to recuperate, though. It’s fun going two months without missing a day’s posting on my mathematics blog. But it’s also work and who wants that?

What’s Needed To Pass This Class (Spring 2016 Edition)

I had thought we were barely entering Final Exam season. But I hear reports many (United States) colleges and universities have already got them started. And I see what people are searching for around my writing here. So let me help folks out here.

Here, under “What Do I Need To Pass This Class”, besides start worrying about grades sooner, is an explanation of how to calculate exactly what score one needs. It allows for the exam to have any weighting possible, and for extra credit points to be considered, to get any desired score. And then since not everyone looking for their grade is actually a mathematics student interested in following equations that, I believe, are well-explained, I put together some tables with the results for some common final exam weightings and target scores. The tables include what scores are needed for finals that are one-fifth, one-quarter, one-third, and two-fifths the total course grade.

Good luck. Read the syllabus and any test preparation sheets the instructor gives. Get a full night’s sleep before and eat well the day of the exam. Don’t pester with e-mails asking for extra credit. Only bother your professor with requests to correct errors of fact, which would be recorded grades or an error in calculation. Have your returned assignment to show, and understand how weighted grades work, before you do.

• Candace 11:33 pm on Thursday, 28 April, 2016 Permalink | Reply

Great info. Hopefully, everyone will do well.

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• Joseph Nebus 7:02 pm on Friday, 29 April, 2016 Permalink | Reply

I would like everyone to do well. Failing that, I’d like them to get their abilities well-measured so they know what they’ve mastered and what they need to work on.

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A Leap Day 2016 Mathematics A To Z: Z-score

And we come to the last of the Leap Day 2016 Mathematics A To Z series! Z is a richer letter than x or y, but it’s still not so rich as you might expect. This is why I’m using a term that everybody figured I’d use the last time around, when I went with z-transforms instead.

Z-Score

You get an exam back. You get an 83. Did you do well?

Hard to say. It depends on so much. If you expected to barely pass and maybe get as high as a 70, then you’ve done well. If you took the Preliminary SAT, with a composite score that ranges from 60 to 240, an 83 is catastrophic. If the instructor gave an easy test, you maybe scored right in the middle of the pack. If the instructor sees tests as a way to weed out the undeserving, you maybe had the best score in the class. It’s impossible to say whether you did well without context.

The z-score is a way to provide that context. It draws that context by comparing a single score to all the other values. And underlying that comparison is the assumption that whatever it is we’re measuring fits a pattern. Usually it does. The pattern we suppose stuff we measure will fit is the Normal Distribution. Sometimes it’s called the Standard Distribution. Sometimes it’s called the Standard Normal Distribution, so that you know we mean business. Sometimes it’s called the Gaussian Distribution. I wouldn’t rule out someone writing the Gaussian Normal Distribution. It’s also called the bell curve distribution. As the names suggest by throwing around “normal” and “standard” so much, it shows up everywhere.

A normal distribution means that whatever it is we’re measuring follows some rules. One is that there’s a well-defined arithmetic mean of all the possible results. And that arithmetic mean is the most common value to turn up. That’s called the mode. Also, this arithmetic mean, and mode, is also the median value. There’s as many data points less than it as there are greater than it. Most of the data values are pretty close to the mean/mode/median value. There’s some more as you get farther from this mean. But the number of data values far away from it are pretty tiny. You can, in principle, get a value that’s way far away from the mean, but it’s unlikely.

We call this standard because it might as well be. Measure anything that varies at all. Draw a chart with the horizontal axis all the values you could measure. The vertical axis is how many times each of those values comes up. It’ll be a standard distribution uncannily often. The standard distribution appears when the thing we measure satisfies some quite common conditions. Almost everything satisfies them, or nearly satisfies them. So we see bell curves so often when we plot how frequently data points come up. It’s easy to forget that not everything is a bell curve.

The normal distribution has a mean, and median, and mode, of 0. It’s tidy that way. And it has a standard deviation of exactly 1. The standard deviation is a way of measuring how spread out the bell curve is. About 95 percent of all observed results are less than two standard deviations away from the mean. About 99 percent of all observed results are less than three standard deviations away. 99.9997 percent of all observed results are less than six standard deviations away. That last might sound familiar to those who’ve worked in manufacturing. At least it des once you know that the Greek letter sigma is the common shorthand for a standard deviation. “Six Sigma” is a quality-control approach. It’s meant to make sure one understands all the factors that influence a product and controls them. This is so the product falls outside the design specifications only 0.0003 percent of the time.

This is the normal distribution. It has a standard deviation of 1 and a mean of 0, by definition. And then people using statistics go and muddle the definition. It is always so, with the stuff people actually use. Forgive them. It doesn’t really change the shape of the curve if we scale it, so that the standard deviation is, say, two, or ten, or π, or any positive number. It just changes where the tick marks are on the x-axis of our plot. And it doesn’t really change the shape of the curve if we translate it, adding (or subtracting) some number to it. That makes the mean, oh, 80. Or -15. Or eπ. Or some other number. That just changes what value we write underneath the tick marks on the plot’s x-axis. We can find a scaling and translation of the normal distribution that fits whatever data we’re observing.

When we find the z-score for a particular data point we’re undoing this translation and scaling. We figure out what number on the standard distribution maps onto the original data set’s value. About two-thirds of all data points are going to have z-scores between -1 and 1. About nineteen out of twenty will have z-scores between -2 and 2. About 99 out of 100 will have z-scores between -3 and 3. If we don’t see this, and we have a lot of data points, then that’s suggests our data isn’t normally distributed.

I don’t know why the letter ‘z’ is used for this instead of, say, ‘y’ or ‘w’ or something else. ‘x’ is out, I imagine, because we use that for the original data. And ‘y’ is a natural pick for a second measured variable. z’, I expect, is just far enough from ‘x’ it isn’t needed for some more urgent duty, while being close enough to ‘x’ to suggest it’s some measured thing.

The z-score gives us a way to compare how interesting or unusual scores are. If the exam on which we got an 83 has a mean of, say, 74, and a standard deviation of 5, then we can say this 83 is a pretty solid score. If it has a mean of 78 and a standard deviation of 10, then the score is better-than-average but not exceptional. If the exam has a mean of 70 and a standard deviation of 4, then the score is fantastic. We get to meaningfully compare scores from the measurements of different things. And so it’s one of the tools with which statisticians build their work.

Reading the Comics, April 24, 2016: Mental Mathematics and Calendars Edition

Warning! I do some showing off in this installment of the Reading the Comics series. Please forgive me. I was feeling a little giddy.

Scott Hilburn’s The Argyle Sweater I had just mentioned to a friend never seems to show up in these columns anymore. And Hilburn would so reliably do strips about anthropomorphized numerals. He returns on the 20th, after a hiatus of some length I haven’t actually checked here, with a name-drop of Einstein instead. I grinned, although a good part of what amused me was the look of the guy in the lower right of the panel. Funny pictures carry a comic strip far. Formulating the theory of relativity is a tricky request. The special theory … well, to do it properly takes some sophisticated work. But it doesn’t take much beyond the Pythagorean Theorem to realize that “how long” a thing is, or a time span is, is different for different observers. That’s the most important insight, I would say, and that is easily available. General relativity, which looks at accelerations and gravity, that’s another thing. I’d be interested in a popular treatment that explained enough mathematics people could make usable estimates but that could still make sense to a lay audience. Probably it’s not possible to do this. Too bad.

Mark Tatulli’s Heart of the City just uses arithmetic because it’s a nice compact problem to give a student. It did strike me that 117 times 45 is something one could amaze people with by doing in one’s head, though. Here’s why. 117 times 100 would be easy. Multiplying by hundreds always is. 117 times 50 would be not almost as easy: that’s multiplying by 100 and dividing by two. 117 times 45 … well, that’s 117 times 50 minus 117 times 5. And if you know 117 times 50, then you know 117 times 5: it’s one-tenth that. And one-tenth of a thing is easy to find.

Therefore: 117 times 100 is 11,700. Divide that by two and that’s kind of an ugly-looking number, isn’t it? But all’s not lost. Let me use another bit of falsework: 11,700 is 12,000 minus 300. Half that is 6,000 minus 150. Therefore, half of 11,700 is 5,850. So 117 times 50 is 5,850. One-tenth of that is 585. Therefore, 117 times 45 is 5,850 minus 585. And that will be … 5,275. Ta-da!

Well, no, it isn’t. It’s 5,265. I messed up the carrying. I still think that’s doing well for multiplying ugly numbers like that without writing it down. It just won’t impress people who want the actual you know correct answer.

Mark Anderson’s Andertoons wouldn’t let me down by vanishing for a while. The 21st is not explicitly a strip about extrapolating graphs. I’ll take it as such, though. Once again the art amuses me. I like the crash-up of charted bars. Yes, I saw the Schrödinger’s Cat thing two days later.

Jef Mallett’s Frazz for the 23rd I drag into a mathematics blog because of the long historical links between calendars and mathematics. But Caulfield does talk about something that’s baffled everyone. There’s seven days to the week. There’s seven classically known heavenly bodies in the solar system, besides the Earth. Naming a day for each seems obvious now that we’ve committed to it. But why aren’t the bodies honored in order?

Geocentrism seems like, at first, a plausible reason. The ancients wouldn’t order the sky Sun-Mercury-Venus-Moon-Mars-Jupiter-Saturn. But that doesn’t help. Geocentric models of the solar system (always, so far as I’m aware) put the Moon closest, then Mercury, then Venus, the Sun, Mars, Jupiter, and Saturn.

The answer that, at least, gets repeated in histories of the calendar (for example, here, David Ewing Duncan’s The Calendar: The 5000-Year Struggle To Align The Clock And The Heavens — And What Happened To The Missing Ten Days, which was the first book I had on hand) amounts to a modular arithmetic thing. The Babylonians, if Duncan is right, named a planet-god for each hour of the day. (We treat the Moon and Sun as planets for this discussion.) The planet-gods took their hourly turn in order. If the first hour of the day is Saturn’s to rule, the next is Jupiter’s, then Mars’s, the Sun’s, Venus’s, Mercury’s, and the Moon’s. Then back to Saturn and the system keeps going like that.

So if the first hour of the day is Saturn’s, then who has the first hour of the next day? … the Sun does. If the Sun has the first hour of the day, then who has the first hour of the day after that? … the Moon. And from here you know the pattern. At least you do if you understand that English derives most of its day names from the Norse gods, matched as best they can with those of the Roman State Religion. So, Tiw matches with Mars; Woden with Mercury; Thor with Jupiter; Freya with Venus. The apparently scrambled order of days, relative to the positions of the planets, amounts to what you get if you keep adding 24 to a number by modulo 7 arithmetic.

That is, at least, the generally agreed-upon explanation. I am not aware of what actual researchers of Babylonian culture believe. Duncan, I must admit, takes a hit in his credibility by saying on the page after this that “recently chronobiologists have discovered that the seven-day cycle … may also have biological precedents”. I’m sorry but I just don’t believe him, or whoever he got that from.

Kevin Fagan’s Drabble for the 24th amuses me by illustrating the common phenomenon. We have all taken out the calculator (or computer) to do some calculation that really doesn’t need it. I understand and am sympathetic. It’s so obviously useful to let the calculator work out 117 times 45 and get it right instantly. It’s easy to forget sometimes it’s faster to not bother with the calculator. We are all of us a little ridiculous.

A Leap Day 2016 Mathematics A To Z: Yukawa Potential

Yeah, ‘Y’ is a lousy letter in the Mathematics Glossary. I have a half-dozen mathematics books on the shelf by my computer. Some is semi-popular stuff like Richard Courant and Herbert Robbins’s What Is Mathematics? (the Ian Stewart revision). Some is fairly technical stuff, by which I mean Hidetoshi Nishimori’s Statistical Physics of Spin Glasses and Information Processing. There’s just no ‘Y’ terms in any of them worth anything. But I can rope something into the field. For example …

Yukawa Potential

When you as a physics undergraduate first take mechanics it’s mostly about very simple objects doing things according to one rule. The objects are usually these indivisible chunks. They’re either perfectly solid or they’re points, too tiny to have a surface area or volume that might mess things up. We draw them as circles or as blocks because they’re too hard to see on the paper or board otherwise. We spend a little time describing how they fall in a room. This lends itself to demonstrations in which the instructor drops a rubber ball. Then we go on to a mass on a spring hanging from the ceiling. Then to a mass on a spring hanging to another mass.

Then we go onto two things sliding on a surface and colliding, which would really lend itself to bouncing pool balls against one another. Instead we use smaller solid balls. Sometimes those “Newton’s Cradle” things with the five balls that dangle from wires and just barely touch each other. They give a good reason to start talking about vectors. I mean positional vectors, the ones that say “stuff moving this much in this direction”. Normal vectors, that is. Then we get into stars and planets and moons attracting each other by gravity. And then we get into the stuff that really needs calculus. The earlier stuff is helped by it, yes. It’s just by this point we can’t do without.

The “things colliding” and “balls dropped in a room” are the odd cases in this. Most of the interesting stuff in an introduction to mechanics course is about things attracting, or repelling, other things. And, particularly, they’re particles that interact by “central forces”. Their attraction or repulsion is along the line that connects the two particles. (Impossible for a force to do otherwise? Just wait until Intro to Mechanics II, when magnetism gets in the game. After that, somewhere in a fluid dynamics course, you’ll see how a vortex interacts with another vortex.) The potential energies for these all vary with distance between the points.

Yeah, they also depend on the mass, or charge, or some kind of strength-constant for the points. They also depend on some universal constant for the strength of the interacting force. But those are, well, constant. If you move the particles closer together or farther apart the potential changes just by how much you moved them, nothing else.

Particles hooked together by a spring have a potential that looks like $\frac{1}{2}k r^2$. Here ‘r’ is how far the particles are from each other. ‘k’ is the spring constant; it’s just how strong the spring is. The one-half makes some other stuff neater. It doesn’t do anything much for us here. A particle attracted by another gravitationally has a potential that looks like $-G M \frac{1}{r}$. Again ‘r’ is how far the particles are from each other. ‘G’ is the gravitational constant of the universe. ‘M’ is the mass of the other particle. (The particle’s own mass doesn’t enter into it.) The electric potential looks like the gravitational potential but we have different symbols for stuff besides the $\frac{1}{r}$ bit.

The spring potential and the gravitational/electric potential have an interesting property. You can have “closed orbits” with a pair of them. You can set a particle orbiting another and, with time, get back to exactly the original positions and velocities. (Three or more particles you’re not guaranteed of anything.) The curious thing is this doesn’t always happen for potentials that look like “something or other times r to a power”. In fact, it never happens, except for the spring potential, the gravitational/electric potential, and — peculiarly — for the potential $k r^7$. ‘k’ doesn’t mean anything there, and we don’t put a one-seventh or anything out front for convenience, because nobody knows anything that needs anything like that, ever. We can have stable orbits, ones that stay within a minimum and a maximum radius, for a potential $k r^n$ whenever n is larger than -2, at least. And that’s it, for potentials that are nothing but r-to-a-power.

Ah, but does the potential have to be r-to-a-power? And here we see Dr Hideki Yukawa’s potential energy. Like these springs and gravitational/electric potentials, it varies only with the distance between particles. Its strength isn’t just the radius to a power, though. It uses a more complicated expression:

$-K \frac{e^{-br}}{r}$

Here ‘K’ is a scaling constant for the strength of the whole force. It’s the kind of thing we have ‘G M’ for in the gravitational potential, or ‘k’ in the spring potential. The ‘b’ is a second kind of scaling. And that a kind of range. A range of what? It’ll help to look at this potential rewritten a little. It’s the same as $-\left(K \frac{1}{r}\right) \cdot \left(e^{-br}\right)$. That’s the gravitational/electric potential, times e-br. That’s a number that will be very large as r is small, but will drop to zero surprisingly quickly as r gets larger. How quickly will depend on b. The larger a number b is, the faster this drops to zero. The smaller a number b is, the slower this drops to zero. And if b is equal to zero, then e-br is equal to 1, and we have the gravitational/electric potential all over again.

Yukawa introduced this potential to physics in the 1930s. He was trying to model the forces which keep an atom’s nucleus together. It represents the potential we expect from particles that attract one another by exchanging some particles with a rest mass. This rest mass is hidden within that number ‘b’ there. If the rest mass is zero, the particles are exchanging something like light, and that’s just what we expect for the electric potential. For the gravitational potential … um. It’s complicated. It’s one of the reasons why we expect that gravitons, if they exist, have zero rest mass. But we don’t know that gravitons exist. We have a lot of trouble making theoretical gravitons and quantum mechanics work together. I’d rather be skeptical of the things until we need them.

Still, the Yukawa potential is an interesting mathematical creature even if we ignore its important role in modern physics. When I took my Introduction to Mechanics final one of the exam problems was deriving the equivalent of Kepler’s Laws of Motion for the Yukawa Potential. I thought then it was a brilliant problem. I still do. It struck me while writing this that I don’t remember whether it allows for closed orbits, except when b is zero. I’m a bit afraid to try to work out whether it does, lest I learn that I can’t follow the reasoning for that anymore. That would be a terrible thing to learn.

• elkement (Elke Stangl) 1:31 pm on Wednesday, 27 April, 2016 Permalink | Reply

That’s an interesting one!! Re closed orbits: I just remember that there are only two potentials that will make sure that every bound orbit is closed: A quadratic (Hooke’s Law, a spring) and a gravitational 1/r potential. Other potentials can have closed orbits, but it depends on initial conditions.
Proofs usually make use of all the constants – energy, angular momentum – to be subsituted in the equations of motion (or the constants emerge from applying Langrange’s formalism) and angular momentum gives rise to an effective ‘add-on’ potential. Then different substitutions are applied that better fit the geometry of the problem, like using 1/r rather than r and angles or polar coordinates … and the statement about closed orbits should be a consequence of calculating the change in angle for moving from maximum to minimum radius.
The procecure felt a bit like so-called early quantum mechanics, where theorems about integer changes in angular momentum were ‘tacked on’ classical theory … and all worked out nicely (and only) with harmonic or 1/r potentials.

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• Joseph Nebus 7:01 pm on Friday, 29 April, 2016 Permalink | Reply

Hm. On reading my copy of Davis’s Classical Mechanics — my old textbook on this — I see he says the kr7 potential allows for closed orbits, but doesn’t say one thing or another about whether every orbit with that potential is closed.

But the section has got that tone like you describe, about early quantum mechanics and other proofs like this, of being ad hoc. Describing where an equilibrium might be is fine. The added talk about what makes it stable? … I suppose that’s more obvious when you’ve got some experience in similar problems, but I remember as a freshman finding it baffling why this should be a calculation. And then the part about apsidal angles, to say whether the orbits are closed, seems to come from a particularly deep field of nowhere.

This does remind me that I’ve got a book I mean to read, partly for education, partly for recreation, that is about introducing the most potent tools of mechanics while studying the simplest orbiting-bodies problems.

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• elkement (Elke Stangl) 2:08 pm on Tuesday, 3 May, 2016 Permalink | Reply

I searched for a reference now – this is the theorem I meant and its proof (translated to English from French): https://arxiv.org/pdf/0704.2396v1.pdf
Quote: “In 1873, Joseph Louis Francois Bertrand (1822-1900) published a short but important paper in which he proved that there are two central fields only for which all bounded orbits are closed, namely, the isotropic harmonic oscillator law and Newton’s universal gravitation law”

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• Joseph Nebus 3:50 pm on Wednesday, 4 May, 2016 Permalink | Reply

Ooh, thank you. This is interesting. And remarkable for being so compact, too! Who knew there’d be results that interesting with barely five pages of work?

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To Apply Both My A To Z Essay Serieses All At Once

I did mean to include a mention of this in yesterday’s post, but I misplaced the link and feel a bit silly about that now. Anyway, over the course of the Summer 2015 and the Leap Day 2016 A To Z I’ve had the chance to talk about a bunch of concepts including duals and Riemann spheres and nontrivial subsets of things. Baking And Math, which combines discussions of mathematics with cooking tips, is as far as I know unaware of my existence. But the blog’s made use of these kinds of things in describing the Poincaré homology sphere. I’m not sure that I can explain why this is an interesting shape to study, especially not this weekend as I’ve been busy with some pinball events. But the shape looks great, and the essay describes some of the making of this wonderful shape. You can appreciate it for beauty bare.

• yenergy 2:07 am on Wednesday, 27 April, 2016 Permalink | Reply

Thanks for linking over to me! I am now aware of your existence. I just published Part II of thinking about the Poincare homology sphere using this construction https://bakingandmath.com/2016/04/26/dodecahedral-construction-of-the-poincare-homology-sphere-part-ii/ so maybe you are interested in it?

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• Joseph Nebus 6:45 pm on Friday, 29 April, 2016 Permalink | Reply

Oh, thank you kindly. I’m glad to know of the follow-up and looking forward to having the time to read.

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