August 2009 - mathfun@mailman.xmission.com

Re: [math-fun] dumb question about general relativity
by Steve Witham 19 Aug '09

19 Aug '09

It seems to me that if we on the outside of a black hole never see things fall past the event horizon, then a white hole is just a black hole with stuff already rushing out from the event horizon. In other words, nothing can escape from inside a black hole, but then, nothing ever gets that far in anyway. An observer falling into a black hole continues past the event horizon onto a piece of paper that branches off from our history; I don't see why white hole ejecta can't have started on a similar side track-- except that both those tracks seem "non-falsifiable". I guess a singularity is a place where a scientific theory continues majestically past science? What I really want are gray holes, randomly absorbing and ejecting large chunks of matter contrary to all probability but consistent with "classical GR". It's hard to imagine a black hole forming after the big bang and *then* ejecting something. You have to imagine a very red-shifted thing accelerating from +.999...c to -.999...c -- how did it get up the gumption in such a short time? Was it actually in a tight chaotic orbit all along? >From: Rowan Hamilton <rowanham(a)gmail.com> > >Classical mechanics is invariant under translations in time - this >results in conservation of energy via Noether's Theorem. Looking at http://en.wikipedia.org/wiki/Noether%27s_theorem it looks like the only kind of symmetries it applies to are transformations that you can have continuous versions of, like translation and rotation. Is this true? It makes me sad! I would have liked a conservation law to pop out of *every* kind of symmetry! >Classical mechanics is *not* invariant under time reversal, as others >have pointed out, since this violates the >Second Law of Thermodynamics. I second the motion to marginalize the second law to statistics and revert to time-reversible physics. Surely a time-reversed black hole is allowed under a time-reversed Second Law. I'm curious how to tell whether the Second Law is working in forward or reverse. I can take two measurements of entropy, but looking at my log, how do I know whether the 1:01pm measurement was taken before or after the 1:03pm measurement? --Steve

7 12

Re: [math-fun] mathematical zombies
by Dave Dyer 19 Aug '09

19 Aug '09

>A team of Canadian mathematicians (and horror film fanatics?) have published a study on how zombification would spread during a zombie apocalypse. The paper covers spread in closed populations and considers scenarios with quarantines and cures. Finally, applied epidemiology we can all use. > ><http://www.mathstat.uottawa.ca/~rsmith/Zombies.pdf>http://www.mathstat.uottawa.ca/~rsmith/Zombies.pdf >(link goes to full paper) > >Susie Tang, MPH >Acute Communicable Disease Control >Epidemiology Analyst >(213) 989-7172

1 0

Re: [math-fun] mapleadvisor
by Axel Vogt 18 Aug '09

18 Aug '09

Alec wrote me that he is not longer running the page since ~ May 2009. > Date: Mon, 17 Aug 2009 20:50:30 +0100 > From: Fred lunnon <fred.lunnon(a)gmail.com> > Subject: [math-fun] mapleadvisor > > Not the place to ask this, perhaps --- but does anybody know what's > happened to Alec Mihailovs' wiki at mapleadvisor.com ? > > I can't find any relevant postings after Dec 2008, the web address just calls > up a blank page titled "hosted by 1+1" (??); and every time I try to log in to > the related site at mapleprimes.com, it has "forgotten" my password. > > Fred Lunnon

1 0

[math-fun] mapleadvisor
by Fred lunnon 18 Aug '09

18 Aug '09

Not the place to ask this, perhaps --- but does anybody know what's happened to Alec Mihailovs' wiki at mapleadvisor.com ? I can't find any relevant postings after Dec 2008, the web address just calls up a blank page titled "hosted by 1+1" (??); and every time I try to log in to the related site at mapleprimes.com, it has "forgotten" my password. Fred Lunnon

2 1

[math-fun] Museum of Mathematics -- another chance to visit it
by George W. Hart 18 Aug '09

18 Aug '09

Dear Math Funsters, The New York Times said: The Math Midway "turned the common conception of mathematics into a circus of square-wheeled tricycles, giant blocks and simulated roller coasters... combining the mobility of a state-fair swath of carny booths with the slick execution of an educational theme park." The New Yorker said: "The idea behind the museum, which doesn't yet have a home, is that math is ubiquitous, [and] supercool..." Come see for yourself. The Math Midway exhibit has two upcoming temporary appearances scheduled: 1) Saturday and Sunday, August 29 and 30, in Manhattan, with a teacher's preview on the 28th. 2) Spring 2010 at the New York Hall of Science Details here: http://www.mathmidway.org/ Please pass this on to anyone who may be interested in attending or helping to further develop this world-class, interactive, hands-on Museum of Mathematics. George http://www.georgehart.com

2 2

[math-fun] Steps, paces & lightning bolts
by Henry Baker 17 Aug '09

17 Aug '09

FWIW: 6'5" Bolt took exactly 41 steps (20.5 paces; 1st & last step on his right foot) to win his new 100m world record (9.58s.); he supposedly topped 30mph at some point in the race. 2nd place took exactly 45 steps (22.5paces); the women's 100m winner took 50 steps (25 paces).

1 0

Re: [math-fun] Fwd: Mathieu group puzzle
by James Propp 17 Aug '09

17 Aug '09

For more on this puzzle, see http://oskarvandeventer.nl/M12/Developing_Topsy_Turvy_and_Number_Planet.doc… Note that Oskar's "pachinko" mechanism (which can be seen on YouTube at http://www.youtube.com/watch?v=Yu3iZDKutCM) implements chip-firing in a lovely way. Jim

1 0

[math-fun] Fwd: Mathieu group puzzle
by Michael Kleber 16 Aug '09

16 Aug '09

Ravi Vakil sent around the mail below about a mechanical realization of the Mathieu group, looking for other people who might be interested in owning one. I can think of a few funster candidates... if so, contact Ravi directly. --Michael ---------- Forwarded message ---------- From: Ravi Vakil <vakil(a)math.stanford.edu> Date: Sat, Aug 15, 2009 at 10:04 AM Subject: Mathieu group puzzle Dear everyone, At Mathfest, before Persi Diaconis' related talk, Ron Graham told me that a very clever Dutch inventor Oskar van Deventer made a permutation puzzle called "Topsy Turvy" centered on the Mathieu group M_12, based on the Scientific American article by Igor Kris. You can see it at http://www.youtube.com/watch?v=H8ZcYvU0sLY It is quite attractive, so of course I was curious about getting one. I contacted van Deventer, and he said that it would be possible to get these made by a professional laser company, but that it would necessarily be expensive. He would check, but he'd expect a single one to perhaps cost more than $500. I asked if it might be cheaper if the company could make more than one (still a small number), he said that he thought the answer would be yes. So I thought I'd e-mail some people (you!) who might have some small chance of being interested. If you are potentially interested (depending of course on the price), and wanted to hear more, please let me know. Please respond just to me, not just the whole list. It might be neat if there are a small number of these in a select few offices/homes around the country and around the planet. Please pass this on to anyone else who you think might be interested. In any case, I hope you are all well! best, Ravi p.s. A few more details: this should be robust enough to have on an office or home wall, and have guests use, but wouldn't be suitable for a public place, as it is easily broken/abused. He also has another puzzle that is more Rubik like involving Mathieu groups, but it is much less robust, and also less attractive, less unique, and not much less expensive. Yes, it's true that these puzzles are expensive, but I'm convinced that this reflects the actual fair cost --- despite van Deventer's expertise in these things (he has lots of neat things that he's made), he has a day job. I am hoping that if a few of us are potentially interested, the price will drop below $500, although well in the triple digits, and then I'd be motivated to dig deep and buy one. -- Forewarned is worth an octopus in the bush.

2 1

[math-fun] Now they tell me: Mathematica for $295
by Robert Baillie 15 Aug '09

15 Aug '09

I wish I had known about this before I shelled out big bucks. But maybe others can benefit from this. For $295, you can get a full-featured 32-bit version of Mathematica for use at home. Commercial uses are prohibited. So is using it for the class you teach at a school. But for true math nerds, like me, who consider Mathematica to be recreational software that we use at home, this sounds like a great bargain! http://www.wolfram.com/products/mathematicahomeedition/

4 3

Re: [math-fun] Plenty of Room at the Bottom, Part II
by Steve Witham 15 Aug '09

15 Aug '09

>Stock Traders Find Speed Pays, in Milliseconds > >by Charles Duhigg >Friday, July 24, 2009 >On Thu, 13 Aug 2009, Steve Witham wrote: > > The question boils down to whether the rules,,,are fair. >From: Jason <jason(a)lunkwill.org> > >Do you mean fair to the privileged class of high-speed traders, or fair to the >market as a whole? If it's the latter, then the claim is rather tautological. What I really meant was voluntary at the individual traction level for players of all speeds. Beneficial as seen by each participant at the time. >If it's the former, then why does "profitable for high-speed traders" imply >"good for the market as a whole"? That would be economics, which is tautological (and mathematical) until you notice it making some correct predictions. >From: Dan Asimov <dasimov(a)earthlink.net> > >Can you define "fair" -- especially in a mathematical sense? > >(Otherwise this may be a political issue and inappropriate >for math-fun.) With that caution in mind, What I was trying to say was that there are mainstream economic models about this stuff, and the NYT article doesn't fit its stories into that picture (nor clash them against it). Henry Baker wrote: >The excursions of a stock price or a stock index during a single day >of trading mean that old-fashioned mutual funds that are priced only >once per day are as dead as a dinosaur. One of the features of these mainstream models is that faster trading benefits people of all speeds except maybe the narrow group of people at the formerly fastest trading speed wishing they could still be traders at that speed. Long- and medium-term investment still make sense & is supposed to get a little better. --Steve

1 0