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Posted in Uncategorized on November 21, 2014
Picture taken from another nice blog post about swarming bees, within a nice blog about bees: http://thelazybfarm.com/why-do-honeybees-swarm
I just read a very interesting book about bees, “Honeybee Democracy”, by Thomas Seeley. The part that I cannot get out of my mind is the end of Chapter 3, describing the way bees probably assess the dimensions of the possible new homes. Therefore I ‘ll not speak about it right away. However let me advertise a bit some really cool other parts from the book.
The whole book describes the dynamics of swarms of bees.
Most of the time bees will be going about their business of collecting nectar and pollen, growing, building up their nest and the reserves of honey, termoregulating, and similar “routine” stuff. This is everyday life has interesting stuff (e.g.: who decides the allocation of bees to different flower spots? how does the dance describing the direction and the distance of a given flower spot encode the information?.. the answers are somewhat surprising! they are answered “en passant” during the book) however the main focus is about the point where, in a moment of high resources, “the swarm splits” and part of the bees has to take off together with a daughter queen..
…Then the swarm will take a momentary position on a branch of a tree or similar, in the open. In this situation it will have to decide where to go settle. The bees at this point have their bellies full of honey reserves and once the weather becomes sunny some of the most active bees decide to explore the surroundings in search of some cavity where the whole swarm could settle (what are the factors discriminting the behavior? maybe it’s just the very full stomach giving her lots of energy? are some bees geneticaly predisposed?).
Then several scouts will go explore cavities around. Once they find something interesting, they will come back and “dance the location” to the other bees, more emphatically and repeatedly, or for fewer times and with less vigour, depending on the overall “quality” they assign to the site. Some of the bees will perhaps get convinced to explore the same cavity (note about the “convincing”: it’s not clear to me how far this was experimentally proved.. could it be just an educated guess? is it the same mechanism which is at work during the search for flowers?). If, after exploring, these new bees get “enthusiastic” themselves, they will “dance” again, if they are not they will not (fascinating fact: once a bee “said its say” about a location, it will “shut up”, leaving the deliberation to the group.. would that be a strategy which could help humans to deliberate too? what about the case where the kind of “deliberation” is the one about scientific truth, and takes place in scientific journals? Probably to some extent differences between bees and humans can be explained on an evolutionary/genetic basis, but that is a topic for another story).
Just when all the returning explorers agree on the same location will the whole swarm take off and go on to settle in that place (how can the whole swarm be directed by just the relatively few bees which really know the location? by the way, how many bees know the location in percentage? how do these swarms cover all the distance wihtout losing cohesion? how about not loosing the queen? ..there is a chapter with the answers).
The crucial qualities of a nesting site
Like a human home, a nesting site does not just have one single parameter to weigh on, rahter there are several, e.g.: 1) the height from the ground will help to avoid predators, e.g. bears; 2) the integrity of the walls will help in the winter months for thermoregulation; 3) a particularly damp site will be rejected too; 4) a wider entrance will allow more predators to enter and more heat to esape, therefore smaller openings are preferred, etc….
There are more factors, and they were tested through experiments, some of which are very clever. I left out one quality of nests, which is quite crucial: The volume of the cavity. It is very important for a swarm to have enough volume mainly because there must be enough space to store reserves of honey for the winter. The principal cause of death of a swarm during winter is precisely the depletion of the honey reserves, and the volume of the cavity is the main constraint this kind of resources (no scientific experiment on this is mentioned in the book, or better, it is said that in traditional honey hives the apicultors leave more space thant necessary on purpose, and it is all filled by honey reserves.. how much space is the limit? is the location playing no role on these matters?or is the location in nice flower-ful regions one of the factors in the nest choice too?).
How do bees measure the volume of a cavity?
You have to immagine that you are a bee, an animal which is optimized to wander around on sunny days and detect flowers using for orientation the sun… (by the way, in rainy periods do the bees always rest? What about just cloudy days? what is the precise factor? maybe the sun visibility? do they loose orientation if they try to fly in these days, or can they find other clues to orient?.. I don’t know now the answers to these questions.. one point which is more or less true is that on cloudy days several kinds of flowers close up, therefore surely they become less attractive..but again: how much? is that a complete answer?)
….and, through a small hole, you enter a very dark cavity which you never saw before. How do you go about measuring the volume? What we know are some scattered facts, and some analogies to other cases. One fact is that bees seem to be just walking around the walls, doing just some small “hopping” flight to move around. Another thing is that the more they walk, the bigger the cavity seems to be from their point of view: this was tested by an ingenious experiment by the author in 1977. We also know that the light inside the cavity is not so strong, basically a good approximation is to say that the cavity is really like total darkness. So let me describe how the book presents the “volume measurement procedure” and then let me give my own small hypothesis to contrast that.
Parenthesis: the Buffon needle problem
Buffon’s needle problem has the following formulation : consider a collection parallel lines in the plane disposed at distance . Then drop a needle of lenght at random in that plane. The expected/average number of crossings between the segment and the set of lines will be .(after you read this paragraph you’ll have the tools for proving this on your own)
If we take tow independent variables choosing two segments of lenght at random then we get the seame result, since expectations of independent variables sum. From there it is a small step to note that even is we impose that the end of the first segment coincides with the start of the second, we still get the same average number of intersections. This is because anyways, the identified points are themselves distributed uniformly at random. We use here the symmetry of the plane.
By iterating this reasoning, we obtain that any “random curve” of lenght will give the average number of intersections again like in the above formula. Note that there isn’t just one notion of “random curve”, and in this case we mean “a limit curve obtained as welding of more and more short independent segments” (another popular notion is that of a “Brownian path”, however I don’t know the correct framework for quantifying the quantity of crossings of a Brownian path of lenght with the set of parallel lines above; it feels like the usual notion of arclenght is not natural in the setting of Brownian motion).
Random walks of bees measure area?
Let’s return to the bees, and to the question of how they measure the volume of pontential nesting sites.
If you are a bee inside a dark cavity, the idea is that you are going to move at random. Then the hypothesis is that bees secrete some kind of pheromone, then check whether they passed through the same point by smelling it around with their antennas. The interesting “prnciple” described in Seeley’s book, and based on analogous ideas regarding scouting ants (where, i have to admit, it is a bit more plausible) by Dornhaus and Franks, is the following: the area of a surface is proportional to the average number of crossings of two random paths of given length. To get more details on this, type “Buffon needle algorithm” on google(.. but don’t expect to find any details or proofs.. at least I didn’t find any!)
I have to put quotation marks around the word principle, because it seems quite far-fetched, and it is probably false in general. But it would be very interesting if any version of this could be proved, i.e. if there were a version of Buffon’s result (i.e. a general formula for the numbers of crossings) where the parallel lines are replaced by another random object (this version is easier) or where the underlying plane is replaced by a general kind of surface (this version is really challenging!).
What I have big doubts about is the extent to which one can hope to find the surface area of an object of unknown shape by looking at crossings of random lines. It would be really cool to find some algorith like that!
Once they know the area, how do bees measure volume?
The next step once the bees calculated the surface of the cavity, is to calculate the volume: the hypothesis is that to do this the bees measure the average mean free path length during their short hopping flights. If you know area and average free path lenght then by a simple calculation you can compute a very good estimate of the volume.
Posted in Uncategorized on November 16, 2014
[While reading this, I recommend listening to: http://www.youtube.com/watch?v=cqgiw3dasEg]
Methane winds violently rush through the higher athmosphere.. vortices form and turbulent thunderstorms, the lightnings crisscrossing the alien sky. The globular lightnings leave smoky dark branching root-like after-images behind, the tremendous energy pyrolyzing the methane into carbon dust..
These root thin drawings of soot are ephemerous – made of burnt methane, they instantly disintegrate through the vorticous action of the storm – and then gradually, imperceptibly, fall for miles down into the hallucinating Saturnian atmosphere. As they disappear from sight within the lower layers of these life-forbearing dark clouds, they begin to condense.
While winds and currents violently perturb the motion of the dust in a chaotic way, lurking behind we find the subtle force of gravity, which is subtle but constant, its accumulated influence becoming preponderant over the course of days and weeks.
..After a some hundred miles of this inexhorable chaotic descent we see small particles of carbon forming, as the pressure increases to that of the bottom of an earth’s lake – the small particles of dust are getting larger, as they fly between sparse crystals of ice, within a dense intoxicating high-pressure and heavy athmosphere of methane and ammonia..
..After 1000 miles and perhaps years of descent we see that this dust has become denser, more compact, and a different, heavier form of carbon – graphite – can now be observed. Yet the attraction of the ringed giant is ever stronger, and descent does not stop, but the carbon’s fate becomes more obscure, farther from the eyes of earth-controlled observers.
What happens to this familiar yet barren dark mineral dust inside the surreal saturnal deep admosphere? Might the dust particles slowly grow and compactify even more? Perhaps, rather realistically, that by a depth of 6,000 miles, these chunks of falling graphite toughen into diamonds – still stronger and unreactive. Such gems which are rare and precious, found on earth’s surface only because of tectonic movements making them re-emerge from the deep layers of the crust where tremendous pressure compactified them, are so special in our blue home planet.. Is it not an uncanny coincidence that “Saturn”, the name of our god of the underworld, was chosen to name this remote planet where the deep-burrowing gems might be raining like hail?..
..At 6000 miles the descent of the carbon-stones does not end: they proceed their bottomless decent for as much as two-and-a-half Earth-spans, meeting a region whose appearance is open to the wildest speculations.. the pressure and temperature are so hellish that even diamonds could not remain solid. It’s very uncertain what happens to carbon down there. Could we imagine a sea of cabon, at these pressures where even diamonds collapse?..
..What is called “eternal” in our world becomes “melting carbon ice” in another.
Posted in Uncategorized on May 10, 2012
This post is about mostly things that I don’t know, but which are very interesting.. The basic topic is threefold:
1) sometimes computers are not capable of doing things, and sometimes it’s things that humans instead can do. I’m not speaking about having feelings or eating, but rather things that computers should be able to do better than humans, namely doing simulations, solving complicated problems, etc. Examples follow below.
2) on the other hand, humans are less easy (or cheap) to convince to do tasks, than computers, more or less by definition of a computer.. they seem to prefer much more playing videogames, looking at Facebook, or downloading illegally music and books.
3) on the other hand (i.e. on the fist hand) one can make those other “unproductive” preferred activities a tiny bit useful for the community (or rather for some precise guys), by using wicked tricks. One can use the same wicked tricks actually also to make the humans more useful to themselves.
A) One example where computers were beaten by humans is in relation to Foldit, a game devised to simulate protein folding. The recreational aspect hides a deep scientific importance, since no efficient way to simulate the folding process of proteins efficiently is yet known, the computational power available being one side of the limitation, and the lack of an algorithm adapted to the problem being the other. However online gamers have an edge on the machines, as shown in this article in Nature.
B) Similar stuff with RNA instead of proteins is called EteRNA.
C) Another project of similar nature is also done to help people place transistors on microchips efficiently, and the game is called FunSAT.
D) If you are interested in the sequencing of human genome, you might also know that the genome contains not all the answers about humans as we liked them. In fact, it looks more like a huge (sequenced) mess than like a nice programming code. One method to get some hints is Multiple Sequence Alignment, and is the topic of this game called Phylo.
E) A more boring (too scientific -looking) one is EyeWire, where you help a program to detect neurons of the retina.
F) During other activities than gaming, like downloading illegally books, or also during legal pseudo-activities like Facebook, you are sometimes asked to “prove that you’re human” by decyphering a text, composed of two words: a so-called captcha. (click link for a picture)
In some Captchas (for example the ones called reCAPTCHA) one of the words looks much more like a real, typed word than the other, and that’s why because the website you are accessing does not know that word, and you are actually helping some company transcribing scanned books from paper to digital format. The true verification is done through the other “artificial-looking” word.
If you try to write a random thing instead of the “book-word” then they will still think you are a human, while doing errors in the other “computer-made-word” is not forgiven (the computer just knows that part o the information). Here is the guy who got the idea. He also got many other nice ideas. And a blog.
I wanted to write more about other aspects of this, but I’ll leave that for another post.
Posted in Uncategorized on May 8, 2012
I just thought of the following question, I’ve no idea if it’s a well-known riddle, but it seems a nice story, and it should not prove too difficult to answer:
God is tired before the 6th day of creation so he actually gives you a task, while he goes to take a nap. Namely you have to build the Amazon forest. (That’s because people in the 21st century are going to need to chop trees in huge quantity in order to make paper, so the plan is that you have to fill the whole South America with trees before midnight). You take an energy drink and prepare to do the job…
Basically the technique to do one of these trees is quite simple: you are given some sticks and you have to glue them at their ends, making a caricature tree, then you plant one of these in the ground, and magically a leaf is going to sprout at the free end of the sticks, while the rest is becoming a realistic wodden tree.
You notice that the sticks become quite nice trunk pieces, while you don’t like so much the way the leaves come out, they seem boring to you (and you immagine that after all 21st century people will look more for wood than for leaves, so you want to concentrate on making that part). So you end up doing trees with many sticks, but as little leaves as possible. The result is not very nice, because you end up gluing all the sticks in a row, and you get a long tree with just one leaf in the end. This makes you wonder what would happen if you start doing a lot of branchings. And here is the question:
If you impose yourself that no gluing is done involving only 2 stick ends (so that your tree has just branch points, possibly with more than 2 branches), what is the least number of leaves that you can get if you use 4 sticks? What about 15 sticks? Is there a formula valid in the case of an arbitrary number of sticks?
Epilogue: Lost in those dreams you fall asleep, leaving an Amazon forest with a total of about 7 trees for the posterity.. but God is forgiving, so he offers you a beer.
Posted in Uncategorized on August 3, 2009
I learned from E. Kowalski’s blog a nice theorem of Kocher and Specker:
There does not exist a function , such that for any which are orthogonal to each other, .
The statement here was at first counter-intuitive, so I tried to prove it. I found some kind of geometrical proof, I think it’s nice (and I hope it’s right!):
Proof: Label all by and identify points of with vectors of . For every othogonal triple one and only one of the labels is . Therefore, for every “zero” label (call it “north pole”), the “equator” will be made of “one”s, and the “south pole” will be a “zero”.
Consider labels “zero” which have an angle between them , there are two intersecting (non-orthogonal!) equators of “one”s. Now while a point moves on one of the equators, there is exactly one point on the other equator which is orthogonal to it: as describes the first equator, will therefore describe the second equator. The third point completing an orthogonal basis , will also describe a curve of “zero”s: it is a circumference having the two “zero”s as diameter!
Iterating the above construction it is easy to see that the set of zeroes is open. But since the set of “one”s depends continuously on the set of “zero”s (through the “equator-pole” construction), it is also open! Contraddiction (since the sphere is connected).
Posted in Uncategorized on July 25, 2009
I want to rewrite here a part of my degree thesis speaking about a very nice way to describe residual sets.. I start with the abstract Bourbakist definition anyway.
Let be a topological space.
A set is called nowhere dense if it fails to be dense in every open subset of : that is, for every nonempty open there is a nonempty open .
is then nowhere dense if and only if its closure has empty interior; a finite union of nowhere dense sets is also nowhere dense; the topological boundary of a closed set or of an open set is nowhere dense.
A set is called of first (Baire) category if is a countable union of nowhere dense sets.
A set that is not of first category is called of second category.
The complement of a first category set is called residual.
Residual sets are then sets that are countable intersections of sets with dense interior. If a set is first category, then all its subsets are. In some sense the sets of first category are “small” (they are also called meagre sets because of this) and the sets of second category are “large”. We will show this using a description introduced by Banach and Mazur.
The Banach-Mazur game
Let be a metric space.
Suppose that there is given a subclass of the subsets of having nonempty interior, such that each open set is contained in some member of . Suppose that there are given two sets and . The game is played according to the following rules: two players (A) and (B) alternately choose sets
from the class , with (A) choosing the ‘s and (B) choosing the ‘s, numerably many times. Such a nested sequence is called a play of the game. The player (B) is declared winner if
while otherwise (that is, if ) the winner is (A).
To be more precise, we will call a strategy for (B) a sequence of functions giving for any sequence of members of as above, a new member
A play of the game is called consistent with a strategy if for all ,
A strategy is winning for (B) if every play of the game consistent with ends up with a victory of B. If such a strategy exists, the game is said to be determined in favour of (B).
Clearly, in order to win, (A) will hope that the set is large, while (B) will have the same hopes for the set . The right meaning for the word “large” in this context was (conjectured by Mazur and) proved by Banach to be the same as “residual”:
The game is determined in favour of player (B) if and only if the set is residual in .
“if” part: Let with open dense sets. We then choose
which can be done for the properties of and of . The resulting strategy is then easily seen to be winning, by the definition of the ‘s.
“only if” part: Let be a winning strategy for (B). We will call a consistent sequence of members of a chain of order $latex n$, and the interior of $latex V_n$ will be called the interior of the chain. We will now inductively construct a sequence of dense open sets whose intersection is contained in the set .
Let be a maximal subfamily of the chains of order 1 whose members have disjoint interiors, and let be the union of the interiors of the members of . This open set is dense by maximality.
Then, among all the chains of order 2 that are continuations of some member of we choose a maximal subfamily with disjoint interiors, and let be the union of these interiors. The open set is again dense by maximality.
Iterating this procedure, we get a sequence of dense open sets . If then there is a unique sequence of chains such that for each , is in the interior of . These chains are ordered by continuation and the limit play is consistent with , so it must be a winning one. Thus , and since was arbitrary, .
As an application, we cite the following, which is usually proved using directly the definitions. We give instead a proof using the Banach-Mazur game:
If the metric space is complete and is a dense set (i.e. a countable intersection of open sets), then is residual.
Let , where the sets are open. We take be the family of all closed balls, and we describe the winning strategy of player (B) in the game : let be the closed ball chosen by (A) at his -th move. Since is dense, int is nonempty. So int is a nonempty open set. By regularity there exists a ball of radius less than whose closure is contained in the above set. We then choose that ball as . We obtain that the sequence refines the Cauchy filter basis and by completeness it has as intersection a single point , contained in each , and thus in .
Posted in Uncategorized on July 23, 2009
I have found today this problem (or conjecture, if you wish):
Does every Jordan curve have 4 points on it which form the vertices of a square?
There is a survey about partial results on the Quomodocumque blog, but I have no access to a library from where I am now, so I think trying to figure it out alone might be a nice idea.
Jordan’s theorem When I mention it I mean this: any Jordan curve has an interior and an exterior, which are open connected by arcs sets such that the one called interior is bounded and together they form the complement of . In particular any path (i.e. continuous curve) starting inside and ending outside it must pass through .
Easy part: the triangle case
I already previously had come in contact with the analogous problem where one looked for inscribed equilateral triangles instead of squares, and that is easier: the answer was yes, by some topological argument, as follows.
There is a (-valued) function with values in depending continuously on such that is an equilateral triangle. Using this and Jordan’s theorem, (with some patience) one verifies that there are couples such that are respectvely in the interior and in the exterior of . Moving the couple in , by Jordan’s theorem again one has a nice equilateral triangle with vertices all on .
The procedure which works for equilateral triangles also works for any kind of triangle (you just have to use the ordering on edge lenghts carefully in each step, and after some trial and error you manage the right combination).
The square case
This case is really square. The problem with the above continuity methods based on Jordan’s theorem seems to be that as 2 vertices wander on the other two may pass independently from one connected component of to the other.
1) If you suppose the curve is smooth (say ) then you might try to take an inscribed “half-square” triangle (i.e. isosceles with one angle in ) which exists by the above theorem, and try to perturb it still leaving it inscribed, seeing what happens to the th vertex of the square. To help intuition, suppose that near and the graph of the curve is a piece of line (recall you assumed so this is not far from true). Then perturbations of give a -dimensional set of perturbations of ‘s if and only if the lines representing near do not form an angle of . If this happens, then by the inverse function theorem one can find for each one such that completing the half square is still on .
Then you could try to reason like before, asking yourself if there are two squares with and in different connected components of . You can construct them as follows:
– if are very close then there are 2 vertices completing the square inside (this uses the regularity of ). Then you can move along until the first time when one of touch .
– if realize the diameter of then you can move towards until the first hitting as above and you get the second square.
Now you move closer together until they become the same point, and let follow them staying on and forming half-squares as discussed earlier, and follow them completing at each moment the respective squares. During all this procedure by Jordan’s theorem have kept on staying on opposite components of the complement of . However nothing tells that there is a general way of moving one onto the other while vertices of the cubes stay all the time on the curve.
2) Let’s use the angle found above to try producing a counterexample. The first idea I get is starting from a regular octahedron and moving two adjacent sides parallel to themselves towards the center, leaving all the other sides supported on the same lines. Exercise: find a square inscribed in this figure!
(there is no square in the original octagon that “survives” on this new one. However:)
3) Lemma: in all convex curves there is an inscribed square: just take any small square completely inside and “blow it up”, immagining that the curve represents a “rigid hole” in outside which the square cannot pass (but it can slide if this helps continuing to “expand”). At a certain point you cannot blow anymore: the square has at least 3 vertices on the curve and has reached the maximal size. Now suppose 3 vertices of are on and isn’t. This implies that the tangent cone to the interior of in and do not contain the sides else by convexity . Then it (is easy to see that it) is not true that you cannot blow anymore.