Earthly Cities Grow Like Galaxies

One of the nifty things about a good mathematical model is that it can reveal hidden connections between two systems that, on the surface, appear to be very different from each other. Two cosmologists, Henry Lin and Abraham Loeb, uncovered just such a surprising correlation, demonstrating that the way galaxies evolve from variations in matter density in the early universe is mathematically equivalent to the way cities grow from changes in population density on Earth.


Their analysis centers on a well-known scaling pattern known as Zipf’s law, observed in everything from personal friendships to the population density of cities. As Gizmodo’s Kelsey Campbell-Dollaghan wrote, “Basically, the city with the highest population in a country will be twice as large as the next most populous city, and three times as large as the third most populous city, and so on.” The same holds true for galaxies, it seems. Loeb and Lin took a mathematical formula describing how galaxies form and evolve and applied it to the evolution of cities on Earth. The two systems proved remarkably similar. The scientists think that similar mathematical tools could be used to better model the spread of epidemics, among other applications. [Paper]

A Grand Theory of Wrinkles


Credit: Denis Terwagne and Pedro Reis, MIT

Wrinkles are found all throughout nature, from the surfaces of planets, to the dimples on a golf ball, and even in the small intestine. But these systems are usually studied on a case-by-case basis, working backwards to create computer simulations to better understand how and why they form. This year a team of engineers and mathematicians at MIT came up with their own grand unified theory of wrinkles, especially applicable to wrinkles that form on curved surfaces.


MIT engineer Pedro Reis has spent years studying how objects wrinkle. While conducting experiments on silicone test spheres, he noted that when he sucked the air out, some of those spheres formed dimples under pressure, but others formed a more squiggly pattern. His MIT colleague, mathematician Jorn Dunkel, noted a similarity between the latter and the patterns that appear when one heats a thin layer of oil. The two departments combined their efforts, pouring over all of Reis’ experimental data.

They found that the kind of patterns that formed depended on just two factors: the curvature of the lower layer in relation to the thickness of the top wrinkling layer, and how much stress was applied to the wrinkling layer. “Our theory you could basically apply to the surface of the moon or Mars, or the surface of a grape,” co-author Norbert Stroop told Quanta magazine. [Paper]


Getting to the Bottom of the Lollipop Hypothesis

It’s a question that featured in a classic candy commercial: how many licks does it take to get to to center of a Tootsie-Pop? This year we learned the answer: about 2500, according to experiments by physicists at New York University. Call it the Lollipop Hypothesis. The NYU researchers used the candy to determine how fluids dissolve solids, a topic that also applies to the erosion of rivers and how pills dissolve in the body.


The NYU team made their own homemade lollipops out of boiled sugar, corn syrup, and water, which they then molded into various shapes. Then they immersed the lollipops in a “water tunnel” (the aquatic equivalent of a wind tunnel) and watched them dissolve, varying the flow speed of the water. They found that there seems to be a preferred shape that objects take on as they dissolve, per Physics Buzz: “a smooth rounded front, a beveled facet in the middle, and a flat back side.” They also found that the dissolve rate depends on flow speed: for example, change the speed from 1 MPH to 4 MPH and the lollipop would completely dissolve in half the time.

As for counting the number of licks, they calculated it would take an estimated 1000 swipes of the tongue per centimeter of candy to reach the center of a Tootsie-Pop. Since the candy measures about 1.063 in diameter, that translates into 2500 licks. [Paper]

Solving the Mystery of How Glass Forms

Glass is a class of materials that has been around for a very long time, yet its deeper secrets still elude physicists — particularly the stubborn mystery of how glass forms at the molecular level. A team of Canadian and French scientists devised a new model for how a liquid turns into a glass by combining, for the first time, two decades-old theories: crowding and cooperative movement.


Molecular crowding basically treats molecules within glasses as people moving about a crowded room. The key element is density. As more and more people squeeze into the room, there is less space, so people (or molecules) move more slowly — although those located near the door are still able to move more freely, just like the molecules on a glassy surface never stop flowing, even at lower temperatures.

That’s where cooperative movement kicks in. As the crowd thickens, people tend to move in conjunction with their nearest neighbors. The scientists found that molecules exhibit similar behavior, forming strings of weak molecular bonds with their nearest neighbors. The new model could prove useful for developing novel glassy nano materials with useful properties. [Paper]


Credit: Evangelidis, V. et al./Journal of Archaeological Science

Slime Mold Builds an Ancient Road Network

Take a moment to marvel at the humble slime mold, an ancient group of organisms that reproduce via spores and get their name from the dimly stuff they excrete. When times get tough, slime molds band together, and exhibit a strange kind of hive-mind, or cooperative intelligence. They can solve mazes, change their appearance, and find the most efficient path between two food sources. And this year they helped reconstruct an ancient road network.


Greek archaeologists used a bright yellow slime mold called Physarum polycephalum to, in essence, redraw the ancient Roman road networks running through the Balkans between the 1st and 4th centuries A.D. They grew the molds on a map of the area made up of agar gel, with oat flakes at strategic locations, representing major Roman cities. The slime molds reproduced the network accurately. Those roads were well known from historical documents; this experiment was proof of principle. The archaeologists hope that they can use slime molds to help reconstruct lesser-known pathways that have been lost. So the best archaeological assistants of the 21st may well be be slime molds. [Paper]


Prat-Camps, J. et al./Scientific Reports

A Magnetic Wormhole Illusion

A team of scientists at Autonomous University of Barcelona, Spain took materials science into stealth mode, creating a “wormhole illusion” that causes magnetic field to move through space undetected. The operative word here is “illusion.” This is not a bona fide wormhole connecting two points in space-time — a staple of science fiction decades, although we’ve never observed any directly. Rather, it’s created using metamaterials to tunnel magnetic fields from one point to another


The device is made of two concentric spheres encasing a spiral of ferromagnetic metal. As Gizmodo’s Maddie Stone wrote,

The ferromagnet transmits magnetic field lines from one end of the device to the other. Meanwhile, a shell of yttrium barium copper oxide (a superconducting material, yellow) bends and distorts the magnetic field lines as they travel. An outer shell composed of “mu-metals” (used for shielding electronic devices, silver) perfectly cancels out the magnetic distortion of the superconductor, rendering the entire thing “magnetically invisible” from the outside. Dunk it all in a liquid nitrogen bath—superconductors only work at extremely low temperatures—and voila, you’ve got yourself a wormhole.


It’s a very cool experiment, with a purpose: it could one day help improve medical scanners. Per New Scientist: “Wormholes could let multiple magnetic imagers work together without interfering with each other, or could be used to put some distance between bulky sensors and patients – all without changing the background magnetic field MRIs rely on.” [Paper]

Top image: Fabrizio Carbone/EPFL.