Bending the light

One of the coolest (and most misunderstood) parts of science is the use of models.

A model is an artificially-created system that acts like a part of nature that might be inaccessible, difficult, or prohibitively expensive to study.  A great many of the models used by scientists today are sophisticated computer simulations -- these are ubiquitous in climate science, for example -- but they can be a great deal simpler than that.  Two of my students' favorite lab activities were models.  One of them was a "build-a-plant" exercise that turned into a class-wide competition for who could create the most successful species.  The other was a striking simulation of disease transmission where we started with one person who was "sick" (each student had a test tube; all of them were half full of water, but one of them had an odorless, colorless chemical added to it).  During the exercise, the students contacted each other by combining the contents of their tubes.  In any encounter, if both started out "healthy," they stayed that way; if one was "sick," now they both were.  They were allowed to contact as many or as few people as they wanted, and were to keep a list of who they traded with, in order.  Afterwards, we did a chemical test on the contents of the tube to see whose tubes were contaminated, then used the list of trades to see if we could figure out who the index case was.

It never failed to be an eye-opener.  In only five minutes of trades, often half the class got "infected."  The model showed how fast diseases can spread -- even if people were only contacting two or three others, the contaminant spread like wildfire.

In any case, models are powerful tools in science, used to study a wide variety of natural phenomena.  And because of a friend and fellow science aficionado, I now know about a really fascinating one -- a characteristic of certain crystals that is being used as a model to study, of all things, black holes.

[Image licensed under the Creative Commons Ra'ike (de:Benutzer:Ra'ike), Chalcanthite-cured, CC BY-SA 3.0]

The research, which appeared last month in Physical Review A, hinges on the effects that a substance called a photonic crystal has on light.  (We met photonic crystals here only a few weeks ago -- in a brilliant piece of unrelated research regarding why some Roman-era glass has a metallic sheen.)  All crystals have, by definition, a regular, grid-like lattice of atoms, and as light passes through the lattice, it slows down.  This slowing effect happens with all transparent crystals; for example, it's what causes the refraction and internal reflection that make diamonds sparkle.  A researcher named Kyoko Kitamura, of Tohoku University, realized that if light could be made to slow down within a crystal, it should be possible to arrange the molecules in the lattice to force light to bend. 

Well, bending light is exactly what happens near a black hole.  So Kitamura and her team made the intuitive leap that this property could be used to study not only the crystal's interactions with light, but indirectly, to discover more about how light behaves near massive objects.

At this point, it's important to clarify that light is not gravitationally attracted to the immense mass of a black hole -- this is impossible, as photons are massless, so they are immune to the force of gravity (just as particles lacking electrical charge are immune to the electromagnetic force).  What the black hole does is warp the fabric of space, just as a bowling ball on a trampoline warps the membrane downward.  A marble rolling on the trampoline's surface is deflected toward the bowling ball not because the bowling ball is somehow magically attracting the marble, but because the marble is following the shortest path through the curved two-dimensional space it's sitting on.  Light is deflected near a black hole because it's traversing curved space -- in this case, a three-dimensional space that has been warped by the black hole's mass.

[Nota bene: it doesn't take something as massive as a black hole to curve space; you're sitting in curved space right now, warped by the mass of the Earth.  If you throw a ball, its path curves toward the ground for exactly the same reason.  That we are in warped space, subject to the laws of the General Theory of Relativity, is proven every time you use a GPS.  The measurements taken by GPS have to take into account that the ground is nearer to the center of gravity of the Earth than the satellites are, so the warp is higher down here, not only curving space but changing any time measurements (clocks run slower near large masses -- remember Interstellar?).  If GPS didn't take this into account, its estimates of positions would be inaccurate.]

In any case, the fact that photonic crystals can be engineered to interact with light the way a black hole would means we can study the effects of black holes on light without getting near one.  Which is a good thing, considering the difficulty of visiting one, as well as nastiness like event horizons and spaghettification to deal with.

So that's our cool scientific research of the day.  Studies like this always bring to mind the false perception that science is some kind of dry, pedantic exercise.  The reality is that science is one of the most deeply creative of endeavors.  The best science links up realms most of us would never have thought of connecting -- like using crystals to simulate the behavior of black holes.

****************************************

Rose-colored glass

My wife, Carol Bloomgarden, is an amazing artist, and participates in art shows all over the northeastern United States.  (Her work is called micrography -- it's drawings made from patterns of tiny handwritten text.  You can, and should, check it out at her website.)  Because showing framed art work requires moving lots of stuff around -- not only the work itself, but the canopies, frames, and stands on which to display it -- I frequently accompany her to her shows.

My usefulness is best summed up in a line from a t-shirt a student of mine used to wear: "I May Not Be Very Smart, But I Can Lift Heavy Objects."

In any case, in between setup and breakdown, I usually have lots of time to wander around the show and see what the other artists are selling.  Last year, one of the booths belonged to a very talented jeweler who made jewelry out of (amongst other things) fragments of Roman glass.

Carol hinted at me that she loved this jeweler's work, so for her birthday I got her a necklace and matching set of earrings made from chunks of turquoise-colored glass dating to about 300 C.E.

The Romans were outstanding glassmakers, and a lot of their work survives (unfortunately, much of it in fragmentary form).  And one curious thing about a lot of Roman glass is that it has a patina -- an iridescent sheen on the surface, sometimes refracting light and creating a metallic or rainbow appearance.  There is nothing in the existing writing from that era indicating that those effects were created deliberately; it seemed to be some sort of byproduct of the aging of the piece.

Fourth century C.E. Roman glass from a glassworks in Syria, showing the gold patina over pale green glass [Image is in the Public Domain courtesy of its creator, Marie-Lan Nguyen]

Researchers in materials science at Tufts University became curious about how these coatings were produced, and did microscopic analysis of the surfaces of pieces of Roman glass.  They came to a surprising conclusion; the gold, silver, or rainbow-colored coatings were (1) naturally produced after the pieces were buried, and (2) were photonic crystals -- regular, periodic microlayers of precisely-arranged molecules, of the same sort used in semiconductors and solar cells, which have the effect of generating light interference and an opalescent or iridescent appearance.

It turns out that the interaction between the glass surface, rainwater, and the minerals in the soil results in a very slow, orderly deposition of thin films on the artifact's surface, and in two thousand or so years, you have something truly spectacular.  "It's really remarkable that you have glass that is sitting in the mud for two millennia and you end up with something that is a textbook example of a nanophotonic component," said Fiorenzo Omenetto, who co-authored the study.  "While the age of the glass may be part of its charm, in this case if we could significantly accelerate the process in the laboratory we might find a way to grow optic materials rather than manufacture them."

"This is likely a process of corrosion and reconstruction," said Giulia Guidetti, also a co-author.  "The surrounding clay and rain determined the diffusion of minerals and a cyclical corrosion of the silica in the glass.  At the same time, assembly of 100 nanometer-thick layers combining the silica and minerals also occurred in cycles.  The result is an incredibly ordered arrangement of hundreds of layers of crystalline material... [so] the crystals grown on the surface of the glass are also a reflection of the changes in conditions that occurred in the ground as the city evolved -- a record of its environmental history."

So here we have another example of the kind of fascinating crossover you see in the very best science -- in this case, between materials science and archaeology.  With possible applications to engineering.  

I know I'll think about this study every time Carol wears her Roman glass jewelry.  

****************************************