Skeptophilia (skep-to-fil-i-a) (n.) - the love of logical thought, skepticism, and thinking critically. Being an exploration of the applications of skeptical thinking to the world at large, with periodic excursions into linguistics, music, politics, cryptozoology, and why people keep seeing the face of Jesus on grilled cheese sandwiches.

Monday, October 30, 2023

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.


No comments:

Post a Comment