Monday, December 30, 2019

Energy scales and crystals in science fiction

Crystals are fascinating.  Somehow, for reasons that don't seem at all obvious at first glance, some materials grow in cool shapes as solids, with facets and obvious geometric symmetries.  This was early support for the idea of atoms, and it's no wonder at all that people throughout history have looked upon obviously crystalline materials as amazing, possibly connected with magical powers.

In science fiction (or maybe more properly science fantasy), crystals show up repeatedly as having special properties, often able to control or direct energies that seem more appropriate for particle physics.  In Star Trek, dilithium crystals are able to channel and control the flow of matter-antimatter reactions needed for warp drive, the superluminal propulsion system favored by the Federation and the Klingon Empire.  In Star Wars, kyber crystals are at the heart of lightsabers, and were also heavily mined by the Empire for use in the planet-killing main weapon of the Death Star.

In real life, though, crystals don't do so well in interacting with very high energy electromagnetic or particle radiation.  Yes, it is possible for crystals to scatter x-rays and high energy electrons - that's the way x-ray diffraction and electron diffraction work.  On very rare occasions, crystals can lead to surprising nuclear processes, such as all the iron atoms in a crystal sharing the recoil when an excited iron nucleus spits out a gamma ray, as in the Mossbauer Effect.   Much more typically, though, crystals are damaged by high energy radiation - if the energy scale of the photon or other particle is much larger than the few eV chemical energy scales that hold atoms in place or bind core electrons (say a few tens of eV), then the cool look and spatial arrangement of the atoms really doesn't matter, and atoms get kicked around.  The result is the creation of vacancies or interstitial defects, some of which can even act as "color centers", so that otherwise colorless Al2O3, for example, can take on color after being exposed to ionizing radiation in a reactor.

Ahh well.  Crystals are still amazing even if they can't propel starships faster than light.

(Happy new year to my readers!  I'm still trying to be optimistic, even if it's not always easy.)


Anonymous said...

Considering the importance of specific crystal orientations for non-linear optics, I would say crystals are actually very important for harnessing the power of radiation even at the scale of multiple electronvolts.

Anonymous said...

I'm not sure I agree that light of electronvolt energies are typically able to kick around atoms. That can happen in organic crystals, but is very rare in inorganic crystals. The primary effect is to create electronic excitations, which don't readily result in atomic rearrangements.

Anonymous said...

Happy New Year prof.

Douglas Natelson said...

Hi - I realize my wording was ambiguous. What I meant was “if the energy scale of the radiation is much larger than a few eV”. Perhaps I should also have emphasized the “much larger” part. Edited now to clarify.

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