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We were taught at school that EM radiation can be categorised as radio waves, microwaves, infrared waves, visible light, ultraviolet radiation, X-rays, and gamma rays, but how arbitrary is this categorisation?

I understand that for humans the 'visible light' category is an obvious, but how did scientists justify drawing a line in the sand between, say, the frequencies constituting radio waves and those constituting microwaves?

Would an advanced alien civilisation divide the EM spectrum in the same way that we do or could they have divided and categorised it in a whole different way than us?

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    $\begingroup$ The "visible light" category may be obvious for humans, but other animals have eyes which respond to different ranges. For example some insects can "see" ultraviolet frequencies invisible to humans, and some plants have evolved "coloured" markings in that frequency range to attract or repel them. Other animals (e.g. some snakes) can "see" heat sources (for example their prey) by "vision" using infra red radiation. $\endgroup$ – alephzero Nov 17 '19 at 1:44
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Any classification of electromagnetic radiation is probably going to have at least a few basic components, which can be derived purely from observing the way radiation interacts with atoms and molecules:

  • Ionizing radiation vs. non-ionizing radiation: Radiation above a certain frequency has the ability to ionize atoms.

  • Radiation emitted by nuclei vs. radiation not emitted by nuclei: Radiation above a certain frequency is generally seen only as gamma rays emitted during radioactive decay.

  • Radiation corresponding to atomic transitions vs. radiation corresponding to molecular transitions: Higher-frequency radiation tends to excite electrons bound to individual atoms, whereas lower-frequency radiation tends to excite electrons in molecular bonds.

So, you can divide the electromagnetic spectrum pretty un-controversially into the following general bands, from lowest frequency to highest:

  • Radiation not energetic enough to excite molecular transitions (radio to lower microwave).

  • Radiation that can excite molecular transitions, but not atomic transitions (lower microwave to upper infrared).

  • Radiation that can excite atomic transitions, but not ionizing radiation (upper infrared to visible/lower ultraviolet).

  • Ionizing radiation not associated with nuclear transitions (ultraviolet to low-energy X-rays).

  • Ionizing radiation associated with nuclear transitions and above (mid-energy X-rays and gamma rays).

The precise location of the divisions between these bands is entirely subjective, but any intelligence that has sufficient experimental data to notice that molecules, atoms, and nuclei all respond differently to different types of electromagnetic radiation will probably have something similar to this classification in place.

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  • $\begingroup$ Isn't the "terahertz gap," between microwave and infrared, a pretty significant division, at least for present-day human technology? My understanding is we have good technology for manipulating "radio frequency" EMR below about 100 GHz, as well as "optical frequency" EMR above about 10 THz, but no good technology for the frequencies in between. It's likely (but not certain) that aliens would have similar difficulties. $\endgroup$ – Tanner Swett Nov 17 '19 at 20:53
  • $\begingroup$ @TannerSwett That's a division that's based on human technology, and there's no guarantee that the progress of some other intelligence's technology would have encountered the same difficulties (in fact, there's no guarantee that future human technology would encounter the same difficulties...). $\endgroup$ – probably_someone Nov 18 '19 at 9:08
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Many of the divisions are arbitrary, the "infrared/visible/ultraviolet" being the most obviously arbitrary. Aliens are virtually certain to use different photosensitive chemicals to see, so their definition of "visible light" will be different from ours (though not by much -- most stars have their peak output somewhere in the near-infrared to near-ultraviolet range).

Our division of the electromagnetic spectrum is mostly an artifact of how portions of it were originally generated or discovered:

  • Radio was first generated by resonant electrical circuits. It was hard to make circuits that resonate at high frequencies, hence names like "Ultra-High Freqency" for frequency bands that are now seen as extremely low.
  • Microwaves, unlike radio waves, were (and often still are) generated by specialized components such as magnetrons.
  • Infrared was first discovered when Herschel observed that a thermometer placed below the red end of the solar spectrum would still heat up.
  • Visible light has been known since pre-history.
  • Ultraviolet was discovered a year later when Ritter observed that the blank area beyond the violet end of the spectrum would still darken silver chloride.
  • X-rays were discovered repeatedly over the course of about 110 years, always in the same way: as a byproduct of an electrical discharge tube. (Röntgen wasn't the first to observe X-rays, just the first to realize there was something new going on).
  • Gamma rays were discovered as a result of investigating radioactive decay.

So, our division of the EM spectrum is basically seven "islands" in the spectrum that have grown to meet each other. The position and size of each of them is a historic accident. The only division an alien civilization is likely to develop is the "IR/visible/UV" split, and even so, the dividing lines are certain to be different from ours.

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  • $\begingroup$ Thanks for your response. I'll look into these discoveries further. $\endgroup$ – VeryCommonName Nov 17 '19 at 21:52

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