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So your friend asks...Part 2: "Other" kinds of astronomy

So your friend asks, "What are radio astronomers listening to? Through what alien locker room walls are X-ray astronomers looking?"

What are photons?
Photons are the massless particles of which light is made. They act like both a particle and wave (a property called the "wave-particle duality"), as illustrated in the results of projects like the double-slit experiment (discussed in this post).

Photons may not have mass, but they do have energy. Every energy corresponds to a specific wavelength and frequency, according the the equation E=hf, where h is a fundamental constant (Planck's constant) that pops up in lots of quantum mechanical relationships.

Rods and Cones
The anatomy of our eyes happens to make us sensitive to visible light, a characteristic which probably evolved for reasons including

  1. The sun radiates the most brightly at visible wavelengths.
  2. Our eyes don't have to be very big to be fairly sensitive to visible wavelengths. The sensitivity of a collector (a telescope, an eye) is based on the relationship between the diameter of the telescope and the wavelength of the light. The collector must be many wavelengths' lengths across in order to be worth anything. So the smaller the wavelength, the smaller the collector can be and still be useful.
  3. Visible wavelengths can travel through the atmosphere (as can radio waves, which is why these are the only two types of telescopes on Earth, and the others are all in orbit). That's good for us and our desire to remain alive.

Atmospheric opacity saves lives. Source.
However, our being able to see nanometer-scale wavelengths does not make sub- or super-micrometer photons somehow not "light."

It's a spectrum, yo
All photons, at all wavelengths/energies/frequencies, are just part of a spectrum, specifically the electromagnetic spectrum. Radio waves have low energy, low frequency, and long wavelength, but they're still light. Just like high-energy, high-frequency, short-wavelength gamma rays are also light.

Since we can't see these photons, we don't think of them as having "color" because a color is a thing you can see, right?

Well, really, a color is just a specific wavelength/frequency, and changing color is just changing the wavelength of photons. So, really, the electromagnetic spectrum is just a bunch of awesome colors that you can't see.

When astronomers make simple radio, gamma ray, infrared, or X-ray images, they say, "Okay, so at this particular wavelength, we saw this brightness here, that brightness there, and that other brightness over at that other place. We'll say that blue means really bright and red means not-so-bright, and we'll fill in the intermediate brightnesses with the intermediate colors. K?"

They are taking visible colors and applying them to nonvisible light to help us understand things we can't see in a way that still makes sense to our senses.

For more complicated images, both frequency and brightness information can be embedded.

Huh. So.
Sometimes, when we think of light like radio waves and X-rays, we think of them in relation to the way we use them on Earth: X-rays allow you to see through people. Radio waves keep you from being so bored in traffic.

However, while X-rays do show you what's inside, they themselves are not what's inside--they themselves are just highly energetic light.

Likewise, while radio waves are used in communication, they themselves are not communication, or sound. Radio waves are used as carriers for the Justin Bieber and enlightening NPR anecdotes that you love to listen to. Sound waves are pressure waves that have to move through a medium (air, in Earth's case). Light waves are electromagnetic waves and don't have to move through a medium (which is why they can travel through empty space). Sound waves have much lower frequencies than radio waves, but remember--this is the frequency air which the pressure in the air is changed every second, and the frequency of the radio wave is how many times a photon travels a full wave-path in one second.

Information that can be turned into sound waves is modulated on top of light waves with radio frequencies--the frequency of the radio station you're listening to. Radio waves, with their long wavelengths, can travel relatively unimpeded and also do not give people cancer like high-energy waves can. But unless a person (or a little green [wo]man) encodes the sound information in radio waves, the radio waves are just low-energy photons, traveling through space and waiting to hit someone's gigantic telescope, or creepily large eye.

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