Happy Monday and welcome to another edition of Beyond The Forecast!
Last week some astonishing news came from our friends at the National Atmospheric and Space Administration: NASA astronomers discovered a sound emanating from the center of the Perseus Galaxy Cluster more than 73,600,000 parsecs away from Earth (which is quite a bit longer than the Kessel Run); to translate that into somewhat more understandable terms, a parsec equals 19,173,511,575,400 miles. Let’s just say it’s a galaxy cluster far, far away.
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But this fact didn’t quite sit right when it hit my ears. As the tagline to the movie “Alien” famously told us, “In space no one can hear you scream,” and on top of that, it’s generally understood that anything that falls into a black hole’s gravity has no hope of escaping. So just how can this sound exist?
As it turns out, there’s more going on than meets the eye in this instance (or ear if you prefer). Let’s start with how sound travels first.
A good contrast to the way sound travels is light. Light particles (photons) travel from light sources all on their own. They don’t need to move through anything to reach us. That’s why light from the sun can reach us even though there’s nothing but empty space to pass through until it reaches the atmosphere.
Sound, on the other hand, needs something to move through. Photons are independent pieces of energy, but sound is a wave of pressure that transfers from molecule to molecule. If someone snaps their fingers the air between their finger and their palm gets compressed as the finger moves down and then rapidly expands outward. That compression and expansion travel through the molecules in the air until it reaches your ears. Specialized bones and nerves are able to detect that change in pressure and our brains interpret that as sound.
The Perseus Cluster goes through a process that in some ways is very similar to a finger snap. The black hole at its center moves and sends pressure waves out. If the black hole was all on its own, it wouldn’t matter how much it moved: if there is no medium for the pressure wave to travel through, it stops when it reaches the end of the medium. The Perseus Cluster is special in that the hundreds or possibly thousands of galaxies within it carry a massive amount of space dust and gases. They are packed closely enough that the wave can travel from the black hole at the cluster’s center out to its extremities. Astronomers can map how the wave moves and turn it into a sound.
The raw sound of those waves is far too low for humans to hear. NASA reconfigured the data and brought the pitch up by almost 60 octaves. That’s high enough that we can actually hear the same pattern that the cluster experiences.
The answer to the first question (how does the sound travel) answered my second question (how did something get transmitted by the black hole). This is less an example of something coming out of a black hole than it is an example of inertia. The black hole moving creates a compression of some particles that then expands and propagates itself along the length of the cluster.
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-- Marshall Downing