Physics: Principles and Problems

 

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The Lowest Note in the Universe
November 2003

Picture a piano keyboard. The farther to the left your hand goes, the lower the note being played. The bottom key isn't the lowest note there is, though. A double-bass in a symphony orchestra could play a note lower than that. And a synthesizer could play a lower note than a double-bass. Beyond that, there are pitches so low that we can't even hear them--they are beneath the relatively small range of frequencies the human ear can pick up. But just because we can't hear such low frequencies doesn't mean they don't exist, any more than pitches that are too high for us. Just ask a dog!


The Note No One Knew Was There

Just recently, a team of scientists in England accidentally ran across a note so unbelievably low that it set a new record--one unlikely to be broken any time soon. What kind of scientists were they? Not acoustical engineers, not recording technicians, but astronomers. The lowest note in the universe is being generated by a black hole.

The astronomers even know what note it is in particular: it's a B flat. Only this B flat is about fifty-seven octaves below the one you find in the middle of a piano. Think of it this way: you can hear Middle C by playing the key that is right in the center of a keyboard; move down twelve keys to hear the same note one octave lower. Now, imagine being able to do that fifty-six more times. That super-low note, many millions of times lower than any animal's ear could register, is occurring out there in space. Way out there--the musical black hole is a good 250 million light years away.


A Singing Black Hole?

A black hole is the remains of a star that has used up most of its nuclear fuel, run through the course of its life, and collapsed under its own gravitational weight. When a black hole forms, it creates a region in space into which anything nearby will be drawn, including light itself. That's why black holes are black.

This particular black hole is at the center of a galactic cluster called the Perseus cluster. A “galactic cluster” is just what it sounds like--many galaxies all huddled together, comprising billions of stars, along with a big cloud of interstellar gas.

The black hole itself only pulls things in, but because it is so powerful, all sorts of effects happen to nearby matter as it is violently hauled down. One result is that huge numbers of particles are actually fired away from the region of the hole in titanic jets. These jets crash into the gas cloud that surrounds the hole and cause a series of pressure waves to ripple outward, much like the way water in a pond ripples out from the spot where someone has leapt in. Pressure waves traveling through any medium are what constitutes sound. And, if the waves have a particular frequency, we call it a note. As unlikely as it seems, the entire Perseus cluster is reverberating like a plucked string.


Imagine This: A Galactic Symphony

Astronomers at the university of Cambridge in England who discovered the distant B flat now speculate that many galactic clusters, and even individual galaxies, could be generating ultra-low pitches in this way. If they’re right, it will help explain how galaxies form, why they have the temperatures they do, and more. To us humans, the idea of an entire galactic cluster acting like a musical instrument is bizarre and somehow charming. More than that, though, the energy released by the steadily out-going pressure waves may be a significant component in the structure and functioning of these enormous systems.

Activity
Find a piano and have someone show you where B flat is. Then, with your eyes closed, listen to the notes each key makes while moving downward, one key at a time. Can you tell when you have come to another B flat, one octave lower?

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