Let’s say you stretch a rubberband between two clamps and twang it. You’ll hear a musical note of some kind. The tighter you string it, the “higher” the note. What’s happening is that the tighter you string it, the faster the band vibrates—that is, the higher the frequency of its vibrations. Correspondingly, the looser you string it, the “lower” the note.
Let’s say now that you have a way of tightening or loosening the band to choose the frequency at which it vibrates. Picking a number totally at random, you set it to vibrate 262—slightly below 262, but never mind—times a second (usually written hertz, or Hz).
That is, if you think of the sound coming at you in waves, 262 of them arrive at your ear every second; alternatively, they are spaced about four thousandths of a second apart.
In any case, with that frequency, what is the note you will hear?
That’s right: the note commonly known as “middle C” on a piano. “C” is of course like “sa” of the “sa-re-ga-ma” scale in Indian classical music, in that it’s the first note of the scale; though the “sa” nearest to middle C on a harmonium, say, usually corresponds to a frequency of 240 Hz.
If you now want to hear the next higher C—or the next higher sa—you need to tighten the band so that it vibrates exactly twice as fast: 524 Hz for C, 480 for sa. That is, the relationship between notes exactly one octave apart is that the higher note’s frequency is double that of the lower note.
On a typical 88-key piano, there are four higher Cs than middle C. Doubling as you go, you’ll find that means the highest C has a frequency of 4,192 Hz. Similarly, there are three lower Cs; halving three times, the lowest one, three octaves below middle C, turns out to have a frequency of 33 Hz.
With me so far? If you’ve heard it played, you know that the lowest C is a deep, rumbling note. Remember that as I tell you about a sound that astronomers discovered in 2003, made by a black hole about 250 million light years away. It’s not that they actually heard it, you understand. That would be somewhat difficult, as you can imagine and will soon see. Instead, they detected the sound waves that a black hole emits. From the frequency of the waves, they knew something remarkable: This is the deepest “sound” we have ever detected anywhere in the known Universe. It is considerably lower and deeper than that lowest C on a piano keyboard.
How much lower and deeper? One more octave? Ten more? Not even close. No, this note is a full—wait for it—57 octaves below the middle C. That is, you would have to halve the middle C’s 262 Hz figure 57 times to calculate the frequency of this black hole’s sonic outpourings. I’ve done it for you: this frequency is about 2 million billionth of a hertz. In other words, these sound waves are spaced about 17 million years apart.
Move your rubberband once, and then again 17 million years from now (okay, pick a Friday). What kind of sound does that generate? What can it possibly mean to call it a sound? Well, that’s what we’re “hearing” from that black hole: sound waves whose crests are separated by 17.5 million years. Or “seeing”, really: From X-ray images of the black hole, we know it is surrounded by a vast cloud of gas, and these sound waves ripple through the cloud.
Still, if strange sounds are your thing, there have been some from right on this planet, though from a spot nearly as mysterious as this black hole.
In 2014 and 2015, submarine probes wandering in the Pacific Ocean near the Mariana Trench—the deepest part of our planet’s oceans—repeatedly detected a strange, haunting sound that was dubbed the “Western Pacific Biotwang”. Just a few seconds long, it features—as the scientists who found it wrote in their paper—“a 38 Hz moan… followed by broad-frequency metallic-sounding sweeps up to 7,500 Hz”: that is, from about the lowest C on a piano to a note nearly an octave beyond the highest C (listen to the Biotwang here).
What was causing this mysterious, beautiful, almost wistful sound? In the paper A Complex Baleen Whale Call Recorded In The Mariana Trench Marine National Monument by Sharon L. Nieukirk et al (Journal of the Acoustical Society of America, September 2016), the scientists remarked that it was “not similar to (the) noise produced by ships or seismic airguns (nor to) earthquakes and ice (or) wind or rain.” It had to be, they believed, “a biological source”: some marine beast, lurking in the Trench.
In fact, they believe it is a sub-species of minke whale, an animal that lives in all our oceans. Minkes are rarely seen, but make regular sounds which we can hear. The problem is that minkes usually call like this only at mating time, in the winter—but the Western Pacific Biotwang has been recorded around the year. Why these steady plaintive appeals, then? Are they in fact plaintive?
We don’t know. But the scientists are planning another trip to the Mariana Trench to find the actual animals responsible and work out why they are calling. “It really is an amazing, weird sound,” says Nieukirk, “and good science will explain it.”
She could, of course, have been speaking of that distant black hole too. Though it is a little harder to plan a trip there.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter Of Numbers explores the joy of mathematics, with occasional forays into other sciences.
Comments are welcome at dilip@livemint.com. Read Dilip’s Mint columns at www.livemint.com/dilipdsouza
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