Posted on 09/22/2003 11:51:44 PM PDT by Aracelis
It's often said that in space, you can't hear yourself scream. True enough, more or less, but rather misleading. Recently, several SPACE.com readers wrote to ask how a B-flat emanating from a black hole could be detected from 250 million light-years away, as we reported earlier this month.
The answer, along with related interesting facts, reveals that silence is in the ear of the beholder, and ears come in a variety of configurations.
Sound can travel through space, because space is not the total vacuum it's often made out to be. Atoms of gas give the universe a ubiquitous atmosphere of sorts, albeit a very thin one.
Sound, unlike light, travels by compressing a medium. On Earth, the atmosphere works well as a sound-carrying medium, as does water. The planet itself is very adept at transmitting an earthquake (news - web sites)'s seismic waves, a form of sound.
Space, though not as efficient, can also serve as a medium.
If a brave and clever astronaut could safely remove her helmet and shout into the cosmos, her voice would carry.
"We wouldn't be able to hear the sound because our ears aren't sensitive enough," explains Lynn Carter, a graduate student in astronomy at Cornell University. Not enough atoms -- if any -- would strike our eardrums. "Maybe if we had an amazingly large and sensitive microphone we could detect these sounds, but to our human ear it would be silent."
An amazingly sensitive microphone, in a sense, was used to discover the constant B-flat coming from the black hole [Story here]. NASA (news - web sites)'s Chandra X-ray Observatory observed gas, compressed by the sound, in concentric rings much like ripples on a pond.
Seeing sound
Andrew Fabian of the Institute of Astronomy in Cambridge, England, explained in an e-mail interview how the sound was generated and why its signature crossed the cosmos.
The black hole under study sits amid a cluster of galaxies, a region of space where gas is denser than the universe on average.
Playing the role of speaker membranes in the galaxy cluster are two huge cavities, filled with gas that is hotter than its surroundings. This heat is generated by energy shot out from the black hole's environment as it accelerates matter to nearly the speed of light, just prior to swallowing it.
"The repetition needed to make the sound into a note is due to the cavities being buoyant -- the ultrahot gas is thinner than the cluster gas," Fabian said. "So the process resembles what happens when a child blows through a straw into a glass of milk."
Every 10 million years, a fresh wave bubbles out of the system.
"Sound waves are waves or ripples of pressure traveling through a gas," Fabian said. "Displace some nearby particles by pushing -- say the membrane of a loudspeaker -- so there's a pressure peak, and those particles will push on particles further out and so on. The result is that the pressure peak moves outward, although no individual particle actually goes very far from its original position."
Ocean waves work similarly. A swell can travel thousands of miles, but it moves through the water rather than packing the molecules along.
As the pressure peaks travel outward from the cavities around the black hole, collisions occur between atoms in the gas, generating X-rays that reveal a concentric ring pattern. Being a form of light, X-rays can traverse the universe sans any medium, and these are what Chandra detected.
The sound waves rapidly die out, their energy converted to heat. So in essence the B-flat was seen, not heard, from 250 million light-years away.
Martian sounds
Our astronaut in space would experience something quite different from one on Mars, where sound could be heard with a modest microphone and some technological help. The Martian atmosphere is less than 1 percent as dense as Earth at sea level. This probably is not enough to carry sounds that the human ear could detect, experts say.
Of course, anyone outside on Mars would be wearing a pressurized suit, so the only sounds they'd hear would have to be electronically delivered. There's no reason a sensitive external microphone could not be used to pick up sounds and amplify them to some terrestrial approximation.
In fact, NASA had plans to listen to the natural sounds of Mars in this manner with a microphone aboard the 1999 Mars Polar Lander. The idea was to hear wind, blowing dust, perhaps even lightning within dust storms.
The microphone was tested under Mars-like laboratory conditions.
"Even at Mars' low pressure, acoustic signals within the frequency range of the human ear can be detected," said Greg Delory, who at the time was a postdoctoral physicist at the University of California, Berkeley.
Mars remains silent to earthlings, because Mars Polar Lander failed to land properly and was never heard from. The microphone idea lives on, though. Another one is slated to fly on the French NetLander mission to Mars in 2007.
And who knows what we'll hear? As Delory said, "The most exciting sounds are likely to be ones that we don't even know about yet."
Only if "Her" name is Hillary....
Not to pick nits, but are earthquake waves really longitudinal/compression, rather than transverse?
This is where I stopped reading. The gender of the pronoun should be determined by the likelihood of an astronaut to be one or the other. Since more than 90% of astronauts (I'm guessing) have been male, using "her" in the sentence is just political correctness.
I knew a male english professor cum bureaucrat who would always do that, and make a big point of casualness when he did. He worked for a woman. He was also privately alleged by several women who worked under and around him to be a serial harasser to the point of being a borderline rapist.
There's a physics reason. Very few males are sufficiently shrill to do this.
Traditionally, the masculine is generic. "Mankind" refers to all humans, not just men ;)
Actually, there are several types of seismic waves:
P waves - Compressional waves, Primary waves, Longitudinal waves
These waves alternate compressions with dilations, and are directed in the direction of propagation (perpendicular to the wavefront). P-wave motion travels fastest in solid materials, and are the first-arriving energies on a seismogram (P-wave). P waves in liquid, gas, or air are pressure waves.
velocity = about 5 – 7 km/s in Earth’s crust; about 8 km/s in Earth’s mantle and core; about 1.5 km/s in water; 0.3 km/s in air
S waves - Shear, Secondary waves, Transverse waves
These waves oscillate in a transverse fashion perpendicular to the direction of propagation either in the vertical or horizontal plane. S-waves do not travel through fluids, air, water or molten rock. A liquid outer core has been inferred from their absence in seismic records. S-waves travel more slowly than P waves in solids, and therefore arrive after the P wave.
velocity = about 3 – 4 km/s in Earth’s crust; about 4.5 km/s in Earth’s mantle; about 2.5 - 3.0 km/s in Earth's solid inner core
L waves - Love waves, Surface waves, Long waves
L-waves display transverse horizontal motion, perpendicular to the direction of propagation and parallel to the Earth’s surface. Love waves are largest at the Earth's surface and decrease in amplitude with depth. Love waves are dispersive, with lower frequencies normally propagating at a higher velocity. Depth of penetration is also frequency-dependent, with lower frequencies penetrating to greater depths.
velocity = about 2.0 - 4.5 km/s depending upon the frequency of the propagating wave
R waves - Rayleigh waves, Surface waves, Long waves, Ground roll
R-waves move both in the direction of propagation and perpendicular to the vertical. Motion is generally elliptical. Rayleigh waves are dispersive, and amplitudes generally decrease with depth inside the Earth. Their appearance is similar to water waves.
velocity = about 2.0 - 4.5 km/s in the Earth depending upon frequency of the propagating wave
And also this kind of WAVE.
I cringe at the specter of gender-free language, wherein we will be forced to refer to that reknowned English soccer team as "Peoplechester United" and those round steel thingies in the sewer sytem as "personhole" covers.
Gyno-American Grammar is evil.
... I'm ... curious about the original article. Consider the introduction:"B-flat flying through space ... 57 octaves below the keys in the middle of a piano."
Wow, 57 octaves below B-flat, eh?
Assuming we are using A-440 Hz (and not some pre-Bach value), the B-flat below middle-C vibrates a medium only 233.0818808 times per second.
57 octaves below that is roughly 1.61733e-15 vibrations per second. This means that the period of the wave is the inverse of that number, or 6.18303e+14 seconds. But, ...
6.18303e+14 seconds
equals
1.71751e+11 hours
equals
7156282833 days
equals
19606259.51 years.Call me suspicious, but how exactly does one make a measurement of a physical process which occurs once every 19 million years? Is there something in the fossil record, perhaps scratchings on a rock thanks to an inquisitive T. Rex?
Granted, if the note in question is 57 octaves below the B-flat which is above middle C, then you only have to wait half that long.
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