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."
ROFL!
As to your inquiry, let's ask the FR resident brainiacs (my background is in carbonate geochemistry). Gentlemen, would you have any comments on Post 19? Thanks!
It does, of course, in 1st and in 2nd person singular. But, our English teachers told us to stop using 2nd person all the time, and we still haven't progressed to the general [affected] use of the pronomial one in place of the 1st or 2nd person. Using one for the 3rd person is nonsense. The astronaut example could be phrased in the pronomial one. One could remove one's helmet.
Yes, in a way. The fossils aren't to be found in Earth's rocks, of course, but in the nebula surrounding the black hole. Per the article:
Every 10 million years, a fresh wave bubbles out of the system.
...
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.
So there you have it: a series of concentric density waves, spaced at 10-million-year intervals, centered on the black hole.
What they are "seeing" are ripples in the hot gas inside the cluster that the black hole resides in. By measuring these ripples and the effects from them, the sound waves can be calculated.
Remember the speed of sound is far less than the speed of light. The size of the ripples are about 30,000 light-years.
And on a unrelated note, I have a question about simulataneous events in relativistic frames of reference. A few years ago, we talked about the seemingly paradoxical nature of the old "ladder speeding through a barn" problem. And how the differing sequence of events in the two frames of reference resolve the question of what happens (or is seen to happen) when the ladder is (is not) fully within the barn, and the barn doors are closed. Namely, in the barn's frame of reference, the doors may be closed simultaneously around the ladder (enclosing it) and the ladder is seen to bursts through the far door, while in the ladder's frame of refernence, the door (not yet behind it) is still open while the ladder blows throught the closed door ahead of it, therefore never being enclosed by the barn. Here's my question: What if some third event is contingent on two events (say the closing of the two doors) occuring simultaneously. In the barn's frame of reference, this third event would occur, while in the ladder's frame of reference it would not. My suspicion is that any experiment which I could set up to point out this paradox would require some kind of "spooky action at a distance." What do you think?
It seems to me that if some process occurs once every 10 million years, then a byproduct travelling at the speed of light will be 10 million light years away at the next occurence.
Indeed if the sound waves were traveling at c, they would be. The waves heat up the gas, which then emit X-rays.
But it is the concentric rings of X-rays which is actually observed, right? Wouldn't these rings grow to be 10 million light years in diameter between cycles?
Yes. :-)
Nevermind, I think I had a revelation in the shower just now (or the retrieval of a suppressed memory from undergraduate days) that simultaneity in time, without consideration of location, is meaningless.
Shades of Bill Clinton, whose 'boss' was the American feminist movement. Seems to be a common theme, that when a guy goes overboard with the window dressing and trappings and affectations of feminism, you know he's a real lout with women in his personal life.
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