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Is it accurate to say 99% of the universe is pure black with no sound?

  1. Oct 4, 2014 #1
    Considering the majority of the universe is intergalactic space and that light gets dimmer by the inverse square law. If you pick a random place in the universe you are going to see no light (Light that's bright enough for the eye) and hear no sound. Is this accurate to say?
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  3. Oct 4, 2014 #2


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    If you're in space, you definitely won't hear any sound as there is no air to transmit any sound. As far as light though, you would probably still see the galaxies around you, unless you were in a particularly "void" area of the universe. Galaxies are very very bright, and the separation between galaxies tend not to be so dramatic as the separation between stars (as long as you're not in a "void" area).
  4. Oct 4, 2014 #3


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    In a true void, there is no sound.

    But, the layman's sense of sound being something that you can hear with your ears, and the physicists sense of sound being coherent waves of motion of particles transmitted electromagnetically are different. The broader physicists sense admits the possibility of sound transmission within a fairly diffuse cloud in hydrogen and helium gas in the interstellar medium far from any star or atmosphere even though you could not hear that kind of sound with your ears.

    My intuition is that the proportion of the universe in which there was no sound in the broader physicists sense might be large (50%?) but probably not 99%.
  5. Oct 4, 2014 #4
    Oh, there is light (photons) everywhere due to cosmology. It is a thermal radiation at 3K called the cosmic microwave background, the light that permeated the universe at the end of the Hot Big Bang era. You can see the problem, it isn't that this radiation is too dim yet, many observatories have studied it. (It gets dimmer as the universe expands.) It is that humans don't have microwave 'eyes' to see it with.

    Also, dark adapted eyes are photon detectors, so you will catch about 1 in 10 photons (IIRC) of the wavelengths that we see with in dim environments. (The rest misses the receptors or don't set off the chemical changes that eventually amplifies to a nerve signal.) Hence you will probably see star light most everywhere. Except perhaps inside dense molecular clouds that absorb it and return it as thermal radiation on wavelengths that we are insensitive to.

    But that is where you can 'hear' sounds. Or at least some type of sensor would:

    "There is a situation in which sound can propagate across space, when sound travel through an interstellar gas cloud. Even though they look thick and puffy, like the clouds after which they are named, a typical nebula (Latin for "cloud") is not really much more substantial than a vacuum. The atoms in the cloud is pretty far apart, but even a few atoms per cubic centimeter adds up when you are talking about a nebula trillions of kilometers thick. These atoms can bump into each other, allowing sound to travel through the cloud. [Goes on to mention that most of those sounds are shock waves, so are more easily seen from afar as sheets and filaments than monitored from within as moving particles.]"

    [ http://books.google.se/books?id=eInnwg77gbkC&pg=PA246&lpg=PA246&dq=bad astronomy sound in space&source=bl&ots=e4KdvW8SeP&sig=2O4Y92GSj7XKvykMRXsXJ-ywsso&hl=sv&sa=X&ei=2-8vVPjjCYX5ywPQ6oKQAw&ved=0CDkQ6AEwAw#v=onepage&q=bad astronomy sound in space&f=false ]

    Ears happen to work at quantum noise levels too. If a human ear could hear those shock waves in practice without a space suit before he/she dies, I'm not sure. The signal is dim, variations between atoms with atom densities of a "few atoms" on scales of cm. The tympanic membrane is ~1 cm2. [ http://www.chicagoear.com/Ear Surgery/tympanoplasty.html ]

    More likely you will hear your own scream, as gas molecules from exhaling lungs will bounce back on cloud atoms. :))
    Last edited: Oct 4, 2014
  6. Oct 4, 2014 #5


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    Without other light sources nearby, the Andromeda galaxy at a distance of 2.5 million light years is nicely visible at an apparent magnitude of 3.44. To fall to an apparent magnitude of 6, it would have to be 8 million light years away. Under excellent viewing conditions (and we would certainly have them if we worry about seeing anything!) we can increase the distance even more. The Andromeda galaxy is nothing special - it just happens to be the largest nearby galaxy apart from our own. A significant fraction of the universe is closer than 10 million light years to some similar galaxy. Some galaxies are signficantly brighter, so they cover much more volume. As a rough guess, I would expect the fraction of the observable universe where you can see something with the naked eye to be somewhere around 5% to 50%. This number increases if you have enough time - supernovae can outshine their galaxy, but very bright versions are rare. It also gets 100% if you use good binoculars.
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