How empty is the universe?

That's a tricky question to answer, because no region of space is really completely devoid of matter, and even regions which would classically have no matter are constantly creating and destroying matter-antimatter pairs.

The regions between stars and galaxies are actually very low density plasmas, rather than a complete vacuum.

And even in dense regions, atoms are mostly empty space themselves.

It's better to talk about the density of the universe, which Google tells me is 2x10^-29 kg/m^3.
That corresponds to 1 proton per 100m^3.
 
Undesired Walrus said:
By 'empty' I mean areas devoid of matter.
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I'm afraid that doesn't make anything clearer. By what standard do you consider an area to be "devoid of matter"? How dilute would a cloud of interstellar gas have to be to pass as an area "devoid of matter"? For example, in the vacuum of nearby interplanetary space, you can still expect to find several million ions in every cubic meter, which is quite a lot compared to some other areas of the universe, so is that empty or not?

Undesired Walrus said:
Is it like 99.999% empty?
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Well, that's not a well-defined question. But because I wouldn't like to answer a question with mere whining without giving any actual facts, I'll try to answer the spirit of your question. You could get an idea about the emptiness of the universe from its average density, which is believed to be on the order of 10-27 kg/m3 - that's a density on the order of several hydrogen atoms per cubic meter.
 
I could say you are nearly empty matter. The distance between atoms is fat greater than the size of the atoms in your body. The good news is that everyone is the same.
 
Are we looking at something in the range of 0.0001%?

I suppose my OP was badly formed if I wanted scientifically literate people to explain it. What I was really looking for was an answer of how much of the Universe is black space (To our eyes).
 
You should be able to calculate it from the number of objects visible to the naked eye and the area you can observe - horizon to horizon - by making some assumptions about the average perceived size of the objects. I don't really see the point, though.

Leon
 
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Undesired Walrus said:
Are we looking at something in the range of 0.0001%?

I suppose my OP was badly formed if I wanted scientifically literate people to explain it. What I was really looking for was an answer of how much of the Universe is black space (To our eyes).
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Define "black space".
If you mean stuff that is not luminous at all then 96% of the energy and mass is dark matter or dark energy. Or do you mean just in the visual spectrum?
Or do you want the volume? In that case we are looking at the range of 0.0001%
 
Undesired Walrus said:
Visual spectrum.
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Then you can calculate it:
Number of galaxies in the observable universe ~ 100 billion.
Assume all of these are like the Milky Way: ~ 200 billion stars.
Assume all of these are like the Sun which has a volume X (you look it up).
Look up the volume of the observable univserse Y.
Then you want 100 billion * 200 billion * X / Y
 
Here's a rough estimate you can do in your head.

age of the universe: 13 billion years -> radius is 10^17 light seconds.
radius of the sun = 2 light seconds.

Volume of sun/volume of observable universe = 10^(-3*17) = 10^(-51)

Multiply by 10^22 stars, and the fraction of the volume taken by stars is about 10^-29. That's 0.00000000000000000000000000001, or 0.000000000000000000000000001%.
 
sol invictus said:
Here's a rough estimate you can do in your head.

age of the universe: 13 billion years -> radius is 10^17 light seconds.
radius of the sun = 2 light seconds.

Volume of sun/volume of observable universe = 10^(-3*17) = 10^(-51)

Multiply by 10^22 stars, and the fraction of the volume taken by stars is about 10^-29. That's 0.00000000000000000000000000001, or 0.000000000000000000000000001%.
Click to expand...

H'ethetheth said:
If by that you mean a small number, then yes. However I'm quite sure that 0.0001% is still too large by at least 5 orders of magnitude.
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...or 20 :D
 
I asked a related question once on the Bad Astronomy forums a while back:

http://www.bautforum.com/space-astronomy-questions-answers/64709-how-much-empty-space.html

There was a lot of interesting discussion including this, reposted from howstuffworks:

howstuffworks said:
This is interesting. I read about this on "Howstuffworks.com" and this is what they had to say:
----------------------------

If you were to move all of the matter in the universe into one corner, how much space would it take up?

It's hard to answer this question exactly because there are some unknowns. But if you are willing to accept three assumptions, we can come up with an answer that sounds reasonable...

The first question is, "How big is the universe?" No one knows, but this Question of the Day assumes that the universe is a cube that is 30 billion light years on each side. That means that the whole universe contains about 2.7E+31 cubic light years.

The next question is, "How much matter does the universe contain?" The mass of the universe is a source of debate right now because there is no easy way to put the universe on a scale. This NASA page and this " Extension, Age and Mass of the Universe" article discuss different techniques that scientists use to estimate the mass. The latter article also includes an estimate of about 1.6E+60 kilograms for the mass of the universe. Other estimates give other numbers, but all are in that ballpark.

The next question is, "What density do you want to assume the mass will have once you push all of it into one corner?" Now, if you were really to do this -- if you actually did move all of the mass of the universe into one corner -- it would condense instantly into a black hole and potentially ignite another big bang. But let's say that you could keep it from doing that, and you were somehow able to keep all of the mass evenly distributed at the density of the sun. According to "Magnitudes of Physics", the density of the sun is about 1,410 kilograms per cubic meter. (For comparison, the density of water is 1,000 kilograms per cubic meter.)

If you are willing to accept these three assumptions, then:

1.6E+60 kilograms / 1,410 kilograms per cubic meter =
1.1E+57 cubic meters of matter in the universe

A cubic light year contains about 1E+48 cubic meters. So all of the matter in the universe would fit into about 1 billion cubic light years, or a cube that's approximately 1,000 light years on each side. That means that only about 0.0000000000000000000042 percent of the universe contains any matter. The universe is a pretty empty place!

http://science.howstuffworks.com/question221.htm
Click to expand...
 
That's interesting. What would be the size comparison of that? A watch in the Atlantic? A sand grain in the Sahara? There's no way I can grasp these things without these examples.
 
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