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speach
2015-Aug-14, 04:41 AM
Now the universe is 1026 meters or 1028 cm to it boundary, and astrophysics say that there is 1 atom of H for every cubic cm of space. The size of the H atom is1.06-12 cm. So between us and the most distance object that we see there must be a least 2cm of solid H, so then why do we see the far off galaxies as well as we do? My simple mind says that we shouldn't see them at all or if we do they should appear as if looking through fog. What have I got wrong?

chornedsnorkack
2015-Aug-14, 05:35 AM
Now the universe is 1026 meters or 1028 cm to it boundary, and astrophysics say that there is 1 atom of H for every cubic cm of space. The size of the H atom is1.06-12 cm. So between us and the most distance object that we see there must be a least 2cm of solid H, so then why do we see the far off galaxies as well as we do? My simple mind says that we shouldn't see them at all or if we do they should appear as if looking through fog. What have I got wrong?
Air has the weight of 10 m of water. Yet we do see heavenly bodies through air. Air does scatter some light, gets blue and dims objects near horizon - but it is not as opaque as fog.
How does the column thickness of intergalactic matter compare against the column thickness of air?

speach
2015-Aug-14, 07:43 AM
Air has the weight of 10 m of water. Yet we do see heavenly bodies through air. Air does scatter some light, gets blue and dims objects near horizon - but it is not as opaque as fog.
How does the column thickness of intergalactic matter compare against the column thickness of air?

No you don't see the point. This is solid Hydrogen not gaseous like air. You see the air in our atmosphere is just empty space between the molecules. when air is cooled to it's liquid or solid form it's opaque.

chornedsnorkack
2015-Aug-14, 08:13 AM
No you don't see the point. This is solid Hydrogen not gaseous like air.
1) Interstellar hydrogen at 1 atom per cubic cm is gaseous, not solid

You see the air in our atmosphere is just empty space between the molecules. when air is cooled to it's liquid or solid form it's opaque.
2) Liquid air is not opaque - it is transparent like liquid water.

malaidas
2015-Aug-14, 09:17 AM
OK firstly, I just want to clarify your terminology, I am guessing this is what you mean btw but we dont actually know the size of the universe itself, we know the size of the observable universe. This is justy a minor niggle

however what I really want to focus on is this, where have you got solid hydrogen from? It is true that the average temperature of objects in space is below the stated melting point of hydrogen, and they don't actually state the pressure because melting point is always given at atmospheric pressure. But critically melting point depends upon pressure. space is a near vacuum and as such there is insufficient pressure for the hydrogen to solidify. there is lots of hydrogen out there and it is all gaseous or plasma.

antoniseb
2015-Aug-14, 12:11 PM
... and it is all gaseous or plasma.
Almost entirely plasma, until you get further than z=6. Also, I think the OP overestimated the amount of Hydrogen. It is about 1 proton per cubic centimeter in the galaxy, but it is much less in the space between clusters, perhaps as low as 1 proton per cubic meter.

malaidas
2015-Aug-14, 12:19 PM
Thanks

Jeff Root
2015-Aug-14, 02:11 PM
A single atom or a single diatomic molecule cannot be and
cannot act like a solid, nomatter how cold it is.

But on the other hand, if nearly all the hydrogen is plasma,
or separate protons and electrons, shoudn't it make space
look pretty foggy? Or even glow like the Sun's corona?

I suppose that to glow, the particles would have to collide
with each other, which would be extremely rare. So almost
no glow at all. But photons, travelling much faster and thus
covering much greater distances in the same time, should
have more frequent interactions with the charged particles.
One would think that would make the sky distinctly murky
compared to how it would look if the hydrogen were nearly
all neutral atoms or molecules.

-- Jeff, in Minneapolis

malaidas
2015-Aug-14, 02:25 PM
A single atom or a single diatomic molecule cannot be and
cannot act like a solid, nomatter how cold it is.

But on the other hand, if nearly all the hydrogen is plasma,
or separate protons and electrons, shoudn't it make space
look pretty foggy? Or even glow like the Sun's corona?

I suppose that to glow, the particles would have to collide
with each other, which would be extremely rare. So almost
no glow at all. But photons, travelling much faster and thus
covering much greater distances in the same time, should
have more frequent interactions with the charged particles.
One would think that would make the sky distinctly murky
compared to how it would look if the hydrogen were nearly
all neutral atoms or molecules.

-- Jeff, in Minneapolis

well atoms are mainly transparent when it comes down to it

Cougar
2015-Aug-14, 05:45 PM
But on the other hand, if nearly all the hydrogen is plasma,
or separate protons and electrons, shoudn't it make space
look pretty foggy?

I welcome correction, but I believe most of the hydrogen is plasma because most of it is tied up in stars, which are hot. ;)

Which brings me back to speach's original post, wherein s/he says....

...astrophysics say that there is 1 atom of H for every cubic cm of space.... [far off galaxies] should appear as if looking through fog. What have I got wrong?

Isn't "1 atom per cubic cm" an account of the {total amount of baryonic matter} spread out into the {total amount of space in the visible universe}? But this isn't how the baryonic matter is distributed throughout the visible universe. By now, most of it has gravitationally collapsed into stars and galaxies. The amount of space between stars is huge compared to the size of the stars, and that's much more the case with galaxies. That "cleaner" space seems to propagate light quite well. But it's even detectable after it has made its way through gas, which leaves absorption line signatures for that gas. As wiki says "Astronomical spectra contain both absorption and emission spectral information. Absorption spectroscopy has been particularly important for understanding interstellar clouds (https://en.wikipedia.org/wiki/Interstellar_clouds) and determining that some of them contain molecules (https://en.wikipedia.org/wiki/Molecular_cloud). Absorption spectroscopy is also employed in the study of extrasolar planets (https://en.wikipedia.org/wiki/Extrasolar_planets)...."

Jeff Root
2015-Aug-14, 06:10 PM
The amount of space between stars is huge compared
to the size of the stars, and that's much more the case
with galaxies.
I'll disagree with that last clause. Galaxies generally
occupy much larger volumes relative to the distances
between their centers than stars do. In other words,
stars in a galaxy are farther apart from one another
than galaxies are, in proportion to their size.

Pop up in a random location in the Universe, and you
have a reasonable chance of being in a galaxy. Pop up
in a random location in a galaxy, and your chances of
being in a star are very, very small.

-- Jeff, in Minneapolis

Cougar
2015-Aug-14, 07:13 PM
Pop up in a random location in the Universe, and you
have a reasonable chance of being in a galaxy. Pop up
in a random location in a galaxy, and your chances of
being in a star are very, very small.

That's an equivalent way of putting it, but I'm skeptical about your point regarding the galaxies and the space between them. It is so in the local group, but when you consider the voids throughout the universe, your odds are going to go way down for landing in a galaxy.

Cougar
2015-Aug-14, 07:16 PM
I'll disagree with that last clause....

Thanks for not disagreeing with the rest of it. :lol:

speach
2015-Aug-14, 11:28 PM
Thanks' everyone, can see all your points.

WayneFrancis
2015-Aug-18, 01:49 AM
I'm going to approach this a bit differently.
OK so what happens when a photon passes a hydrogen atom?
It can get absorbed and excite an electron into a higher orbital. That electron then can fall back down to a lower energy state and emit one or more photons. But this requires the photon to either be in a very precise set of energy levels or high enough to knock the electron completely out of the atom. So most photons will ignore the atom. Note that the energy levels for exciting an electron is in the range of the visible light we see ...so there are a lot of these photons around. But there are a lot more that aren't the right frequencies.
It can hit a charged particle and cause the Thompson/ Compton Effect which sends the photon off in a random direction and can change the frequency of the photon as energy is transferred.
Otherwise it keeps going.
Even if you had a foot of solid hydrogen what is the odds that any one photon would interact? It would largely be dependent on the frequency of the photon first.

Remember "Solid" does not mean "Opaque". A solid is where the atoms can not move freely. Obviously with distances of even centimeters between them any random hydrogen atom is freely able to move. You can't just say there is enough atoms to make a "solid" of n thickness and treat it as having the same properties because the forces involved are not the same.

So the problem is 2 fold. Even if you could treat that column of material as a "solid" of much shorter length it doesn't mean it is opaque.

John Mendenhall
2015-Aug-18, 02:35 AM
Which is another way of saying the Universe is really, really transparent.

malaidas
2015-Aug-18, 09:32 AM
Yes :)

Amber Robot
2015-Aug-18, 05:59 PM
The neutral hydrogen existing in space does not absorb light at all wavelengths. If you look out into space at 121.6 nm, you will indeed see almost nothing because all the intervening hydrogen is blocking the light. If you look at 500 nm, you see everything because the photons of that wavelength pass by the hydrogen atoms unimpeded.

malaidas
2015-Aug-19, 08:52 AM
The neutral hydrogen existing in space does not absorb light at all wavelengths. If you look out into space at 121.6 nm, you will indeed see almost nothing because all the intervening hydrogen is blocking the light. If you look at 500 nm, you see everything because the photons of that wavelength pass by the hydrogen atoms unimpeded.

Very good point.

chornedsnorkack
2015-Aug-19, 08:30 PM
Even if you had a foot of solid hydrogen what is the odds that any one photon would interact? It would largely be dependent on the frequency of the photon first.

Remember "Solid" does not mean "Opaque".

Solid hydrogen freezes into hexagonal close packed crystals... which means they should be birefringent, like ice. Does anyone know how strong the double refraction in solid hydrogen is?

Amber Robot
2015-Aug-20, 05:53 PM
Think about looking at the Moon. How many water molecules are between you and it? Yet we see it very clearly. However, look at it at the wavelength of a water resonance line and you won't see it. Transparency of materials is wavelength dependent. That's true for every state of matter.