# What is in space?

I was recently on a chat server having a random discussion about science stuff and someone I was talking to then made the comment that "space is not a complete vacuum and it's full of plasma / matter".

That got me thinking ... ok so I don't expect that the bulk of space is totally empty but if I took a "cube volume of the space outside the ISS" (or further out for scientific accuracy) ... do we know what would be observed inside that cube in terms of "real particles" from the standard model?

I'm thinking that there would be some amount of photons (light) and possibly some other stuff, by the other guys claim of "plasma being everywhere" raised some weirdness in my head I couldn't resolve.

He followed that up with "we live in an electric universe".

Does anyone have a professional / academic viewpoint on this (i'm no physics grad though so go easy on me)?

EDIT:

I've had time to go deeper in to this concept with the person I was talking to earlier and he cited NASA as a source and linked me to this ...

https://science.nasa.gov/science-news/science-at-nasa/1999/ast07sep99_1

"99.9 percent of the Universe is made up of plasma," says Dr. Dennis Gallagher, a plasma physicist at NASA's Marshall Space Flight Center. "Very little material in space is made of rock like the Earth."

... surely this is a contextual statement but what is the context and does this literally mean as is worded or is there something else here?

• Near Earth, space contains about 5 particles per cubic centimetre. See en.m.wikipedia.org/wiki/Interplanetary_medium Beware of the electric universe "theory". Electromagnetism is certainly important, but the electric universe stuff that's currently being promoted is pseudoscientific rubbish. Nov 19 '18 at 16:01
• I figured as much ... but usually with the conspiracy theory stuff there's a grounding in real science first that gets twisted so I figure it was worth asking about the "real science" first and then going back to my conversation with some scientific facts before making wild claims that I can't back up with real evidence.
– War
Nov 19 '18 at 16:14
• There's some info here physics.stackexchange.com/questions/18950/… Nov 19 '18 at 16:18
• Nov 19 '18 at 17:49

1) To begin with, space or the interstellar medium if that is what you are refering to, accounts for all the matter that is not in stars, neither in planets or asteroids, and there is actually a lot of matter in there.

However, from an human point of view it is legitimate to consider that space is "empty", given that the average density of particles is tremendously lower than what you experiment on Earth. For example, physicists who try to obtain the emptiest environment for their ground based experiments manage to go down to millions of molecules per cubic centimeters, which you have to compare with average densities of tens of molecules per cubic centimeters in interstellar atomic gas clouds. This is a whole different level of emptiness.

So, if you were to take a human-sized "cube volume of space outside the ISS", indeed you would observe a very little amount of molecules (like carbon monoxide or dihydrogen), atoms (like hydrogen), ions, electrons, dust particles, some cosmic rays but also photons and neutrinos.

On one hand, the number of neutrinos and photons that would be crossing your cube at each seconds is nowhere near negligible, as the flux of these guys is very important everywhere in the universe, mainly due to the thermal emission from stars and from the ubiquitous cosmic microwave background.

On the other hand, you are right when you guess that the amount of baryonic matter contained in that cube would be ridiculous, but this is by no mean negligible if you start to consider a cube of $$1 \times 1 \times 1~\mathrm{pc}$$, where $$\mathrm{pc}$$ stands for parsec, with $$1~\mathrm{pc} \approx 3,1 \cdot 10^{16}~\mathrm{m}$$.

In a cube this size, assuming a density of 10 molecules of hydrogen per cubic centimeters this would add up to a total mass of hydrogen of the order of $$10^{23}~\mathrm{kg}$$, which is close to the mass of Mars or Pluton. Now, realize that there are many interstellar clouds which sizes range from sub-parsec to hundreds of parsecs, with densities ranging from tens of atoms per cubic centimeters to $$10^7$$ molecules per cubic centimeters for the densest molecular clouds. This is a lot of matter, actually way more than what you find in planets and stars.

What I wanted to demonstrate here is that if the universe may appear filled with emptiness, it is important to notice that when you sum up things, you find out that most of the mass is contained in these vast and apparently empty spaces.

For that reason, the statement that "space is not a complete vacuum and it's full of plasma / matter" makes sense. I would just add that it is also massively filled with photons.

2) Concerning the "plasma is everywhere" statement, this can be considered somewhat true as well. Let's return to the definition of plasma. According to google, plasma is "an ionized gas consisting of positive ions and free electrons". Let's make a list of a few astronomical objects which are in a moderate or highly ionized state:

That already represents a large portion of what is found in space. In fact, all things considered by using a rough estimate you can show that up to $$99,9~\%$$ of the observable universe is made up of plasma.

3) About the "electric universe", I won't comment as this statement sounds a bit vague to me.

• An excellent answer. But I think you should emphasized how much photon there is in space. Nov 19 '18 at 18:05
• Even in the deepest intergalactic space, there are about 400 photons per cubic centimeter, part of the cosmic microwave background from the Big Bang. Nov 19 '18 at 18:53
• @Maxter True, there are a lot of photons. OTOH, they don't contribute a lot of energy compared to the matter particles in space. Eg, the rest mass of a single electron is roughly equal to the energy of 770 million CMB photons at their peak frequency of 160.4GHz. Nov 20 '18 at 6:31
• Nov 20 '18 at 13:04