This is one of those questions I should have asked when I was about six, but never really got around to it. (Isn’t it odd how you can trundle through life quite happily with these huge holes in knowledge?). Please bear in mind I have no physics education whatsoever.
Anyway, here’s where I’ve got to: Planets (or some of them) have atmospheres, meaning that they are surrounded by various different gases. And some planets/stars, like our Sun, are mainly composed of burning gases.
So what is the dark stuff beyond the atmospheres and between the planets and stars composed of? It can’t be ‘nothing’, can it? Just as my brain will not grasp infinity, it won’t grasp ‘nothing’. Is it some sort of very dilute gas? But diluted in what, though? Oh dear…
Are you asking what fills the space between stars, or why the night sky looks mostly black?
The answer to the first question is “almost nothing.” Interstellar space is a better vacuum than anything we can make in the lab. The density can be as low as 2 atom per cubic inch. If you are wondering why gases from planets and stars don’t spread out to fill this void, the answer is gravity. If you have a small cloud of gas (say, a party balloon) and release it in vacuum, it has a lot of pressure and almost no self gravity so it will disperse quickly. But if you have a huge gas cloud tens of times more massive than the sun and larger than the solar system, gravity will win over pressure. Instead of dispersing into space, it will collapse on itself and form tight little clumps of gas. These eventually become stars. So it’s actually difficult to spread gas over a large void - you need to break it up into diffuse patches which have too little gravity to collapse. Even then, one good shock wave from a nearby supernova can sweep up the gas and compress it into a self-collapsing cloud.
The answer to the second question is “because the universe is finite.” If the universe were infinite in space and time, everywhere we look in the sky you’ll see a star. The night sky should be bright white, not black. But since the universe is only about 12 billion years old, you can only see stars and galaxies within 12 billion light-years distance, which isn’t much. So the night sky is mostly empty and looks black.
Depending on where you are space can be filled with differing things.
Solar system
Protons and other elementary particles flowing through the region, lots of dust and the odd mountain sized rock.
Interstellar
Much more rarifies stuff here basically your looking at 1 hydrogen atom per cubic centimeter.
Molecular clouds
High density regions with a million or so particles per cc. Tends to have dust and simple molecular compounds like alcohol or water.
Now if you zoom in enough to a region of space we expect to see a frothing foam of virtual particles appearing and disappearing. So you never really wind up with a pure vacuum.
That’s not exactly accurate. Here are some images of distant galazies and other objects, taken by the Hubble telescope. Without the Hubble, we would never have noticed some of these things, because they are so faint. Because of the immense distance, only a relative handful of photons come our way in the first place, and many of them get absorbed enroute by nebulae and dust clouds and such.
Thanks for this.
So it is not nothingness? Some areas are denser than others (like the areas of molecular cloud) and once the atoms reach a certain density threshold gravity will take over and force them into a clump/star?
So there is always something in interstellar space? Scr4 says interstellar space is composed of 2 atoms per cubic inch. Atoms of what, though - different types of gas?
Re: finiteness of universe - presumably 13 billion years or so ago there were just a lot of dispersed atoms floating about that had not got dense enough for gravity to do its thing? So although the universe was not there, in terms of solid stars and planets, there was still something there?
Quick cosmology lesson.
14 Billion years ago the universe came into existence. It expanded faster than the speed of light (space can do that) and then slowed down and the energy released cooled down and formed protons and electrons.
13 Billion years ago some protons and electrons slow down enough that they capture one another and form hydrogen atoms. Now we have a universe filled with hydrogen that has small irregularities in the distribution of that gas. Generation I stars form.
A few million years later the Gen I stars explode creating all the more massive elements like Helium, Sodium, Oxygen etc.
4 Billion years ago a Gen III star forms and you and I show up to tell lies about the universe to one another.
Basically when scr4 and I discuss particles/atoms were typically discussing hydrogen. It could be a single uranium atom in that cubic centimeter but it is much more likely to by hydrogen.
As to what was there for the universe to expand into, that is a meaningless question the universe is the space it expanded into.
Usually (in traditional, backwards astronomy fashion) we do this the other way around. Population III stars are the very very old ones, Population II stars are the very old ones, and Population I stars are the ones like our Sun. I don’t think PopIII stars exist anymore.
Cite for Gen III stars Ask the Astronomer. I was sure that there was a recent press release on almost detecting the first stars and I remember them being refered to as Gen I. I’ll go digging.
Actually, I don’t know that we’ll need a name for them. The big difference between the stellar populations is their metallicity (ie, the fraction that’s made up of elements heavier than Helium). The sun, a Pop I star, has a metallicity of about 2%. The very old Pop III stars had almost zero metallicity, because these elements just didn’t exist back then. As time goes on, I guess metallicity will continue to increase, but it may hit a limit. Stellar astrophysics was never my strong suit.
Now, there are definite differences between a star with 0.01% metallicity, and a star with 2% metallicity. There may not be so much of a difference between 2% and 4% or 6%, though. So maybe there will never be a need to classify a new population.
Thanks for cosmology lesson Grey. All of it makes sense, except your first paragraph and the last sentence. How can something ‘come into existence’ from nothing, an effect without a cause? And what was it that suddenly expanded faster than the speed of light? How come people (like astrophysicists) find this (to me) incomprehensible idea perfectly easy to talk about? Do you just accept that it is weird and then get on with looking at the history of the universe, dating stars and measuring their composition etc? I’m inclined to think the human mind is not equipped to think about concepts that cannot be related to the boundaries, edges, beginnings and endings that we live with. But maybe it’s just that my mind is limited.
PS Don’t worry I’m not about to drag God into this…
Actually, scr4’s post was entirely accurate. It is something that has been known for hundreds of years as Olber’s Paradox. I think the your mistake stems from your failure to grasp the implications of a universe infinite in space and time (not just very large). Yes, Hubble found some very faint objects that would never have been seen otherwise, but, since they were seen, some photons must be reaching us from them. In a universe infinite in space and time, a view in any direction must eventually (even if only in a hundred million billion trillion lightyears) land on the surface of a star, and hence a photon must enter the eye from that direction. Thus the night sky (and the day) would be solid photons. Even if there were intervening material to block the photons, that material must eventually heat up (assuming infinite past time), and will also be emitting photons. The fact that the sky is not solid white is proof that the universe is not homogenous, not infinite in time, not infinite in space, or some combination of the above.
Welcome to the realm of philosophy. As I understand it, the laws we call “physics and math” break down when trying to look back beyond the original singularity or whatever it was that formed our universe. As such, not only do we not know what happened before (and therefore how, exactly, “something came from nothing”), we can’t know. However, we’ve done amazing work figuring out everything shy of that.
While I’m not willing to sell us short just yet, I do think there’s something to the notion that the human mind has operational biases that make it more comfortable to think in certain modes than in others. Consider how the bilateral symmetry of our physiology has given rise to the either-or paradigm: right/left, male/female, good/evil, east/west, yin/yang, profit/loss, white/black, open/closed, dead/alive, dog/cat… It’s so deeply ingrained we don’t even think about it. We constantly try to stick things in pigeonholes — something is either “this” or it’s “that” — and we’re not very good at “fuzzy” categories. Consider wet/dry, for example. Is an ice cube wet or dry? If you hold it, it feels wet, but that’s because your warm hand is turning it into water. If you hold it with an insulated glove and manipulate it with a cold piece of metal, it behaves like a dry object. That’s just one example.
So yes, while you do have a point, that the human mind has difficulty working with certain concepts (e.g., infinity; look at the recurring debate about .999… = 1), it’s possible to overcome those deficits with experience and training. While it may be outside our ability to easily visualize a four-dimensional cube (a tesseract), with work and imagination it’s possible to grasp the concept enough to manipulate it mentally.
Population III stars are the very very old ones, Population II stars are the very old ones, and Population I stars are the ones like our Sun. I don’t think PopIII stars exist anymore.
As I understand it, the laws we call “physics and math” break down when trying to look back beyond the original singularity or whatever it was that formed our universe. As such, not only do we not know what happened before (and therefore how, exactly, “something came from nothing”), we can’t know. However, we’ve done amazing work figuring out everything shy of that.