Which difference is greater, the size of atoms compared to humans; or the size of galaxies compared to the universe? I get that this varies depending on, say, which human or which galaxy; I’m just looking for a general answer. Similar comparisons would be appreciated too 
all the length scales you could want…
An atom has a diameter of around 1 x 10^-8 cm, depending of stuff. I’m 183 cm tall, so I’m about 18,000,000,000 times larger than an atom.
The diameter of the Milky Way galaxy (where I keep my stuff) is about 1,000,000,000,000,000,000 cm. The universe, assuming it expands at the speed of light, is about 137,500,000,000,000,000,000,000 cm in diameter or about 137,500x larger.
Observable universe 3x10[sup]10[/sup] billion light years across
Milkyway galaxy 1x10[sup]5[/sup] light years across
Human 2x10[sup]0[/sup] m
Hydrogen atom 1x10[sup]-10[/sup]m
So galaxy to observable universe factor of 5
Atom to human factor of 10
As a way to concieve of the relative size of an atom, I’ve always found this example useful:
If you take a glass of water and throw it in the ocean, then wait, say 30 years (or whatever it takes) for the water from that glass to disperse through all the oceans. If you go to the ocean again and take a glass of ocean water, it will probably have a molecule of water that was in the original glass. That’s because there are more water molecules in a glass of water, than there are glasses of water in all of the oceans.
Wow, there really are a lot of variables… I forgot that the universe is expanding and that some atoms are a zillion times bigger than others. Now I feel really silly for asking in such a simple way. Thank you for your answers!
Here you go, the Scale of the Universe. Takes a little while to load. Once it loads hit play and then move the slider.
So I guess we can say the world of the very small is “infinitely” larger than the very large.
Yes, atoms are very small. To go with the glass of water analogy:
If you removed all of the empty space from all the atoms that make up every person on earth (so the nucleus and electrons were touching, and the adjacent atoms also touched) you could fit every person on earth into space the size of a sugar cube.
In a book I was reading:
“The period at the end of this sentence is about halfway in size between an atom and the planet Earth.”
(It was phrased less clumsily. Don’t know how accurate it is.)
Does anyone know where these images originated?
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Contrary to popular belief, there is no empty space in atoms, or between them for that matter. All that space is filled with electromagnetic fields, which have just as strong a claim on being “something” as do the protons, neutrons, and electrons.
Blows my mind! Thanks to all especially ghardester. I appreciate these analogies. I was surprised with these answers because I always imagined that I could understand the smallness of atoms, but not the vastness of the universe. For me this is based on feeling like I can comprehend the smallness of living cells and then seeing atomic-scale diagrams of cellular structures in textbooks and such (if that makes any sense). This information really puts it in perspective.
Here’s a similar example I got in another thread that is interesting (sorry if I Godwinized this thread ;)).