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Asked by elliop to Clare, Mariana, Pedro, Robert, Susanne on 14 Nov 2012.
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Clare Taylor answered on 14 Nov 2012:
Apparently it’s 100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000, 000,000,000,000 atoms!
A lot! To write this in more simple terms, we would use 1E+80
Can you imagine how long it would take to count!?
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Robert Insall answered on 14 Nov 2012:
Wikipedia has everything (and I should know, I edit some of the pages). Look here for some info:
http://en.wikipedia.org/wiki/Observable_universe#Mass
That says the universe is about 10^52 Kg (1 with 52 0’s after).
I’m going to assume that most of the atoms in the universe are hydrogen, with a molecular mass of 1. Then each 1 gramme of universe will contain 1 x the Avogadro number of atoms.
http://en.wikipedia.org/wiki/Avogadro_constant
Avogadro’s number is 6 x 10^23 (6 with 52 0’s)
And that number is in grams, not kilos, so you need to multiply by another 1000.
I make that about 10^79 (1 with 79 0’s). Sounds similar to Clare, right, but actually its a whole tenfold different, like comparing £1 with £10.
Of course, nobody’s actually counting, are they? It’s just a fairly educated guess…
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Susanne Muekusch answered on 14 Nov 2012:
Hi elliop,
an interesting follow up question on that: How did anyone end up with that number? Obviously, noone counted.
It can be good to question where the information comes from that you get. Thinking about that will sometimes uncover “scientific facts” as not really trustworthy. I am not saying the number wikipedia has is nonsense. I just wanted to make a general point here, because this number is a very good example for information, where you don’t immediately see where it comes from.
Keep on asking (at least yourself) even when you got an answer! I am curious how they know the number. I need to have a look on wikipedia now, to see if they explain it in the article.
Comments
Susanne commented on :
@Robert: I am curious how you derive the number of atoms from the mass of the universe and Avogadro’s number?
Avogadro’s number is the number of molecules (or atoms) per mol. The mass of the universe is in kilo. To derive the number of molecules and atoms in the universe it is necessary to know the molar mass of the universe.
Avogadro’s number is not in grams, but in 1/mol.
Let me know if it is me overlooking something or what happened 🙂
Susanne commented on :
“Let me know if it is me overlooking something”
what I wanted to say: “Let me know if I miss something here”. At least I think that’s what I want to say…
Robert commented on :
Ahh, hello Susanne, I didn’t realise what you were talking about, hadn’t seen this.
I assumed that most of the mass in the universe was hydrogen, which is not a bad assumption. Then 1 mole equals one gram. But I should have included that assumption, shouldn’t I?
All the hydrogen that’s been through a star will have fused to heavier atoms. But I don’t think that’s a very significant proportion of the atoms in the universe, so 1 is not a bad approximation.
Susanne commented on :
aha, now it makes more sense! Thanks for explaining. And I think adding the assumption is a good idea. Not to confuse students about Avogadros constant being in g 🙂
Robert commented on :
but I will add my assumption…