
Scale of the Universe - spcmnspff
http://uploads.ungrounded.net/525000/525347_scale_of_universe_ng.swf
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dansingerman
Can someone explain to me if the Universe is supposed to be 14,000,000,000
years old, how can it be estimated to be 93,000,000,000 light years wide?

Assuming the Big Bang, doesn't that imply the 'outer matter' must have an
average velocity of about 3.3 times the speed of light?

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gjm11
The answers from benihana and cromulent are correct, but the following may
possibly be a useful thought experiment.

Imagine a balloon (our universe) with lots of ants crawling on it (photons
moving at the speed of light). The ants can't walk any faster than 2mm/s. They
can't take a message from any part of the balloon to any other faster than
that. (And, let's suppose, there's nothing living on the balloon that moves
faster than an ant.)

Now someone blows up the balloon; it gets much bigger. Its expansion moves the
ants apart much faster than 2mm/s. But it's still true that if you have two
points 10mm apart, no ant can get from one to the other in less than 5s.

Suppose the balloon starts off rather small, and then is blown up very
rapidly: perhaps it grows abruptly from 100mm across to 1000mm across in less
than a second. (The corresponding phenomenon for the universe is called
"inflation"; it explains many otherwise puzzling things but no one knows for
sure whether it really happened.) Then it may happen that the only ants that
have been able to reach one particular place on the balloon (our
observatories) have come from a smallish fraction of the balloon (the
observable universe).

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phlux
Epanding into what???

This is what I cant wrap my head around... into what is the universe
expanding?

What if the universe is not a balloon - but a doghnut where the outer edges
fold back into itself, the overall space it is in can be finite yet the
surface is curved away. The light would travel along the curved surface of
space-time and thus travel a greater distance than is linear.

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gjm11
It's not expanding into anything (so far as we know), just expanding. A curved
spacetime doesn't need to be, and so far as we know isn't, embedded inside a
higher-dimensional not-curved thing in the way that a balloon is embedded in
(more or less) flat three-dimensional space. (That sort of embedded picture is
useful for the intuition, though.)

So, you might be asking, whatever does it _mean_ to say that space, or
spacetime, is curved? It means, e.g., that if you measure the radius and
circumference of a small circle very very very accurately, the relationship
isn't C = 2 pi r. If the circumference is "too small" then space is positively
curved there, like a balloon. If it's "too large" then space is negatively
curved there, like a saddle. The curvature might actually be different
depending on the orientation of the circle, but if space is the kind of thing
we think it is then you basically only need six numbers to tell the whole
story at each point of space. (In two dimensions -- the surface of that
balloon -- there's only one possible orientation for the circle, so you only
need one number to describe the curvature at each point.) Note that you can
measure this thing without needing to go "outside" space: the radius and
circumference are measured within the space. Distances along the surface of
the balloon.

Yup, the universe could be curved and topologically nontrivial in all kinds of
interesting ways: it doesn't have to be like the surface of a balloon.
(There's some intriguing but inconclusive evidence suggesting that the large-
scale geometry of the universe is a "Poincare dodecahedral space": take a
dodecahedron, and apply some magic to its faces so that when you try to leave
it by one face you come back in by the opposite face.) And yes, light travels
along the shortest paths it can within the universe, and doesn't take out-of-
the-universe short cuts even if there are any.

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jcfrei
it's still nice but it's been here before:
<http://news.ycombinator.com/item?id=1835460>

BTW: since there's no search feature to check for older posts, I suggest you
use google: site:ycombinator.com

~~~
ronnier
I've also created an API to look up URLs that have been posted to HackerNews.

[http://api.ihackernews.com/getid?url=http://uploads.unground...](http://api.ihackernews.com/getid?url=http://uploads.ungrounded.net/525000/525347_scale_of_universe_ng.swf)

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scalyweb
And I was getting big-headed this morning for solving a silly problem for a
client. Thanks for reminding me of my place. :)

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heed
If there is a minimum physical length, that is a planck length, does that mean
that nothing is technically continuous? Or, is it possible to imagine 'half a
planck length'?

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Devilboy
The quantum world is indeed discreet, and (as far as we know) half a planck
length doesn't make any sense physically.

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brianpan
I think of it as: Newtonian physics makes down to a certain scale where it is
no longer accurate. Quantum makes sense down to a Planck length where beyond
that, we may need different theories.

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bungula
Try doing a fast zoom out from the smallest to the largest. If you do it at
just the right speed, it will give you serious goosebumps.

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latch
Always been amazed at how really small things (atoms) look a lot like really
big things (solar systems) (ya, ya, same forces...)

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gjm11
They don't really -- in particular, the picture you might have from high-
school physics lessons of electrons orbiting a nucleus like planets orbit a
star is really wrong. And it's not really the same forces; the way an atom
looks is mostly down to the electromagnetic force, whereas the way a solar
system looks is mostly because of gravity.

On the other hand, it _is_ true that Newtonian gravity and the electrostatic
force (i.e., what the electromagnetic force becomes when you make the
approximation that there are no moving charges) have very similar mathematical
forms, with the same sort of differential equation governing them. What
Newtonian gravity and electrostatics have in common is mostly that they are
convenient simplified approximations; you could say that they're so similar
because there's a limited repertoire of differential equations simple enough
to make good approximations. (There's a bit more to it than that; for
instance, inverse square laws fall naturally out of the geometry of a
3-dimensional world.)

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jerf
(Elaborating:) Electron shells "look" more like this:
[http://www.absoluteastronomy.com/topics/Azimuthal_quantum_nu...](http://www.absoluteastronomy.com/topics/Azimuthal_quantum_number)

They really don't resemble gravity-based orbiting in solar systems at _all_.

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Aron
I would point out that the ant is precisely in the middle of the scale between
small and large. Think about that, grasshopper.

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CallMeV
Aw, they changed the theme tune.

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pixdamix
How not to feel sooooo small and insignificant after that !?

~~~
hugh3
_Some people get depressed when they find out how huge the universe is... What
is the big deal about bigness? A cow is much bigger than you, but it is a
ridiculous animal and you are a valuable person. You know it's a cow. It
doesn't know anything. it just stands there eating grass (grass!) and mooing.
And if it were bigger, that would only make it more ridiculous._

\- David Deutsch

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erikig
David Deutsch, while making valid philosophical/metaphysical points, exhibits
symptoms of a serial cow-tipper.

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Marticus
That... is spectacular...

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SZW
It is! I don't have particularly clever thoughts about it, but I want to say
it's amazing. Thanks a lot to the creator!

