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Scale of the Universe

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Re: Scale of the Universe

#21

Post by retube »

latch wrote: post #11

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

No: not the same forces. The structure (and motion) of the macro-verse is governed entirely by gravity. The structure of the micro-verse is governed by the EM, weak and strong forces. Gravity, being about 10^25 times weaker than the next weakest force (the weak) has no influence at quantum scales. And as gjm11 points out below, the "solar system" model of the atom is a old classical model of convenience, hypothesised before the work of bohr, fermi, dirac et al worked out the quantum model. It's still taught in schools because the QM model requires fairly advanced math.

Re: Scale of the Universe

#23

Post by gjm11 »

dansingerman wrote: post #17

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?

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).

Re: Scale of the Universe

#24

Post by ntownsend »

cromulent wrote: post #20
dansingerman wrote: post #17

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?

Because the distance has also grown since the light travelled. It's expanding. http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN

And on that note, the creator has made the common mistake of performing the calculation speed_of_light * age_of_universe to determine the radius of the observable universe.

Space has expanded during the lifetime of the universe so the objects that emitted light at the edge of the observable universe have moved further away since emitting the light. The estimated distance to the edge of the observable universe is more like 47 billion light-years: http://www.astro.ucla.edu/~wright/cosmology_faq.html#DN

The visible universe, however, is a bit smaller since we can only see as far as the surface of last scattering: http://en.wikipedia.org/wiki/Cosmic_microwave_background_rad...

Re: Scale of the Universe

#25

Post by jerf »

gjm11 wrote: post #13
latch wrote: post #11

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

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 for…

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

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

Re: Scale of the Universe

#28

Post by 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'?

Re: Scale of the Universe

#29

Post by phlux »

gjm11 wrote: post #23
dansingerman wrote: post #17

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?

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…

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.

Re: Scale of the Universe

#30

Post by baddox »

heed wrote: post #28

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'?

That's a great question. I'm not sure how motion (and thus transfer of information) would be possible in a discrete universe. A somewhat related question is whether the universe is the result of computation. Many thinkers have chimed in about the physics and philosophy of this sort of thing:

http://en.wikipedia.org/wiki/Fredkin_finite_nature_hypothesi...

http://en.wikipedia.org/wiki/Digital_physics

http://en.wikipedia.org/wiki/Digital_philosophy

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