Warp wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>>>   No. Space expands, the brick doesn't.
> 
>> That doesn't really make sense.
> 
>   It makes sense when there's a force opposing the expansion.

Warning: Underinformed rambling ahead... :-)

Hmmm.  Maybe it makes sense if you're assuming that the "expansion of 
space" is caused by "adding space" between other bits of space. It 
doesn't make as much sense if you think of "expansion of space" in the 
same terms as you think of "contraction of things going near light speed".

I.e., it's like saying "time slows down, so you get more seconds per 
minute, but the seconds are the same length."  How would the universe 
know to keep the seconds the same but make the minutes different?

>   Think about it this way: The distance between atoms in a molecule is
> determined by the bounding forces between atoms. 

It's determined by electrons exchanging photons, which gives rise to 
what can be interpreted as forces, yes. It certainly depends on distance 
(amongst other things) or you'd stick just as readily to glue on the 
other side of the room as you do to the glue on your chair. :-) But the 
underlying numbers stay the same ("n" and "j", basically, with "n" being 
related to the mass of the electron (equal, if random spontaneous 
"virtual particles" didn't interfere), and "j" being related to the 
"charge" of the electron, which is proportional to that electron's 
likelihood of absorbing or emitting a photon). But the formula using 
those numbers is also based on distance, due to the inverse-square law. 
(Or inverse-cube, I forget which.)

E.g., it's much more likely for an electron to wind up stuck to an 
adjacent atom in a molecule than it is for that electron to wind up 
thirty feet away.

 > These are proportional to the charge of the electrons, or whatever.

Charge (which is the probability of an electron absorbing or emitting a 
photon), mass (not sure if this includes relativistic mass), and distance.

 > It's a constant. It doesn't change.
 > If you tried to separate the atoms of a molecule, this force would
> resist it.

Right. But would the molecules themselves be getting bigger? Would the 
protons and electrons be larger? Would the photons that mediate that 
force be getting red-shifted in both directions?

Or would new space be getting inserted between the protons and the 
electrons? If so, I'd think your chemistry would change, in just the 
same way that chlorine is more chemically active than oxygen.

>   Not everything gets longer. Only intergalactic space, far away from
> the galaxies, gets longer.

That's what has me curious.  How does space far from galaxies "decide" 
to get larger while space between stars doesn't?

>>>   AFAIK space doesn't expand inside galaxies.
> 
>> OK. More weirdness. Have you heard any postulates on why that would be 
>> the case? Space only expands where the gravity is sufficiently low? 
> 
> http://en.wikipedia.org/wiki/Hubble%27s_law
> 
> "In using Hubble's law to determine distances, only the velocity due
> to the expansion of the universe can be used. Since gravitationally
> interacting galaxies move relative to each other independent of the
> expansion of the universe, these relative velocities, called peculiar
> velocities, need to be accounted for in the application of Hubble's
> law. The Finger of God effect is one result of this phenomenon
> discovered in 1938 by Benjamin Kenneally. In systems that are
> gravitationally bound, such as galaxies or our planetary system, the
> expansion of space is (more than) annihilated by the attractive force
> of gravity."

OK. That still doesn't make sense to me. As space expands between two 
stars within a galaxy, the gravity between those stars gets *weaker*, 
meaning they orbit more slowly and are less likely to hold together. 
Maybe that's why galaxies turn from globular clusters into spirals or 
something, tho.

It makes sense if what it's saying is "the expansion of space isn't 
rapid enough to keep a collapsing galaxy from collapsing", but if you 
have a stable orbit and then start adding "space", the orbit won't 
somehow adjust to take account of that - the sign of the change is the 
wrong direction.

It also makes sense if what it's saying is "galaxies moving relative to 
each other due to gravity are moving at a significant speed compared to 
the speed caused by the expansion of space, and thus we must account for 
that." That seems to be what the "finger of god" effect is talking 
about, regardless of the quote.
(http://en.wikipedia.org/wiki/Fingers_of_God)

I'm not clear on two things: (a) why, if space expands, don't the things 
in the space expand, and (b) why would gravity "overcome" the expansion 
of space. There's no reason for the earth, for example, to have a 
"preferred distance" from the sun, gravitationally-speaking. If extra 
space gets added, either the earth will spiral away from the sun or 
it'll slow down (as larger orbits are slower orbits).

Now, for (a), maybe stuff in the expanding space *does* get larger. A 
red-shifted photon is, in many senses, "bigger" than a blue-shifted 
photon, so maybe the expansion is small/slow enough that the expanding 
things *do* get bigger. But if that was the case, atoms would also be 
getting bigger in proportion to "space", and your rulers would show that 
space isn't getting bigger after all, so the *only* way to notice might 
be from red-shifts? But if "space expands" in the same sense that "time 
slows down near the speed of light", then it doesn't make sense to say 
"the galaxies will hold together", as everything will nevertheless 
expand too - I expect the expansion of the matter in the galaxy (if it 
leads to increased mass) might exactly balance (via increased gravity) 
the increased distance between objects. (I'd have to do more thinking 
about this than I want to at this hour to check.) Alternately, it's 
possible that the "G" in "F=G m_1 m_2 / r^2" is fractionally higher than 
thought, just enough to offset the expansion of space? Dunno.

To address (b), you could be inserting "new space" between other "bits 
of space", but then I can't imagine the mechanism (other than "new 
space" only appearing where curvature from gravity is flat enough) that 
would create "new space" between galaxies but not inside galaxies.

I don't think saying "gravity holds them together" would keep galaxies 
from expanding at the same rate as the rest of the universe, and indeed 
I'd expect them to expand faster as the gravity holding things together 
got weaker with increasing distance and stuff started "flying apart".


-- 
   Darren New / San Diego, CA, USA (PST)
     It's not feature creep if you put it
     at the end and adjust the release date.

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