confused about spacetime expansion.

[insane_alien]

this question was getting to me last night and its annoying the hell out of me.

 

i get that space is expanding and all that but my question is, why doesn't matter expand with it?

 

like if you had a length of spacetime(okay, so, not physically possible) and a length of matter. initially they start at the same length. eventually the space time bit gets bigger but the matter doesn't.

 

why doesn't it? it would seem to me that if space-time was expanding then everything(right down to sub-atomic particles) would expand also.

[RyanJ]

A very good question!

 

The answer is quite simple actually. The expansion is only apparent over large distances and on a smaller scale the force created by the spacial expansion is quite minimal, it is easily overcome by the gravitational attraction between objects as well as the other forces which are significantly stronger than gravity.

 

So in effect the fundamental forces are far strong enough on small scales to hold matter together but on larger scales the gravity and so forth are inherently weaker due to the larger distances between all the objects thus allowing the space there to expand without being overcome. Does that make any sense?

[Royston]

i get that space is expanding and all that but my question is, why doesn't matter expand with it?

 

I'm probably missing something, but the forces keeping matter, and galaxies together prevent this from happening, so the expansion is limited to areas where the forces are too weak to stop the expansion i.e between galaxies. Though it's obvious you already know this, so maybe you're thinking of something else ?

 

EDIT: Heh, RyanJ beat me to it.

[insane_alien]

yeah, that makes sense and i knew that already. i was meaning the actual size of the particles. umm its hard to explain.

 

okay, try this. if space expands so it is double the origional size why wouldn't a meter be double the origional size. i think, maybe something like that. as i said, i'm kindof confused probably because i've got somethng pretter basic wrong.

[RyanJ]

okay, try this. if space expands so it is double the origional size why wouldn't a meter be double the origional size. i think, maybe something like that. as i said, i'm kindof confused probably because i've got somethng pretter basic wrong.

 

Well the problem with measurement is that everything is relative to something else. In this case the rule that you use to measure the length of the object with you ascertain its size. As the matter its self doesn't get "larger" as the space expands due to the reasoning above then the meter before will be the same as the one after - thus the lengths remain unchanged with the expansion.

 

On the bright side, even if everything were getting bigger you wouldn't be able to tell because everything would be increasing by the same proportion anyway so everything would seem the same

[insane_alien]

ahh right i see my problem. i was assuming some sort of absolute scale thingy.

 

yeah it all clears up after that. thanks.

[BenTheMan]

Also remember---particles are pointlike, at least in normal QFT. And when you expand a point, you get...a point.