[Physics Dilettante (OP)]

Due to the universe's expansion, that is. And, if it is getting bigger, could it be detected? Wouldn't anything that might be used in the attempt also be getting bigger and, thus, the mutual expansions would cancel?

[Kryptid]

Molecular forces would prevent your nose from growing due to the Universe's expansion. It's not powerful enough to do that.

[Physics Dilettante (OP)]

On TV and in print sources intended to explain such things to the public, the universe's expansion is described typically as an expansion of space. We're told the galaxies aren't flying apart in the way, say, pieces of an exploded cannonball do. Instead, space is expanding and the galaxies are going along for the ride. At this scale, I think I can understand how gravity could slow, cancel, or reverse the galactic expansion, since general relativity says matter produces what we perceive as gravity by warping space. Apparently, though, gravity isn't powerful enough to do this at small scales -- hence, my question.

Given my mental model, your answer implies that molecular forces (are these the electromagnetic force?) warp space, which is an idea I haven't encountered. Maybe you didn't mean that. Either way, how can molecular forces overcome an expansion of space?

[Soul Surfer]

Very easily because atomic and molecular forces are vastly stronger than gravity. about 10**40 times on an atomic scale.

[Physics Dilettante (OP)]

Do atomic and molecular forces cancel space expansion (at small scales) perfectly? Do they overcome the expansion by warping space, like matter, or in some other manner?

Isn't there an inconsistency in the idea that bosons would, in this way at least, behave like fermions?

[Soul Surfer]

No  they just stay where the intraatomic electromagnetic fields balance out and lets space expand around them a bit like trying to inflate a coarse fishnet like a balloon

[Physics Dilettante (OP)]

I'm having trouble visualizing this.

I infer from what you've said that, if I placed a ruler between two matter particles, I'd find that their locations WRT the ruler stay the same. Furthermore, the ruler wouldn't expand, either, because it's made of matter too.

But since space is expanding, the measured distance must increase, and the only way this could happen given the visualization above would be for the ruler's scale markings to magically change.

Since we're in imagination land anyway, I could substitute a laser-based or other measurement technique and I think I'd still end up with an equivalent mystery.

I suspect my problem has something to do with trying to differentiate between the measured distance and what the distance "really is," which would be a mistake. When performing experiments, we know only what happens to our measurements when we manipulate things and have no way of knowing what's "really" going on. If I'm making a mistake like this, please explain.

[JP]

You have to make a basic assumption in your ruler case: the two matter particles must not be interacting with each other or the ruler (or anything else).  In that case, there's nothing holding them together, so as the universe expands they drift apart.  The ruler is being held together by chemical bonds and so it doesn't stretch.  What you would see is that the two matter particles drift away from the ends of the ruler, and so the distance between them increases.

More practically, distance is defined as the distance light travels in a given amount of time.  This is (to the best of our knowledge) not changing in time, but if we measure distances to distant galaxies, they are moving away from us due to the universe's expansion.  The light-year here takes the place of your ruler, which isn't changing, and the galaxies are like the matter particles.

[Soul Surfer]

Consider I have two magnetic metal balls stuck together and freely suspended in a sealed cylinder full of air I put this into a large evacuated room and open the seal to let the air rush out.  What happens to the balls? very little.  They may be moved a little as the air rushes out and expands to fill the the whole volume but they rapidly settle down to be exactly as they were before the air expanded.  The effect of expanding space on normal solid and gravitationally bound materials is just like that.

[Physics Dilettante (OP)]

Please pardon my pause. I've been thinking.

You have to make a basic assumption in your ruler case: the two matter particles must not be interacting with each other or the ruler (or anything else).  In that case, there's nothing holding them together

Actually, in my thinking, they could all be interacting. From a godlike frame of reference, the 2 particles would be moving apart and the constants governing their interactions would, necessarily, be changing too. We, however, could never discern these things because our rulers, whether wooden, based on redshifts, or whatever, would be compensating perfectly.

The problem I have with the metal-balls analogy is that the balls don't contain any air, which is playing the role of space. If we substituted 2 balloons, they'd both expand, just as we're told space is doing. This result raises an implication of my thinking I haven't voiced yet: Not only should matter particles be moving apart, they should be getting bigger, too.

This reminds me of the demonstration used so often to illustrate the universe's expansion: Points marked on a balloon move apart as it's inflated. If a ruler was drawn on the balloon, we'd see that the scale expands also. Thus, distances among galaxies as indicated by the ruler would remain unchanged. This screws up the expanding-universe explanation (i.e., it raises the question of how we're managing to measure it), but illustrates pretty well why I'm having trouble understanding why space expansion doesn't affect distances among atoms, among mesons, of electrons from their nuclei, etc., and how we could detect it if it were happening. Space is space, whether we're talking about "outer" space or space separating particles.

I hope I don't sound as though I'm arguing -- no doubt, you're both right. I'm just trying to convey my flawed mental model in the hope you'll think of an explanation that will click for me.

[JP]

This reminds me of the demonstration used so often to illustrate the universe's expansion: Points marked on a balloon move apart as it's inflated. If a ruler was drawn on the balloon, we'd see that the scale expands also. Thus, distances among galaxies as indicated by the ruler would remain unchanged. This screws up the expanding-universe explanation (i.e., it raises the question of how we're managing to measure it), but illustrates pretty well why I'm having trouble understanding why space expansion doesn't affect distances among atoms, among mesons, of electrons from their nuclei, etc., and how we could detect it if it were happening. Space is space, whether we're talking about "outer" space or space separating particles.

You're thinking about a ruler as behaving just like space, and thinking that it should stretch just because space stretches.  This isn't the case, since the ruler is something that lives in space and is held together by forces that act within space.  A "better" way to think of the balloon model is to think of your ruler as a small string sitting on the balloon.  If you blow up the balloon, it gets bigger, but the string doesn't really get stretched.  If you chop the string into pieces, these are now acting like noninteracting particles in space, and those pieces do get moved apart as the balloon expands.

[Physics Dilettante (OP)]

I think my conceptual problem -- indeed, my original question -- stems from confusion re. what it's supposed to mean when we say that space is expanding. So, I'm going to start a new thread on that topic and see where that leads.

Thanks to all for your help.

[yor_on]

PD I also wondered over that, and seen that same answer as you have.
But as this expansion comes 'through' dimensionless 'point particles', then shouldn't it come 'everywhere', right?
That 'expansion' would then be 'gravity'?
Not necessarily.

But there is one point to notice here.
To stop 'matter' from growing that expansion needs to be 'weaker' than those 'bondings' holding 'matter' together.
And as waves doesn't seem to grow either, the same rules should be valid there.

So what are growing then?
Not 'waves', not 'matter'.
Spacetime?

Or rather whatever there is still left when we taken away those two.
But is spacetime 'empty' then.
what about spontaneous matter creation, real and virtual particles.

So the question is good, and the answer is, as far as I can see, unknown.
As far as I know this is just a idea, not a 'truth'.

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What one could notice though is that gravity, if it is as i believe.
Is a field with a propagation speed obeying 'c', but still being a 'field'.
Reacting instantly to your spaceships rudder movements sort of:)
Then one might want to take away the name space and call it gravity instead.
As they seem to be interchangeable here, at least as a 'thought experiment'

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[LeeE]

...A "better" way to think of the balloon model is to think of your ruler as a small string sitting on the balloon.  If you blow up the balloon, it gets bigger, but the string doesn't really get stretched.  If you chop the string into pieces, these are now acting like noninteracting particles in space, and those pieces do get moved apart as the balloon expands.

I think that's a very good way of illustrating it.