[joesc (OP)]

Okay, I'm not really worried about disintegrating.  Just wanted to get your attention.  I asked the following question on a more general forum and got a single, not-so-satisfactory answer.  So here goes:

If the very fabric of space is expanding, then I would assume that such expansion is resulting in increasing distances between all physical objects -- not only macroscopic objects like galaxies, but also between the very particles that make up atoms.  I get that at that scale, such increases in distance would be infinitesimally minuscule -- but I also assume not zero.

My layperson understanding of particle physics tells me that the strength of the forces governing subatomic interactions are very critical -- and measured to many, many decimals of precision.  Furthermore, if the constants were reduced by the even the slightest amount, one result would be that nucleonic particles would not be able to adhere (or whatever the term of art is) to form stable nuclei.

So (I'm sure you see where this is going), if the forces have remained constant, and the distances, however infinitesimally, are increasing, then at some point in the expansion of the universe, won't atoms cease to exist?

[Halc]

So (I'm sure you see where this is going), if the forces have remained constant, and the distances, however infinitesimally, are increasing, then at some point in the expansion of the universe, won't atoms cease to exist?

The small distances are not increasing.  A water molecule has the same distance between the various atoms as it did billions of years ago since that's the distance where all the forces are in balance.

Likewise, if Earth is pulled a millimeter away from the sun due to expansion, it immediately falls back down to its old orbit since it does not have the energy (kinetic + gravitational potential) to orbit at the larger radius.

[Janus]

Okay, I'm not really worried about disintegrating.  Just wanted to get your attention.  I asked the following question on a more general forum and got a single, not-so-satisfactory answer.  So here goes:

If the very fabric of space is expanding, then I would assume that such expansion is resulting in increasing distances between all physical objects -- not only macroscopic objects like galaxies, but also between the very particles that make up atoms.  I get that at that scale, such increases in distance would be infinitesimally minuscule -- but I also assume not zero.

My layperson understanding of particle physics tells me that the strength of the forces governing subatomic interactions are very critical -- and measured to many, many decimals of precision.  Furthermore, if the constants were reduced by the even the slightest amount, one result would be that nucleonic particles would not be able to adhere (or whatever the term of art is) to form stable nuclei.

So (I'm sure you see where this is going), if the forces have remained constant, and the distances, however infinitesimally, are increasing, then at some point in the expansion of the universe, won't atoms cease to exist?

As of this time, Atomic radii are not increasing due to the expansion of the universe.  The bonds holding them togehter are stronger than the effect caused by the expansion. 
To explain, I'll use a somewhat loose analogy.   Imagine you and someone else are standing on a waxed tile floor in your stocking feet,  each standing in the center of am adjacent tile.  The tiles all begin to expand.  You and your friend will move apart. But if you grip hands, the floor will still expand but you won't move apart.  The strength of your grip is stronger than the friction between your socks and floor.  This is somewhat similar to what is happening with the universe expansion. The expansion, if it could, would separate adjoining atoms, but the force holding the atom together holds them in place.
This is true not only for atoms, but galaxies and groups of galaxies.  The gravitational bound is stronger than the effect of the expansion and holds them together against it and keeps them from expanding along with the universe.  Galaxies and groups of galaxies retain their size, while moving apart from other galaxy groups.

Now, having said that, this may not always be the case.  A while back we learned that the expansion of the universe is accelerating and the that the rate at which it expands is increasing.
So let's go back to the tiled floor.  If we keep increasing the rate at which the floor expands, the tendency for the soles of your feet being dragged along increases.  Eventually, it will exceed the strength of your grip and you will be pulled apart.
In cosmology, this eventual result due to an ever increasing rate of expansion is call the "Big Rip".
As the rate continues to increase, the ability for structures to hold together against it begins to fail.  First groups of galaxies will begin to pull apart,  then later galaxies themselves will pull apart into independent star systems. Down the road, the planets of those star systems will be ripped away.   Going further forward more, the planets and stars will be ripped apart as their gravity will no longer be enough to hold them together.  This will continue as smaller and smaller structures succumb.

If this turns out to be the eventual fate of the universe, it turns out that the end comes fairly quickly. For one model, which has the end coming in 22 billion years, galaxies would be pulled apart ~60 million years before the end, and our solar system would hold together until just three months until the end, planets and stars wouldn't go until minutes before the end, and atoms just moments prior to the end.

And we really don't know if the the Big Rip scenario is the actual fate of the universe.

[yor_on]

I liked your analogy Janus, but there is one thing I wonder about. The vacuum expanding isn't a 'force', or is it? What I mean by it is that I'm not sure how the idea of the big rip came to the conclusion of it being able to rip apart atomic bonds, etc. When it comes to galaxies 'separating' from each other then isn't that a geometrical result of a space 'growing'?
=

Syntax

Thinking of it, it seems to become a question of if gravity is a 'force' or not. Then again, why should it? Two things separate from each other but no instrument used, anywhere, will show any force involved into it. The gravitational potential of each object is the same although when considered a system it will have changed between them, becoming 'weaker' due to a increased distance, if I'm now thinking right here?

Maybe we can exchange it to. Do the vacuum do work when it expand, accelerating?

One might be able to argue that cosmological redshift shows us that there are energy lost as they separate? But then we can take two uniformly moving stars separating from each other, showing us a similar type of redshift although now not locked to a expanding vacuum. The 'uniform motion' I'm using isn't a 'force' in itself as far as I know, only when connected to other uniformly moving objects, as in two objects colliding. And then only as a result of the combination of their 'relative speeds' versus each other. But it is the same there, there are no local experiments proving a 'force' existing for this uniform motion. And you're also free to define what is moving relative what in such a case. In a black box scenario (Earth), ignoring spin, there is no way to define if any uniform motion exist at all as far as I know.

We tend to see patterns, to see them we need to connect different things. So we look out on the universe and we define things moving relative ourselves. Each one of them is locally defined in a same situation as Earth, non existing provable motion, but from our global interpretation of the universe everything moves. A acceleration is definitely a (local) force though, by Einstein under specific conditions (constant uniform acceleration) defined to be a equivalent to gravity. Meaning that one will find oneself to have a weight using a scale. That's GR, and looking at it that way gravity is equivalent to a 'force', possibly?

But I'm not sure on that one at all. Another way to define it is by using SpaceTime and then defining gravity as a distortion of the same. That one is GR too, and have several proofs of its validity as I remember.

Alternatively accelerations are misunderstood? (thinking of the first example, defined as a local force)
Heck, Maybe they are? After all, Earth is also defined to accelerate at one uniform constant gravity, in perpetuity.

There is one thing differing between Earths acceleration and a rockets as it seems. You can't get that rocket to accelerate without transforming 'energy', making the rocket do 'work' burning fuel of some sort. Although there are also 'gravitational accelerations' to consider in where no 'energy' needs to be transformed, all as I see it?

But still, do the vacuum do work as it expands accelerating?

Better add this too. I can't accept that everything must be 'forces'. Dark energy is nothing measured but it seems we need to look at both gravity and accelerating expansion as 'driven' by 'forces', even if we can't find their constituents. If gravity would be a force then I don't think we would be able to hold it up as a 'distortion', would we? One would need to connect this 'contracted/folded space' for example to ones constituents and in effect define it either as belonging to whatever force one define as creating it, or create even more 'forces' contributing to this contracted vacuum. So in effect a contracted vacuum would then be f.ex 'gravitons', or a combination of those and new 'forces/force carriers/particles'.

Another way is to define it primarily to be a geometry, containing 'force carriers/virtual particles etc'. Personally I like that one better and it seems to me to connect better to the idea of SpaceTime as a whole. And yes, the one with forces I won't call 'virtual' because then a vacuum is particle defined, in the other version a space is a geometry.

[Hayseed]

On that kind of level, you're pretty safe.  Your atoms will out last you.  Worry about flesh and bone first, then keep an eye on your molecules' health, for they are under constant attack.

[yor_on]

Ok, keep wondering about this question. Looked it up and found this paper

" In an expanding universe, what doesn't expand?

The expansion of the universe is often viewed as a uniform stretching of space that would affect compact objects, atoms and stars, as well as the separation of galaxies. One usually hears that bound systems do not take part in the general expansion, but a much more subtle question is whether bound systems expand partially. In this paper, a very definitive answer is given for a very simple system: a classical "atom" bound by electrical attraction. With a mathemical description appropriate for undergraduate physics majors, we show that this bound system either completely follows the cosmological expansion, or -- after initial transients -- completely ignores it. This "all or nothing" behavior can be understood with techniques of junior-level mechanics. Lastly, the simple description is shown to be a justifiable approximation of the relativistically correct formulation of the problem. "  https://arxiv.org/pdf/gr-qc/0508052

But that one seem to be understood from a classical point of physics, as differing from a quantum mechanical definition.
https://physics.stackexchange.com/questions/8301/can-the-big-rip-really-rip-apart-an-atomic-nucleus

[yor_on]

As a simple response I find it very hard to define a expanding 'space' as something made of 'particles', which I think we need to introduce in some way to make it a 'force', just as we need with 'gravitons'. If one instead call a dimension/dimensions a result of something else one can avoid that as I naively think about it. It goes back to what a vacuum 'is'. A geometry or something else?
=

Then again, thinking of a 'space' as something containing some sort of 'energy density'. Then presuming that the reason for a inflation / expansion being a influx of 'particles' from 'somewhere else' 'forcing' space to expand. I don't like that one. Then you have the one in where a 'infinite universe' still must have boundaries in some way as the inflation and expansion both are presumed to 'lend' from something inside our universe creating this phenomena. I'm not that happy with that one either.

Another argument would be that both a inflation and a expansion must be time dependent. As soon as we introduce them time must exist for them to evolve. If we now presume that a inflation is ftl, what would that do time dependently to a influx of particles? It would set a border for this 'speed', wouldn't it? Presuming that no 'particles' by themselves can exceed the speed of light in a vacuum.  For that one you will need to imagine this influx to happen everywhere, in all 'points', old and new.

When it comes to the idea of space being a geometry containing a energy density, equivalent in every point, old as new we find that virtual particles don't have the same time dependency as 'real particles' though. In other words, particles need time to become real to us. You can rip them apart before annihilating each other as in spontaneous pair productions, using for example gravity, and in such a case they are also a result of what energy density you define to exist for them to appear in.

What one seem to be able to state is that gravity actually will create a higher energy density in a vacuum, f.ex surrounding a black hole, but it doesn't change virtual particles time dependency as far as I get, only the amount of them appearing. Chalmers university made a experiment in where they created a energy density, making 'light' https://www.chalmers.se/en/news/Pages/Chalmers-scientists-create-light-from-vacuum.aspx

What is interesting to me there is this " The physicist Moore predicted way back in 1970 that this should happen if the virtual photons are allowed to bounce off a mirror that is moving at a speed that is almost as high as the speed of light.

The phenomenon, known as the dynamical Casimir effect, has now been observed for the first time in a brilliant experiment conducted by the Chalmers scientists."

It's about what those 'time dependencies' are, where the border between 'virtual' and real is broken.

Another way to look at it would be to define 'c' as the best clock existing, then that could be called everythings local 'time keeper'. Then we combine this with 'energy densities' to define where and how a particle becomes real.

[CPT ArkAngel]

Space expands only when you have more energy of expansion than contraction. Space does not expand by itself.

There was no problem until it was demonstrated to expand at an increasing rate at the cosmological distances. The extra energy needed for this accelerated expansion is called Dark Energy. Forget models, just stick to observations. Whatever Dark energy is, it does not expand space at a galactic scale or smaller. Everything else is hypothetical.

[yor_on]

I'm not sure CPT. Thinking of energy as a coin of exchange doesn't make 'energy' existing on its own. It's transformations, isn't it? The best analogy I know of is 'photons', having a momentum but no proper mass.
=
And those 'photons' are funny things, they are presumed to propagate but in no way observable, and the experiment defines the way they will express themselves.

[CPT ArkAngel]

Photons have a mass but no proper mass. A proper mass is just a mass going slower than C, with everything that comes with it.

Photons are detectable. What is real might not be all detectable directly. For example, gravitons exchanged between the Earth and the Moon may not be detectable because it is a symmetrical exchange, the total flux is null except for the energy lost in tides and changes in the trajectories. Furthermore, it is probably a fundamental relation between all particles which can only be influenced by gravity. If you try to measure the gravitons between two bodies, you can only influence them by adding a mass in between. What you will measure is gravity...

When a particle is measured, it is only the EM field and possibly the inertial mass which are detected. Does this mean only the EM field is real?

[evan_au]

When a particle is measured, it is only the EM field and possibly the inertial mass which are detected. Does this mean only the EM field is real?

We have successfully detected neutrons and neutrinos, which don't have an electromagnetic field.

However, our technology is most advanced in managing electromagnetic fields, so we usually convert whatever is detected into an electromagnetic pulse so it can be captured as an image and/or analysed by a computer.

See: https://en.wikipedia.org/wiki/Fundamental_interaction

[CPT ArkAngel]

Look how neutrons, neutrinos and other interactions are measured... You will see I am correct. This is all inference from EM field and momentum exchanged.

[yor_on]

Nice link Evan. One of the reasons for it CPT is that there are other ideas than the one connecting whatever we see and find to some specific force. I'm not saying that 'forces' doesn't exist, and act, but I don't think we need a 'force' for, f.ex, being a vacuum. It's a subtle difference and arguable but it's the same idea as we see with gravity. A question of definitions maybe, as I gather the Higgs boson does not explain all of gravity, instead it presents one mechanism but leave other parts of it unexplained. If we now want to look at everything as forces, then we will need to create plausible mechanisms for those others too, and in the end create some sort of net of forces possibly explaining everything leading to a universe.  Against it we have ideas as holographic universes, where one might argue that they too need forces but where the representation of a vacuum isn't a 'force' in itself.

[CPT ArkAngel]

Gravity not being a force is Einstein's interpretation of his own theory. It was not fundamental in his theory. He doesn't not describe matter and particles but its behavior in space and time as if they were centers of mass in a gravitational field. It is his interpretation of his fields equations in the absence of a description of how the energy is exchanged. He did not do the maths alone. In the end, maybe it is not a force but energy must be conserved somehow. When you consider QM, there is no energy without particles. And there is no limit to how large a particle can be but the size of the Universe.

Imagine you look from the Earth at two large stars. These two stars come closer together. According to GR, their emissions are redshifted from your point of view. Why? Energy must be conserved. If you have particles mediating gravity and the distance between the two stars is shorter, the wavelength is shorter and the frequency is higher for particles mediating gravity. If they have a single wavelength in-between, the gravitational energy increases in the field while the wavelength of the particles inside the stars increases and their energy decreases. These particles, or rather their oscillations travel at the speed of light. The energy is transferred from the stars particles to the gravitons in-between relative to you. Not only energy is conserved but you can't eliminate gravity by blocking it because you need to put an object with a size larger than the distance in-between to stop or dampen the gravitational field, which is impossible. And in fact, the angular size observed is larger by the same amount as the gravitational redshift in real observations of stars and other celestial objects... Not only that but the speed of light becomes a true constant for all observers... It is the wavelength which truly changes according to the gravitational potential in agreement with GR. Integrated Sachs Wolfe effect included. GR speaks of redshifts of bodies, QM speaks of wavelength of particles but no gravity is included.

[yor_on]

Yes, it was fundamental CPT. also revolutionary and still is. In his theory gravity is a result of space becoming 'warped' or 'distorted'.  It's a force globally defined but locally non existent, aka in a black box scenario. He did not define it as, for example, equivalent to EM although he searched for it.
=


What I think he searched for was 'fields' unifying it all, but that also will question how we define a force.

[yor_on]

Also, be careful when discussing the idea of 'energy'. JP who has a very good education called it a 'coin of exchange', that was as close as he wanted to go to it.

[yor_on]

I can give you some problems if you like to think of a vacuum as being 'energy' btw. A 'distorted space aka Einstein must then contain more energy? Globally defined or locally? How come they differ?

A inflation then?
And a accelerating expansion?

[yor_on]

What people seem to miss is that SpaceTime is a unification of dimensions, making 'time' one of them. From that it makes sense.

[CPT ArkAngel]

Ok, I must admit, I made a mistake by saying that curvature of spacetime is not fundamental. It is, but when you look at Einstein's field equation, the curvature of spacetime equals energy. E=MC^2. Is it possible that particles are curved spacetime? In QFT, the energy of the vacuum is just too high. When you look at the calculation of the energy of a single particle in QFT, it gives a sum of positive and negative terms. Maybe the real potential of the vacuum is just the remnant of the calculations and all the vanishing terms are not real... The problem is they calculated all terms for the total vacuum energy of the universe as if they were all real.