Some people’s concept of nothing is so sometho-centric.
Damn Nothingists.
Sorry if I’m coming off a little overly sensitive, but, well, you know…I Know Nothing.
Thanks for the % estimate.
Which force are we talking about? (Some forces repel, others attract, right?) None of the confirmed forces cause space to expand, right? It just does. …right?
Quoth Corner Case:
A far better question than you realize. There’s some mysterious phenomenon, which has been given the label “dark energy”, which is causing the expansion of the Universe to actually speed up. While we really don’t have any clue what this phenomenon is, one possible explanation would be a “substance” with particular properties: To wit, negative pressure which is as large in magnitude as its energy density. Well, as it happens, most models of particle physics predict that the foam of virtual particles should have just this property.
Wow, that ties everything up neatly, then, right? Well, not really. You see, while particle physics models aren’t yet sophisticated enough to be able to calculate what the density of the virtual particle foam should be, they can at least provide an estimate. And meanwhile, we can also measure, from our observations of the Universe, what the density of the mysterious dark energy ought to be. And the particle physics estimate is 120 orders of magnitude larger than the dark energy. Note, that’s not a factor of 120; that’s 120 orders of magnitude. As in, a factor of a hundred billion billion googol. And since it beggars belief that any estimate could ever conceivably be that bad, we’re left, if anything, even more puzzled than we were before.
Right. Space expanding is not directly related to the forces that hold materials together. Anything that clumps together to begin with is made up of stuff that attracts one-another. Sure, some things repel (anything with electrical charge of the same sign, for instance), but this has nothing to do with the expansion of space. Space itself is what is expanding. Nobody yet has a very good theory for exactly why space is expanding the way it does. This is part of the mystery called ‘dark energy’. But the fact is that it does. And when it does, it ‘adds space’ in between the particles in things. But if these things are staying together by some attractive force, they will not expand. Pretty much everything you see around you is held together by electromagnetic attraction (the force that matters in chemistry). Take two magnets, for example, stuck together. If the universe expands, and some space gets added in-between the two magnets, they won’t stay apart for long! They will quickly bridge the gap and snap back together.
Just to get an idea of magnitudes…
Does the space between the Earth and the Sun increase, or does gravity overcome that? Does the milky way galaxy increase in size, or does gravity overcome that? Is this effect mostly limited to inter-galactic interactions?
I disagree space expanding is directly related to the stuff that’s in it. In standard big bang cosmology it’s a consequence to the how Einstein’s field equations describe the dynamics of space under the assumption of spatial homogenity and isotropy. Dark energy is postulated to explain the observation that the rate of expansion appears to be accelerating.
I think it’s not correct to think about expansion on a small scale (such as two magnets) as what’s happening on a small scale is far more complex. This is simply because on a small scale the assumptions of homogenity and isotropy generally are not good ones, only when you get on to a vast scale do they become an excellent fit with reality.
See my post above. Basically expansion should only be thought about when examining the dynamics of the universe on a very large sclae. The sort of scale we’re talking about is so large that even galatic clusters are modelled as having essentially zero-volume.
Right. Otherwise, if those magnets have a stable equilibrium at a distance r (which simple magnets generally don’t, but it’s too early Saturday morning to come up with a better example, you get the idea), then if space were to uniformly expand, that equilibrium distance would simply be taken to a larger one, and we’d arrive at the ‘everything expands in size, which nobody notices’ conundrum again.
So, as you point out, it’s probably better to say that a FLRW universe (i.e. a homogeneous, isotropic one) uniformly expands (or contracts), but our universe is only FLRW on a (very) large scale.
I’ve enjoyed this thread, very thought-provoking!
I agreee with you, though I’ll explain the exact thinking behind what I said:
Take for example the solar system, you will get a far better approximation for this physical system using the Schwarzchild solution than approximating it as a patch of space in a FLRW solution. Someone might say, “but hang on the Schwazrchild solution is a static solution in GR [whereas non-degenerate, non-finely tuned FLRW solutions are not], couldn’t we just add expansion by hand to this system by viewing it as a [slightly] non-static small peturbation of the Schwarzchild solution?” We could do this, but IMO that would not make it any better approximation than the Schwarzchild solution as there are far bigger peturbations to the staticity of the spacetime representing the solar sysetm coming from the distribution of matter both within and without the solar system.
The ‘noise’ from these much larger peturbations essentially drown out the very small peturbation caused by the large-scale expansion of the universe. It’s only when you get to scale such that the peturbation from the approximations/assumptions of FLRW cosmology (i.e. absolute homogenity and isotropy) is sufficiently small that it makes sense to start talking about the metric expansion that is a result of FLRW cosmology.
So basically what I’m saying is that the dynamics of a small scale system can be very different from the dynamics of large scale system of which it is part. This is mainly because of the approximations that we can make for large scale systems become very poor approximations for small scale systems.
I think measure for measure was asking whether or not the expansion was really just a repelling ‘force’ between particles. My answer is: no. The space itself is expanding in between the particles, and the expansion of the space is not directly related to the forces between the particles. The expansion of the space is related to the solutions to the field equations for configuration of space-time itself, which is related of course to the force of gravity. The expansion of space is also related indirectly to forces that contribute to the vacuum energy.
You are right, but I think the example correctly explains in spirit the answer to measure for measure’s question. I hope you don’t confuse the guy. The fact is that the expansion of space is far, far, far smaller than the forces holding any materials/planets/solarsystems together on scales smaller than our own galaxy. The point is that the electromagnetic or gravitational forces holding a system together totally overwhelm the expansion, so that objects maintain their prior configuration with totally negligible perturbation. For galaxies, which are often far enough away from each other that gravity is very small between them, there is not enough force acting to hold them together, and the expansion of space therefore effectively moves those galaxies apart.
[QUOTE=Half Man Half Wit]
Right. Otherwise, if those magnets have a stable equilibrium at a distance r (which simple magnets generally don’t, but it’s too early Saturday morning to come up with a better example, you get the idea), then if space were to uniformly expand, that equilibrium distance would simply be taken to a larger one, and we’d arrive at the ‘everything expands in size, which nobody notices’ conundrum again.
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The change in equilibrium distance will be, however, totally negligible, no?
Gravity overcomes the expansion easily. The effect is limited to inter-galactic interactions because:
- the expansion is so small per meter (you need a lot of meters between objects before you notice the expansion)
- galaxies are often so far apart that the effect of gravity is small
(continuing above post)
To give a feeling for 2):
The acceleration between two typical galaxies that are, say 10 million light years from each other (really not very far), as per Newton’s theory of gravity, is approximately 10^-14 m/s. Compare this to the acceleration between the earth and the sun due to gravity, which is about 6*10^-3 m/s. In other words the gravity between two galaxies is typically over 10 orders of magnitude less than the gravity between the earth and the sun.
I don’t know if this is related or not…
In my old cosmology class, the professor tried to deal with a very, very, VERY simple hypothetical cosmos: it consisted of a planet, much like the earth…and nothing else. Nothing else AT ALL. No sun, no moon, no stars, no space dust.
He taught us that this planet could “rotate” even in the midst of…nothing. Everyone, of course, asked, “Rotate relative to what?” There isn’t any way to tell. No stars, no ecliptic, no reference point. But he said that the planet would develop an equatorial bulge – just as the earth has – from its rotation, and that would be the proof.
i.e., the meta-rules of the universe, according to my old cosmology prof, say that there is an intrinsic “metric” or “reference” or “direction” or “space-time” even when there is “nothing” material in it to make it visible.
I think this is all implied in Einstein’s (and Mach’s, and other people’s) equations. Anyway, is this relevant to the discussion of “nothing?”
(And…my old prof could have been wrong. Not like that never happens! Had one teacher who insisted that the Elbe River was the border between Germany and Poland…)
Trinopus
I think what I’m saying is that is probably a better to see metric expansion as an emergent property of the universe on a large scale, a bit like temperature is a property of a system only when the system is large enough or you examine it for a long enough time.
Expansion is not a force is in the sense that it is not something that can be represented as a four-force vector like for example the electromagnetic force. However gravity isn’t represented by a four-force vector in general relativity. The repellant ‘force’ of metric expansion the attractive force of gravity are both properties of the spacetime metric, so it depends on how you define force. That said I would say I agree it’s better not to see it as a force as it exists way outside of the Newtonian limit.
The forces between particles contribute directly to the stress-energy tensor so the metric properties of spacetime cannot be said to be independent of fgorcfes that exist between particles. Of course with the exception of gravity, even very on a fairly small scale most forces cancel out so can be ignored.
Vacuum energy/dark energy is needed to explain the accelerating expansion of the universe, but it isn’t needed to explain the expansion of the universe. It’s only in the last few years that terms that could be described as descrbing vacuum energy have re-appeared in cosmological equations of state.
I think this is my point, I don’t think that’s a particualrly great way to see things as the ‘truth’ is just so much more complex. What I’m saying basically is basically the best way is to completely ignore expansion until you get on to a large enough scale.
Yes rotation is not relative in Newtonian physics and it’s not relative in relative either (or at least it’s only in the very broad sense of diffeomorphism invariance in general relativity). Mach’s principle (or perhaps it should be seen as Mach’s conjecture) is that even, for example, rotation is relative, but only appears to be due to creation of a set of ‘preferred’ inertial frames by the large scale distribution of matter.
Personally, I prefer to say that the phenomenon driving the acceleration isn’t a distinct force. It’s just another manifestation of gravity, which appears to be following all the same rules as “normal” gravity, just as applied to a very peculiar (from our point of view) sort of substance.
iamnotbatman, These are my own pants: The discussion has been helpful: thanks.
I don’t think so – unless I’m getting something mixed up, if everything expands by a certain factor, so should this equilibrium distance – it’s the reason the whole ‘local forces overcome dark energy’ explanation never rang true with me. It does work for gravity, though, because in this case, you simply have a metric that’s locally different from FLRW.
Maybe I’m misunderstanding you, but that factor – the rate of expansion – is so small that the change in equilibrium distance is totally negligible for scales less than the size of our solar system. It’s an easy calculation that I have done before; model the expansion by an effective repulsive force, and this force (serving to perturb an equilibrium distance) is ridiculously small compared to the forces between the magnets that determine the unperturbed equilibrium distance. We are talking about dozens of orders of magnitude smaller in the case of forces on the scale of centimeters.
Oh yes, of course, it’s going to be ridiculously tiny. But, if we were to use this equilibrium distance as a ruler, and measured the distance to some distant galaxy with it, and then have the universe expand a little, and then measure again – we wouldn’t notice any change, would we? If at first the distance was some huge nr, and both that distance and r have grown by the same (tiny) factor, we would still measure the same nr… No? Only if there is actually no (or less) expansion on the small scale will we actually observe a difference.