nothingness

March 10, 2006 8:52 AM

Does space expanding require more nothingness inbetween?

I am told that in modern physics, space is different from nothingness, because space contains vacuum energy, can split into matter/antimatter and because it can be bent and stretched.

My question is this; if what causes the universe to expand is space stretching/getting less dense, does this mean that there's bigger gaps of nothingness within the space, or is it that more space 'grows'? Is the universe completely full up with space/matter/energy, or is there nothingness as well? Or what? Am I thinking of all this too mechanistically?

posted by leibniz to science & nature (14 comments total)

If you draw dots on a baloon, and blow up the baloon, does that mean that there's bigger gaps of nothingness between the dots, or is it that more baloon 'grows'?
posted by Jairus at 9:20 AM on March 10, 2006

Am I thinking of all this too mechanistically?
posted by leibniz

heh. That should be Newton's line, I think.

Leibniz was well read in the mystical literature, and many of its proponents were his personal friends. Its influence clearly shows in many of Leibniz’s writings, and the Monadology is a particularly good example, with its almost mystical vision of universal animism, a form of reincarnation, worlds within worlds, and the special relation between God and humanity.

/nothelpful
posted by metaculpa at 9:25 AM on March 10, 2006

Could you clarify what you mean by "nothingness"?

The vacuum between galaxies is increasing in volume, but that vacuum isn't truly empty as you point out.
posted by justkevin at 9:32 AM on March 10, 2006

Scientific American had an in-depth article on this very issue a year ago.
posted by Netzapper at 9:43 AM on March 10, 2006

The whole idea of 'nothingness' is pretty much bogus when it comes to space-time. It's possible for particle/antiparticle pairs to spontaneously appear and disappear, not even necessarily simultaneously (since they can exchange photons both backwards and forwards in time). I'd suggest reading some of Feyman's stuff on quantum electrodynamics if you're interested in this... the little QED paperback can be read in an afternoon and is very enlightening.
posted by unSane at 10:00 AM on March 10, 2006

IANAExpert, but it seems like the density of matter in an expanding universe would decrease over time.
posted by grateful at 10:36 AM on March 10, 2006

As physics is currently understood today, the space which "appears" between two distant points due to the cosmic expansion is exactly the same as the space that was there before. It's not like space is viewed as a collection of discrete points that are flying apart as the Universe expands, leaving "nothingness" in between them.

This is actually at the root of a significant problem in modern physics: vacuum energy should behave as a certain energy per volume of space, exactly like Einstein's cosmological constant. This means that as the Universe expands, the effects of the vacuum energy will become greater and greater, until eventually the large-scale behaviour of the Universe is completely dominated by the effects of the vacuum energy.

That's actually not a problem per se — in fact, that's what the current thinking on the ultimate fate of the Universe is. The problem is that the actual measured cosmological constant is much, much, much, much smaller than our best understanding of physics says it should be. As in, the predicted energy density due to vacuum energy is 10¹²⁰ times larger than the actual measured value. Some wags have gone so far as to describe this as "the worst theoretical prediction in the history of physics", which might be overselling it a little, but it ain't too far off either.
posted by Johnny Assay at 10:57 AM on March 10, 2006

Brian Greene's latest book addresses this topic as well. its called "The Fabric of The Cosmos"... of course almost any book on cosmology talks about it. a good summary is the "very short introduction to Cosmology". you can find either of these books on amazon.
posted by joeblough at 1:54 PM on March 10, 2006

I was JUST about to recommend "The Fabric of the Cosmos"... joeblough beat me to it.
posted by Decani at 8:20 PM on March 10, 2006

Grab yourself a pair of strong bar magnets, one in each hand, north poles facing each other. Shut your eyes and move the magnets together, and apart, and around each other.

So, what's that shape you can feel between your hands? Is it space or nothingness, or is it a thing of some kind?

It seems to me that what your question is actually about is some imaginary properties of imaginary gaps between imaginary billiard balls in an imaginary storm of virtual particles; that is, you're asking questions about a physically intuitive model of an inadequately understood mathematical model of the finest scale of reality. God knows I enjoy doing the same thing :)

But since, as far as we know, the real world is NOT in fact constructed from submicroscopic hard-edged plastic billiard balls with featureless gaps in between, you may pick any answer that pleases you.

As I understand it, "expansion of space" simply refers to the tendency for things to get further apart over time. The question of what should be considered a "thing", in this context, is left as an exercise for the reader :)

General Relativity is our best mathematical model for describing these effects, and General Relativity is a classical model; the "shape" of space, in the terms of that model, is akin to the "shape" you feel between your hands in the magnet experiment. Space is modelled as smooth and continuous, with properties characterized by fields - mathematical objects whose values are well defined at any arbitrary point.

GR has nothing to say about the fine structure of space - the whole virtual-particle-storm thing is simply missing from that model. Conversely, quantum models (which do include storms of virtual particles) have nothing to say about the expansion of the Universe.

A proper answer to your question might be derivable from a decent theory of quantum gravity, which we don't currently have. But it's my guess that any attempt to answer your question using a QG model will cause you to reject one or more of your underlying assumptions.
posted by flabdablet at 12:47 AM on March 11, 2006

Could it not be merely likened to the dissipation of an aerosol-sprayed scent in a room?
posted by vanoakenfold at 4:20 AM on March 11, 2006

thanks for the answers and sources guys. It looks like there's no straight answer to this one yet. By nothing, I really do mean 'not anything', except that if it's allowing expansion, it must at least have dimension. Even this would be unlike the nothingness 'outside' the universe, for which it's meaningless to talk in those terms.

I don't buy into infinite smoothness of space, and don't see how space could grow without some material to grow from. If matter/space is just converting into another form, there still seems that what makes it different is different proportions of nothing to something.

Anyway, any more ideas are welcome. I'm going to at least read that sciam article and maybe get the Greene book.
posted by leibniz at 5:02 AM on March 11, 2006

The problem with thinking about stuff like this is that humans are generally inclined to think in metaphors from the physical world we are familiar with at a scale we are familiar with -- balloons, aerosols, billiard balls and so on. However all the lessons of physics of the last century are that these metaphors are entirely useless when it comes to thinking at larger or smaller scales. This is why subatomic physics is so math-oriented, because many of its properties are completely counter-intuitive and the only way to make predictions about a situation is to calculate.

There is an even more fundamental problem in generalising about the universe, which is this: almost all of the universe is completely beyond mankind's horizon. That is to say, it is so far away in space-time that information (photons, gravity) from it will never reach human beings during the lifetime of the universe.

It might as well be labelled 'here be dragons'.

For instance, we have no evidence whatever about the value of fundamental physical constants in these uncharted parts of the universe. It is quite possible that, for example, the speed of light or the cosmological constant are different. If so, it is possible that there are regions of the universe which are radically different in terms of the physical existence than the one we inhabit.
posted by unSane at 5:58 AM on March 11, 2006

I don't buy into infinite smoothness of space, and don't see how space could grow without some material to grow from.

The universe isn't "infinitely smooth"; there are all kinds of lumps in it. At some degree of lumpiness, it becomes mathematically infeasible to keep modelling the shapes of the lumps with fields, and we get better answers by treating certain kinds of exceedingly lumpy regions as particles.

If you're not doing maths - if all you're doing is handwaving, as we are here - then the point at which you stop thinking "space" (fields) and start thinking "stuff" (particle clusters) is pretty much arbitrary.

Both particle clusters (stuff) and fields (space) have shapes. The shape of stuff is generally apparent by inspection. The shape of a field is a slipperier idea; it's characterized by the forces that would be experienced by a probe particle that couples to the field (charged particles for electromagnetic fields; massive particles for gravitational fields).

This is why you absolutely have to actually grab those magnets and have an actual eyes-closed feel around. You really need direct bodily experience of a shaped field to get any kind of gut understanding of this stuff.

To get the expanding-universe idea clear, think about working up a map of a patch of universe at least a few thousand light-years wide. Your map will need to be good enough to let you point to a bunch of features and name those for later reference. Those labelled features don't have to be Stuff (i.e. particle clusters like hydrogen atoms or dust grains or billiard balls or stars or galaxies); they could as easily be distinctively shaped regions in assorted fields.

The expanding-universe model predicts that if you measure the distances between every pair of labelled features and add them all up, and then do the same thing again a bit later, the second result will be bigger than the first. It says nothing at all about what's going on in places you haven't labelled.

That prediction is based on mathematical relationships that have been shown to model, with good accuracy, the physical relationships between massive objects and gravitational fields. There is no "underlying mechanism" posited at all. There's just a testable prediction based on mathematics based on abstraction of observables.

In particular, the expansion of the universe is not caused by space doing anything at all. In particular particular, space doesn't stretch despite all those college textbooks depicting it as a rubber sheet, nor does it grow. Things just tend to get generally further apart over time.

Thinking about a universe expanding because its interior spaces are doing something to force it to is absolutely putting the cart before the horse. You might as well think of your chest expanding as you breathe in because of being pushed out of shape by all that inrushing air. Expanding is something that the whole universe is just doing; the effects of that on local bits of space are just that - effects - not causes.

If matter/space is just converting into another form, there still seems that what makes it different is different proportions of nothing to something.

No, what makes the new form different is that the total of the distances you'll measure between things is larger. And as I mentioned earlier: exactly what you consider to be a "thing" is up to you.

Just remember that even where it's hard to find particles, you'll find fields - not "nothings" - and fields can have arbitrarily complicated shapes; they're not necessarily anything you'd think of as smooth, even if they do have well-defined values (by definition) at any arbitrary location. To think of fields as "nothings" is merely particle chauvinism :)
posted by flabdablet at 9:23 AM on March 11, 2006