@taboo:
…i expect a big post about this subject so i can learn about it, 'aight??? i'll be waiting...
Well, I've left you waiting for over a month, but the time for your "big post" has finally arrived. I really do hope you will enjoy it.
And, free-yow, is it a big post. Amateur astronomers probably saw it coming with their telescopes for about a week.
Since I don't know what your background in physics is, I tried to cover as many of the bases as I could manage. That said, the whole post is pretty wordy. Also, I may just really suck at explaining things…! So, I will totally understand if you simply lose interest before you get even 1/10 of the way through it. Good luck.
Disclaimer:
[hide]Hailing all fellow nerds. I'm in my second year of graduate school in physics at UCSD. I've been focusing on plasma physics for a little while, so I know a fair bit about what I'll be saying in the following post. But, please, please do take it all with a grain of salt. In studies, my main focus has been applied fusion techniques, not astrophysics. Still, I'll do what I can to verify what I'm saying and provide (hopefully simple) references as I go. Unfortunately, I'm super lazy, so you'll probably just have to be satisfied with links to Wikipedia.[/hide] Astronomical Attraction: Just How Weighty is Gravity, Really?
It's common knowledge that, when it comes to celestial mechanics, gravity is the king of all forces. If there is one singular force that seems to play the most massive role in the clockwork of the universe as we currently understand it, gravity is certainly it. Gravity seems to give everything in the world just as much weight as it needs, and with it the whole universe stays in motion. But it can't be all that really matters, now can it?
In the following, I am going to try to show how our current knowledge also suggests that the most intriguing objects seen in the night sky are not formed solely by gravity, but rather by the interplay of this force with one or more of the other forces of nature. I hope to show that by studying the details of the various types of interplay, we can begin to comprehend the multitude of shapes that we see out in space. Ultimately, I will try to focus on the subject of cosmic rings.
First, let's review the fundamental forces of nature: [hide]In its simplest interpretation, modern physics says that only four fundamentally different natural forces exist in the universe. Gravity is an obvious choice for one, as well as the Electromagnetic force for another. In addition, there appear to be two additional types of forces that bind things together on the tiniest of scales. For lack of better names, they are called the Weak and Strong nuclear forces. To compare the strength of these forces, we might compare how much each one individually causes two arbitrary objects held at a constant distance to feel attracted to one other (in the complete absence of all other objects). In this highly specialized case, we'd have the following relationship:
Strong > Weak > Electromagnetic >> Gravity
Gravity, perhaps surprisingly, comes in last place by a huge margin.
If we are to believe that gravity really is the weakest of the natural forces by the above comparison, then it seems bizarre that it would also be the ultimate driving force of all astronomical physics. So, what makes gravity so special, exactly? We can begin to answer this question by acknowledging the impossibility of a scenario where two objects can be completely isolated from the rest of the universe.
[hide]The nuclear forces seem to operate under the framework of quantum chromodynamics (QCD). In QCD, every interacting object is assigned one of three different charge types (usually denoted by the colors red, green, or blue) which then determines how every object interacts with all other QCD charged objects. The force itself is communicated by little quantized bits of nuclear field called gluons.
These strong and weak forces are both incredibly powerful, enough to hold the very fabric of our matter together. For example, protons are built out of subatomic particles called quarks which are bound into an R-G-B triplet by the strong nuclear force.
The electromagnetic force operates under the framework of electrodynamics. In electrodynamics, every interacting object is assigned one of two charge types (usually denoted by positive or negative) which then determines how every object interacts with all other electrically charged objects. The force itself is communicated by little quantized bits of electromagnetic field called photons. In the static (non-moving) case, this force is pretty easily understood with just three heuristics: "opposites attract", "likes repel", and "closer means stronger". When charges are moving, things become a little more complicated because of the addition of a magnetic field. Any single charge in an area of nearly constant magnetic field will be forced to orbit the local magnetic field lines. In this way, individual charges can sometimes be thought of as beads strung onto the magnetic field lines, easily sliding along them, but dragging the lines with them when pushed in a transverse direction.
Electromagnetism is what drives the biological processes of arguably every living thing on the planet. It's also the reason you don't fall through the floor when you stand up. This force is so strong that the tiniest number of electrons in the soles of your feet are pushing your entire weight upwards via the repulsive force from electrons on the floor's surface. When someone hits you over the head with a frying pan, its the electromagnetic force that leaves you with a painful lump.[/hide]As a force, gravity is quite unusual because it seems to be associated with only one charge type: Mass. In this case, there is no "negative mass", as far as we can tell. Even things like antimatter are commonly thought to have mass in a positive sense, so that a proton and an antiproton would still feel attracted to each other via gravity. The only heuristics for gravity are "all things attract" and "closer means stronger", since there is no antigravity yet to be found (ignoring dark energy, but this is another topic entirely).
With the nuclear and electromagnetic forces, when you start to gather a great many objects of varied charge together, the average charge of the group tends to even out to zero (Add enough red, green, and blue and you'll end up with white/colorless, enough positive and negative and you are left with neutral). Charges prefer to be with things they are attracted to, and they are generally attracted to different charge types, effectively cancelling out their own. In this way, the collective effects of surrounding charges cause the strongest forces to vanish very quickly as you gain distance from a single charged object due to the "quasineutrality" of the surrounding collective. We can start to see, though, how we might run into problems if quasineutrality isn't always strictly maintained.
With gravity, you can try to diminish a single particle's force on you by getting very far away, but then you're still left to contend with the gravity of all the other surrounding objects. At an alarming rate, the collective gravity of many objects can integrate to forces enormous enough to cause rising and falling ocean tides, or even cause a star to undergo gravitational collapse into a black hole. Unfortunately, due to quasineutrality, the incredibly massive Moon probably doesn't have a significant net electric charge to be taken advantage of. But, if it did, then getting to the moon would be super easy: Build up a lot of the opposite charge on something and then grab hold real tight.[/hide]Now, I hope that my argument about interplaying forces is starting to come into better focus. From my description of the four natural forces, we should notice that gravity's premier status is something that is maintained very tenuously. For example, if a significant perturbation in the net electric charge of a material were to occur, then the resulting electromagnetic force might quickly become an important influence and may even under the right circumstances exceed the strength of gravity. As I've said, all biological processes seem to be driven by electromagnetism, and indeed, it's the chemical bonds within hard plant cell walls that give a tree the strength to hoist up all its bulk, even under the enormous influence of Earth's gravitational field. These chemical bonds are nothing more than intricately arranged localized electric charge distributions.
Before we continue, I'd like to review some concepts from plasma physics, fluid mechanics, and thermal radiation: [hide]Nearly 100% of the material in outer space is in the so-called fourth state of matter: Plasma. More energetic than even a gas, a plasma can often be thought of as two ionized fluids occupying the same space at the same time; one composed of heavy positive atomic nuclei, the other composed of lightweight negative electrons. For a partially ionized plasma, we simply add to this description a third neutral fluid.[hide]And we can continue to tack on additional fluids if more species of particles (elements) need to be added to the model.
One vital property of plasmas (that may sound a little familiar) is quasineutrality; powerful electric forces appear with the slightest disturbance to the local electric charge density of the plasma. The resulting forces typically have a restoring effect (bringing the plasma back to neutrality), but can often result in erratic behavior and turbulence when driven hard/fast enough or at the appropriate resonant frequencies.
Plasmas are among the most complex things in the known universe, and are more susceptible to instabilities and turbulence than one might first suspect. To get a sense of just how crazy they can be, think of the last lightning bolt you witnessed up close and personal.
. While quasineutrality is a very important property of plasmas, it isn't enough to stop the formation of strong eddy currents within the fluids, building enormous magnetic fields in a phenomenon called a
dynamo.[/hide]Since most astronomical bodies are composed almost entirely of plasma, we can get a very good sense of how they behave by applying some of the basics of fluid mechanics. For our purposes, I'd like to quickly review the fluid properties of
pressure and
viscosity as well as the extremely pervasive phenomenon of
turbulence.
[hide]Pressure can effectively be thought of as a delocalized force, distributed over some area of a fluid. Any force that is somehow shared throughout the body of a fluid will cause a corresponding pressure distribution which may then even induce collective motion/flow within the fluid. In liquid water, pressure (as well as surface tension) is created because of the hydrogen bonds between the individual water molecules, and is further enhanced/modified by the presence of gravity and existing currents within the fluid. Water flows across the Earth the way it does in an attempt to balance its internal pressure with its main external force, gravity. Even the ocean tides can be seen as resulting from the perturbation of the Earth's gravitational field by the Moon's own gravity, enough to change the altitude at which the water pressure and the net force of gravity effectively balance. In plasmas, pressure can be an even more interesting phenomenon, because it is sometimes
anisotropic in nature.
Viscosity is a term that refers to the strength and/or frequency of interactions (collisions) between the particles that constitute the fluid. Highly viscous fluids are semisolid in consistency, will readily agglomerate, and are very resistant to shearing forces and flows. Higher viscosity typically means more direct and/or immediate sharing of momentum/energy/temperature among the fluid particles, causing more fluid to be dragged around and mixed together when exposed to an external stimulus. One way to picture this is that you'd expect more drag when paddling a canoe through honey than you would when paddling through water. A subtle thing about viscosity is that it can also be influenced by the dynamics of the prevalent particle-particle collision process itself. In a plasma, particle-particle collisions are occurring all the time due to the electromagnetic force between nearby charged particles. Thus, the viscosity of celestial plasmas results directly from the existence of the electromagnetic force!
Turbulence is something we're all familiar with. It happens everywhere around and inside us, and IMO it's one of the most beautiful things to observe in nature. Every fluid exhibits turbulence when it is stirred vigorously enough. Large whorls or eddies within the fluid are formed due to the initial stimulus, but these quickly and violently rip themselves apart into smaller and smaller eddies. This turbulent cascade can even be maintained in a more or less constant state, creating what is called a fully developed turbulent flow. Even Da Vinci found this stuff fascinating. Turbulent fluids, while omnipresent, are also often limited to certain spacial scales, depending on the strength and size of the stirrer as well as the dynamics of the dissipating turbulent cascade, which vary from fluid to fluid. Suffice it to say that most of the complexity you see in the world around you is the result of turbulence of some form or another. Fluids with low viscosity are sometimes easier to make turbulent, since the fluid does not overly resist shearing forces, allowing for non laminar flows to develop. With continued stirring, turbulent eddies can quickly fill a volume of fluid that was initially quite still or laminar. An important consequence of turbulence is that mixing of the fluid is enhanced to the extreme, and can cause turbulent fluids to behave as if they have viscosity many orders of magnitude higher than would be expected during nice, laminar flows.[/hide]Lastly, we should also note that the incredible temperatures, speeds, and accelerations that celestial bodies undergo can all result in radiation unlike anything we can begin to imagine. In the hot, swirling plasmas of galactic and stellar accretion disks, radiation of photons, electrons, and other massive particles by thermal emission is so commonplace that it can become a significant portion of the total fluid body forces that interplay during the celestial body's formation.[/hide]Alright, now we seem to be getting somewhere! Thanks for sticking it out if you made it this far!!! It sure took me long enough to write it…
Let's collect up some of the evidence that I've gathered, so far. Accretion disks are probably the most common example of celestial rings that we can see from our planet. They are composed of (relatively) hot, dense gas and plasma, all falling into a central star, which I am insisting can be understood at a fundamental level with simple fluid mechanics. Furthermore, the existence of fluid behavior, including traits like pressure and viscosity, are born out in the first place because of the electromagnetic force. Finally, even if the plasma in the forming disk shouldn't have enough viscosity to explain the agglomeration, if there is a method for stirring the plasma up (like an externally applied magnetic field from a nearby star), then you may have enough turbulence to artificially enhance the effective viscosity of the plasma.
Imagine a huge, sparse collection of slowly rotating gas in space. Collective gravity will cause all of the particles to orbit in towards the center, but should fling them right back out to their initial positions (like a high eccentricity comet in our solar system) if not for the existence of viscosity in the pervading plasma/gas. The constant, weak electromagnetic "collisions" of the particles in the cloud will perturb them from their initial orbits, especially when they collect into dense locations, ultimately redistributing their angular momentum and orbital energy among the rest of the particles in the collective. Over time, the combined effects of attractive gravity and viscous agglomeration will cause the cloud to take on a spherical shape as it continues to shrink while conservation of angular momentum requires that the cloud's angular velocity steadily rises. Eventually, the appreciable angular velocity will result in the appearance of the non-inertial Centrifugal and Coriolis effects in the fluid behavior of the plasma, both of which will tend to stretch the spinning sphere out into a wider disk. Finally, in this shape, the accretion disk can do its job of collecting more and more material in the center via the turbulent viscous diffusion of angular momentum from plasma in the center outward toward the edges of the disk. In the end, so much material will have bulged up in the center of the disk that a new star will be born, eventually burning away most of the gas left behind with its newly enhanced radiation pressure.
Now, what can be said, given my story so far, about rings made of largely solid stuff, like the asteroid belt, and the planetary rings of our very own gas giants? My answer is that there is probably at least one of three things happening with these types of ring systems: (1) It could be simply left over from the original accretion disk, captured in planetary orbit before the radiation pressure pushed it all off to infinity. (2) Some other major debris-making event created the rings, like the pulverization of a large satellite, or relatively frequent and repeated volcanic eruptions from the surface of the planet or one of its satellites. (3) The collective behavior of the rocky/solid stuff is such that it displays characteristics similar in nature to viscosity (strong mixing among the objects constituting the ring).
Going back to that article about the ring discovered around the asteroid, I really don't know which explanation seems most legit. Who really knows what is up with that?
But, I'm still pretty sure that there is some sort of a viscous nature to the way that the asteroid belt seems to collect other objects entrained to Jupiter's orbit.
Anyway. It was a long post. Hope you made it, soldier. I don't know how I got through it, honestly! Feel like I lost a lot of steam towards the end, there. I'm happy to keep discussing the topic if anyone's interested, though.