Note: Until co-bloggers return to blogging duty, I'll be posting one blog from the archives during the week, and one fresh new blog over the weekend. This week's special from the archives: a 2006 rumination on plasmas -- the fourth state of matter -- and how physicists are learning cool things about them by recreating tabletop versions in the lab.
Jen-Luc Piquant and I meant to post earlier this week, but a little thing called jury duty intervened. Normally this entails sitting around a dank and dreary waiting area for a day, catching up on my reading and occasionally shuffling into a courtroom long enough for the respective attorneys to take in the black leather jacket and profession of science writer, and summarily reject me out of hand as a potential juror. But this time, I was selected to serve on a personal injury trial, dragging the whole thing out and seriously cutting into quality blogging time. Judges tend to take a very dim view of jurors who don't pay close attention to the case at hand during a trial, and laptops with decent wireless connections are strictly verboten in a sequestered jury room.
As trial experiences go, it was pretty satisfying, with all the required elements for solid courtroom drama: competent counsel, expert testimony, photos and diagrams, lots of "Objections!" and lawyerly sniping back and forth, plus a character witness burst into tears on the stand, forcing a brief recess while she composed herself -- all over a minor knee injury. (Jen-Luc Piquant somewhat uncharitably dubbed the plaintiff "Little Miss Ow-My-Leg.") It was like a tiny microcosm of a major trial (or an episode of Law and Order), scaled down to manageable size.
The days weren't entirely wasted from a blogging standpoint, thanks to my uncanny ability (some might say astounding self-absorption) to tie my most mundane daily experiences to the world of science. It just so happens that several physicists -- on hand at the APS April meeting in Dallas to present their latest research results -- are finding lots of ingenious ways to cut the universe down to a more manageable size, by simulating exotic astrophysical phenomena at much smaller scales in the laboratory.
For instance, lab-based experiments can offer insight into space plasmas -- those ionized clouds of gas that make up a fourth state of matter in modern physics. Outer space is filled with plasmas. But it's difficult to come up with decent mathematical models to predict their behavior, because plasmas are a bit like tornadoes or the jet stream: they are "fluids" that exhibit "turbulence." It's easy to predict the behavior of just a couple of individual fluid particles. But there are huge numbers of particles in even a small-ish plasmas, and said particles constantly moving and reconfiguring into wildly unpredictable shapes. They are also charged particles, not neutral, so their motion is affected by electric and magnetic fields, especially the magnetic fields of nearby stars and galaxies.
Why do scientists care? Well, among other things, plasma-related phenomena are tied to the sun's periodic solar flares, which eject powerful bursts of charged particles so strong, they can sometimes interfere with earth-bound communications -- they've even been known to knock out power grids. Michael Brown, a physicist at Swarthmore College, has figured out a way to create scaled-down experimental versions of solar phenomena in the laboratory, most notably magnetic reconnection. That's what happens when magnetic fields are forced together, like two strands of a rope. These fields are oppositely oriented (like repels, opposite attracts) so they annihilate each other and produce a burst of excess energy, which accelerates the plasma outward to produce a solar flare. You can get more information on the technical details from Brown's excellent online FAQ. It's a bit like squeezing a tube of toothpaste: the magnetic fields form the "walls" of the tube and if they are forced together powerfully enough, the plasma squirts out at the top in a bright burst.
Brown's laboratory plasmas are about one foot tall whereas those on the sun are about five times the diameter of the earth, but the temperature of the gas and the strength of the magnetic field are about the same in both cases. So he has successfully recreated solar conditions in the laboratory. (He also involves his undergraduates in his research, giving them a taste of what it's like to be a practicing scientist.) Among other things, such studies may help solve the mystery of why the temperature of the sun's corona is so much hotter than the core.
Somewhat similar in concept to solar flares are the mighty jet plumes of plasma emitted by certain galaxies. That's the focus of the laboratory simulations produced by CalTech's Paul Bellan. These long thin plasma jets shoot out of newly formed stars, black holes the size of our sun (sometimes known as microquasars), and those super-massive black holes believed to serve as active galactic nuclei. Even small jets would be roughly the size of our solar system; the sheer scope makes them very difficult to study. Bellan's lab-based jets are a mere 20 inches tall, and while he admits this is "really pushing the size scale," and it is not an exact model, nonetheless they exhibit the same underlying physics, and can therefore provide insight into the mechanisms at work in the large-scale jets.
It just so happens that both the large and small scale jets are produced through magnetic lines of force, generated via lots and lots of power: on the order of 200 million watts in the laboratory simulations. Even if gas prices weren't topping $3 per gallon in certain geographical regions, this would still lead to some pretty steep electricity bills. Bellan's lab cuts down on those costs by using short, extremely powerful magnetic pulses every few seconds, thereby minimizing their power usage. These magnetic forces are essentially a larger version of those that cause opposite poles of two adjacent magnets to attract, or the same poles to repel each other. As Bellan put it, "Parallel currents mutually attract, so anti-parallel currents will mutually repel."
Here's how scientists like Bellan create plasmas in the laboratory. Bellan uses a copper disk and an annulus to simulate the accretion disk that surrounds a black hole. (Jen-Luc explains that an "annulus" is best pictured as a donut; the space in between the inner and outer edge of that deep-fried donut-y goodness is the annulus. Yes, we had to look it up. Plasma astrophysics isn't really our specialty.) A coil provides the initial "seed" magnetic field for the confined gas, which can be broken down to form a plasma by the judicious application of a strong electric current. The current first flows along the path created by the seed magnetic field, creating a pattern that looks for all the world like "spider legs." The attractive forces concentrate the current into a jet, and the repulsive magnetic forces speed it up so the spider legs get bigger and bigger. Eventually the jet undergoes a "kink instability," coiling up like a twisted telephone cord (see image) and shooting outward.
It's all very complicated and taxes the brain a bit, but the end result is pretty cool. (Undergraduates would be all over these experiments, if they weren't too busy shoving Mentos into soda bottles to cause said bottles to explode.) You can read more about this work on Bellan's Website, which also features a totally awesome movie of these mini-jets in the laboratory that might prove more illuminating than the above technical description. (There's also an enlightening June 2005 talk he gave at the Kavli Institute of Theoretical Physics on this very topic.)
Lab-based simulations could also teach physicists a great deal about the interiors of neutron stars, formed whenever massive stars explode at the end of their very long lives. Astronomers believe that the cores of neutron stars may contain a state of matter not known to exist anywhere else in the cosmos, at least not since the Big Bang itself -- possibly even that elusive quark-gluon-plasma (in which the normally tightly-bound quarks are allowed to roam free), a liquid version of which has already been experimentally observed at the Relativistic Heavy Ion Collider. NASA scientists at the meeting reported on the latest analysis of data gleaned from "starquake" explosions by a class of neutron star known as a magnetar. You can read up on the specifics here and here, but basically, these explosions generate the equivalent of seismic waves that travel along the neutron star's crust. Not only does this tell astrophysicists something about the thickness and composition of that crust, it may also provide clues to the exotic matter that may lie at the core.
The problem is that these types of "starquake" events are extremely rare; only three have been observed, most recently in December 2004. They also feature extremely intense X-ray bursts that have a tendency to cripple astronomical instruments. The December 2004 explosions knocked out several satellites. NASA's Rossi X-Ray Timing Explorer proved more resilient and gathered much useful data, but while neutron stars might be a great cosmic laboratory for studying extreme physics, it's not like physicists can actually crack one open to take a peek inside. Bringing the whole thing down to earth might provide further insight into what types of matter make up neutron stars.
To that end, University of Iowa physicist John Goree is creating small-scale versions in the lab of so-called "dusty plasmas," and using them to simulate and study the propagation of waves through the plasma -- which appear to behave much like the shock waves generated along the crusts of neutron stars. Dusty plasmas are the things that make up the tails of comets and the rings of Saturn, although there is a sense in which they can also be found closer to home. Carbon flowing out from stars will cool into dust, giving rise to carbon-based life forms like, well, us -- which is why Carl Sagan once famously observed, "We are all starstuff."
Goree creates his mini-dusty plasmas by dropping polymer microspheres -- the "dust" -- into a glow discharge plasma (essentially the same thing one finds in a neon light). The dust can absorb electrons and ions because its particles are so much heavier, giving the Dusty plasmas a negative charge and changing their behavior. Goree then videotapes the particles' behavior, which mimics that of dusty space plasmas.
So tabletop physics is a burgeoning field. There are now mini-particle accelerators that use ultrafast laser light to focus and speed up electron beams to very high energies over short distances -- a few centimeters, compared to the two miles required for large-scale particle accelerators. They're called wakefield accelerators. Intense, short pulses of laser light are fired into a plasma, producing a wave that ripples through it, leaving a wake of charged particles, much like a motorboat racing across a lake churns up a water wake in its path. A second laser pushes even more electrons into the plasma, and these can "surf" the wake, picking up speed by draining extra energy from the wakefield. We won't be producing top quarks any time soon with a wakefield accelerator, but we can simulate certain conditions on a smaller scale, thus saving the "big guns" for the big questions.
Tabletop physics isn't without its controversies either, most notably the claims of producing "tabletop fusion," including recent experimental results announced by researchers at UCLA in 2005, and just this past February by scientists at Rensselaer Polytechnic Institute. Those findings also have their naysayers, however, and it will take many more such experiments to determine whether this is truly fusion. Regardless of how that particular physics brouhaha plays out, Brown, Bellan and Goree are just three of a growing number of physicists looking to make one little lab an everywhere, perhaps solving a few niggling cosmic mysteries in the process.