Author: dabacon

Fermilab and SLAC have come together to produce a free magazine called Symmetry. Last week I received my first issue. In this day when science writing is usually pretty dumbed down, Symmetry seems to have quite a few well written articles.
I especially enjoyed the article on the Pierre Auger Observatory for cosmic rays located in Argentina (Some of you may remember studying the Auger effect in physics where a vacancy of an electron in an inner shell of an atom is filled not by radiating, but instead by ejecting energetic electrons from the outer shells.) Comsic rays are very high energy particles which strike the Earth’s atmosphere and produce spectacular showers of billions of electrons, muons, and other particles. The great mystery, of course, is what produces these highly energetic particles. The first thing you might think is, well just use the shower to locate where in the sky the cosmic rays come from. For low energy cosmic rays, this indeed has been done. But what you find is that they are pretty much randomly distributed across the sky. There’s a simple explanation for this: the galactic magnetic field is strong enough to singificantly bend the direction of the charged particles which produce the cosmic ray showers. Thus you’re not getting a true indication of where the particle is coming from by tracking its direction when it strikes the Earth.
What’s nice about the Auger observatory is that it will be able to detect significant numbers of really high energy cosmic rays. At something like greater than 10^19 electron Volts, the galactic magnetic fields are not able to significantly bend the charged particles. And here is the really neat thing: nobody has any real good idea of what could produce cosmic rays with energies of 10^19 or 10^20 eV. (The world record for such a particle had 3×10^20 eV, 300 million times more powerful than the our most powerful particle accelerator!) One constraining effect that we do know is that these particles must come from somewhere in the local galactic neighborhood. This is because of the GZK cutoff: the cosmic background radiation looks pretty nasty to a particle with greater than 5×10^19 eV. Almost no particles above 5×10^19 eV can survive long at these energies and within something like a few hundreds of millions of years almost all particles will be reduced down to the cutoff. Thus we know that these particles must be coming from our local neighborhood of galaxies (ruling out active quasars, everybody’s favorite explanation for all things energetic.)
The cool thing is that the Auger observatory (which consists of 1600 detectors covering 1200 square miles!) will be able to really begin to pen down where these high energy cosmic rays are coming from. Which is particularly good, considering that there are nearly as many theories about cosmic rays as there are theorists who have studied them. When will we know more? Sometime around August the observatory will be reporting its first results. Exciting stuff!

Today, thanks to some very beautiful cosmology, we think we know quite a bit about the matter content of our universe. The observed universe is, according to these studies, 1 part ordinary matter, 5 parts dark matter, and 14 parts dark energy. One of the interesting gaps in our understanding of this picture, however, is that when we add up all the numbers, we find that we are missing between 30 to 40 percent of the ordinary matter. One possibility for where this matter may be found is in hot (10^6 K) low density gas in the intergalactic medium. At these high temperatures, atoms like oxygen and nitrogen retain a few bound electrons. But because these are heavy elements with a few bound electrons they will absorb only at very high energies. In order to see this absorbtion, you need to look in the ultraviolet or X-ray regime of the spectrum. Since it’s impossible to test this theory from ground-based instruments, this idea has floated around, but never really been verified.
Now there is news today, published in Nature by Nicastro et. a (vol. 433, p.493), that the Chandra space telescope has indeed detected evidence of this absorption and, with admittedly still large uncertainty, the calculations suggest that indeed this indeed makes the calculations for ordinary matter add up.

The journal Nature has finally discovered physics! For a while now there have been specialized Nature journals for different disciplines. Now, starting in October 2005, they’ve discovered physics: Nature Physics.
Oh and look what at the list of what they will cover:

* quantum physics
* atomic and molecular physics
* statistical physics, thermodynamics and nonlinear dynamics
* condensed-matter physics
* fluid dynamics
* optical physics
* chemical physics
* information theory and computation
* electronics, photonics and device physics
* nanotechnology
* nuclear physics
* plasma physics
* high-energy particle physics
* astrophysics and cosmology
* biophysics
* geophysics

Clearly not alphabetical and quantum physics is number one!

Ken Brown suggests the following solution to the Physical Review Letters “problem”:

I think a possibl[ity] is to make it so no one can submit to PRL. Instead the editors/refs can choose to bump your paper up from PRX. Then the PRX paper would be published in full and the PRL would be a short summary(intro and conclusions) with the details left in PRX.

This is a really intriguing idea. There must be something wrong with it, but I can’t see it right now. One small problem, I think, is that as the system is currently set up, when I submit a paper to Phys. Rev. A, there is usually only one referee for the paper and it seems a bit much to put all of the discission making in two peoples hands (one ref, one editor.)