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!
O brave new quantum world!
I take it you don’t put much stock in the idea that the particles are coming from farther away, but can still reach us because of modified dispersion relations in quantum gravity?
If by stock you mean I’d buy stock and mortgage my future on it. No. But if by stock you mean that I’d like to keep the idea in the arsenal of ideas, then sure it’s an intriguing idea.
Here is case where I think the traditional physics approach of experiment first, is very appropriate and if I wanted to work on cosmic rays, I’d be more inclined to work with the experiments and not the theory.
Thank you for the link to symmetry, I have signed up for a free copy.
This is an interesting blog, I’ll be adding you to my favourites. As an undergraduate physics student i’ll be reading this quite frequently, as an educational supplement. 🙂