Friday, November 09, 2007

Cosmic rays

I was reading the astronomy news this morning when I came across this article with the title proclaiming "Cosmic rays believed to start in black holes." Now, we know that once something is in a black hole, it can't escape. That's the definition of a black hole. So, I read the article to find out what was really intended by the story. As I suspected, this headline was just a little artistic license. But the story is interesting nonetheless.

Cosmic rays are bits of radiation that are always passing through us. Most of the time they do no harm, and there is no way to shield yourself from the radiation, so life has evolved to live with it. Most cosmic rays are protons or electrons, occasionally other stuff like helium or even iron. These atoms are accelerated to near the speed of light. When they hit Earth's upper atmosphere, the collision (sometimes stronger than our biggest particle accelerators) releases lots of energy, including some faint light (too faint for our eyes to see) and other subatomic particles.

Cosmic rays come in a variety of energies, from pretty wimpy to some true monsters. The largest cosmic rays ever measured are strong enough that, if all of their energy could be delivered as motion energy to a 2 pound block of lead, that block would jump a foot into the air. And all of that energy comes from a single atom!

Most cosmic rays are through to come from supernovae, the explosion of a dying star. As the remnants of the star push out into space, it is possible for protons, electrons, and other atoms to get trapped at the boundary between the explosion and the gas of outer space beyond. While trapped in this boundary, it gets bounced back and forth, slowly picking up energy until it is travelling near the speed of light and it escapes.

The physics here is quite complicated, but it seems to work, and we have seen X-ray glow coming from these particles, so most people think this mechanism works. The problem is that this mechanism can't produce the monster cosmic rays. They'd escape the supernova long before they became so ultra-energetic. So, we need something more monstrous than an exploding star to make these.

Some astronomers have suggested that the monster black holes at the center of most galaxies could make massive cosmic rays. As material is falling toward the black hole (but not yet inside!) it is possible to give a tiny bit of the incoming material some ginormous amount of energy.

But, in order to prove this, we need to figure out where cosmic rays are coming from. A world-wide collaboration of scientists has used several telescopes in Argentina to image the faint flashes of light coming from cosmic rays hitting the atmosphere, and also to trace that light back to the direction the particles that caused the flash came from. And all the monster cosmic rays come from nearby galaxies known to host giant black holes.

While this news is not Earth-shattering, nor was it unexpected, it is a big triumph of both theory and observation. A lot of people spent a lot of time to develop a very complex experiment, and it worked. That is highly satisfying. And they may have cleared up one of the longest-standing mysteries in astronomy -- cosmic rays were first discovered in 1912, and we only know are certain where all the cosmic rays come from.

As for coming from inside of black holes, that isn't right -- some editor mis-interpreted the article. If you'd like to read a more technical version (but not too bad) of this story, Science Magazine has a nice summary. But I'm not sure if you'll be able to read it or not -- many (but not all) of their articles require a subscription to the magazine.

No comments:

Post a Comment