# Cosmic rays and the 2.7K background radiation **URL:** \ **Category:** Factual Questions\ **Created:** [October 14, 2001, 5:38am UTC](https://boards.straightdope.com/t/cosmic-rays-and-the-2-7k-background-radiation/87337 "2001-10-14T05:38:36Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [October 14, 2001, 5:38am UTC](https://boards.straightdope.com/t/cosmic-rays-and-the-2-7k-background-radiation/87337/1 "2001-10-14T05:38:36Z")
In [this article](http://mist.npl.washington.edu:80/av/altvw76.html), the following statement is made: > [@](#): > > The enormous energy of these (10[sup]20[/sup] electron volt) cosmic ray events is perplexing because the universe has a built in energy barrier, the 2.7 K black body photons from the Big Bang, which suppresses such ultra-energetic particles. These microwave background photons are the most abundant particles in the universe, and fill all of space. When an energetic proton moves with an energy of 10[sup]20[/sup] electron volts, its collisions with these photons can be violent enough to produce pi mesons. This collision process will slow down ultra-energetic cosmic rays until their energy drops below 10[sup]19[/sup] electron volts. From this, we know that the rare 10[sup]20[/sup] electron volt cosmic rays that are being detected must have been produced somewhere in our galactic neighborhood, no more than 100 million parsecs (330 million light years) away. If I have understood this correctly, a 10[sup]20[/sup] eV proton interacts with a background radiation photon to _invariably_ produce a pi meson. Do 10[sup]19[/sup] eV (or less) protons interact at all with such photons? If yes, doesn’t that screw up the study of cosmic rays (making their energy determination problematic)? If no, why not (why shouldn’t cosmic ray particles with less than 10[sup]20[/sup] eV interact with background photons to produce particles, albeit less massive particles than pi mesons?) ---
**Author:** ![MattTheCroc](https://avatars.discourse-cdn.com/v4/letter/m/439d5e/32.png) [@MattTheCroc](https://boards.straightdope.com/u/MattTheCroc)\ **Post date:** [October 14, 2001, 9:11am UTC](https://boards.straightdope.com/t/cosmic-rays-and-the-2-7k-background-radiation/87337/2 "2001-10-14T09:11:52Z")
IIRC, 1 Electron Volt (eV) is equal to 1.6x10^-19 Joules… that is, 10^20 eV is somewhat less than 10 Joules… not exactly enormous energy… It might heat a litre of water a thousandth of a degree or something, but not a whole lot else… ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [October 14, 2001, 12:25pm UTC](https://boards.straightdope.com/t/cosmic-rays-and-the-2-7k-background-radiation/87337/3 "2001-10-14T12:25:24Z")
Actually, it’s a HUGE amount of energy. From the [same site](http://mist.npl.washington.edu:80/av/altvw76.html), > [@](#): > > (10[sup]20[/sup] eVT) is a truly macroscopic amount of energy, enough to lift a one kilogram mass (2.2 lb) by half a meter (20 in) against the force of gravity! ---
**Author:** ![KarlGauss](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/karlgauss/32/3713_2.png) [@KarlGauss](https://boards.straightdope.com/u/KarlGauss)\ **Post date:** [October 14, 2001, 12:27pm UTC](https://boards.straightdope.com/t/cosmic-rays-and-the-2-7k-background-radiation/87337/4 "2001-10-14T12:27:40Z")
Actually, for a proton it’s a HUGE amount of energy. From the [same site](http://mist.npl.washington.edu:80/av/altvw76.html), > [@](#): > > (10[sup]20[/sup] eVT) is a truly macroscopic amount of energy, enough to lift a one kilogram mass (2.2 lb) by half a meter (20 in) against the force of gravity! ---
**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\ **Post date:** [October 14, 2001, 6:39pm UTC](https://boards.straightdope.com/t/cosmic-rays-and-the-2-7k-background-radiation/87337/5 "2001-10-14T18:39:35Z")
The key aspect here seems intrinsically technical to me, so apologies to anybody hoping for a non-technical answer. It’s called the Griessen cut-off and it’s entirely down to kinematics. While 100 billion GeV is a lot by subatomic standards, a 2.7 K photon isn’t carrying a lot of energy; the mismatch between the two particles in the proton-photon collision is about 24 orders of magnitude. As a result, the centre-of-mass energy in the collision is comparitively small, only 0.17 GeV. That’s however just enough for the collision to produce a pion in addition to the proton. Hence the proton-photon cross section jumps up at this energy, producing the effect. At various higher energies, you’ll be able to produce hadrons other than pions and there will be corresponding jumps in the collision cross section. Since pions are the lightest stable hadrons, this is the first energy the effect can kick-in at. There are lighter stable leptons (notably the electron), but they would have to be produced in an electromagetic process rather than a strong force one. The EM force is much weaker than the strong force and so the cross sections for such processes are much smaller. Indeed protons with less energy do interact with such photons via the Compton effect, which is electromagnetic, but the cross section is significantly smaller and the energy losses less. It doesn’t make much difference even over inter-Galactic distances. Yes, the effect does screw up what we can know about cosmic rays: very energetic ones that are produced far away don’t look energetic by the time they get here. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 14, 2001, 7:52pm UTC](https://boards.straightdope.com/t/cosmic-rays-and-the-2-7k-background-radiation/87337/6 "2001-10-14T19:52:26Z")
It’s the GZK limit (I think that the G might be Griessen, but there’s a couple of other guys, too), and it’s a mostly absolute cutoff. Below the GZK energy (10[sup]18.5[/sup]eV or so), the process will not occur at all. Above that energy, there’s still no guarantee it’ll occur over any particular distance, but give it a long enough distance and it’ll happen eventually. The real problem in cosmic ray physics is not that we don’t see any such high energy cosmic rays, but that we _do_. Not very many, admittedly, but it’s more than anyone would have expected, and we’re not sure why. The simplest explanation is that ultra high-energy cosmic rays (UHECRs) just come from someplace close enough that a few of them can survive the GZK process, but there don’t seem to be any potential sources close enough in the right direction. M87, a particularly active galaxy, might fit the bill, but there don’t seem to be any more UHECRs coming from the vicinity of M87 than from anywhere else.