[Alan McDougall (OP)]

Hi,


2 Ultra-energetic cosmic rays

FOR more than a decade, physicists in Japan have been seeing cosmic rays that should not exist. Cosmic rays are particles - mostly protons but sometimes heavy atomic nuclei - that travel through the universe at close to the speed of light. Some cosmic rays detected on Earth are produced in violent events such as supernovae, but we still don't know the origins of the highest-energy particles, which are the most energetic particles ever seen in nature. But that's not the real mystery.

As cosmic-ray particles travel through space, they lose energy in collisions with the low-energy photons that pervade the universe, such as those of the cosmic microwave background radiation. Einstein's special theory of relativity dictates that any cosmic rays reaching Earth from a source outside our galaxy will have suffered so many energy-shedding collisions that their maximum possible energy is 5 × 1019 electronvolts. This is known as the Greisen-Zatsepin-Kuzmin limit.

[LeeE]

I don't see why all low-energy photon collisions result in lowering the the energy of the particles.  If the photons can come from any direction, surely just as many will add energy as drain it?  If such a cosmic-ray particle is formed in a super-nova event I would expect it to be boosted by the EM radiation from the SN, after it's creation and initial ejection, until the radiation from the SN fell below background levels.

[Soul Surfer]

The energy would not be a problem if the particles were much more massive for example the neucleus of an iron  or other heavy atom.

[lightarrow]

I don't see why all low-energy photon collisions result in lowering the the energy of the particles.  If the photons can come from any direction, surely just as many will add energy as drain it?  If such a cosmic-ray particle is formed in a super-nova event I would expect it to be boosted by the EM radiation from the SN, after it's creation and initial ejection, until the radiation from the SN fell below background levels.

Right, but in the particle's frame of reference, photons coming from the SN direction are extremely red-shifted, while those coming from the opposite direction are extremely blue-shifted, so those photons against the particle's movement are extremely more energetic = have extr. more momentum than the others.

[lightarrow]

Einstein's special theory of relativity dictates that any cosmic rays reaching Earth from a source outside our galaxy will have suffered so many energy-shedding collisions that their maximum possible energy is 5 × 1019 electronvolts. This is known as the Greisen-Zatsepin-Kuzmin limit.

Ah, thank you. Now I've understood why particles with energy = 3*10²⁰ eV are called "Oh my God particles"  []
http://www.fourmilab.ch/documents/OhMyGodParticle/
http://en.wikipedia.org/wiki/Ultra-high-energy_cosmic_ray
http://www.thenakedscientists.com/HTML/content/news/news/1179/

[LeeE]

Yeah - umm...

When a photon is absorbed and re-emitted by a single particle can we be sure that it will be re-emitted back in the direction it came from?  When we have a reflective macro-structure we can say yes, but can we say so for a single particle?

[lightarrow]

Yeah - umm...

When a photon is absorbed and re-emitted by a single particle can we be sure that it will be re-emitted back in the direction it came from?  When we have a reflective macro-structure we can say yes, but can we say so for a single particle?

At those energies the main mechanism is Compton scattering. It's a real collision, as if the photon were a real "hard" particle; it's not an absorption and re-emission.

[LeeE]

Hmm... not something I'm familiar with but doesn't Compton Scattering just refer to photon-electron collisions?  Does it also apply to protons or nuclii?

[lyner]

Google Neutron Compton Scattering

[Soul Surfer]

The problem is that if the particles were so light at that energy they would be going so close to the velocity of light that even interacting with the microwave background radiation photons would represent a significant drain of energy and there are a hell of a lot of them around.  As someone pointed out  if we could see it the energy level of the microwave background would be a bit like bright moonlight.  it.s not a tiny signal but a real humdinger

[Alan McDougall (OP)]

These particles are enormously more energised than they should be when the reach earth thus our diemma whith them

[Soul Surfer]

These particles come very rarely and are only detected and measured by the massive air showers they generate.  Most cosmic ray particles are probably protons but there is no reason why they couldn't be  a heavy nucleus shattered by a major collosion with the nucleus of an air molecule.  one of the possible sources of such particles is the collapse of a supernova in our galaxy.