  It is now widely acknowledged that cosmic rays experiments can test possible
new physics directly generated at the Planck scale or at some other fundamental
scale. By studying particle properties at energies far beyond the reach of any
man-made accelerator, they can yield unique checks of basic principles. A
well-known example is provided by possible tests of special relativity at the
highest cosmic-ray energies. But other essential ingredients of standard
theories can in principle be tested: quantum mechanics, uncertainty principle,
energy and momentum conservation, effective space-time dimensions, hamiltonian
and lagrangian formalisms, postulates of cosmology, vacuum dynamics and
particle propagation, quark and gluon confinement, elementariness of
particles... Standard particle physics or string-like patterns may have a
composite origin able to manifest itself through specific cosmic-ray
signatures. Ultra-high energy cosmic rays, but also cosmic rays at lower
energies, are probes of both "conventional" and new Physics. Status, prospects,
new ideas, and open questions in the field are discussed. The Post Scriptum
shows that several basic features of modern cosmology naturally appear in a
SU(2) spinorial description of space-time without any need for matter,
relativity or standard gravitation. New possible effects related to the
spinorial space-time structure can also be foreseen. Similarly, the existence
of spin-1/2 particles can be naturally related to physics beyond Planck scale
and to a possible pre-Big Bang era.
