THE STORY OF COSMIC RAYS — SWANN 263 



Suppose now that we imagine the molecules to be reduced in number 

 so that the chances of molecules colliding with one another are very 

 much less than their chances of colliding with the spheres. The mole- 

 cules may become "ambitious" and seek, through their collisions, to 

 accommodate themselves to the kinetic energy of the spheres, acquir- 

 ing velocities much greater than those of the spheres. This ambition, 

 though at first sight fantastic, is not indeed illogical under the spe- 

 cialized conditions we have assumed — dynamical laws require it. 



In the Fermi mechanism our steel spheres are replaced by bounded 

 magnetic fields, associated with moving masses of gas in the galaxies. 

 A magnetic field of this kind is representative of a hard elastic body, 

 because an electric particle entering it is turned back to the region 

 from which it came, without any loss of energy as measured in the 

 frame of reference in which the magnetic field is at rest. 



However, there are certain difficulties in the Fermi mechanism. 

 A fast-moving charged particle loses energy by ionizing the other 

 atoms in its path; and while the density of matter in space is very 

 small, the loss of energy by this process more than offsets the Fermi 

 gain for low energies, where the ionization probability is greater. 

 Only when a particle has attained the lower range of cosmic-ray 

 energies is the Fermi mechanism capable of taking hold to increase 

 further the energy of the particle. In this matter, heavy particles 

 are at a disadvantage with respect to light particles. Thus, the Fermi 

 mechanism requires a kind of injector process to get it started, such 

 as the suggestion of Spitzer already cited. 



Then, since the average energy gained by the mechanism is only 

 about 10 electron volts per collision, and since each particle would 

 have only about one collision per year, about 60 million years would 

 be necessary for a particle to acquire cosmic-ray energy. During 

 this period, it would have opportunities for collision with the nucleus 

 of some particle in the surrounding space, and such a collision, as 

 we know from the evidence presented by protons entering our atmos- 

 phere, would result in destruction of the particle and its conversion 

 into mesotrons, which disappear because of their finite life. 



As our quantitative knowledge becomes more complete, the strength 

 of the evidence against the Fermi mechanism increases also, one of 

 the most potent difficulties arising from the time necessary for the 

 particle to acquire cosmic-ray energies and the chance of its destruc- 

 tion during that time. 



Electromagnetic methods. — The science of electricity and magnet- 

 ism suggests many processes by which cosmic-ray energies can be 

 realized. Phenomena and quantities that are of negligible importance 

 in experiments on a laboratory scale can grow to very fimdamental 



412575—57 18 



