ABSOLUTE ZERO — SIMON 263 



temperature, we can surpass in the laboratory the conditions found 

 in nature, whereas, in the opposite direction it is extremely unlikely 

 that we will ever reach the high temperatures and big densities to be 

 found in the stars. 



There is not much point in generating low temperatures simply 

 for the fun of playing among the low figures. How can one make 

 investigations on substances other than the paramagnetic salts? 

 The most simple way is to press the paramagnetic salt and the 

 substance to be investigated together, so that they form a solid pill. 

 In cooling down the salt, the substance cools with it. In this way 

 we examined a lot of metals to see whether they became supracon- 

 ducting or not, and three new supraconductors were found in the 

 new region. Some other metals, however, did not become supra- 

 conducting down to 0.05°. 



At the same time these measurements showed us still another thing. 

 Cooling an additional substance with the paramagnetic salt and see- 

 ing how far the temperature is lowered in demagnetizing, compared 

 with the temperature reached in demagnetizing the pure salt, one 

 can measure the specific heat of the additional substance, and see 

 whether anything happens within the new temperature region. If 

 there is any change of energy within the substance that is equal to 

 the thermal agitation in the new region, then it should be seen in 

 these specific heats. And there is a very definite thing that must 

 be expected to happen in this region. As is known from the analysis 

 of the atomic spectra, there is an interaction of the magnetic 

 moments of the nucleus with that of the electrons, chiefly the valency 

 electron. As a result of this, the ground state of the atom is split 

 up, and the energy difference between the different levels is of such 

 an order of magnitude that the new temperature region is charac- 

 teristic of it. We have already got some results in this direction, 

 but it would be proceeding too far now to go into details. 



You saw that within this new temperature region there really 

 are things that still happen, and it is not only a game with low 

 temperature figures. One can also say that it will be both necessary 

 and interesting to investigate temperatures lower than those yet 

 reached. 



What imposed the limit to the temperatures reached above? It 

 should be possible to get clown to any temperature if the dipoles 

 in the salt were perfectly free, because then, even as near to absolute 

 zero as one likes, there would be a random distribution of the mag- 

 netic dipoles, and we would always still have a means of lowering 

 the temperature. But here the same thing happens as in the case 

 of gases. If we had still a gas at these very low temperatures, we 

 could certainly use it for lowering the temperature, but no more 



