ABSOLUTE ZERO — SIMON 253 



not SO, because, unlike our analogy where the order depends only 

 on one variable, in a real physical system it depends on many more. 

 The most important quantity it still depends upon is the volume. 

 Taking the probability of finding an atom within a certain region as 

 a measure for the state of order respective to the positions in space, 

 it is quite evident that this probability decreases on enlarging the 

 space the atom has at its disposal. So in a diluted gas we have a 

 great disorder; if it is compressed to a small space, its order is 

 increased. 



Now thermodynamics has given us a quantitative measure for this 

 state of order of which we have to speak so much now, since it is 

 necessary for understanding things later on. This measure is called 

 the entropy. I will not trouble you with this quantity, as I know it 

 is not a very popular one. I only want to remind you that it is a 

 measure for the state of order, and that there is a law, namely, the 

 second law of thermodynamics, which tells us that within a closed 

 system during any change the entropy can only increase, or at the 

 best by making a reversible change (that means avoiding unnecessary 

 disorder) it can remain the same. Speaking in our terms now, this 

 second law means that in a closed system the state of order can only 

 decrease or at the best remain constant. 



Let us see now what this has to do' with generating low tempera- 

 tures. We will take a cylinder with a piston, containing a gas, the 

 whole system being perfectly isolated from its surroundings. The 

 state of order in this system consists of two parts, one depending on 

 the temperature and the other on the volume. Compressing the gas, 

 we increase the state of order corresponding to the volume. As the 

 whole state of order must remain the same, the disorder due to the 

 thermal motion has to increase; that means the temperature rises, 

 and you all know that in compressing a gas it heats up, this heat 

 being called the compression heat. 



Bringing this system into thermal contact with its surroundings, 

 it will cool down to the initial temperature, and so its disorder 

 becomes smaller. But, of course, that does not contradict the law I 

 spoke of before, because heat is transmitted to the surroundings, so 

 increasing the disorder of the particles there. 



Now we will isolate the system again and pull the piston out. 

 The part of the disorder due to the volume increases again. The 

 whole state of order must remain constant, so the part of the dis- 

 order due to the temperature must fall, and that means the tempera- 

 ture itself falls. 



That is a characteristic example, and one of the most important 

 cases of how to generate a low temperature. Generally speaking, 

 whenever one wants to lower the temperature, one must have a 



