Here's a question my thermodynamics teacher was unable to answer. I present it to you and see where the flaw of my argument is. Is it possible to derive the 3rd law of thermodynamics from the 2nd law? Carnot's theorem (no other machine than Carnot's has maximum efficiency) tells us that when the process is reversible (Carnot engine) Q2/Q1=T2/T1 and therefore it is possible to define an absolute temperature. Now if there were a thermal reservoir at T2=0, it would be possible to have a more efficient engine than Carnot's. T2=0 -> Q2=0 and efficiency=1 This argument holds the other way around, it is impossible to have Q2=0 (2nd law) therefore T2=0 cannot be zero. Our teacher argues that Einstein tackled this problem and demostrated that it has no physical sense to talk about ADIABATIC and ISOTHERMAL processes at the same time (I mean simultaneously). This, I do not understand. What do you think? Why do we need a 3rd law? Thanks for your help.