Expert: James Gort - 4/3/2010

Question
James,

Thank you VERY much for your answer, it was VERY helpful, and I have just order the Kip Thorne book from Amazon! The Technology Review blog you linked to is also immensely interesting.

I have a related question. I am actually still grappling with understanding entropy in full, so I thought you might be able to answer this question:

What if the given closed system is a very hot one, and you introduce a cold object which is then heated up until it reaches an equilibrium with the environment? Would we still describe that as an "increase" in entropy?

You see, I am trying to imagine what entropy would be like in a hypothetical universe that was heating up instead of cooling off. A universe that was, so to speak, "running uphill" because additional energy was being introduced from some outside environment (which, I guess, wouldn't technically make it a closed system, but we might have to think of it as closed, because we simply don't know any details about the exo-cosmic state that would be supplying the additional energy).

I'll be very grateful indeed if you have helpful comments. :-)

Thanks a lot,
- Tue

Answer
Hi Tue,

I just want to understand your question properly.  If the cold object enters the hot universe, and absorbs heat from the universe (heating it up), but cooling the rest of the universe (until it reaches equilibrium), then, yes, entropy would increase.  

But if your cold object receives additional energy from "outside" to heat it up, then entropy would decrease in the "local universe" (which excludes the source of the additional energy).  

Conversion of energy from one form to another is not usually reversible.  That irreversibility is the reason for physical processes increasing entropy.  When you bring in an "outside energy source", that affects the reversibility of the process.

In your hypothetical universe (which was heating up - decreasing entropy), processes would be more than reversible.  If you put in 100 Joules of energy, you'd get 110 Joules back.  Which is fine - because your extra energy comes from the outside source.  

Taking it a step further, in your "ultra-reversible" universe, a cloud of ash might come together and "unburn" to produce a house (which had burned down in a normal universe).  Or a junk yard full of cars would become "unrusty" and the cars would appear brand new and drivable - if you just wait awhile.

Cool Universe!

Prof. James Gort