22
$\begingroup$

I was reading a book review by Freeman Dyson at http://www.nybooks.com/articles/2018/05/10/the-key-to-everything/ and he makes the following statement at the end of the 13th paragraph (3rd dropcap) and beginning of the 14th:

Any object whose motions are dominated by gravity will have energy decreasing as temperature increases and energy increasing as temperature decreases.

As a consequence of the second law of thermodynamics, when energy flows from one such object to another, the hot object will grow hotter and the cold object will grow colder. That is why the sun grew hotter and the planets grew cooler as the solar system evolved.

Could someone help me understand that statement? Doesn't the second statement imply a violation of the first law of thermodynamics? What are the "objects" here? Clouds of gas? Planets? Balls falling from towers?

$\endgroup$
16
$\begingroup$

There's no contradiction. Heat still leaves the hot star, but that raises its temperature because it has a negative heat capacity. See here.

| cite | improve this answer | |
$\endgroup$
  • $\begingroup$ Thanks, I guess I just needed a search term. Makes perfect sense. $\endgroup$ – Andrew Apr 22 '18 at 23:42
18
$\begingroup$

Smolin refers to gravitationally-bound dynamical systems as "antithermodynamic" for this reason: when you add kinetic energy to, for example, an object in orbit around a larger object, it moves out to a larger orbit and slows down. If you subtract kinetic energy from it instead, its orbit shrinks and it speeds up.

| cite | improve this answer | |
$\endgroup$

Not the answer you're looking for? Browse other questions tagged or ask your own question.