# Q & A: entropic doom?

Q:
can someone please explain the "entropic doom theory".
- Gary (age 13)
Old Buckenham Hall, Suffolk, England
A:
I guess this is a reference to the Second Law of Thermodynamics, according to which the entropy of the universe keeps increasing. Things tend toward thermal equilibrium, in which temperatures are uniform and energy dispersed among many tiny modes of motion (like those associated with heat) rather than concentrated in a few large-scale modes (like all the mechanical motions that you can see). At some point, it looks like the universe will be too run down to support any life.

That's not really worth worrying about since life on Earth will undoubtedly be wiped out long before that, as the Sun will expand into a smallish red giant some billions of years from now.

Mike W.

(published on 10/22/2007)

## Follow-Up #1: smallish red giant

Q:
Just as a matter of course "smallish red giant" is a misnomer. A red giant is ~100 times the diameter of our sun. It THEN evolves into a small white dwarf.
- Michael (age 19)
South Africa
A:
You’re right that red giants are big compared to the sun.  By ’smallish red giant’ I’m sure we meant compared to typical red giants. We were grading size on the curve.

Mike W.

(published on 10/22/2007)

## Follow-Up #2: entropy and cosmic expansion

Q:
This question ties together two threads, so I didn't know where it fit best. We are studying entropy and the tendency of the universe toward thermal equilibrium (sometimes referred to as the "heat death" of the universe because at equilibrium, there will be no temperature differences, and therefore no heat engines will be possible). When that equilibrium is reached, will the universe stop expanding?
- Lambert Li (age 16)
Normal, IL USA
A:

Nice question. At least within our current understanding, the expansion will keep on going. The equilibrium will never quite be reached, but we don't have any reason to think that reaching it would stop the expansion.

This does raise all sorts of other interesting questions. How can you combine a picture where the entropy always increases with some sort of cosmic picture without a weird  something-from-nothing start? Sean Carroll discusses this issue some in From Eternity to Here. I'll  try to give his picture, to the extent I understand it. A cold, flat, nearly empty, almost-in-equilibrium universe should still have quantum fluctuations giving birth to baby universes. That increases overall entropy, counting the old and new universes. From the point of view of someone growing up in one of those baby universes, however, its own entropy started out very low and just increased ever since.

Mike W.

posted without checking by Lee due to timely question for class

(published on 10/21/2013)