# Q & A: Are there closed systems?

Q:
is there a such thing as a closed systems or is entire universe the only closed system? please define the closed system if possible thank you and have a great day.
- John MArtin (age 27)
Corpus Christi, Tx, US
A:
Great questions. We  don't know the answers.

Certainly for many purposes the galaxy is almost a closed system, but not quite since light enters it and leaves it, etc.

What about the whole universe? For starters, we don't know if it's finite. We aren't sure what's happening at all the horizons. So we don't know if there's a way of defining any finite exactly closed system. I guess you could say "everything" is a closed system, but that would be a nearly meaningless claim, since we don't have a clear idea of what "everything" is, and we're sure we can never observe most of it.

Perhaps this is a good thing for our understanding. There's a theorem (Liouville) that in any fully closed system obeying laws of physics like the ones we know, entropy never changes. But perhaps our most general observation is that entropy always goes up. It turns out that this is what's expected from the known laws so long as there's always some "outside" for things to get more "entangled" with. So it's nice not to have to worry about systems without outsides.

Mike W.

(published on 02/26/2013)

## Follow-Up #1: will the universe become a closed system?

Q:
I've read that, because of the expansion of the universe, there will come a time when the light from distant galaxies fades away. There will be so much expanding space between us and them that light from them will no longer catch up to us, and they will appear to redshift and then vanish completely. My question is, does that qualify the visible universe as a "closed system"? Since nothing can get in or out?
- Matt (age 34)
Inverness, FL
A:

That's a very well-thought-out question. Here's a somewhat sloppy answer. Once there's pretty much nothing else within our horizon, it will be approximately true that nothing is coming in. That won't stop a significant outward flow, especially of light. So I don't think it would quite qualify as a closed system.

Mike W.

(published on 09/18/2013)