Peter Kirby

I read a science book a long time ago, and it said that one problem with the idea of a universe that has expanded and contracted for all eternity is that this would be a perpetual motion machine, which is ruled out of court. Is there anyone who has attempted to respond to the "perpetual motion machine" objection to an eternally pre-existent universe?

best,
Peter Kirby

Jesus Tap-Dancin' Christ

Simple. Just because something exists for eternity doesn't mean it has to do anything interesting, much less anything at all. Further, during the collpase, it would break into a singularity, where the laws of physics are indescribable in their entirety. As such, a perpetual motion machine may be possible--but only within the context of singularities.

bleubird

Quantum laws are not the same as newtonian laws.
bleubird

GunnerJ

Off to Science and Skepticism.

theyeti

That's kind of an odd way of putting it. But I do believe that there is some merit in this objection.

The reason why perpetual motion machines are impossible is the 2nd law of thermodynamics. Any machine which does work will lose some heat to its surroundings. Heat has greater entropy than the energy used to do work, and can therefore never be recaptured and used to do work itself. So the large-scale outcome of this is that the entropy of the universe is always increasing. If we were to assume that this has been going on for an enternity, then we would expect the universe to be of uniform temperature, which of course it's not. Now as far as an expanding/contracting universe is concerned, there are two reasons I can think of off-hand why entropy wouldn't matter. The first is that time's arrow may reverse itself when expansion is complete and we head towards a "big crunch". The 2nd law of thermodynamics will work in reverse and everything will go back in time until we reach the singularity from which the big bang originated. While I've heard of this as a speculative hypothesis, I don't know if there is any reason to think it might be true. The second reason is that no one knows what will happen when the big crunch occurs and everything is shoved back into a singularity. Given that the laws of physics do not behave the same on such a tiny and massive scale, it may be that entropy will be "reset" or something to that effect. It may matter greatly whether or not singularities result in a decrease of information in the universe -- something which is currently not sovled. If it turns out that they don't decrease information, then I suspect that there's no reason why the universe can't expand and contract forever.

However, all of this might be moot because I believe that current models suggest an ever expanding universe, so that the big crunch will never happen. If the goal is to fluster theists with a model of the universe that's unfriendly to their metaphysics, you might be better off going with mulitverse models that are looking more and more likely.

And I'm no expert when it comes to this stuff. You'll get a better answer from Tim Thompson or Jesse.

theyeti

Demosthenes

and the physicists aren't even sure if the laws of thermodynamics apply to the universe in its entirety. It's still far from a settled question of whether the laws can be used to describe situations at a greater than supercluster scales.