• Question: Can entropy be reversed?

    Asked by shreyasharma to Dave, David, Jack on 27 Jun 2013.
    • Photo: David Freeborn

      David Freeborn answered on 27 Jun 2013:


      Entropy is just a statistical effect (but a statistical effect of enormous significance for literally every process in the Universe).
      The second law of thermodynamics is often phrased to say that the entropy always increases, but what is really says is this:
      high entropy states are more probable than low entropy states, so any change to a system will in general lead to higher entropy.

      Its perfectly possible (but statistically unlikely) for systems to tend towards lower entropy. For example, if we have a container with two compartments, it is possible that all the air molecules will fly into one compartment. This would be a lower entropy state than the air even distributed. But it would be very, very unlikely. A lot of devices, not least the airlocks on spaceships, and a lot of chemistry experiments, rely on this not happening.
      If there are a lot of air molecules, the probability would become increasingly small.

      So entropy can reverse itself. It’s not possible for humans to do anything in particular to increase entropy though. For example, we could try to pump all the molecules into one compartment, but that would involve doing mechanical work, and mechanical work increases entropy by creating heat.

      You might have read about Maxwell’s Demon, a thought experiment in which a Demon is able to decrease entropy by sorting fast and slow molecules (http://upload.wikimedia.org/wikipedia/commons/8/8b/Maxwell%27s_demon.svg). But Maxwell’s Demon can only do this by gaining information about the system: and in so doing, Maxwell’s Demon increases information entropy.

      Some machines like fridges and biological cells work by decreasing entropy in a local environment. But in so doing, they increase the entropy around them. This is what makes life possibly: biological cells create highly ordered states in which low entropy activities can happen. But they excrete heat and waste products to such an extent that they increase the entropy around them.

      So, fluctuations against entropy are possible, but improbable and we can do nothing to increase their likelihood.

      That doesn’t mean they’re not important though. The whole Universe was in some sense caused by a fluctuation against entropy, and it is this fluctuation that may give the “direction of time” to our Universe.

      The early Universe, at the point of the Big Bang and just after was (for some reason, and we don’t yet know why) in an extremely low entropy state. Every process in the Universe since has been driven by a tendency towards higher entropy ever since, and the two possible final states of the Universe, either a Big Crunch or a Big Freeze both represent equivalent “maximal entropy” states.

      The second law of thermodynamics is derived from time-reversible physical laws. That means the following: if we are ever in a low-entropy state, we should expect entropy to increase, both forwards *and* backwards in time. The point is this: low entropy states are always very improbable, whenever they occur in time. So the fact that time has a direction: the fact that entropy seems to increase forwards in time but not backwards, is probably a result of that very mysterious, low entropy early state of the Universe. In a sense, the entire Universe was created by a statistically very improbable fluctuation to a low entropy state!