If our three molecules did jump back into the bottle, the entropy change would be reversed but time would not change. The announcement relies on the assumption that our sense of time is due to the law of entropy, which is commonly used to explain the 'arrow of time. However, if there are millions of molecules (or more) in the bottle it would be statistically so unlikely that this reversal would take place that we would never ever see. Even less likely is one that will reverse time. It is now not impossible that we would observe the three molecules jumping back into the bottle, if we were patient. However, imagine that there are only three molecules in the bottle. It has never been observed that the gas goes back into the bottle and we might be tempted to say that this will never happen and that it cannot happen. The bottle is opened to the laboratory and the gas spreads out. That answers your question.įollowing the work of Boltzmann it was realised that the second Law of thermodynamics is really only statistical in nature. If it were to go back then changes in thermodynamic state functions would reverse, including entropy. This means that once a spontaneous process has come to its end, it never goes back to the initial state. Spontaneous processes are regarded as irreversible in classical physics. Suppose we measure the entropy change of a spontaneous process that takes place within an isolated system. genuine logical contradictions cannot arise in physics, otherwise they could not be experimentally observed. Loschmidt's own name for it is "reversal objection" (umkehreinwand), not "paradox". In summary, we have a second law of thermodynamics simply by dint of the exquisitely low entropy state of the early universe. Sir Roger Penrose calls this notion the "Thermodynamic Legacy" of the big bang and you could read the chapter entitled "The Big Bang and its Thermodynamic Legacy" in his "Road to Reality". Likewise, in the everyday world, things "happen" when a systems are not in its maximum entropy state: they spontaneously wander towards these maximum entropy states, thus changing their states and undergoing observable changes. found itself at the time of the big bang) in an exquisitely low entropy state, so that almost any random walk in the universe's state space tends to increase entropy. The fact that this understanding is manifestly against experimental observation can be explained if we observe that the universe began ( i.e. You're hitting here on an idea known as Loschmidt's Paradox: given that microscopic laws are time reversible, entropy should have the same tendency to increase whether we run a system forwards or backwards in time, exactly as you understand. Simple answer: in our universe, definitely no.
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