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- Thread starter greendog77
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The second law is considered by most physicists to be a statistical. It is not impossible for entropy to decrease, but it is overwhelmingly likely, for large systems.

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Andrew Mason

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When you say "possible" do you mean something that will actually happen somewhere in the observable universe for long enough to observe? If that is the definition of possible, then violation of the second law is not possible.

Think of the likelihood of as about the probability that the Democrats will win all the seats in the House and Senate next election and continue to do so for the next 1000 years and then divide that by the number of atoms in the earth. That is about the probability that one gram of air will violate the second law for about 1 ms. once since the Big Bang.

AM

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jtbell

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Next, suppose that there are 2 gas molecules in the container. The probability is (1/2)

Next, suppose that there are 3 gas molecules in the container. The probability is (1/2)

See the pattern here?

Next, suppose that there is a mole of gas (6.02 x 10

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Well I understand it's extremely small, but after a long amount of time it would happen. That being said, it just seems to me that the 2nd Law is not 100% rigorous, that it's statistical like stevendaryl said?

Next, suppose that there are 2 gas molecules in the container. The probability is (1/2)^{2}= 0.25 that both molecules are in the left half of the container at any particular time.

Next, suppose that there are 3 gas molecules in the container. The probability is (1/2)^{3}= 0.125 that all three molecules are in the left half of the container at any particular time.

See the pattern here?

Next, suppose that there is a mole of gas (6.02 x 10^{23}molecules) in the container. The probability is ##(1/2)^{(6.02 \times 10^{23})}## that all molecules are in the left half of the container at any particular time. Exercise: evaluate that probability.

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I think the probability would be much less then that, because once you have enough atoms to make collisions likely, such collisions would always be more likely to send an atom to whatever side had fewer atoms at that moment.Next, suppose that there is a mole of gas (6.02 x 10^{23}molecules) in the container. The probability is ##(1/2)^{(6.02 \times 10^{23})}## that all molecules are in the left half of the container at any particular time. Exercise: evaluate that probability.

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Khashishi

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Jano L.

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In classical thermodynamics, 2nd law is a basic assumption derived from experience with macro-systems changing their equilibrium macrostates. It is thought to be a physical law, unless a contradicting evidence is found (which was not to this day).Well I understand it's extremely small, but after a long amount of time it would happen. That being said, it just seems to me that the 2nd Law is not 100% rigorous, that it's statistical like stevendaryl said?

In statistical physics, the statement of the 2nd law is no longer an assumption, but a statement that is true with probability very close to 1 for system of large number of particles describable by macrostate, provided certain probabilistic assumptions are assumed. If the number of particles is too small, the statement of the 2nd law loses meaning, as there is no macrostate for such system.

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anorlunda

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You might ask, "A statistical fluctuation in what?" But physics can give no answer. Particles, energy, space, and time may or may not exist outside the universe. We don't know. We can only speculate. In PF we don't do metaphysics.

So wasn't this creation event a massive violation of the second law? Well, we only say that the second law applies post-creation so it dodges the question.

So how improbable is the event you postulate? I say this tongue in cheek -- at least once in this universe's eternity.

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