Can entropy decrease?

[HoloYolo]

Let me lay out the scenario here. Let's say I had a vaporizing gun and a piano. I fired the gun at the the piano and vaporized it. All it's atoms and matter disappear. Where did it go? Did I just successfully decrease entropy? Does nature have a few tricks up its sleeve? Tell me what you think, in a not-rude matter.

[SargeRho]

Entropy can decrease locally, but has to increase somewhere else to compensate for it. In the case of Earth, the Sun is "somewhere else".

[HoloYolo]

Entropy can decrease locally, but has to increase somewhere else to compensate for it. In the case of Earth, the Sun is "somewhere else".

But the Sun isn't gaining more entropy. It's a ball of hydrogen fusing helium.

[Bill Phil]

Locally, sure.

But the only way to decrease the Universe's entropy would be to bring another Universe......

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But the Sun isn't gaining more entropy. It's a ball of hydrogen fusing helium.

When energy is released, some of it is unusable, or, entropy.

[HoloYolo]

Locally, sure.

But the only way to decrease the Universe's entropy would be to bring another Universe......

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When energy is released, some of it is unusable, or, entropy.

So what you're getting at is that entropy is unusable energy?

[Bill Phil]

So what you're getting at is that entropy is unusable energy?

Very basically. It's a bit more complicated....

I don't like calling it "disorder" because that's difficult to explain in and of itself.

[Deutherius]

Vaporization is the change from liquid state to vapor by either evaporation or boiling, so

1) the piano cannot be vaporized

2) even if it could, the molecules that make it up would not disappear, they would just spread around in a gaseous form - the entropy of the piano itself would increase

I believe the word you meant is dematerialization, which, if I'm not mistaken, is a topic of science fiction, and for a good reason - you can't just make energy, and therefore matter, disappear.

[Skyler4856]

Entropy simply describes the number of ways in which something can be arranged, it may only decrease locally with the help of outside energy.

[Brotoro]

Hey! Why the hell did this piano just appear in my back yard?

Ahhh....Nature is pulling a fast one on Kaboom to prevent entropy decrease.

[ZetaX]

Vaporization is the change from liquid state to vapor by either evaporation or boiling, so

1) the piano cannot be vaporized

2) even if it could, the molecules that make it up would not disappear, they would just spread around in a gaseous form - the entropy of the piano itself would increase

Nitpicking: "The term vaporization has also been used to refer to the physical destruction of an object that is exposed to intense heat." (http://en.wikipedia.org/wiki/Vaporization).

Obviously still not vanishing matter into nothingness.

[Deutherius]

Nitpicking: "The term vaporization has also been used to refer to the physical destruction of an object that is exposed to intense heat." (http://en.wikipedia.org/wiki/Vaporization).

Obviously still not vanishing matter into nothingness.

Oh, fair enough. The second point should still stand though, right?

[ZetaX]

Oh, fair enough. The second point should still stand though, right?

Sure; you would maybe (by that linked article) even have chemical splitting into single atoms, but that still won't be dematerialisation, just intense heating.

[RainDreamer]

If hypothetically you truly have something that can absolutely removing something from existence together, then I guess, yes?

[ZetaX]

If hypothetically you truly have something that can absolutely removing something from existence together, then I guess, yes?

That power is significantly greater than just reducing entropy. It breaks about every law of nature we know.

[RainDreamer]

That power is significantly greater than just reducing entropy. It breaks about every law of nature we know.

I don't think we can possibly reduce universal entropy without breaking laws of nature.

Relevant old thread of mine: http://forum.kerbalspaceprogram.com/threads/112086-How-may-entropy-be-reversed

[ZetaX]

I don't think we can possibly reduce universal entropy without breaking laws of nature.

There is a difference between "something breaks every known law of nature" and "something breaks at least one law of nature".

[kerbiloid]

The second law of thermodynamics

https://en.wikipedia.org/wiki/Second_law_of_thermodynamics

tells that entropy can only increase.

Time reversibility principle

http://en.wikipedia.org/wiki/Time_reversibility

tells that any physical law acts in both time directions.

Both are proven.

So:

If entropy increases only in one direction (from "past" to "future"), this contradicts "time reversibility".

If entropy increases in both directions, this contradicts "the second law of thermodynamics" and common sense.

As far as I know, nobody yet has given a clear answer how to live with this.

The solution which seems to be closest to the true: stochastic fluctuations and self-organization of matter could locally drop entropy down, keeping total entropy of the Universe constant forever.

[ZetaX]

The second law of thermodynamics

https://en.wikipedia.org/wiki/Second_law_of_thermodynamics

tells that entropy can only increase.

Time reversibility principle

http://en.wikipedia.org/wiki/Time_reversibility

tells that any physical law acts in both time directions.

Both are proven.

So:

If entropy increases only in one direction (from "past" to "future"), this contradicts "time reversibility".

If entropy increases in both directions, this contradicts "the second law of thermodynamics" and common sense.

As far as I know, nobody yet has given a clear answer how to live with this.

The solution which seems to be closest to the true: stochastic fluctuations and self-organization of matter could locally drop entropy down, keeping total entropy of the Universe constant forever.

To quote from your very own second link: "Modern physics is not quite time-reversible". And "proven" is not a word you should use here.

Apart from that, one way is seeing entropy as the arrow of time.

[K^2]

To quote from your very own second link: "Modern physics is not quite time-reversible".

And it goes on to note that it exhibits CPT symmetry instead. For our purposes, that's the same thing as perfectly time-reversible.

If entropy increases only in one direction (from "past" to "future"), this contradicts "time reversibility".

This is why typical explanations of entropy, these not amed at people who spent many years studying mechanics, chaos, thermodynamics, and field theory, well, frankly, suck.

Loosly speaking, there are two types of entropy we consider. There is the fine grain entropy, which is defined for the universe as a whole, and it is a conserved quantity. It's boring, just like the universe as a whole is boring. It's an object that just is throughout all of the time, and any dynamics within it is fully reversible. What's far more interesting is considering a small corner of the universe, which, nonetheless, interacts with the rest of the universe. If I drop a glass on the floor, I do not need to know anything other than composition of the glass and maybe small region of the floor near impact to predict what happens. And yet, the details of the impact and how the glass shatters, will depend on everything that's happening. This is where irreversibility comes in.

Precisely, this is described with coarse entropy. It's the quantitiy that doesn't just concern itself with total number of states, but rather the rough volume these states occupy. There is a way to state this with mathematical precision, but I don't think it will be useful. The important part is that if the system exhibits no chaos, and all trajectories flow together, then the coarse entropy never changes. The processes are reversible. A glass falling on the way to the floor is a good example. Once the impact happens, chaos. Small, tiny changes in original states of particles in the glass or floor make huge differences in where the cracks head and how the glass flies into shards. The trajectories diverge. As they do, the phase space volume occupied by these states increases. The corase entropy has increased, and the process is irreversible.

Now, if you happen to have full information about all of the universe with precise state of every particle, you can still reverse it. The CPT symmetry still makes everything fundamentally reversible. But if you are existing within the universe, trying to gather the information you need would cause more irreversible processes to happen. Once coarse entropy has increased, you cannot make it decrease again. It is a strictly increasing quantity. But the actual amount of coarse entropy is strictly subjective. It depends on what you call time zero, what you consider as your sub-system, and so on. It's a useful quantitiy, but it's useful precisely for determining what can and cannot be reversed, as well as a few other things related to determining direction of certain processes. (Why things dissolve, for example.) But it's not a fundamental quantity. It's not inherent to the universe as a whole, but rather enters into our discription of small parts of it as means of accounting for the infinite vastness we have no access to.

[ZetaX]

And it goes on to note that it exhibits CPT symmetry instead. For our purposes, that's the same thing as perfectly time-reversible.

I was thinking about things like nuclear decay there, which, if I remember correctly, are asymmetric in regard to time.

In regard to newtonian physics we obviously have time-reversibility.

[lajoswinkler]

This is like asking if god can make an object so heavy that not even he can lift it or whether Santa Claus can move at the speed of light, etc.

The question is useless as it's trying to come up with a scientific explanation of a phenomenon that's purely fictional (no empirical or theoretical data).

[1of6Billion]

Entropy can only increase? So there is now more entropy around us than in that chaotic gas and dust cloud that formed the solar system?

[cicatrix]

Entropy can only increase? So there is now more entropy around us than in that chaotic gas and dust cloud that formed the solar system?

Yes, it is now. And it increases as I type.

I have another question:

I fired the gun at the the piano and vaporized it. All it's atoms and matter disappear.

How can one make something 'disappear'? You can convert all the mass of the piano into energy but I can hardly call it 'disappear'. The energy will not go. Energy can't disappear.

[BlueCosmology]

Entropy can only increase? So there is now more entropy around us than in that chaotic gas and dust cloud that formed the solar system?

Yes. Much more.

[cicatrix]

Yes. Much more.

I sometimes use this concept as an excuse for me doing nothing

I'm not just lazy I simply try very hard not to increase the world's entropy.