# Reversing Entropy

1. Aug 12, 2008

### avant-garde

Do you think there is a way to reverse entropy? If humans obtain the power to reverse entropy, then they will gain the power to do anything in this universe.

This reminds me of The Last Question by Isaac Asimov.

2. Aug 13, 2008

### stewartcs

No.

CS

3. Aug 13, 2008

### Staff: Mentor

To make it a little more vivid, what you are actually asking the following question: "Can I throw a bunch of broken shards on the floor and have them spontaneously bounce back up as completely intact dinner plate?"

4. Aug 13, 2008

### ZapperZ

Staff Emeritus
Get some water, and cool it. There! You've "reversed" the entropy of that water.

Zz.

5. Aug 13, 2008

### Topher925

It all depends on how you define your system. If you create an open system, sure you can. However if you choose an isolated system, then no.

6. Aug 13, 2008

### stewartcs

The entropy in this case would be negative, not reversed. The entropy generation is always positive per the Second Law of Thermodynamics.

Of course I may have misunderstood what the OP asking. I'm taking "reversed" to mean that the entropy generation is negative, which cannot happen.

CS

7. Aug 13, 2008

### stewartcs

Entropy generation always increases.

For example, here is an open system (control volume) with entropy generation increasing:

Take a tank with a valve on it that is connected to a high pressure air line. Open the valve, the air goes into the tank, the temperature of the air in the tank increases, and the entropy increases.

CS

8. Aug 13, 2008

### Topher925

Ok, lets define a system of H2O and sand mixed together to create a homogeneous mixture in an closed system (I know I said open above). By mixing creating the mixture we create entropy. Now if we apply work to the system in the form of a large centrifugal force we separate the mixture removing entropy from the system. True, you are not actually reversing entropy as there is no such thing as an entirely reversible process. But it would be negative and removed from your system. I guess the real question is can you destroy entropy, which like you said the answer is no.

9. Aug 13, 2008

### stewartcs

There is no entropy transfer associated with energy transfer as work. In other words, no entropy is exchanged during a work interaction between a system and its surroundings. There are only two mechanisms of entropy transfer, heat and mass flow. Entropy generation can occur due to a degradation of the quality of energy - which obviously will increase the entropy. For example, take a closed system composed of an adiabatic rigid container with a paddle wheel in it. The only entropy is that which generated due to the paddle wheel disorganizing the gas molecules and friction. Even though work isn’t causing the transfer of entropy across the system boundary, it is responsible for the increase in entropy (entropy generation in this case) due to the dissipation of that energy into a less useful form of energy. The energy being dissipated is the energy used in performing the work on the shaft.

By applying work to separate the mixture you will increase the entropy generation within the system boundary. Hence, the entropy increases.

CS

10. Aug 13, 2008

### Topher925

Assuming this closed system is perfectly insulated like you said, how is this so? No heat is being transfered and the system is being "organized", therefor removing the mixture entropy.

As an example of what I attempted to convey in my last post, heres an example using ideal gases. Although originally attempted to explain quantum behavior I think it still applies.

http://web.mit.edu/lhenders/www/papers/final.htm

Last edited by a moderator: Apr 23, 2017
11. Aug 13, 2008

### stewartcs

Like I said in the previous post (#9), the entropy generated within the system boundary will increase for the following reason: The quality of the energy used to provide work to the shaft to mix or separate the system is degraded due to the irreversibilities associated with the system (friction for example). Since the quality of the energy is reduced due to the irreversibilities of system, the entropy generated is increased.

The quote you provided comes with the statement that the separation is reversible and that they are ideal gases. In reality, this will not be the case since neither one really exist. However, I’m not all that familiar with quantum mechanics so perhaps there is some way to have a process in that realm that is so close to reversible that they can assert that.

CS

EDIT: I just read the article and think that they may be saying that the net entropy change is negative, which of course is possible as pointed out in post #6. But that doesn't mean entropy is reversible.

Last edited by a moderator: Apr 23, 2017
12. Aug 13, 2008

### ZapperZ

Staff Emeritus
Until the OP comes back and clarify him/herself, I'd suggest we ease of on making any further assumption on what the initial question was on. Or else whatever we're discussing might be completely moot.

Zz.

13. Aug 13, 2008

### map19

Take a large amount of interstellar gas with very high entropy and throw it into a black hole. The entropy of the universe is decreased.

14. Aug 13, 2008

### stewartcs

Exactly how does one throw entropy into a black hole?

CS