# Second law of thermodynamics

1. Jun 7, 2006

### francisco

can the second law of thermodynamics be defied? in other words, can mother nature be fooled somehow? have we (homo sapiens) accomplished such a thing with religion? for example, the seventh day of the week is nothing but a positive entropy change from sunset to sunset. can we somehow use that positive entropy change to defy the positive entropy change of the first six days of the week?

"if you observe a system in which the entropy appears to decrease, you can be sure that somewhere there is a change in the entropy of the environment large enough to make the total entropy change positive." (physics by resnick, halliday, and krane, 4th ed. p 585)

can we somehow change this statement? for example,

if you observe the first six days of the week in which the entropy appears to decrease, you can be sure that there is a change in the entropy of the seventh day large enough to make the total entropy change positive.

i am searching for something, but it is like searching for a needle in a hay stack.

i need some feedback, criticism, and exchange of ideas regarding this new thread. thank you.

2. Jun 7, 2006

### Gokul43201

Staff Emeritus
The thing with science is that it can not address human misperceptions of nature, so it really doesn't have anything to say about entropy "appearing to decrease", other than "I'm sorry Francisco, but your measurement is probably wrong".

It's actually like searching for an elephant in a haystack. A very large elephant...in a very small haystack.

If the elephant were there, someone would have been bound to see it.

3. Jun 7, 2006

### rcgldr

Maybe someone can explain this. A lot of human activity, such as construction, seperation of chemicals, and reproduction ... seem to be decreasing entropy. Where's the "waste" product of these activities to compensate and result in an overall increase of entropy?

4. Jun 7, 2006

### vanesch

Staff Emeritus
In short, no you can't. If you observe the first six days of the week a decrease in entropy (AND YOU ARE SURE THAT YOU HAVEN'T FORGOTTEN A CONTRIBUTION SOMEWHERE), then you've falsified the second law of thermodynamics.

If you observe apples fall up from an apple tree, and you're sure it is gravity which does so (and not some joker with some nylon wire, say), then you've falsified Newton's theory of gravity.

The point is that nobody has ever witnessed a decrease of entropy during six days. That's why we still think that the second law of thermodynamics is valid. We have had (and have more and more ) people who THOUGHT that they witnessed a decrease in entropy during six days, but that's simply because they forgot to take all contributions into account.
A common example is "human constructive activity", or "life" or...

The second law doesn't say that entropy has to increase EVERYWHERE. You can have a decrease here, on the condition that you have SIMULTANEOUSLY an increase elsewhere. The most common increase is the photon flux: low-entropy photons (high-energy, such as visible light) from the sun IN, high-entropy photons (low energy, thermal infrared radiation) OUT. It's our main "absorber of entropy".
Anything that contributed in this transformation can have a local decrease in entropy (such as a growing plant) because it is set of by the increase in entropy in the photon flux.

5. Jun 7, 2006

### Gokul43201

Staff Emeritus
Vanesch, he does not conjecture that you observe an absolute decrease but only an apparent[/i] decrease.

6. Jun 7, 2006

### vanesch

Staff Emeritus
Ah. Well, then there is an apparent violation of the second law

7. Jun 7, 2006

### LURCH

My biology textbook in highschool defined life as the ability to reverse entropy (however innefficiently or temporarily). so in that sense, entropy can be reversed within a system, but only at the expanse of increased entropy outside (but somehow connected to) that system.

Jeff brings up another example of appearent reversal (using the construction sight as an example), but the food energy used up by the workers and the feul burned by their equipment constituted as much energy as was required to organise and assemble the structure they've built, (and then some). Entropy within the system was reversed only if we define "the system" as that part of the process wich built up the complex organised structure, and excluding the energy expediture that went into building it.

8. Jun 7, 2006

### vanesch

Staff Emeritus
Mmm, then my fridge's alive

This is a strange definition of life, no ?

9. Jun 7, 2006

### Andrew Mason

Ok. That would explain all green stuff at the bottom under the Christmas meatloaf.

To verify the hypothesis, try turning the fridge off for a few days to increase the entropy and see if it stops begin alive.

AM

10. Jun 7, 2006

### LURCH

Yeah, it's a bit oversimplified, but it's the best definition I know of for answering the question of why flame is not considered a living thing. You've heard it before, right?
"Flames eat, breathe, grow, move and reproduce, but they're not alive; why?"

Because fire is enegry being released as things break down. Life takes in energy and builds itself up into more and more complex and organised structures. Your fridge can delay the decay of a slab of beef inside it, but it cannot reverse that decay. Now, if your fridge were capable of taking raw materials from nature, seeking out its own power source without the need of an outside intelligence to plug it in, and use that power source to build those raw materials into beef, and continue doing so as older bits decay, so that the overall amount of beef it contained were constant, or even increased as time went by, and could also use that same manufactured beef to form and maintain its own operating parts, and use the surpluss to make other refridgerators out of beef that could carry on the process, then it would be alive.

Of course, it wouldn't be a fridge anymore, it'd be a cow. (Like that delightfull quote from the new Dr Who, where he defines life as it looks to medical nanobots as, "nature's way of keeping meat fresh").

11. Jun 7, 2006

### rcgldr

But most of the expediture of the energy used during contruction results in heat (from the machinery), much of which is captured by the Earth's atmoshpere, heating up the earth relative to the the rest of the universe. Since much of the heat energy is trapped, and takes a long time to dissipate via radiation, how is this an increase in entropy?

For a better example, what if alll of the constuction was done by robots and machinery powered from solar energy? Where is the increase and decrease in entropy under these circumstances?

What if the heat energy from the sun is simply captured? It's my understanding that entropy increases when the earth radiates heat, cooling off the earth, and eventually heating up other objects in the universe. If instead, the heat were captured, the earth would get hotter, and the rest the universe would end up cooler due to less radiated heat from the earth, and this would be a decrease in entropy.

Last edited: Jun 7, 2006
12. Jun 7, 2006

### 3trQN

A human is a complex chemical fire :P

13. Jun 7, 2006

### Rach3

Theological thermodynamics... :yuck:

14. Jun 7, 2006

### Ivan Seeking

Staff Emeritus
..heat....

15. Jun 7, 2006

### Data

As above, mainly heat probably. There'll be increases from other things too, of course. For complicated systems, obviously it gets hard to do real calculations. But in situations like this, physics is more useful as a predictive tool. If you can build a system like that and get perfect efficiency, then you'll be famous! If you don't have perfect efficiency then there's an entropy increase somewhere.

16. Jun 7, 2006

### rcgldr

In my last situation, I'm only talking about heat and temperature. Entropy increases when the temperature of everything in the universe starts approaching the same value. In the situation I described, the average temperature of the earth would be getting hotter, and everything else would be getting less hot than it would if the earth was radiating the heat instead of capturing it. Where's the increase in entropy in this situation?

If there was an earth bound process that converted this captured heat into another form of energy, like kinetic, such as changing the rotational rate of the earth, then the earth remains at is current temperature while radiating less heat to warm up the rest of the universe. Again, where's the increase in entropy in this case?

17. Jun 7, 2006

### Ivan Seeking

Staff Emeritus
Sorry, I had skipped right over this somehow. It has to do with useful energy. When the photons from the sun impact the earth and get used to do work, the energy becomes less accessible. We could make a heat engine that runs using the earths temp compared to the temp of deep space, but this would yield less work - it would be less efficient - than an engine [or in this case, a motor] designed to run directly from the solar panels discussed. This loss in "useful" energy is represented by the increase in entropy. See also Carnot.

Last edited: Jun 7, 2006
18. Jun 7, 2006

### Gokul43201

Staff Emeritus
All the heat from the Sun gets to the earth and the earth doesn't radiate to the rest of the universe, right?

That rate of change of the Sun's entropy is:

$$\frac{dS_{Sun}}{dt} = \frac{1}{T_{Sun}} \frac{dQ_{Sun}}{dt} ~~~~---(1)$$

And the rate of change of entropy of the earth is :

$$\frac{dS_{Earth}}{dt} = \frac{1}{T_{Earth}} \frac{dQ_{Earth}}{dt} ~~~~---(2)$$

Also, since all the heat from the Sun gets to the Earth,

$$dQ_{Sun} = -dQ_{Earth} \equiv dQ<0~~~~~~~~~~----(3)$$

So, the total rate of change of entropy is just the sum, given by:

$$\frac{dS_{total}}{dt} = \frac{dS_{Sun}}{dt} + \frac{dS_{Earth}}{dt} = \left(\frac{1}{T_{Sun}} - \frac{1}{T_{Earth}} \right) \frac{dQ} {dt} ~~----(4)$$

The assumption that the Sun radiates to the Earth (one we can make without loss of generality, since "Sun" and "Earth" are merely labels) and not vice versa establishes that $T_{Sun}~>~T_{Earth}$. This makes $1/T_{Sun}~-~1/T_{Earth}~<~0$ and hence, from (3) and (4), $dS_{total}/dt~>~0$.

19. Jun 7, 2006

### Cyrus

Simple, and elegant post Gokul. Nice job.

20. Jun 8, 2006

### rcgldr

Why is it that some chemical reaction produce a reduction in temperature while other produce an increase in temperature.

I've read a few web sites, but some issues like the creation of stars after the big bang, are confusing. Is matter in a higher or lower state of potential energy than actual energy itself? Some texts states that matter is a lower state, but noted that stars convert matter back into energy, this is my confusion.

21. Jun 8, 2006

### vanesch

Staff Emeritus
Let's hope none of it is trapped and takes a long time to dissipate via radiation !
In fact, the earth dissipates as much heat (at her night side mainly) as it receives from the sun + generates from radioactive decay, and that better remain so, or it would VERY QUICKLY get VERY HOT over here.
(and indeed, in the latter case, there wouldn't be much entropy-lowering construction anymore).

I don't think earth is much heating up the universe
The point is simply that we're a "plug" in an energy flux from the sun into the blackness of space, which is essentially at ~4 Kelvin.
The hot spot of the sun (~5000 Kelvin) versus the coldness of space is what allows us to transport energy (essentially in the form of photons) from a hot place to a cold (which is the essence of entropy generation) ; and in doing so, we may locally have small decreases in entropy as long as this is (over) compensated by the entropy increase in this hot-> cold flux.

22. Jun 8, 2006

### vanesch

Staff Emeritus
Because there's also an entropy content in the material part: it might be overall entropy creating if the decrease in entropy which happens when the temperature of the material lowers is set off by the increase in entropy of the reaction products.

Once you take gravity into account, things get slightly more subtle. For instance, without gravity, the highest entropy is usually reached with a homogeneous distribution of intensive parameters (like pressure, composition,...). However, when gravity is taken into account, this is not so anymore. The state of highest entropy in gravity is a black hole (or, slightly below it, a gas of black holes). This makes that when gravity is taken into account, a homogeneous distribution of matter is in fact an extremely LOW state of entropy. It is high wrt to its other interactions, but it is extremely low wrt the same stuff in a big black hole. This comes about because the variations of entropy we're used to (in chemistry, engineering thermodynamics and so on) are PEANUTS compared to the entropy of the material at hand in a black hole.
In fact, it is the extraordinary uniformity in the early stages of the universe + gravity which makes that there is so much "room" for entropy increase, which makes all the marvels (such as shining stars, life, ...) of our universe possible.
A contracting hydrogen cloud in a cold universe, forming a star which can sustain a nuclear burning for over 10 billions of years, is our local source from which we can pump entropy and which allows us to do all kinds of fun things.

23. Jun 9, 2006

### TeTeC

Humans are capable of locally decreasing entropy (that's what I understood from the preceeding messages) but they increase entropy as a general case. We are constantly struggling against this phenomenon. May I say that on Earth, global warming could be explained by the increase in entropy? Global warming could be overcome by technological means, at the cost of more energy and a more important increase in entropy. However, sooner or later, mankind will be annihilated by an insoluble 'increase in entropy' problem, won't it?

I don't know if the following question is either pertinent or properly posed, but I would like to make things clearer if what I stated before is enough right to make sense to my question:

Is there any possible way to quantify the time when our civilization will reach a peak in development followed by an insurmountable fall? In other words, where is the physical barrier to our development?

Be indulgent, I pertinently know that it is quite confused. :shy:

Thank you.

(Be also indulgent for syntax or spelling mistakes, English is not my mother tongue)

24. Jun 9, 2006

### LURCH

That much seem s quite certain.

Not really. We could learn an awefull lot in the future, maybe even find a way to tap the overall flow of energy through the universe. But even in the unlikely event that we could do that, there would still come a time (in the unnimaginably distant future) when there simply isn't an energy flow to tap.

Then again, we may blow ourselves to ellemental particles before the decade is out!

25. Jun 9, 2006

### vanesch

Staff Emeritus
Yes, but we don't increase the entropy so much locally. We send it off into the blackness of outer space. As long as the sun shines (still for about ~5 billion years) and outer space is black and cold, we don't really have an "entropy problem" on earth.

This really has nothing to do with it.

This will start to be an issue when the sun will near the end of its existence. We might have many many other problems to solve before!
I would say, don't spill your nights over it, for the moment it is not our worry.

If we manage not to destroy ourselves in a "stupid" way (by fighting eachother, by doing something silly like changing our environment in such a way that we are not fit anymore to survive in it) or by biological competition (an illness that erases humanity, the ants that attack us ) or by meteor impact or by geological change (vulcanic activity...) or by natural climatic change, or by evolutionary reasons, and we would become one of those rare species which exist for more than a few 10s of millions of years, then the first really big hurdle will be when the sun will become a red giant and swallow the earth, in about 5 billions of years.
We should leave earth by then.

The next, bigger, hurdle, will be when stars stop shining in or galaxy, when they've all become black holes, neutron stars or cooling white dwarfs. I don't know exactly when that will be, say, 20 billion years (just guessing).
We might devellop technology to extract energy from rotating black holes and so on, but we'll think back nostalgically to the good old days when we could stroll on the surface of a nice lush planet, with a sun in the sky.