Reversing entropy with unlimited energy

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peregrine_crow
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Reversing entropy with unlimited energy

Postby peregrine_crow » Sun Jun 12, 2016 6:53 pm UTC

I was recently in a discussion over at r/askscience about whether we could reverse entropy given a limitless source of usable energy. That discussion didn't really manage to make it click for me, so I figured I'd ask about it here.

The context for the discussion was the idea that since energy is not conserved in an expanding universe you could get effectively limitless amounts of energy by tying one end of a thousands of light-years long rope to a dynamo and the other end to some far away star (we're ignoring some minor engineering difficulties here :) ). As the universe expands the star moves away and powers the dynamo. So first question: is this setup at least theoretically possible?

Then the second question: supposing this works (or we have some other source of limitless energy) can we use this to reverse entropy without increasing it elsewhere? The way I understand it, entropy corresponds to how much work can be done with the energy in a system. My thought is that since moving a dynamo definitely counts as work being done, assuming the dynamo-rope-star system doesn't increase in entropy somewhere else we should be able to use that work to reverse entropy in some other location, is this correct or am I missing something ?

I should probably mention that I don't actually think we figured out a way around the heat death of the universe in a Reddit thread, I'd just like to know exactly where I'm going wrong.
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Soupspoon
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Re: Reversing entropy with unlimited energy

Postby Soupspoon » Sun Jun 12, 2016 7:25 pm UTC

My thoughts: If we have a way of pumping energy into a closed system, then it isn't a closed system. Local entropy can be reversed, but only by the energy source succumbing so that the whole whole system (including the source of 'enough energy to do this') goes in the only accepted direction known to physics...

Or am I missing something?

(With the dynamo, I suspect inefficiencies dominate. The rope is not perfectly inelastic so stretches and does not provide energy enough to counter the related expansion-induced energy-diffusion.)
Last edited by Soupspoon on Sun Jun 12, 2016 7:30 pm UTC, edited 1 time in total.

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untitled
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Re: Reversing entropy with unlimited energy

Postby untitled » Sun Jun 12, 2016 7:27 pm UTC

peregrine_crow wrote:The context for the discussion was the idea that since energy is not conserved in an expanding universe you could get effectively limitless amounts of energy by tying one end of a thousands of light-years long rope to a dynamo and the other end to some far away star (we're ignoring some minor engineering difficulties here :) ). As the universe expands the star moves away and powers the dynamo. So first question: is this setup at least theoretically possible?

Interesting proposition. I wouldn't be amazed if this is how dark energy is created (i.e. as space expands, it converts the "something lost/gained/whatever in expansion" into energy inaccessible from space itself).

PsiCubed
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Re: Reversing entropy with unlimited energy

Postby PsiCubed » Sun Jun 12, 2016 10:33 pm UTC

peregrine_crow wrote:I was recently in a discussion over at r/askscience about whether we could reverse entropy given a limitless source of usable energy. That discussion didn't really manage to make it click for me, so I figured I'd ask about it here.


Sure. Life on earth has been doing something very similar for the past 4 billion years without any trouble, by utilizing the "limitless" energy of the Sun.


Then the second question: supposing this works (or we have some other source of limitless energy) can we use this to reverse entropy without increasing it elsewhere?


No, but that's not really a problem.

Because in a world with an "unlimited source of usable energy", total entropy can increase without bounds: The maximum entropy of a system is proportional to the total energy in that system (and inversely proportional to temperature).

Actually, it is possible for this to happen even if there's only a finite amount of usable energy: Since the universe is getting colder with time and since entropy is inversely proportional to temperature, there are cosmological models where entropy does indeed increase without bound and there is no "heat death" at all.

Unfortunately, since the discovery that the universe's expansion is accelerating, it became almost certain that we do not live in such a universe.

I should probably mention that I don't actually think we figured out a way around the heat death of the universe in a Reddit thread, I'd just like to know exactly where I'm going wrong.


You aren't wrong in your conlcusion.

IF there existed an unlimited source of usable energy, then - indeed - it would have been possible to avert the heat death of the universe.

The only problem is that such a source does not exist, as far as we know. So there's no outside source to power your star-dynamo for all eternity (and the dynamo certainly cannot power itself for all eternity, because that would be a classical perpetual motion machine and we already know that these are impossible).
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Hypnosifl
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Re: Reversing entropy with unlimited energy

Postby Hypnosifl » Mon Jun 13, 2016 1:20 am UTC

peregrine_crow wrote:The context for the discussion was the idea that since energy is not conserved in an expanding universe you could get effectively limitless amounts of energy by tying one end of a thousands of light-years long rope to a dynamo and the other end to some far away star (we're ignoring some minor engineering difficulties here :) ). As the universe expands the star moves away and powers the dynamo. So first question: is this setup at least theoretically possible?

This question is addressed by a physics Ph.D. on quora here, he says that in a universe with no cosmological constant it wouldn't work, but in one with accelerating expansion it might be possible, although the rope would need to be continuously lengthened and it's possible that doing so would take more energy than you could extract. Also, as you say, energy is not globally conserved in general relativity (though this page notes that you can come up with a new non-local definition of energy using "pseudotensors" and this new type of energy will be conserved).

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Minerva
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Re: Reversing entropy with unlimited energy

Postby Minerva » Tue Sep 13, 2016 2:02 am UTC

In a localised system, yes, we do it all the time. That's how your air conditioner or refrigerator works. Or the clean patch on my desk that allows me to reach past junk to reach the keyboard, or essentially anything else in the universe that works with expended effort to generate a local reduced-entropy bubble. But the energy has to come from somewhere else outside, and the entropy of the universe doesn't decrease, it increases.

One of those classic Internet quotes also speaks to the issue you're thinking about. :)

One of the most basic laws in the universe is the Second Law of Thermodynamics. This states that as time goes by, entropy in an environment will increase. Evolution argues differently against a law that is accepted EVERYWHERE BY EVERYONE. Evolution says that we started out simple, and over time became more complex. That just isn’t possible: UNLESS there is a giant outside source of energy supplying the Earth with huge amounts of energy. If there were such a source, scientists would certainly know about it.
...suffer from the computer disease that anybody who works with computers now knows about. It's a very serious disease and it interferes completely with the work. The trouble with computers is you play with them. They are so wonderful. - Richard Feynman

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pogrmman
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Re: Reversing entropy with unlimited energy

Postby pogrmman » Fri Sep 16, 2016 1:36 am UTC

Minerva wrote:
That just isn’t possible: UNLESS there is a giant outside source of energy supplying the Earth with huge amounts of energy. If there were such a source, scientists would certainly know about it.


I know! It's not like there is a giant, extremely hot mass of gas and plasma flooding the planet in all sorts of radiation or anything... :lol:

Sandor
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Re: Reversing entropy with unlimited energy

Postby Sandor » Mon Sep 19, 2016 2:50 pm UTC

pogrmman wrote:
Minerva wrote:
That just isn’t possible: UNLESS there is a giant outside source of energy supplying the Earth with huge amounts of energy. If there were such a source, scientists would certainly know about it.


I know! It's not like there is a giant, extremely hot mass of gas and plasma flooding the planet in all sorts of radiation or anything... :lol:

The Earth radiates away just as much energy as it receives, don't you know*, so I don't see how that will help.

(*) In fact it radiates away many more photons than it receives, even. And in many more directions. But I'm sure that's not significant.

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Eebster the Great
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Re: Reversing entropy with unlimited energy

Postby Eebster the Great » Mon Sep 19, 2016 5:11 pm UTC

Growth and reproduction are also patently absurd "theories" that don't stand up to thermodynamics.

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pogrmman
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Re: Reversing entropy with unlimited energy

Postby pogrmman » Mon Sep 19, 2016 9:01 pm UTC

Sandor wrote:
pogrmman wrote:
Minerva wrote:
That just isn’t possible: UNLESS there is a giant outside source of energy supplying the Earth with huge amounts of energy. If there were such a source, scientists would certainly know about it.


I know! It's not like there is a giant, extremely hot mass of gas and plasma flooding the planet in all sorts of radiation or anything... :lol:

The Earth radiates away just as much energy as it receives, don't you know*, so I don't see how that will help.

(*) In fact it radiates away many more photons than it receives, even. And in many more directions. But I'm sure that's not significant.


The raw number of photons doesn't matter because they are of much lower energies than the ones we receive [citation needed]


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