# Thread: Can entropy be negative?

1. As said above, I'd like to know if entropy can be negative. I'd be glad too if I get a clear explanation
I have only got very little background in "Science", or I should say I don't actually know anything. Can someone still explain it to me?
Thanks a lot

2.

3. Entropy is defined relative to a baseline. That baseline is the entropy of a pure crystalline substance at absolute zero (this is the lowest possible entropy a system can have) which is arbitrarily called zero. According to this definition entropy cannot be negative. Entropy changes for a given process can be negative1 but the absolute entropy of a given substance cannot.

You might find these links useful:

Entropy - Wikipedia, the free encyclopedia
Absolute zero - Wikipedia, the free encyclopedia
Third law of thermodynamics - Wikipedia, the free encyclopedia

1. As long as there is a corresponding larger increase in entropy coupled to it to satisfy the second law of thermodynamics

4. Hi Emika

Nothing stops an individual component of an isolated system disobeying the trend of a system towards maximum entropy (but it is very unlikely), but for the closed system as a whole (as a classical macroscopic system), it MUST obey the trend towards maximum entropy. Note there has been suggestions that a microscopic system can violate this principle, but I am doubtful whether this has been validated. In fact this appears to be just a statistical fluctuation that real systems experience at microscopic scales. The principle of entropy is fundamentally tied to the energy of systems and how that energy of a closed system dissipates over time. The dissipative force is fundamental to all non-conserved forces of nature. Entropy is inherently probabilistic in nature. Nothing has ever yet been described and validated that breaks the 2nd Law of Thermodynamics. If there was we would have perpetual motion machines. It's a pretty formidable rule. PhDemon's links are good. Here is a less technical one that may help you.

Entropy is a very powerful principle in physics that is tied to notions of 'the arrow of time' and why systems evolve in a particular direction. It is very useful in both thermodynamics and information theory and relates to how systems evolve from a constrained state (can only be described a few ways due to that systems restricted degrees of freedom) to a less constrained state (can be described many more ways due to more available degrees of freedom in that state). This is what is meant when we refer to trending towards a state of equilibrium. :-))

A good example. Consider a magazine with 4 pages of text and a few pictures ordered in a particular way iso you can read it and understand it. As a magazine it is in a high energy state as the order that is described by that magazine in that state is high.

Now let's assume we leave that magazine for thousands of years where it slowly decays and the type, the pictures and the pages disintergrate into the surrounding closed system. The same information is there but it is distributed throughout the closed system. As a disintegrated magazine it is in a very disordered state and the energy of that disintegrated state is low. Note however that the total energy of the closed system has never actually changed. It is just how that energy has been constrained in a local area of that closed system and then how it becomes more distributed throughout the system. You can see this by imagining how much energy you would have to draw from that closed system to gather up all that diffuse information from the entire system and re-assemble it back into that ordered state.

All systems follow this tendency as it is energy's way of dissipating from a constrained (ordered state) to an unconstrained (disordered state or state of equilibrium). You may be lucky in that a letter on a fragment of paper reassemble but the chances of this are infintesimal, let alone the entire magazine reassembling in pristine condition.

PS Now in open systems (several sub-systems within a greater connected system) things can get interesting where locally sub-systems can ratchet up their energy state by drawing on greater energy from the greater system for productive purposes. This is an example of how life exploits entropy to create order. Note however that in it's entirety the system still evoles towards maximum entropy. This is not surprising considering metabolic processes are at the heart of living things.

When you look at living systems at the microscopic scale you can see how we have level upon level of semi-contrained system such as an organelle, the nucleus, the cell membrane, the organ, the blood vessels etc. These are all embedded in each other to create an energy gradient that allows the entire system to work in a corodinated fashion by harvesting the principle of entropy. Energy flows throughout the system in cascades and waterfalls between these constrained yet interconnected systems. The ordered structure of this system allows this process to occur in a managed way and it takes the useful conversion of energy (by metabolism) to create this ordered system.

If you superimposed a map over a living organism and put points all over the organism and defined these points by the potential energy starting with the energy inputs (high potential energy) into the living system. Then joined the points by contours to represent points of equal potential you could see how all this energy flowed through the connected sub-systems like a river and identify where it was locally used (by life's use of structures to ratchet up local energy against the general flow) to create higher potential energy states (eddies and whirlpools) within this greater system and then all out again into the external environment in a lower potential energy state.

5. You may find this useful. It's my old tutor talking about entropy:

Peter Atkins - Entropy - YouTube

And a slightly different take on it from MC Hawking:

MC Hawking - Entropy - YouTube

6. Originally Posted by PhDemon
It's my old tutor talking about entropy:
Peter Atkins is a living legend. You lucky lucky Demon. *bows humbly with offerring in hands*

I read his pop science work Galileo's Finger and came away from it all with a totally different perspective on things. He was the guy that nailed it for me in regards to seriously treating physics in terms of systems due to the power of the 2nd Law. A seriously talented scientist who knows how to write and convey information to his audience in a coherent way. (yeah I know he is pigeon-holed as a chemist but he is across all disciplines and IMO a master of physics from his understandings of how natural systems actually work) :-))