Is it possible to create entropy? Is it possible to destroy it? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Entropy, unlike energy or momentum, is not a local property. It is a macroscopic quantity, valid for a collection of molecules. Although you can take such a collection and calculate the entropy per molecule, that really has no meaning by itself. Entropy gives a number that represents the likelihood that a collection of molecules or other particles will be configured in a particular way.

You can certainly lower entropy locally. When you grow a plant, you are taking the high entropy of rain and nutrients and converting them into a relatively low entropy (highly organized) state: the plant.

But, you say, entropy always increases! That’s not the entropy of any object; the thing that increases is the entropy of the universe. The plant grows, but in doing so it throws off photons and oxygen and other things that increase the entropy of the universe.

Here’s a simpler example, taken from my book Now—The Physics of Time. Sit a cup of hot coffee on the table. Watch it cool. You are seeing the entropy of that coffee reduce, all by itself! Of course, if you include the entropy of the infrared radiation that is emitted, and the entropy of the air that is conducting heat away, then the entropy of the universe is increasing.

Every night, the entropy of the biosphere decreases, due to the cooling. In the day, it increases.

The entropy of the Earth is constantly decreasing, mostly because of the cooling of the core (think of the coffee cup). The entropy of the Sun is also decreasing, as its core becomes more highly organized. But the Sun’s activity increases the entropy of the universe by filling it with lots of newly generated photons. Similarly for the Earth.

If all this sounds inconsistent with what you have read elsewhere, it's because many popular writers have over-simplified the concept of entropy in their books. That really annoys me, which is part of the reason that I wrote Now. Ever since this book came out, it has been number one on Amazon’s list of books in the category of entropy, even though the book is aimed at understanding time. (Actually, the hardcover is listed as #1 and the paperback as #2.) But so many people have mistakenly taken the “arrow of time” to be a result of entropy increase (rather than the other way around) that I had to demolish that argument before moving on to the more intriguing and difficult issue of: why can we stand still in space but not in time? I recommend my two chapters on entropy as a good introduction (including for non-math people) to a subject that is not so much not understood by most non-scientists as it is misunderstood.

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