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# Entropy – Why Everything

(Last Updated On: April 6, 2019)

### Did You Know?

A broken cup never re-assembles itself. A cube of ice melts if kept outside, but the water doesn’t freeze back to ice. Nature has a tendency to go from a state of order to chaos. And this tendency has a name – Entropy.

It is a thermodynamic function that is a measure of disorder of a system. Also, it denotes the amount of useful Energy available in a system. And the law of physics work such that the overall entropy of the Universe always increases.

It was first hypothesized by Rudolf Clausius, who argued that entropy was just the amount of heat lost in an engine due to friction.  Later, scientists such as Boltzmann and Gibbs gave a statistical definition. It plays an important role in studying the thermodynamic properties of a system.

It is a state function. In simple words, it means that over a reversible cycle, the net change in entropy is always zero. It  is denoted by the letter S. However, for an irreversible cycle, the net change in entropy is always positive.

### What is it?

It is more than just disorder. In statistical mechanics, it is related to the number of possible micro states of a system. A micro state is just a particular arrangement of particles and energy in a system. For every micro state, there is a different entropy. And for the possibility that there are multiple probable outcomes, the system moves to the state with the highest entropy.

Entropy is probabilistic, which means that it is not necessary that the system will always proceed towards highest possible entropy. This means that there is a non-zero, but infinitesimally small possibility, that your glass of water just might freeze into ice by itself. However, such a system would be highly unstable and the ice would melt back before you could tell other people about what happened. In statistical terms, every micro state has a probability that the system will evolve into that micro state. And the micro state with the highest entropy will have the highest probability.

But entropy doesn’t just melt ice. Entropy works on global scales, throughout the Universe. A main sequence star, at the end of its life, becomes a white dwarf.. A white dwarf cannot sustain fusion and thus, releases no energy. The high entropy also denotes absence of any energy to do any useful work. Stars burn their hydrogen and helium into heavier elements because the entropy of the star must increase. Since you can no longer obtain energy from fusion of elements heavier than Iron, there is no energy the star can generate.

### Want to know more?

The fact that entropy of the Universe must increase is a holy rule of physics. All other theories, including relativity and quantum mechanics, must abide by this law. Entropy can decrease on a local scale, but always increases on a global scale. Physicists have so much confidence in this law that it is our best guess on how the world will end.

If the entropy of the world will keep increasing, it will ultimately reach a maximum value. Then, there will be no energy left in the Universe to do anything. Stars will die out, planets will fade away in the absence of gravity. Neutron stars and black holes, however, are very stable, and will last for trillions of years.

However, a neutron eventually decays into a proton and electron, which means that even neutron stars won’t last forever. And one day, even the black holes will evaporate away. The Universe will be a cold, dark place, without any energy, and only a few particles scattered everywhere – which too will eventually decay into energy. The Universe will eventually die a Heat Death. It will simply exist forever without any activity.

However, there are a few other models about the eventual demise of our Universe. And every one of them predicts something different. Some of them are re-assuring. Others are pretty violent.

When we talk about these models, we will also have to talk about dark matter and dark energy. More on that later.

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