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The Restless universe
Introduction to The restless Universe

1 The lawful Universe

2 The clockwork Universe

3 The irreversible Universe

3.1 Thermodynamics and entropy 1/3

3.1 Thermodynamics and entropy 2/3

3.1 Thermodynamics and entropy 3/3

3.2 Equilibrium and irreversibility 1/2

3.2 Equilibrium and irreversibility 2/2

3.3 Statistical mechanics 1/2

» 3.3 Statistical mechanics 2/2

4 The intangible Universe

5 The uncertain Universe

6 Closing items

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Other titles in the Physical World series

Describing motion

Predicting motion

Classical physics of matter

Static fields and potentials

Dynamic fields and waves

Quantum physics: an introduction

Quantum physics of matter

3 The irreversible Universe

3.3 Statistical mechanics

Part 1 of 2 | Part 2

For a printable version of '3 The irreversible Universe' click here

There is a special branch of physics, called statistical mechanics, which attempts to bridge the gap between descriptions on the scale of molecules and thermodynamics. It recognizes that our knowledge of a complicated system, such as a glass of water, is inevitably incomplete so we are essentially reduced to making guesses. This may seem to be a terrible weakness, but statistical mechanics actually turns it into an advantage. It replaces precise knowledge of the motion of molecules by probabilities indicating how the molecules are likely to move, on average. It then goes on to estimate the probability of measuring a particular pressure, energy or entropy in the system as a whole.
Figure 1.15 Ludwig Boltzmann
(1844-1906)
Click here for larger image (16.95kb)

The statistical interpretation of thermodynamics was brought to fruition by Austrian physicist
Click here to learn more about Boltzmann
figure 1.15
This is rather like the trick pulled by opinion pollsters when they predict the result of a general election without knowing how every individual in the country intends to vote. Pollsters have a mixed reputation, but the calculations of statistical mechanics are much more clear cut. They turn out to provide predictions that are overwhelmingly likely to happen - so much so, that they appear to be laws of Nature. The second law of thermodynamics is a case in point. From the viewpoint of statistical mechanics, the entropy of the Universe is not bound to increase, it is just overwhelmingly likely to do so. Perhaps 'heat death' will not be the end after all. After countless years of dull uniformity, a very unlikely (but possible) new fluctuation may occur with a lower than maximum entropy, and interesting things will start to happen again.
Continue on to 4 The intangible Universe, part 1 of 4

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A note on powers of ten and significant figures

Some highlights of physics

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Acknowledgements

Index

S207 The Physical World
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