CSW's Ideal Gas

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The physics of this two-dimensional gas (below) are reversible. Time can be run forwards and backwards. Energy is perfectly conserved.

Please press 'GO'.

T=,  pause=ms,  Entropy= 
(this version 29/6/2017 Lloyd Allison)
'T' is the current time of the simulation.
'fwd', that is 'forward', makes 'T' increase.
'rev', that is 'reverse', makes 'T' decrease.
'pause' is the delay, in milliseconds, between showing adjacent states of the simulation.
'faster' and 'slower' decrease and increase 'pause' (within reasonable limits).
'Entropy' is the entropy of the velocity distribution of the particles,
that is the amount of information (in nits, natural bits) required to state the velocity, (u, v), of one particle.
'reinit' reinitialises the simulation.

Chris Wallace showed that this artificial gas obeys many of the properties of real gases – see the Feathers on the Arrow of Time chapter in the "MML book."


The entropy that is displayed is the entropy of the velocity distribution. It can be seen that from a randomly selected state, the entropy is equally likely to be higher or lower in the next (or previous) state. However, if the gas is observed to be in a low entropy (ordered) state at time t, it will almost certainly be in a higher entropy state at time t+1, and the time-reversibility implies that it was almost certainly in a higher entropy state at time t-1.

Determinism: 1-1 State Transitions

The physics of the gas are deterministic. Therefore the states will cycle  - over  a  v e r y  l o n g  period, N. So from this point of view the entropy of every state is the same, log(N).

© C.S.Wallace & L.Allison, 2007, 2017.

(The figures in the "MML Book" that illustrate the gas were generated by a C programme – which cannot be run via the web.)


© L. Allison   http://www.allisons.org/ll/   (or as otherwise indicated),
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