chaos and uncertainty (was seti@home is SORTA WORKING)

Ron Kean (
Thu, 15 Jul 1999 13:08:11 -0400

On Thu, 15 Jul 1999 12:56:18 +0100 Rob Harris Cen-IT <> writes:

>> >A system with a load of variables that we can't track at this tech
>>> >level is>> >not "unpredictable", it is "very hard to predict", if
you want to >be>> >correct.

>> I think N-body systems are 'chaotic', meaning that extremely small
>> differences in initial conditions can result in immense differences
>in>> outcomes, over sufficient time. While the computation may be
>> impractical, I suppose that in principle the motions of an N-body
>> gravitational system may be predictable from a classical
>perspective. >> But that does not take into account quantum
fluctuations, which I >suppose>> are in principle unpredictable.

>Does anyone know about the principle of true uncertainty? Often when
>I >discuss determinism and causality with people, they pull out the old
>"Quantum uncertainty" thing. I don't know what this is, or if it
>really >describes true uncertainty (i.e. truly spontaneous occurrences).
>doubt it. >Ron, can you expand the uncertainty detail at all? Anyone

If I have a cup of hot coffee sitting on the table, the water molecules in the cup, being hot, are jiggling randomly. There is a slight, vanishingly small chance that the coffee will leap out of the cup and fly up towards the ceiling. In practice, I am highly confident this will not happen, but in principle I cannot predict what the coffee will do with absolute certainty.

But suppose that I knew the position and momentum of each molecule in the cup at the present time. Then, in principle, could I predict the future behavior of the coffee? Even then, I don't think so. An errant driver could lose control of his car, smash through the wall, hit the table, and spill the coffee. But neglecting the case of unpredictable external influences, there is still a problem with predicting the future behavior of a system. That is that the uncertainty principle limits the precision of measuring both the position and momentum of a particle at the same time. The product of the uncertainties of position and momentum is at least Planck's constant.

So the precise initial conditions needed to make a prediction cannot be obtained, given the uncertainty principle.

Ron Kean




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