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Popper Mistaken About Physical Determinism

_Objective Knowledge_ by Karl Popper, p 221
physical determinism implies that every physical event in the distant future (or in the distance past) is predictable (or retrodictable) with any desired degree of precision, provided we have sufficient knowledge about the present state of the world.
This is false. Physical determinism does not imply that we can calculate what the past was like based on the present.

The reason is that some functions are not reversible. Knowing the function used, and the output, does not let you calculate the input.

An example is addition. If you know two numbers were added, and the result was four, you cannot work out what the original numbers were. The output of addition has less information than the input.

To predict the past based on the present, one needs to posit both physical determinism and that all the laws of physics are reversible.

Elliot Temple on February 18, 2009
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