In quantum physics, a unitary transformation (the quantum
representation of application of a force) evolves a closed quantum
system deterministically forward in time according to the Schrodinger
equation. von Neumann called this evolution "causal." (See von
Neumann's Mathematical Foundations of Quantum Mechanics.) By this he
meant that the time evolution of a local region in space is
determined by the initial state and the forces applied to that
region. Like the classical laws of physics, Schrodinger evolution is
time-reversible, which means that the laws of physics are indifferent
as to whether the past causes the future or the future causes the
past. (01)
Schrodinger evolution is not the whole story of quantum theory. More
precisely, no one has yet found a way to get the predictions of
quantum theory from just unitary evolution, and there is controversy
over whether it will be possible to do so. What has been confirmed
to such stunning accuracy is the irreversible, stochastic theory
described in von Neumann's Mathematical Foundations. In this theory,
deterministic, unitary evolution is sometimes punctuated by events
which have been called, picturesquely, "collapse of the wave
function." von Neumann used the more neutral term "Process 1," with
unitary evolution being "Process 2". There is no theory governing
when these events occur or which kind of event they are (i.e., what
the possible outcomes are). However, given that an event occurs at a
particular time, and given the possible outcomes of the event and the
state just prior to the event, the probability of each outcome is
quite precisely specified. These probabilistic predictions have been
confirmed to as many decimal places as you like. (02)
von Neumann called Process 1 events "non-causal" because he thought
of the cause and effect relationship as deterministic. But quantum
theory with reductions can be interpreted as a stochastic
interventionist causal theory of the kind recently popularized by
Judea Pearl. See http://ite.gmu.edu/~klaskey/papers/Laskey_QCN.pdf. (03)
K (04)
At 4:20 PM -0500 8/10/07, Pat Hayes wrote:
> >Pat,
>>
>>> Do you know anything at all about quantum physics, or indeed physics
>>> generally? Find me a place in, say, Feynman's 'lectures on physics'
>>> (or choose your favorite text from the last 50 years) where causality
>>> plays any kind of explanatory role in modern physics.
>>
>>Causality slips in in the guise of force, which is central.
>
>Nah, not classical causality. The cause is always before the effect,
>right? Well, suppose you push against a wall. The wall pushes back
>(Newton 2nd law), which is why nothing moves. Did your force cause
>the reaction force? Seems intuitively clear that it did. But it can't
>possibly precede it, unless the wall moved and then moved back.
>
>BTW, I didn't know you were reading this stuff, Jerry. Id better be
>more careful what I say :-)
>
>Pat
>
>>-- Jerry
>
>
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