Is concurrency hard?

David Hopwood <>
Wed Nov 2 18:20:29 CET 2005


Marc van Woerkom wrote:
>> On dear - who is right? - both - it is the question that is silly -
>> you cannot ask questions about the simultaneity of event occurring at
>> different places (basic physics).
> 
> Each one has his own correct point of view about the events happening,
> and it is possible to find out what is going on in another observer's
> local coordinate frame (one knows the proper transformations).
> 
>> Now in physics there is no concept of sharing and no concept of
>> simultaneity at a distance. We can only say that two things occur at
>> the same time if they occur at the same place. 
> 
> The theory of relativity has abandoned absolute time, that is true.
> 
> One the other hand we have that strange world of quantum mechanics.
> There you have the odd phenomenon of coupled states.
> E.g. certain radioactive isotopes have an event, where two photons are
> emitted in opposite directions. The polarisation of the photons is
> undetermined at first. But if you measure one photon, and thus force
> nature to make a choice, instantaneously the other photon takes the
> opposite polarisation.

More precisely, the two photons will not both be measured as having the
same polarisation. It's an overspecification of what is observable (and
itself a subtle example of sequential thinking) to describe this as
though measuring one photon causes a change to the other.

But enough of the armchair physics. I'm sure we're giving any real
physicists in the audience fits.

-- 
David Hopwood <>




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