On 4 January 2014 00:10, Bruno Marchal <marc...@ulb.ac.be> wrote:

> We cannot observed something like time reversibility. We can only inferred
> it from a finite number of observations, and then assume a theory which
> either assumes it at the start, or explains it from other assumptions, or
> perhaps refute it.  We can observe facts, not theories. I guess you were
> just in the hyperquick mode of talk :)
>
> Hmm. Weeeeeeeeeeell, there are inferences involved, of course, but I'm not
sure what you mean by "not observe". We observe emission and absorption
spectra, for example, and we deduce that atoms emit and absorb photons -
but we are only inferring from observation. But we are inferring all the
time, e.g. we assume the spectra we observe are real and not implanted in
our minds by malevolent scientists keeping our brains in vats. At that
level, of course, everything is inference, but...

Emission and absorption spectra indicate that the process of an atom
absorbing a photon and emitting one are time symmetric processes. Kinetic
theory explains the properties of gases by assuming that they engage in
time-symmetric collisions (once they reach thermal equilibrium). The
equations of Newtonian dynamics are time-symmetric and appear to accord
very well with observation (modulo dissipative processes and the existence
of an entropy gradient, hypothetically due to boundary conditions on the
universe).

I agree that from a philosophy of science viewpoint we have to make
caveats, but they apply to everything, not just this one feature of physics!

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