On 03 Jan 2014, at 21:35, LizR wrote:
On 4 January 2014 00:10, Bruno Marchal <[email protected]> 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.
"emission" and "absorption" are already "interpretation" done in a
theory, in which we assume atoms and photons. In experimental physics
we observe numbers and infer relations from number's correlation.
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...
At that level too. But if you show a spectra to Og in the cave, even
if it does not cross his mind that he might be dreaming, he will not
observe emission, atoms, etc.
Emission and absorption spectra indicate that the process of an atom
absorbing a photon and emitting one are time symmetric processes.
I agree, but this is due to my background in physics, and in my
tendency to take the current theories seriously.
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).
OK.
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!
Yes. That is what constitute our belief. My remark was general. Many
people believe that the existence of the moon is a fact, but it is a
theory. The existence of our parents is a theory. It works well for
all practical purpose, but in the fundamentals, we have to be more
strict on those kinds of things.
Bruno
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http://iridia.ulb.ac.be/~marchal/
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