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S16c. Can good theories be falsified?
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The philosopher Karl Popper claimed that falsifiability is the
hallmark of scientific theories. But scientific practice speaks
against him. 

A correct theory cannot be falsified, and in this sense is not
falsifiable, in spite of Popper. (Falsifiability can  be asserted
only in a contrafactual sense, that there are _conceivable_ situations
that, according to the theory, are excluded. But for a correct theory,
these situation will never happen, hence are completely ficticious.)


What happens with good theories is, at worst, that their region of
validity or accuracy gets restricted as new data about more remote
instances come in.

In today's understanding, people are careful to indicate the
limits where a theory is claimed to be valid, and the accuracy
to which its answers are to be trusted. 

For example, the Standard Model is claimed to be valid whenever 
gravitation is negligible, accuracies conform to present possibilities, 
and energies are well below a putative unification scale. 
Failures outside this domain are not counted as falsifications. 

While limits and accuracy claims are not necessarily part 
of the theory proper, they are part of the theory as actually taught
and applied. Indeed, although people try to extrapolate, one can 
never be sure whether a theory is correct outside the domain where 
the data were collected. 

But one can be reasonably sure within the domain where enough data 
are available. Good scientific practice requires that a good theory
agrees with the data within the tolerances claimed.
Once this is the case, these theories can never be falsified. 
Rather, if people find disagreement in experiments, the
theory falsifies the experimental arrangement or analysis. 

All science students who ever did experiments in the lab know
very well that this is common practice.


The degree of caution and care at the highest 
level of quality has been increasing through the centuries.
It is now too late to ask Newton whether he believed his theory was 
valid without restrictions. (Or are there any hints in the Principia 
Mathematica?) Certainly Newton's theory as taught today is taught 
(i.e., with the restriction that it is valid at speeds small compared 
to c and at distances large compared to the radius of the largest atom).
But we nevertheless believe that it is the 'same' theory, and if 
Newton would live today, I think he would agree with that.

And Newton's theory will never be falsified, unless God suddenly 
decides to change the physics of the Universe.

(That the observed advance of Mercury's perihelion did not match 
Newton's theory was known as a limitating condition already before
relativity was born.)