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... I think that measuring a distance is REAL as long as I have
a tape or yardstick, (or a protractor, if triangulation is involved).
These instruments are so simple that nobody questions the
results, except for the accuracy. Naturally, the brain and retina
are involved. We do not understand everything about these
biological components but we, in physics, are not bothered
by this. Using brains and retinas does not mean that we are
imagining things. We believe measurements are REAL.
Sometimes a sophisticated set up, such as an interferometer,
is used, instead of a simple tape or ruler. Or a spectrometer
to measure the magnitude of red shift. Or an atomic clock
instead of a simple pendulum. We do not understand our
brains and retinas but we do understand how the added
components work.
Suppose I hire a laboratory assistant who follows instructions
without understanding optical components. To him optical
components are like retina and brain. He believes that
counting fringes, for example, is a REAL measurement of
distance. ...