MOTION MOUNTAIN

observations and their collection 295
Physical observables are tools to communicate observations. Is it possible to talk about
observations at all? Yes, as we do it every day. But it is many a philosopher’s hobby to
discuss whether there actually is an example for an ‘Elementarsatz’ – an atomic fact –
mentioned by Wittgenstein in hisTractatus. There seems to be at least one that fits:Differences
exist. It is a simple sentence; in the final part of our walk, it will play a central
role.
Today, all physical observables are known.The task of defining tools for the communication
of observations can thus be considered complete. This is a simple and strong
statement. It shows that the understanding of the fundamentals of motion is near completion.
Indeed, the BIPM,theBureauInternationaldesPoidsetMesures,hasstoppedadding
newunits.Thelastunit,thekatal,wasintroducedin1999asanabrreviation of or mol/s.
The full list of physical units is presented in Appendix A.
No new observables are expected to be found. In the past, the importance of a physicistcouldbe
ranked bythenumber ofobservables he or she had discovered. Discovering
obervables had always been less common than discovering new patterns, or ‘laws’
of nature. Even a great scientist such as Einstein, who discovered several pattern of
nature, only introduced one new observable, namely the metric tensor for the description
of gravity. Following this criterion – as well as several others – Maxwell might be
the most important physicist, having introduced several material dependent observables.
For Schrödinger, the wave function describing electron motion could be counted as an
observable (even though it is a quantity necessary to calculate measurement results, and
not itself an observable). Incidentally, the introduction of any term that is taken up by
others is a rare event; ‘gas’, ‘entropy’ or ‘kinetic energy’ are such examples. Usually, observables
were developed by many people cooperating together. Indeed, almost no observables
bear people’s names, whereas many ‘laws’ do.
Given that the list of observables necessary to describe nature is complete, does this
mean that all the patterns or rules of nature are known? Not necessarily; in the history of
physics, observables were usually defined and measured longbefore the precise rules connectingthemwerefound.Forexample,allobservables
used in the description of motion
itself – such as time, position and its derivatives, momentum, energy and all the thermodynamicquantities
–weredefinedbeforeorduringthenineteenthcentury,whereas
the most precise versions of the patterns or ‘laws’ of nature connecting them, special
relativity and non-equilibrium thermodynamics, have been found only in the twentieth
century. The same is true for all observables connected to electromagnetic interaction.
The correct patterns of nature, quantum electrodynamics, was discovered long after the
corresponding observables. The observables that were discovered last were the fields of
the strong and the weak nuclear interactions. Also, in this case, the patterns of nature
were formulated much later.
In summary, all observables about the fundamentals of motion have been discovered.
We are, at this moment of history, in a fortunate situation: we can talk with precision
about all motion observed in nature. The last part of our adventure will explore the tiny
possibility for errors or loopholes in this statement.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net