MOTION MOUNTAIN

322 10 classical physics in a nutshell
Challenge 321 e
samespotat thesametime. This is thefirst statementthat humans encounter about
electromagnetism.Itisduetotherepulsionofchargesofthesamesignfoundinmatter.
More detailed investigation shows that electric charge accelerates other charges,
that charge is necessary to define length and time intervals, and that charges are the
source of electromagnetic fields. Also light is such a field. Light travels at the maximum
possible velocityc. In contrast to objects, light and electromagnetic fields can
interpenetrate.
2. Everyday motionconserves mass, electric charge, energy, linear momentum and angularmomentum.Forthesequantities,nothingappearsoutofnothing.Conservation
applies to all kinds of motion: to linear motion, to rotational motion, and to motion
of matter, radiation, space-time and horizons. Energy and momentum are similar
to continuous substances:theyare never destroyed, never created, but always redistributed.
Noteven heat, growth, transformations, biological evolution or friction are
exceptions conservation.
3. Everyday motion isrelative: motion depends on the observer. Not even the firm floor
below our feet contradicts relativity.
4. Everyday motion isreversible: everyday motion can occur backwards. Not even friction,thebreaking
ofobjectsordeathareexceptionstoreversibility.
5. Everyday motion ismirror-invariant: everyday motion can occur in a mirror-reversed
way. In short, we found that the classical motion of objects, radiation andspace-time
is right–left symmetric. Human-made objects, such as writing, are no exceptions to
mirror-invariance.
6. Everyday motion islazy: motion happens in a way that minimizes change, i.e., physicalaction.InGalileanphysicsandelectrodynamics,actionisthetimeaverage
of the
difference between kinetic and potential energy. In general relativity, action takes into
account the curvature and elasticity of space-time.The principle of leastaction – or
cosmiclaziness–holdforall cases.
In short, our exploration of classical physics showed us that motion is predictable and
limited: nature follows patterns and rules and there are no surprises in nature. Nature
cannotdowhatitlikestodo.
Wewilldiscover later that some rare examples of non-everyday motion violate reversibilityandmirror-invariance
in a subtle way.The subtle violations disappear if the terms
are properly extended in their meaning. Also mass conservation is violated separately,
but becomes, in relativity, part of energy conservation. In short, the general statements
about motion, suitably corrected, remain valid across the nature.
Above all, we saw that motion minimizes action. This deep result remains valid
throughout our adventure. In other terms, the universe has no freedom to determine
what occurs inside it.
After completing the classical parts of the adventure, you might think that
you know classical physics well. If you do so, read the excellent collection by
Friedrich Herrmann, Historical Burdens on Physics, available for free download,
at www.physikdidaktik.uni-karlsruhe.de/index_en.html. If the topics presented there –
all simple to understand – are clear to you, you have become a real expert on classical
physics.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net