MOTION MOUNTAIN

302 9 concepts, lies and patterns of nature
Challenge 314 s
did not mean either morality or spirituality; every scientist is a friend of both of these.
By religion, Berkeley meant that the standard set of beliefs for which he stood is above
the deductions of reason. This widely cited statement, itself a belief, is still held dearly
by many even to this day. However, when searching for the origin of motion, all beliefs
stand in the way, including this one. Carrying beliefs is like carrying oversized baggage:
it prevents one from reaching the top of Motion Mountain.
Does the void exist?
“Teacher: ‘Whatisfoundbetween thenucleus
andtheelectrons?’
Student: ‘Nothing, onlyair.’
”
In philosophical discussions ‘void’ is usually defined as ‘non-existence’.Itthenbecomes
agameofwordstoaskforayesornoanswertothequestion ‘Doesthe void exist?’ The
expression ‘the existence of non-existence’iseitheracontradictionoftermsorisatleast
unclearlydefined;thetopicwouldnotseemtobeofgreatinterest.However, similar questions
do appear in physics, and a physicist should be prepared to notice the difference
of this from the previous one. Does a vacuum exist? Does empty space exist? Or is the
world ‘full’ everywhere, as the more conservative biologist Aristotle maintained? In the
past, people have been killed for giving an answer that was unacceptable to authorities.
It is not obvious, but it is nevertheless important, that the modern physical concepts
of ‘vacuum’ and ‘empty space’ are not the same as the philosophical concept of ‘void’.
‘Vacuum’ is not defined as ‘non-existence’;onthecontrary,itisdefinedastheabsenceof
matterandradiation.Vacuumisanentitywithspecificobservable properties, such as its
number of dimensions, its electromagnetic constants, its curvature, its vanishing mass,
its interaction with matter through curvature and through its influence on decay, etc. (A
table of the properties of a physical vacuum is given on page 133.) Historically, it took a
long time to clarify the distinction between a physical vacuum and a philosophical void.
People confused the two concepts and debated the existence of the vacuum for more
than two thousand years.The first to state that it existed, with the courage to try to look
through the logical contradiction at the underlying physical reality, were Leucippus and
Democritus, the most daring thinkers of antiquity.Their speculations in turn elicited the
reactionary response of Aristotle, who rejected the concept of vacuum. Aristotle and his
disciples propagated the belief about nature’shorror of the vacuum.
The discussion changed completely in the seventeenth century, when the first experimentalmethodtorealizeavacuum
was devised by Torricelli.* Using mercury in a glass
tube, he produced the first laboratory vacuum. Can you guess how? Arguments against
the existence of the vacuum again appeared around 1900, when it was argued that light
needed ‘aether’ for its propagation, using almost the same arguments that had been used
two hundred years earlier, but in different words. However, experiments failed to detect
any of the supposed properties of this unclearly defined concept. Experiments in the field
of general relativity showed that a vacuum can move – though in a completely different
way from the way in which the aether was expected to move – that the vacuum can be
* Evangelista Torricelli (b. 1608 Faenza, d. 1647Florence), physicist, pupil and successor to Galileo. The
(non-SI)pressureunit ‘torr’ isnamed after him.
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net