International Relations. A self-Study Guide to Theory

physics can only be alternatives (wave or particle), can actually be con-
sistent, complementary aspects of reality: wave and particle. This property, 
the dual nature of matter and light, was difficult to understand; how can 
something be both particle and wave at the same time? (Capra 2012: 66). 
Ultimately, quantum theory and the formulation of the new laws of quantum 
mechanics solved this perceived contradiction (Capra 2012: 66). These laws 
will be briefly described in the next step. Nevertheless, please be aware that 
it might be difficult to follow this description if you are not familiar with the 
paradigms on which contemporary physics is built. There will therefore be 
recommended reading at the end of the unit that – depending on your famili-
arity with physics – you might already begin to consult while reading this 
text. 
As mentioned above, it was Max Planck who paved the way in quantum 
physics with his findings that thermal energy is not emitted continuously but 
rather in “energy packages” called quanta (Capra 2012: 66). Based on ex-
tended versions of Young’s double slit experiment, in 1926 and 1927 Erwin 
Schroedinger, Max Born, Niels Bohr and Werner Heisenberg formulated the 
basic laws of quantum theory. They can be briefly summarized as follows 
(drawing on Bedenig 2011: 161-178): 
In his famous Schroedinger equation, Erwin Schroedinger, an Austrian 
physicist and Nobel laureate for physics in 1933, described the development 
of the quantum state of a physical system (that is for example, atoms, mole-
cules, sub-atomic particles) over time when there is no measurement and no 
experiment. The Schroedinger equation is equal to Newton’s second law for 
the motion of a mechanical system in classical mechanics. However, in quan-
tum mechanics, it is a linear partial differential equation that describes a 
wave function of the physical systems (that is of atoms, molecules and sub-
atomic particles as well as of macroscopic systems/the universe).
This quantum mechanical wave function was formulated by Max Born, a 
German mathematician and physicist and a 1954 Nobel laureate, as a statisti-

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94 
cal interpretation. It is known as the Born rule or Probabilistic Interpreta-
tion. This function is a law of quantum mechanics and describes the proba-
bility with which the measurement of a quantum system will bring about a 
specific result. In other words, the descriptions of nature, of physical objects, 
and of the laws of nature are interpreted as essentially probabilistic (and not 
deterministic, as in classical mechanics). 
Then Niels Bohr, a Danish physicist and a 1922 Nobel laureate, formulat-
ed his famous Complementarity Principle. Another law of quantum mechan-
ics, it demonstrated the principle of fundamental complementary aspects of 
physical phenomena. One example is the previously mentioned “particle and 
wave” aspect of physical objects, emissions and matter: “particle” and 
“wave” are two complementary phenomena (and not alternatives). However, 
experimental measurement can only demonstrate either one or the other as-
pect, but never both phenomena in the same measurement process. The 
demonstration of one aspect necessarily precludes the other. They cannot be 
simultaneously measured, even though there are always two aspects of phe-
nomena in one and the same process. As complementary aspects, they belong 
together and form a “whole”. Nevertheless, which aspect will be measured 
depends on the observational practices of the experiment.
With the principle of complementarity, paradoxes have thus been shown 
to be a feature of reality. Each phenomenon has an “other side”, an aspect 
contrary to the one being currently observed. Observation thus never results 
in “facts”, but only in aspects of perception, aspects of reality. 
In addition, Niels Bohr demonstrated that the measured properties of a 
physical object are affected by the measurement process, by the process of 
experimental observation. That is, objects governed by quantum mechanics 
do not have intrinsic properties that are independent of the process of meas-
urement. There are no “given” properties of objects independent of the ob-
server or the observation. The measurement process itself has an impact on 
what is measured. This effect was discovered through extended double-slit-
experiments, which were constructed to find out what happens when a wave 
passes the double slit. Normally, it would result in the classical interference 
patterns due to the nature of waves, for example of light. However, the ex-
tended Young experiments proved that when observed, the wave appeared as 
a particle when passing the double slit. In other words, under observation, 
there were no interference patterns. Without observation, however, the classi-
cal interference pattern of the wave-nature appeared. This might sound 
“spooky” to you. I therefore recommend that you do your own search on the 
extended double slit experiment and its interpretation. Knapp (2011: 65-79) 
provides a good and readable overview. 

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95 
Werner Heisenberg, a German physicist and 1932 Nobel laureate, found 
out that uncertainty always exists when measuring quantum systems. This 
uncertainty is a fundamental limit to the precision with which pairs of physi-
cal properties of particles (such as position and momentum) can be known 
simultaneously. Heisenberg’s discovery is well-known as the Heisenberg 
Uncertainty Principle. For example, the more precisely the position of a par-
ticle can be determined in an experiment, the less precisely the particle’s 
momentum can be known and vice versa. This observation applies to other 
pairs of physical properties, too. Hence, uncertainty is an important aspect of 
the property of matter, due to the wave-nature of physical objects. “Uncer-
tainty” is therefore a statement about a fundamental property of a quantum 
system (not a statement about defective measurement devices), a principle not 
compatible with the deterministic view of classical physics.
The laws of quantum mechanics briefly described above form the basis of 
the famous Copenhagen interpretation of 1927 – an attempt to explain the 
mathematical formulations of quantum mechanics and the experimental re-
sults (see Heisenberg 1977: 28-40).
All laws of quantum physics have been experimentally proved. Among 
these experimental proofs, the 1981 experiments of John Bell deserve closer 
attention, as they highlight another aspect of quantum phenomena that is im-
portant for our discussion of the transition of the Cartesian-Newtonian world 
view. Bell provided the experimental proof of quantum entanglement (a term 
from the German physicist Schoedinger; in German, Verschränkung). He 
demonstrated that a fundamental connectedness of interacting particles such 
as photons, protons, or molecules exists even when those particles are subse-
quently separated. The two or more particles in question stay entangled; they 
cannot be described as single particles with clearly defined properties, but on-
ly as a whole system. One object cannot therefore be fully described without 
considering the other. For instance, this entanglement occurs in atomic decay, 
where pairs of particles can be generated in one and the same process. Addi-
tionally, physicists have proved that the measurement of the properties of one