International Relations. A self-Study Guide to Theory

particle and a wave (see the overview in Bedenig 2011: 129-134). There are 
phenomena that can only be explained by the particle-nature of light and oth-
er phenomena that can only be explained by the wave-nature of light. Thom-
as Young, Michael Faraday and James C. Maxwell paved the way for these 
ground-breaking insights, starting with the double-slit-experiment by Thomas 
Young in 1804 in which he first demonstrated the wave-nature of light 
through interference patterns. The wave-nature of light could explain phe-
nomena that were not compatible with the corpuscle theory of light as formu-
lated by Newton. Young’s experiment later played an important role in the 
development of quantum mechanics. Faraday, with his work on light as a 
wave-movement in an electromagnetic field (1846) and Maxwell with his 
famous Maxwell equations (1861-1864) were additional forerunners of dis-
coveries to come. The Maxwell equations provided the mathematical descrip-
tion for Faraday’s electromagnetic field and thus the proof that light is a phe-
nomenon of electromagnetic waves. In 1900, Max Planck then demonstrated 
that light is not disseminated in continual waves but rather in “energy pack-
ages” called “quanta” (later “photons”). Max Planck is therefore seen today 
as the father of quantum theory. In 1919, he was awarded the Nobel Prize in 
physics for his discovery of Planck’s constant. 
In addition to his demonstration of the dual nature of light, Einstein is fa-
mous for showing that the speed of light is a universally valid constant, inde-
pendent of the state of movement of the observer: nothing can move faster 
than light. 
Time, space, matter and energy 
However, Einstein is above all well-known for his revolutionary ideas about 
time and space. Before Einstein, classical physics/classical mechanics ac-
cepted time and space as absolute – they existed everywhere in the universe 
and independently of the observer. For example, this belief was reflected in 
Kant’s philosophical thought about space and time as absolute categories of 
human thought that are “a priori” given, not to be questioned and with no 
need to be explained; they are axioms. With his special theory of relativity, 
Einstein was able to show that space and time are not absolute, but flexible 

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and relative. Beyond that, space and time are inextricably linked with each 
other, forming spacetime. The new physical laws formulated by Einstein in 
his special theory of relativity thus replaced the ideas held by Newton and 
classical mechanics that an absolute time and an absolute space exist in 
which all physical phenomena happen (Bedenig 2011: 137-148). 
The incompatibility of Einstein’s special theory of relativity with New-
ton’s law of gravitation provided a driving force for additional insights in the 
field (see the overview in Bedenig 2011: 150-157). Einstein tried to extend 
the framework of his special theory of relativity to include gravitation. This 
forms the core of his general theory of relativity (1915): the idea that the 
same linkage as exists between space and time also holds true for matter and 
spacetime. The two are inextricably linked to each other and this interde-
pendency is called gravitational force. Einstein demonstrated that gravita-
tional force causes the curvature of spacetime. Curved spacetime is due to the 
distribution of matter within spacetime. The idea of curved spacetime was 
important in that it replaced the old ideas of geometry that dated back to Eu-
clid’s Elements and which had been formulated for a homogenous, not 
curved space. As proven by Einstein, Euclid’s geometry was no longer valid 
for curved space. However, it had been accepted as the truth for 2500 years!
Consequently, Einstein replaced Newton’s s laws of gravitation with his