  Theory of Relativity (Special and General) is one of the most influential
theories of the 20th century and has changed the way we view the world. It is
part of many undergraduate curriculums and it is often suggested that it should
be integrated into an upper secondary curriculum. Special Theory of Relativity
combines time and space whereas General Theory of Relativity describes gravity
as a geometric property of spacetime. As it describes abstract phenomena,
students encounter several difficulties understanding its basic principles and
consequences. In this paper, we present the research that we conducted in order
to detect the aforementioned difficulties. This research constitutes a part of
a more general study concerning the integration of Special and General
Relativity into an upper secondary and undergraduate curriculum. The sample
consisted of 45 non-major physics undergraduate students. The purpose of our
study was to determine and categorize the difficulties students face when they
study the principles of the Theory of Relativity and its consequences. The
results of our research indicate that student face many obstacles when trying
to interpret phenomena described by the Relativity and confuse Special and
General Relativity principles. These results dictate us to create an
educational approach that tackle the difficulties found.
