 
Summary: ASTR 1120 Section 1 (3 credit hours): Spring 2006
SUMMARY OF KEY CONCEPTS: WEEK #5
Lecture #9 textbook Chapter S2 `Space and Time'
We began discussing the Theory of Relativity, which was developed in two parts. The Special
Theory of Relativity describes how space and time, which we normally perceive to be completely
different concepts, are actually closely linked. The Special theory does not include gravity. The
General Theory of Relativity goes much further, and explains how mass curves spacetime.
Special relativity is based on two concepts:
1) The laws of physics are the same in all inertial frames (an inertial frame is one in which
the experimenter is not subject to any external forces, such as gravity). In particular, this
means that there is no preferred frame as far as velocity is concerned observers moving
at different velocities relative to each other are in equally valid positions.
2) The speed of light is the same constant when measured in any inertial frame, regardless
of how that frame is moving relative to the source of the light. This is very weird, but it's
an experimental fact!
We showed that these concepts imply some strange behavior moving clocks appear to run
slow (we actually derived this one) and moving objects appear to be length contracted. The
effects are only significant once we are moving close to the speed of light, hence we don't notice
them in everyday life.
We outlined the twin paradox suppose one twin flies in a superadvanced spaceship to a
