relative
to the ether, so any moving observer would see light travelling
faster or slower than
depending
on whether the were moving in the same direction or not not as the
light.
A Level Physics Notes: Special and General Relativity – The Need for Special Relativity
At the end of the nineteenth century, all the important questions
of science seemed to have been answered. The were only a few puzzles
and anomalies that did not fit into the scheme. The Michelson –
Morley experiment for example, showed that the speed of light is the
same in all inertial frames. This contradicts the assumption that
speeds in different inertial frames are related by the Galilean
transformation. In the nineteenth century, scientists proposed the
existence of a substance called the ether, which pervaded all of
space and acted as a medium for the transmission of light in the same
way that air acts as a medium for the transmission of sound.
Scientists thought that light would travel at a speedrelative
to the ether, so any moving observer would see light travelling
faster or slower thandepending
on whether the were moving in the same direction or not not as the
light.
The speed of light was also predicted to be a constant because of
physics consideration. The value ofarises
naturally out of Maxwell's equations. From them we can construct a
wave equation for electromagnetic waves:
The speed of lightis
constructed from known physics constant
and
constants
of electromagnetism, which do not vary at all, implying thatshould
not vary. In fact
Another problem arose out of the Lorentz force law,
This
supposed law of physics contained a velocity term
The
Galilean transformation implied the same acceleration and the same
force in different inertial frames. The velocity of an object is not
the same in different inertial frames, hence by applying the Galilean
transformation to both sides we obtain a contradiction, since the
right hand side will be changed but the left hand side will not.
Either the Principle of Relativity – that the Laws of Physics can
be written in the same form in all inertial frames – or the
Galilean transformation must be modified or discarded.
The Galilean transformation was in fact modified, and became the Lorentz transformation. The Principle of Relativity survived unscathed, and by surviving, became much more of a bedrock of physics. Becoming also one of the two postulates of the theory.