Relativity
One of the guiding principles of physics since Galileo's
time, the principle of relativity states that all inertial frames
of reference are equally valid. In other words, an observation
made by one person should be the same when made by a different
person, and what is valid science here is valid science there.
This fundamental principle was called into question
by Maxwell's equations, which implied a constant speed of light.
This was taken to be an indication that all light must therefore
be a wave in some stationary medium, dubbed "ether"
by Maxwell. Continuing this chain of logic, it followed that the
frame of reference of the ether was "privileged," being
fixed with respect to the speed of light, while other reference
frames were not.
Thankfully for the foundations of physics, the Michelson-Morley
experiment failed to detect ether. While other scientists were
unable to reconcile this result with the constant speed of light,
Einstein took it as evidence of the validity of the principle
of relativity.
He was able to explain the observed constant speed of light by
using the Lorentz transformations which modified definitions of
"observed" velocity, length and time to explain why the
ether was not detected. Unlike Lorentz however, Einstein did away
with the concept of ether entirely, instead assuming the observed
measurements to be the actual measurements and thus taking the daring
step of assuming Newtonian physics was wrong. By Einstein's postulates,
addition of velocity was no longer as simple as adding the vectors,
but now required a complexnonlinear formula. In addition, some of
the consequences of the Lorentz equations implied that an object
shrinks in length when traveling at high speeds, that moving clocks
slow down, that simultaneity at non-coincident points is impossible
to determine, and that nothing can travel faster than the speed
of light.
However counterintuitive, Einstein's leap of intuition
proved to be correct: later observations were able to detect the
tiny changes predicted at high velocities. Both Maxwell's Equations
and the principle of relativity turned out to hold true, leading
us to a more complex, but contradiction-free universe.
Next article:
Quantum weirdness, it only gets stranger... 
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