“There
was no absolute standard of rest. There exists a relativity principle that
applies to all of physics, but it's not Galilean relativity. Laws of mechanics
must be revised. Perhaps laws of electrodynamics can survive intact.”
In the year 1905, Albert Einstein published the theory
of special relativity.
Einstein explained that when two objects are moving at a
constant speed, relative motion between
the two objects was essential, instead of appealing to the ether as an absolute
frame of reference that defined what was going on. For instance, if you and
some astronaut, Amber, are moving in different spaceships and want to compare
your observations, all that matters is how fast you and Amber are moving with
respect to each other and nothing more.
What Einstein
does is to extend the concept of the old
Galilean relativity to include light too. He asserts that when this be done the
laws of mechanics has to be modified. We must note here, that this holds only
as long as the system being considered is moving with relative velocity. We
thus have the special case (hence the name Special Relativity) .
The motion it explains is only if you’re travelling in a
straight line at a constant speed. As soon as you accelerate or curve — or do
anything that changes the nature of the motion in any way — special relativity
ceases to apply. That’s where Einstein’s general theory of relativity comes in,
because it can explain the general case of any sort of motion.
My apologies that, I presented the notes on basics of General Theory of Relativity first.
Anyway, Einstein’s theory has two key postulates:
The principle of relativity:
“The laws by which the
states of physical systems undergo change are not affected, whether these
changes of state be referred to the one or the other of two systems in uniform
translatory motion relative to each other.”
The principle of the speed of light:
· “As measured in any inertial frame of
reference, light is always propagated in empty space with a definite velocity c that
is independent of the state of motion of the emitting body.”
The genius of Einstein’s discoveries is
that he looked at the experiments and assumed the findings were true. This was
the exact opposite of what other physicists seemed to be doing. Instead of
assuming the theory was correct and that the experiments failed, he assumed
that the experiments were correct and the theory had failed.
The belief in ether had caused a mess of
things. In the latter part of the 19th century, physicists were searching for
the mysterious thing called ether — the medium they believed existed for
light waves to wave through. However, Einstein just removed the ether entirely
and assumed that the laws of physics, including the speed of light, worked
the same regardless of how you were moving and that was the genius of the Einstein.
In the next notes we will be discussion on the concept of the LorentzTransformation.
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