Relativity
is a term almost guaranteed to bring a blank look to the face of most
of the population. It also brings a certain uneasiness to many
scientists. Something that is acknowledged and respected, yet with an
air of foreboding, a foreboding that comes from a lack of confidence
in our understanding of it.
Why?
Just
what is this thing called Relativity?
It
has the reputation of being mathematically complex, and esoteric.
That it can only be understood in the abstract realms at the
boundaries of science. Despite this, it is in fact very simple.
Imagine
two passengers sitting in trains on opposing platforms in a station.
Each will see the other as stationary. Until one of the trains starts
to move. Then, for a moment, each passenger is convinced that his
train is moving. He automatically assumes that what is outside the
window is stationary. Only the one train is moving relative
to the railway track; yet each train, and the observer within it, is
moving relative
to the other train.
Take
this a step further and imagine two trains passing one another.
Observers seated on those trains will each measure the same speed for
the other train, relative to themselves for each observer will deem
their own train stationary and the other to have all the movement.
In
fact we can expand this to say: If, relative to system K,
K'
is a uniformly moving co-ordinate system devoid of rotation, then
natural phenomena run their course with respect to K'
according to exactly the same general laws as with respect to K.
This statement is called the principle
of relativity
(in the restricted sense).
(The
secret is to imagine one train is stationary, taking all measurements
relative to that train. The other train is then moving with velocity
v relative to the stationary train.
Yet
in the same way, if we were to take the second train as stationary
then the first train would no longer be stationary but would be
travelling with velocity -v with respect to the now stationary second
train.
And
it this been with us from the distant realms of history, before being
defined in those terms by Galileo. First when man learned to sail
the seas, using maps to Navigate by and reckoning in the winds and
currents obtaining in the seas. Then similarly in the days of
aviation, accounting for winds and weather systems, right up to the
modern day, when the need to account for the velocity of the orbiting
satellites used in GPS navigation brought new challenges to be
catered for. Challenges predicted by the genius of Einstein and his
follow pioneers.
In
one chapter of his little book, Einstein described: 'the
theorem of the addition of velocities employed in Classical
Mechanics'.
In it he stated that a man walking with velocity w along a train
travelling at velocity v, would be travelling with velocity, v + w,
relative to the track. He termed this the Galileian Transformation.
Essentially,
relativity is about how the locations and measurements from one
observer's perspective are transformed to become those of another
observer, moving with respect to the first.
At
low speeds, simple Relativity, where Relative measurements can be
calculated using simple addition and subtraction, Gallilei
Transformations, is all we need. Such calculations being sufficiently
accurate for all practical purposes. But Einstein went on to point
out, that using light shining along the railway track instead of the
man, the speed of the light relative to the train would be c ±
v. This, of course, is contrary to the many experimental
measurements that have all shown that the speed is in fact 'c'.
Thus
the need for a new theory of relativity was born and that new theory
was Einsteins Theory of Special Relativity.
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