Einstein's Relativity
Distance and time has a very close relationship, the main link being the
speed of light. Distance is measured in the unit of the metre, which itself
is based upon the distance light travels in a unit of time. This section
will deal with this simple relationship and the consequences if ideas first
put forward by Albert Einstein in 1905.
The Constancy of the speed of light
The basis of Einstein's ideas is that the speed of light (in empty space)
is a constant for all observers. However fast you travel, light still shoots
away from you - or towards you- at the same speed of 2.99792458 x 108
ms-1. You can never ever gain on the light wave in front. Light
does not obey the ordinary laws of relative motion. This led Einstein to
call his theory the principle of relativity.
The consequence of the principle is that distances,
time intervals and the time at which an event occurs will be different
for observers who move at a steady velocity with respect to each other.
In time dilation, the processes take longer to happen
in objects that are moviing relative to us. The effect is most dramatic
at relative speeds close to the speed of light. Time dilation leads to
other effects: objects shrink in the direction of travel -- the Lorentz
contraction - and their mass increase. From these ideas, comes the formula
: E= mc2 .
Einstein's Theory
Albert Einstein produced the theory that explained the null results of
the Michelson- Morley experiment. The Lorentz - FritGerald theory was seriously
flawed since it was invented to explain just one effect. Einstein produced
a formula for contraction with a simpler assumption about the constancy
of light.
Einstein took account of the nature of light and
the fact that it was an electromagnetic effect. His theory was based on
two simpe assumptions. The first was that physical laws - mechanical, optical,
electromagnetic - are the same in all uniformly moving frames of reference.
The second is that speed of light in a vacuum is the same for all observers,
in all uniformly moving frames of reference.
General Relativity
Spacetime grips mass, telling it how to move: mass grips spacetime,
telling it how to curve.
This single sentence carries the seeds of general
relativity. It began with Einstein thinking of the simple relative situation
in which he imagined being in a closed box, like a lift. He would feel
a force at his feet, interpreted as "weight". When he dropped a pen it
would fall to the floor with contant acceleration. From experiments inside
the lift, no one could tell if these effects were causerd by a downward
all-pervading force or if the box was being accelerated upwards by an unknown
agent. He then applied his theory of special relativity to this situation
and borrowed some advanced geometry to extend it to the Universe at large.
First, Einstein abolished gravity as a 'force that
acts through space'. He replaced it with geometry - a simple consequence
of curvature of spacetime. Einstein showed that a lump of matter caused
the spacetime around it to be curved.Two masses travelling near each other,
such as the Earth and the moon, will move in 'straight lines through spacetime'
- but we see them as moving in curved paths. The earth has more mass, so
its spacetime curving effect is greater. Both bodies orbit around the same
point, but the moon orbits in a larger, less curved path. The geometry
works and the theory predicts the orbits of planets more accurately than
Newton's theory of gravitation. The elliptical orbit pf the planet Mercury
swings around the sun in a way that only Einstein's theoty could explain.
The general theory of relativity is the basis of large scale cosmological
theory-- including the black hole.
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