[GRLUG] GRLUG meetings

[Godwin]

...hmm, I wasn't aware of the "top post" issue.   I bet this is
worse...(deleting the whole email and leaving just my response)..
Haaaa!!!!

G-

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Special relativity

    Main article: Special relativity

Special relativity is a theory of the structure of spacetime. It was
introduced in Albert Einstein's 1905 paper "On the Electrodynamics of
Moving Bodies". Special relativity is based on two postulates which
are contradictory in classical mechanics:

   1. The laws of physics are the same for all observers in uniform
motion relative to one another (Galileo's principle of relativity),
   2. The speed of light in a vacuum is the same for all observers,
regardless of their relative motion or of the motion of the source of
the light.

The resultant theory has many surprising consequences. Some of these are:

    * Time dilation: Moving clocks tick slower than an observer's
"stationary" clock.
    * Length contraction: Objects are observed to be shortened in the
direction that they are moving with respect to the observer.
    * Relativity of simultaneity: two events that appear simultaneous
to an observer A will not be simultaneous to an observer B if B is
moving with respect to A.
    * Mass-energy equivalence: per the relationship E = mc², energy
and mass are equivalent and transmutable.

The defining feature of special relativity is the replacement of the
Galilean transformations of classical mechanics by the Lorentz
transformations. (See Maxwell's equations of electromagnetism and
introduction to special relativity).

[edit] General relativity

    Main article: General relativity

General relativity is a theory of gravitation developed by Einstein in
the years 1907–1915.

The development of general relativity began with the equivalence
principle, under which the states of accelerated motion and being at
rest in a gravitational field (for example when standing on the
surface of the Earth) are physically identical. The upshot of this is
that free fall is inertial motion: In other words an object in free
fall is falling because that is how objects move when there is no
force being exerted on them, instead of this being due to the force of
gravity as is the case in classical mechanics. This is incompatible
with classical mechanics and special relativity because in those
theories intertially moving objects cannot accelerate with respect to
each other, but objects in free fall do so. To resolve this difficulty
Einstein first proposed that spacetime is curved. In 1915, he devised
the Einstein field equations which relate the curvature of spacetime
with the mass, energy, and momentum within it.

Some of the consequences of general relativity are:

    * Time goes slower at lower gravitational potentials. This is
called gravitational time dilation.
    * Orbits precess in a way unexpected in Newton's theory of
gravity. (This has been observed in the orbit of Mercury and in binary
pulsars).
    * Even rays of light (which are weightless) bend in the presence
of a gravitational field.
    * The Universe is expanding, and the far parts of it are moving
away from us faster than the speed of light. This does not contradict
the theory of special relativity, since it is space itself that is
expanding.
    * Frame-dragging, in which a rotating mass "drags along" the space
time around it.

Technically, general relativity is a metric theory of gravitation
whose defining feature is its use of the Einstein field equations. The
solutions of the field equations are metric tensors which define the
topology of the spacetime and how objects move intertially.
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