General Relativity

A few years after publishing his first work on special relativity, Einstein wanted to apply his theory
more broadly; to do so he turned his attention to gravity.  Between 1905 and 1914 Einstein
attempted to unify his theory of special relativity with the classical concepts about gravity, but to do
so he had to create a more complete theory of gravity than Isaac Newton had.  The formulas and
laws that Newton derived quite elegantly described gravity and allowed physicists to predict very
precisely how different objects will fall, orbit and attract one another. However, Newton’s formulas
failed to describe how the force operates at a more fundamental level.  By introducing his theory of
general relativity Einstein showed the world that there was more to gravity than was thought, and
he also allowed physicists to increase the precision of the predictions that they could make about
gravitational effects.

Newton knew that every massive body has a gravitational “field”, and that the strength of a field
increases with the mass of the body.  He also helped physicists understand that the strength of
attraction between two bodies increased with proximity.  But Newton did not try to explain the
force in terms of particles or waves- quite honestly, he didn’t know how gravity created its effects.  
Einstein, however, proposed something new and very exciting.  It was his insight that the force of
gravity actually warps space.  Before Einstein time was thought to be a universal constant, but
special relativity changed that- similarly the theory of general relativity showed us that space is not
immutable either.

Any object that is not acted on by a force will travel in a straight line, and in flat space the shortest
path between two points is a straight line.  But in curved space the shortest path between two
points is not a straight line- this path is called a geodesic.  So the question arises: “do planets
travel in straight or curved orbits?”  Well, a physicist would hold that planets are traveling in a
curved path relative to our 3 dimensional coordinate system, yet are traveling in a straight line
relative to the curved space that surrounds them.  This may sound counter intuitive but Einstein’s
insight allowed us to make more precise predictions about the paths of massive bodies and this
lead to improved navigational systems and the creation of global positioning systems, among
other things.  

Einstein also put a speed limit on the force of gravity.  Previously Newton had believed that the
effects of gravity were instantaneous- that it took no time for gravitational waves to propagate
through space.  To use a thought experiment- Newton thought that if the sun disappeared then the
planets would immediately discontinue their curved paths (relative to our coordinate system) and
continue in straight lines away from the spot where the sun once was (like a lasso that is released
after being swung around a cowboy’s head).  But Einstein showed that the force of gravity travels
at the same speed as light, therefore, because it takes 8 minutes for light to get to us from the sun,
it would take 8 minutes for the sun’s gravitational, attractive force on our planet to cease.  In other
words if the sun disappeared, it would take 8 minutes for the spacetime continuum to flatten out.  
Because the planet Saturn is further from the sun than the earth is, it would take more than 8
minutes for this flattening effect to reach it- and thus it would continue to orbit around a sun that isn’
t there for a few minutes longer than the earth would.

Using physical formulas, math and originality Einstein developed an entirely new concept of
gravity.  His new theory suggested that the attractive forces of gravity can be thought of as
curvature in space and time.  This echoed his conceptions about the effects of motion on
spacetime that were laid out in his theory of special relativity.  It is interesting to note that even
though the special and general theories of relativity work well together, the general theory does not
reconcile with our theories about quantum physics.  A grand unified theory that incorporates
general relativity along with quantum physics is needed to unite these dissonant fields.  After 1915
Albert spent the rest of his life searching for a grand unified theory, and even today the same
search is taking place around the world.

Dissonant: adj.
Inharmonious or discordant.  Disagreeing or producing dissonance.

Geodesic: noun
The shortest path between two separate points on any mathematically defined surface.  When the
path between two points is not a straight line, it is a geodesic.

Spacetime: noun
The four dimensional continuum (three dimensions of space and one of time) in which physical
events take place.  A four dimensional coordinate system.