Physics 101 Introduction to Mechanics 

February 15 Gravity  



Einstein's
theory of general relativity and curved spacetime
A dramatic example of noninertial frames can been seen in accelerating rockets. When a rocket accelerates, an occupant
feels "gravity".
Consider a rocket with a small "window" in one side
and a major league pitcher with amazing accuracy.
An outside observer sees a horizontally
thrown ball travel in a straight line. Since the ship is moving upward
while the ball travels horizontally, the ball strikes the wall somewhat
below a point opposite the window.
To an inside observer, the path of the
ball bends as if in a gravitational field.
From this thought experiment we realize that we cannot
tell the difference between an accelerating frame and one that is at rest
and has a gravitational pull. This is the "equivalence principle"
and is the corrnerstone to Einstein's theory.
Now replace the ball with a light beam. Again, to the observer in the rocket it will apear to have it's path "bent." This tells us that gravity "bends" light beams.
Stars, including our sun are massive enough to bend light from distant
stars.
But, wait a minute... doesn't light travel in a stright
line? How can this be?
Perhaps space is somehow "curved" If this is
the case then light still travels in a stright line, but now the idea of
"striaght" has to change since our surface is no longer flat. Think
about how we define parallel lines on a flat surface and one a sphere.
Einstein's theory of special
relativity talks about how space and time are connected. This
theory deals with objects and reference frames moving at constant speeds.
His theory of general realtivity deals with accelerating objects (which
includes gravity). The mathematics of special relativity isn't too
bad, but the mathematics of general realtivity is a matter for a graduate
level course. Both theories are well establish experimentally.


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