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Albert Einstein
A New Vision of the
Cosmos
Einstein’s Vision
• Space + Time = Spacetime
• Four dimensions: 3 of space
and 1 of time
• Cosmos has only these 4
dimensions
• Must know where and when
Natural Motion
• Newton: Natural motion is
constant speed along a straight
line (constant velocity, no
acceleration)
• Straight line: Shortest distance
between two points
• Assumed: A flat (Euclidean)
geometry
Einstein
• NO assumption about local or
global geometry of cosmos
(flat or curved)
• Options: flat (Euclidean), open
(saddle), closed (sphere), in
four dimensions (spacetime)
• Natural motion: Freefall
(weightlessness)
General Theory of Relativity
• Gravity not a force! Natural
motion in a curved spacetime.
• Objects follow straight
(4-dimensional!) lines in
spacetime
• “Matter tells space how to
curve, and space tells matter
how to move.”
Gravity-Two Views
• Newton: Forces
(interactions at a distance)
• Einstein: Courses
(local paths in spacetime)
–Predicts bending of light
(eclipses, gravitational lenses),
expansion of universe; used in
GPS satellites
General Theory Cosmology
• Physics of cosmological models
• Explains past, predicts future
–Big Bang (any geometry)
–Big Bore (flat or open)
–Big Crunch (closed)
• Compare actual density to
critical density (open, flat ≤
critical; closed > critical)