Page 23 - GALIET PHYSICS BLOSSOMS III

Basics of GR23
• Special Relativity posits spacetime where the three dimensions of space and the
one dimension of time form a unified four-dimensional space.
• GR includes gravity because mass-energy shapes spacetime’s fabric.
o We must think of spacetime as a rubber fabric or as a trampoline. Heavier weights distort the taut 2D rubber fabric more, just as heavy mass-energies distort spacetime more (even though we can’t really visualize distortions in spacetime!).
• Mass-energy tends to warp or curve spacetime in its vicinity generating a gravity field. Planets and stars and light beams and waves react to this curvature, changing their pathways. In this way, these curvatures determine how other objects move through spacetime. In GR, gravity is no longer a force that acts at a distance in Newton’s sense, but a part of spacetime curvature. Simply, objects move as they do and they are deflected as they are because they follow the spacetime curvature determined by the object’s mass-energy size. Succintly, “spacetime tells matter how to move (gravity) and matter (energy) tells spacetime how to curve” (Wheeler).
• In gravitational fields, time runs slowly. When gravity is stronger, time runs more slowly.
• Black holes can exist in spacetime because curvatures and wells increase that much more when objects are compressed (that is, the more an object is compressed, the more curvature). These curvatures and wells can be so immense that they can create an infinite singularity or a black hole capable of relentlessly pulling all things to its bottomless pit, thus swallowing all matter and things nearby, even light. If one were to fall into it, one would perish and cease to observe the universe.
• The universe is boundless and without a center, yet it may have finite volume.
• Large mass-energies that undergo rapid changes in their motions and/or
structures emit gravitational waves travelling at the speed of light.
23 From Dr. Stamp’s class notes, lectures and from Bennet, Donahue, Schneider, Voit. The Cosmic Perspective. 6th Edition. What are the major ideas of General Relativity. 428-429.
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