S3 general relativity Flashcards
What are the major ideas of general relativity?
General relativity tells us that gravity arises from the curvature of spacetime and that the curvature arises from the presence of masses. This idea leads us to a view of gravity in which time runs more slowly in gravitational fields, black holes can exist in spacetime, and the universe has no center or edges. It also predicts the existence of gravitational waves propagating through space.
What is the fundamental assumption of general relativity?
The starting point for general relativity is the equivalence principle, which states that the effects of gravity are exactly equivalent to the effects of acceleration.
What is spacetime?
Spacetime is the four-dimensional combination of space and time that forms the “fabric” of our universe.
What is curved spacetime?
Spacetime can be curved much like a rubber sheet but in more dimensions. We can recognize spacetime curvature from the rules of geometry. The three possible geometries are a flat geometry, in which the ordinary laws of flat (Euclidean) geometry apply; a spherical geometry, in which lines that start out parallel tend to converge; and a saddle-shaped geometry, in which lines that start out parallel tend to diverge.
What is gravity?
Gravity arises from the curvature of spacetime. Mass causes spacetime to curve, and the curvature of spacetime determines the paths of freely moving masses.
What is a black hole?
A black hole is a place where spacetime is curved so much that it essentially forms a bottomless pit, making it like a hole in spacetime.
How do we test the predictions of the general theory of relativity?
Observations of the precession of Mercury’s orbit match the precession predicted by Einstein’s theory. Observations of stars during eclipses and photos of gravitational lensing provide spectacular confirmation of the idea that light can travel curved paths through space. Gravitational redshifts observed in the light of objects with strong gravity confirm the slowing of time predicted by general relativity, a prediction that has also been confirmed with clocks at different altitudes on Earth.
What are gravitational waves?
General relativity predicts, and observations confirm, that accelerating masses produce gravitational waves that travel at the speed of light. Indirect evidence for gravitational waves comes from observing the orbital decay of binary neutron stars. Direct detection has come from the instrument called LIGO, which has detected pulses of gravitational waves produced when orbiting neutron stars or black holes ultimately merge.
Where does science end and science fiction begin?
No known physical laws prevent hyperspace, wormholes, or warp drive from offering “loopholes” that could allow us to get from one place to another in less time than we could by traveling through ordinary space. However, if any one of them prove to be real, then cause and effect might not be absolute, a proposition troubling to many scientists.
How has relativity changed our view of space and time?
Prior to Einstein, space and time were viewed as separate and distinct. We now know that they are deeply intertwined as spacetime and that understanding spacetime is crucial to understanding many aspects of astronomy, including black holes and the overall geometry of the universe.
