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• General Relativity is a physics theory invented by Albert Einstein.

• General relativity is just a fancy 20th-century name for gravitythe force that pulls

• stuff to the ground and keeps the planets in orbit around the sun .

• General relativity is the idea that gravity happens because space is curvedlike how

• when you walk along the surface of a ball, you end up curving downwards.

• Actually general relativity is the idea that space and time aren't separatetime

• is a physical dimension, and together they form a single geometry called spacetimeand

• gravity is caused by curvature in spacetime.

• Except, curvature in spacetime on its own doesn't explain gravity – I mean, just

• because you're on a curved ball that doesn't mean you follow a specific path on the ball.

• General relativity is the combination of the ideas that spacetime is curved AND that stuff

• in spacetime obeys laws of motion : an object in motion stays in motion along a “straight

• path in curved spacetime . Like following a straight line along the surface of a ball.

• But spacetime can't be curved any which way.

• Just like how a ball looks flat when you're close enough, curved spacetime is locally

• flat (rather than crumpled, or something).

• Of course, flat space makes sense, but what does it mean for spacetime to be flat?

• That it obeys the rules of special relativity!

• Finite speed of light, time dilation and length contraction, relative addition of velocities,

• and all that.

• Basically, if general relativity is like a globe, special relativity is being on the

• surface of the globe (and mathematicians call the globe a pseudo-Riemannian manifold with

• Lorentzian signature).

• Except we still haven't said anything about what determines the curviness, or shape, of

• spacetime in the first place!

• Or why objects tend to follow straight paths in that spacetime.

• General Relativity is actually the idea that all the stuff in spacetimematter, radiation,

• pressure, energy, momentum, particles, and so onall that, together with spacetime

• itself, obeys a set of equations called the Einstein Field Equations.

• These equations look simple if you write them in a clever way, but they're actually a

• very complicated set of ten nonlinear differential equations [2nd order] that you have to solve

• in order to make predictions about how spacetime will curve and how the stuff in it will move,

• depending on how spacetime is curving and how the stuff is moving .

• The solutions to the Einstein Field Equations of General Relativity describe gravity around

• solitary objects like black holes or the sun or the earth, and they facilitate very accurate

• predictions of orbits around these objects . But these equations aren't limited to

• describing just energy and matter and spacetime around the earth or sunthey can be used

• on the universe as a whole to describe and understand the past and present and future

• of the cosmos.

• So.

• General relativity is the idea that the universe can be described by a pseudo-Riemannian manifold

• representing spacetime and an energy-momentum tensor representing all matter and energy,

• which together obey the Einstein Field Equations.

• For our 3+1 dimensional universe, the predictions generated by this idea have been experimentally

• verified by many many incredibly precise observations, ranging from the precession of the orbit of

• Mercury, to the slight drift of the moon's orbit away from the earth, to the gravitational

• lensing and redshift of starlight, to time dilation of atomic clocks and precession of

• gyroscopes orbiting the earth, to observations of the polarization of the cosmic microwave

• background radiation, to gravitational wave detections of black hole mergers, to direct

• imaging of the black hole at the center of the Milky Way.

• Wait but how does general relativity explain the everydayforceof gravity we experience

• on earth?

• Well, you know how when a vehicle turns but your body's inertia makes you want to go

• straight and it feels like you're being pulled sideways because you're being accelerated

• away from your straight-line path?

• In general relativity an object's straight-line inertial path is actually to fall towards

• the center of the earth, and since the surface the earth accelerates us away from that straight-line

• path, we feel that acceleration as a weight or force that we call gravity [equivalence

• principle].

• If you're in free fall (or in orbit), then you are following a “straight path through

• curved spacetime”, that is, you're not accelerating in spacetime, so you feel like

• you're floating or experiencing “0 g”.

• And that explains gravity!

• Oops.

• General relativity doesn't explain quantum mechanical phenomena, and has problems jiving

• with the theory of quantum mechanics in certain extreme situations.

• Physicists have been working for over 90 years to reconcile general relativity and quantum

• mechanics in those situations, and while we've learned a lot, we still haven't come close

• to solving everything!

• It's hard.

• General Relativity works so well in most cases where we can test it, and quantum mechanics

• works so well in most cases where we can test it, and they're both so mathematically sophisticated

• and constrained and distinct from each other, that imagining a different mathematical model

• that encompasses both while being just as accurate where they're already accurate

• and better where they're in conflict isvery high level stuff.

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General Relativity is a physics theory invented by Albert Einstein.

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# General Relativity Explained in 7 Levels of Difficulty

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Summer posted on 2021/02/18
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