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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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B2 spacetime relativity general relativity general curved gravity

General Relativity Explained in 7 Levels of Difficulty

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    Summer posted on 2021/02/18
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