What is Einstein's central idea in General Relativity about the nature of gravity?

Gravity is a force carried by particles called gravitons that travel between massive objects Gravity is not a force — it is the curvature of spacetime caused by mass and energy, and objects follow the straightest possible paths through this curved spacetime Gravity is caused by the pressure of the aether medium that fills all of space Gravity is a modification of Newton's laws that only becomes significant at high speeds

In 1919, Arthur Eddington led expeditions to observe a total solar eclipse and confirm a key prediction of General Relativity. What did they measure?

They measured the exact distance to the Moon to verify the curvature of space They observed the Sun's corona to detect gravitational waves for the first time The positions of stars near the Sun during the eclipse — the Sun's gravity bent the starlight, making stars appear slightly shifted from their true positions, exactly as GR predicted They measured the speed of light near the Sun and confirmed it slows down in stronger gravity

What is gravitational time dilation, and what everyday technology depends on it?

Clocks run slower near the Earth's poles because the planet bulges at the equator Clocks run slower in stronger gravitational fields — GPS satellites in weaker gravity high above Earth tick faster than clocks on the ground, and this must be corrected for accurate navigation Clocks run faster in stronger gravity because energy is concentrated in those regions Time runs the same everywhere but instruments malfunction near large masses

Mercury's orbit has an unexplained behavior that stumped astronomers for decades before GR. What is it, and how does GR explain it?

Mercury sometimes speeds up unexpectedly when solar flares hit it Mercury's orbit gradually expands because the Sun slowly loses mass by shining Mercury's perihelion (closest point to the Sun) shifts forward by an extra 43 arcseconds per century beyond what Newton predicts — GR explains this because spacetime near the Sun is significantly curved Mercury wobbles perpendicular to its orbital plane in a way that Newton's laws forbid

What is gravitational redshift?

Light appears red near massive objects because the Doppler effect shifts wavelengths when gravity attracts photons Light loses energy climbing out of a gravitational well, shifting to longer (redder) wavelengths — the deeper the gravity well, the more the light is redshifted Gravity slows light down, and slower light appears redder to a stationary observer Dense objects absorb blue light preferentially, leaving only red wavelengths to escape

What is gravitational lensing, and what has it allowed astronomers to do?

Massive objects absorb light from behind them, creating dark patches in observations Massive objects bend light around them like a lens, allowing astronomers to see distant galaxies that would otherwise be hidden behind massive galaxy clusters, and even detect dark matter through its lensing effect Gravity focuses light into a beam directed toward Earth when aligned correctly Massive objects reflect light from behind them, acting as mirrors that reveal the distant universe

What does it mean that the universe is expanding according to General Relativity?

Galaxies are moving through space away from a central explosion point like the Big Bang Spacetime itself is stretching — the distances between galaxies increase not because galaxies are moving through space but because the space between them is growing The universe expands because new matter is constantly being created at the edges The universe appears to expand because light from distant galaxies is absorbed on its journey to us

Einstein's GR is extremely precise, but physicists believe it must eventually break down. Where does it fail?

GR fails whenever speeds exceed 10% of the speed of light — Special Relativity must be used instead GR is completely correct and has no known limits or failures At the centers of black holes and at the Big Bang singularity, where densities become infinite — GR breaks down because quantum mechanics must also apply, but no quantum theory of gravity yet exists GR fails at large scales because dark energy disobeys its field equations

The equivalence principle is the cornerstone of General Relativity. What does it state?

All masses fall at the same rate regardless of their size or composition The force of gravity equals the force needed to keep an object moving in a circle Being in a gravitational field is locally indistinguishable from being in an accelerating rocket — gravity and acceleration are physically equivalent Energy and matter are equivalent and can be converted back and forth

If you could somehow stand on the surface of a neutron star (which you could not survive), you would experience gravity about 100 billion times stronger than Earth's. What prediction of GR would be most dramatically evident?

Length contraction — you would appear compressed to a flat disk to a distant observer Mass-energy equivalence — your body mass would be converted to energy by the intense gravity Gravitational time dilation — a clock on the neutron star surface would tick noticeably slower than one far away; light escaping from the surface would be strongly redshifted The uncertainty principle — the intense gravity would violate quantum mechanical rules for measuring your position