general-relativity

What if gravity vanished for one second? The math shows orbits shift by millimeters to meters, and the ground may carry you along. A thought experiment.

A wormhole — a shortcut through spacetime connecting two distant points — would revolutionize physics, transportation, and our place in the universe. But the physics suggests it would also be unstable, possibly lethal, and almost certainly a one-way trip.

Gravitational lensing, the deflection of light by mass, was predicted by general relativity and first measured during the 1919 solar eclipse. Strong lensing produces Einstein rings and multiple images, weak lensing maps dark matter statistically, and microlensing reveals planets through brief brightenings of background stars.

General relativity describes gravity at cosmic scales. Quantum mechanics rules the subatomic world. They are mutually incompatible — and unifying them is the deepest unsolved problem in all of physics.

The event horizon is the point of no return — beyond it, not even light can escape. But what actually happens inside? The physics gets strange fast: time and space swap roles, the singularity may not be what we think, and quantum mechanics breaks everything.

Space and time aren't separate things — they're a single four-dimensional fabric that can bend, stretch, and warp. Massive objects curve spacetime, and that curvature is what we experience as gravity. Here's what Einstein actually discovered.

Einstein predicted them in 1916. We detected them for the first time in 2015. Gravitational waves let us "hear" cosmic events that produce no light. Here's how they work and what they reveal.

Black holes aren't made of "stuff" in the normal sense — they're regions of spacetime where matter has been crushed past every known state. Here's what physics actually says is inside.