This puzzle is known as the problem of time, and it remains one of the most persistent obstacles to a unified theory of ...

Time feels like the most basic feature of reality. Seconds tick, days pass and everything from planetary motion to human memory seems to unfold along a single, irreversible direction. We are born and ...

Scientists pushed photons across the universe to test the speed of light—and found Einstein’s cosmic speed limit still holds firm.

A newly detected gravitational wave, GW250114, is giving scientists their clearest look yet at a black hole collision—and a powerful way to test Einstein’s theory of gravity. Its clarity allowed ...

General relativity describes gravitation in a geometrical framework generalizing special relativity and classical mechanics. This term includes theoretical studies of gravity, experimental studies of ...

Will two rare supernovas finally tell us how fast the universe is expanding? Perhaps, but we'll have to wait for it for them ...

Even given a set of possible quantum states for our cosmos, it's impossible for us to determine which one of them is correct ...