General Relativity: What It Is and How It Explains Gravity

From special relativity to gravity

In 1905 Einstein had published special relativity, the theory linking space and time for observers in relative motion. That theory only holds for an observer far from any source of gravity: it is the limit that E = mc²: What Einstein’s Special Relativity Really Says cannot cross, and it took Einstein another ten years to get past it.

By late 1915, Einstein wrote that he had finally managed to complete the theory.

Gravity as the curvature of spacetime

General relativity hands gravity over to the curvature that massive bodies create in the spacetime around them. A mass placed in a region of space distorts the surrounding geometry, and other bodies move by following that distortion.

Where mass is present, spacetime is curved. Curved spacetime has no straight lines, just as the surface of a sphere has none: bodies follow geodesics, the straightest lines available in that geometry. Gravity, in this picture, does not deflect bodies from a straight path they would otherwise take; it redefines what moving “straight” even means once space itself is curved.

Practical example: picture a bowling ball resting on a stretched rubber sheet: the sheet dips under its weight, and a marble rolled near the ball follows the shape of that dip instead of rolling straight. It is the image NASA’s Gravity Probe B mission uses to make visible how a mass deforms spacetime, though it remains a teaching analogy, not a physical fact in itself. Physicist John Wheeler summed up the mechanism this way: mass shapes the geometry of spacetime, and that geometry in turn guides how mass moves.

Anyone wanting to compare this description with the older picture of gravity can start from Newton’s Law of Universal Gravitation: From Kepler’s Laws to the Formula, where gravity was treated as a force acting at a distance rather than as curved spacetime.

The 1919 test: an eclipse puts the theory on trial

The theory predicted that the Sun would bend the light of stars passing close to it. Testing that required a solar eclipse, the only moment when stars very near the solar disk can be observed without the Sun’s own light hiding them.

According to ESA, that eclipse ranks among the most important in the history of science: it was the moment Einstein’s theory, published only a few years before, moved from paper to the sky. ESA traces this idea about gravity back to 1915, from the same mind that had rewritten the concept of time in special relativity only years earlier. Arthur Eddington, of Cambridge University, was one of the few people who grasped the meaning of the freshly published theory almost on his own, and the measurements he led returned a bending of starlight near the Sun matching what the theory had calculated in advance. The effects of curved spacetime, pushed to their most extreme, also describe objects like Black Holes, What They Are and How They Form: a separate deep dive that starts from the same geometry covered here.

Effects measured today: from GPS to frame-dragging

General relativity has not stayed a chapter of science history. GPS satellites orbit at a different altitude and speed than a clock on the ground, and the combined effect of special and general relativity makes their clocks gain about 38 microseconds a day compared with a clock on Earth’s surface. That sounds tiny, but left uncorrected, the resulting position error would grow by about 10 km a day.

The Gravity Probe B mission, run by NASA together with Stanford University, was designed to detect another effect the theory predicts: spacetime being slightly dragged into rotation by a planet like Earth, a phenomenon called frame-dragging. Anyone interested in following the thread of time, including how orbiting clocks measure it differently from clocks on the ground, finds another angle on the same subject in Time Travel: Is It Possible? What Physics Really Says.