What is the difference between Newton's gravity and Einstein's general relativity?

Newton's law of universal gravitation (1687) describes gravity as a force between two masses: F = GMm/r². It works extraordinarily well for most everyday situations — calculating planetary orbits, spacecraft trajectories, and projectile motion. But it treats gravity as an instantaneous action at a distance, which conflicts with special relativity, and it fails for extreme conditions. Einstein's general relativity (1915) reframes gravity entirely: mass and energy curve spacetime itself, and objects follow the straightest possible paths (geodesics) through that curved spacetime. This predicts effects Newton's theory cannot: gravitational time dilation, the bending of light, gravitational waves, and the existence of black holes. For weak gravitational fields, GR gives the same predictions as Newton. Differences become significant near compact objects and at cosmological scales.

Why does gravity bend light?

In Newtonian gravity, light has no mass and should not be affected by gravity. But in general relativity, light follows geodesics — the shortest paths through curved spacetime. Mass curves spacetime, so light travelling near a massive object follows a curved path — it is deflected. Arthur Eddington's 1919 solar eclipse observations confirmed this: stars near the Sun's edge appeared shifted by exactly the amount GR predicted (1.75 arcseconds), twice Newton's prediction. This gravitational lensing is now a powerful tool in astronomy: massive galaxy clusters act as natural telescopes, magnifying and distorting background galaxies. Astronomers use lensing to map dark matter distributions and discover extremely distant objects.

What are gravitational waves and have they been detected?

Gravitational waves are ripples in spacetime caused by accelerating masses — predicted by general relativity in 1916 but considered essentially undetectable for decades. They stretch and compress space as they pass, by an almost unimaginably small amount. On 14 September 2015, LIGO detected the first gravitational wave event — the merger of two black holes 1.3 billion light-years away. The signal (GW150914) caused a stretch in LIGO's 4-km arms of about 1/1000th the diameter of a proton. The discovery earned the Nobel Prize in Physics in 2017. Since then, dozens of events have been detected — black hole mergers, neutron star mergers, and mixed systems. See the gravitational wave dictionary entry for full details.

Why is gravity the weakest of the four fundamental forces?

Gravity is the weakest of the four fundamental forces by an enormous margin — about 10³⁶ times weaker than electromagnetism. A small magnet can pick up a paperclip, overcoming the gravitational pull of the entire Earth. Yet gravity dominates at astronomical scales because: (1) it has infinite range; (2) it is always attractive (unlike electromagnetism, which can cancel); (3) it cannot be shielded. The hierarchy problem — why gravity is so much weaker than the other forces — is one of the deepest unsolved problems in physics. Some hypotheses involve extra spatial dimensions (gravity leaking into them) or the Higgs mechanism. A complete quantum theory of gravity — quantum gravity — does not yet exist, making gravity the only fundamental force without a working quantum description.

How Gravity Works — From Newton to Einstein

From Newton to Einstein

Isaac Newton formulated the law of universal gravitation in 1687: every object in the universe attracts every other object with a force proportional to the product of their masses and inversely proportional to the square of the distance between them — F = GMm/r². This single equation explained the motion of falling apples, the Moon's orbit, and the trajectories of planets. For over two centuries, it was the complete description of gravity.

Albert Einstein's general theory of relativity (1915) superseded Newton's description with a deeper one. Mass and energy do not simply exert a force — they curve spacetime. Objects moving through curved spacetime follow geodesics — the equivalent of straight lines in curved geometry. What we perceive as gravitational attraction is actually objects following the geometry of the spacetime curved by nearby mass. On Earth's surface, the "force" we feel is spacetime curved so strongly we cannot follow its geodesic without the ground pushing up against us.

Key predictions of general relativity

Gravitational time dilation — Clocks run slower in stronger gravitational fields. GPS satellites must correct for this to maintain accuracy.

Bending of light — Light follows curved spacetime; confirmed by Eddington in 1919. Gravitational lensing is now a precision cosmology tool.

Precession of Mercury — Mercury's orbit precesses by 43 arcseconds per century more than Newton predicts; GR explains it exactly.

Black holes — Solutions to Einstein's equations include regions of infinite spacetime curvature. See the event horizon entry.

Gravitational waves — Ripples in spacetime from accelerating masses, first detected by LIGO in 2015. See our gravitational wave entry.

Gravity is the force that governs orbital mechanics — understood in our how orbits work guide — and the large-scale structure of the universe, explored in our dark matter and dark energy guide.

How gravity works

From Newton to Einstein

Key predictions of general relativity

Frequently asked questions