Gravity

Weight, free fall, and falling sideways

NASA Space Place’s classroom line: gravity is the pull by which a planet (or any mass) draws other objects toward its center. It is why you land when you jump, why dropped things fall, why the air stays on Earth, and why the Moon stays with us. Anything with mass has gravity; more mass means a stronger pull; the pull gets weaker with distance.

Weight is that pull on your mass, not the mass itself. On the Moon you have the same mass and less weight. In free fall — a dropped apple, an elevator cable cut, the International Space Station — you and the floor accelerate together, so “weightlessness” is falling, not escaping gravity. An orbit is the same idea with sideways speed: you keep missing the ground. Newton’s cannonball thought-experiment is the ancestor of every circular cartoon on this page.

Inverse square, good enough, then not

Newton’s law: two masses attract with a force proportional to the product of the masses and inversely proportional to the square of the distance between centers — F = G m₁ m₂ / r². NASA’s Imagine the Universe classroom notes put G near 6.67 × 10⁻¹¹ N·m²/kg², and surface gravity on Earth near 9.8 m/s². Drop a hammer and a feather in vacuum and they fall together; weight scales with mass, acceleration does not.

That inverse-square picture still flies rockets, sites dams, and predicts most solar-system motion to engineering accuracy. Limits show up when you need high-precision clocks, when light itself is the test mass, near compact objects, or when you ask what gravity is instead of how it pulls. Mercury’s leftover perihelion, gravitational lensing, and GPS are the public reminders that “good enough” is not “complete.”

Spacetime curves; clocks and light notice

Equivalence, in plain words: you cannot tell a closed elevator that is accelerating in empty space from one that is sitting in a gravitational field. Einstein used that to argue that gravity is not a special force layered on flat space — it is the geometry of spacetime responding to energy and mass. Free-fall paths are the straightest lines that geometry allows (geodesics). A planet’s ellipse is that “straightest line” drawn on a curved map.

Light bending. If space is curved, a light ray grazing the Sun is deflected. The 1919 solar-eclipse expeditions made that a public fact; Hubble and later surveys turned the same idea into gravitational lensing — mass as a lens. NASA’s black-hole and Hubble desks treat lensing as observed astronomy, not metaphor.

GPS time dilation is the everyday receipt. NIST’s public clock writing: GPS satellites move fast enough that special relativity would slow their clocks by about 7 microseconds per day relative to the ground, while weaker gravity in medium-Earth orbit would speed them by about 45 microseconds per day — a net +38 μs/day if you did nothing. The constellation is built with that offset. Your blue dot is a relativity instrument, not a Newtonian afterthought.

The map gravity actually runs

At planets and moons, gravity is the scheduler. The solar map (day and local time) is a schematic of that schedule — compressed orbits, not a JPL ephemeris. Stars live and die by gravity versus pressure; a high-mass path can leave a black hole, where escape speed meets the speed of light. Exoplanets are found in part because gravity tugs (radial velocity) or because a star’s gravity and a planet’s silhouette line up (transit). The force the Standard Model omits is the one that draws the sky.

Closer to home, the same pull is why loose, wet sand can fail when shaken — see Bay Area earthquakes and liquefaction. Gravity does not cause the rupture; it is why the ground and the building still have weight after the shear strength is gone.

Why a collider can ignore it

CERN states the luck plainly: on the size of a particle, gravity is so weak next to electromagnetism, the weak force, and the strong force that you can omit it and still have a working collider theory. Two protons in the LHC feel electric repulsion and QCD far more than they feel each other’s weight. Gravity dominates only when matter is in bulk — a body, a planet, a star.

That is why the Standard Model desk can be honest about three forces without pretending the fourth is a missing tile on the grid. The outsider card there points here on purpose.

No observed graviton, no weekend patch

A quantum carrier is sometimes nicknamed the graviton. CERN’s Standard Model page: it has not been found, and fitting gravity into the quantum-field framework has not been done. Gravitational waves (LIGO/Virgo) confirm Einstein’s classical prediction of ripples in spacetime — they are not a detection of individual gravitons. Extra-dimension or microscopic-black-hole searches at the LHC remain speculative; this page will not narrate them as results.

There is no public, experimentally confirmed theory of everything that swallows both general relativity and the Standard Model. Saying so is the literacy. Anything that claims the gap closed last Tuesday belongs in a blog, not on this desk.

Inventory questions, not slogans

Dark matter is inferred because galaxies, lensing, and the cosmic microwave background behave as if most matter does not shine and does not match leftover Standard Model species. Neutrinos have mass but are too light and too fast to be the cold dark matter those maps require. NASA’s dark-matter desk is the honest door — already cited on the Standard Model page.

Dark energy is the name for the dominant term in the present cosmic energy budget, inferred from supernovae, baryon acoustic oscillations, and the CMB — an accelerating expansion whose microphysical cause is not a measured particle. NASA’s dark-energy desk stays labeled unknown. Both are gravity-related accounting problems: we watch how spacetime and motion respond, and the books do not close with atoms alone.