What Is a Tidal Force? Gravity Pulling Harder on One End Than the Other

What matters is not the strength of gravity but how much it varies across an object. Consider the Earth in the moon's gravitational field: the near side is pulled slightly more strongly than the centre and the far side slightly less, so relative to the centre the near side is pulled towards the moon and the far side is pulled away from it, which stretches the whole body along the line joining them and squeezes it perpendicular to that line. That is why there are two tidal bulges rather than one, which is the part people find counterintuitive, and why high tides occur roughly twice a day rather than once. The effect falls off with the cube of distance rather than the square, which means it weakens far faster than gravity itself, and it also means that a nearby small object can produce stronger tidal effects than a distant large one, which is why the moon dominates Earth's tides despite the sun's far greater gravitational pull.

The same mechanism appears in a wide range of circumstances:

• •Ocean tides, modified enormously by the shape of coastlines and basins, which is why tidal range varies from centimetres to over fifteen metres in different places
• •Tidal locking, where the deformation dissipates rotational energy until a body always shows the same face to its partner, which is why we never see the far side of the moon
• •Tidal heating, where continuous flexing of a body in an eccentric orbit generates enough internal friction to melt rock, which makes Io the most volcanically active body in the solar system and maintains liquid oceans beneath the ice of Europa and Enceladus
• •The Roche limit, a distance inside which tidal stretching exceeds the gravity holding a body together, so a moon approaching too closely is torn apart
• •Planetary rings, which occupy the region inside that limit and are generally thought to consist of material that could never coalesce or that was disrupted
• •The gradual recession of the moon, currently about three and a half centimetres a year, as tidal friction transfers rotational energy from the Earth into the moon's orbit and slows the day

The Earth rotates faster than the moon orbits, so the tidal bulge is dragged slightly ahead of the line to the moon by friction, principally in shallow seas. That displaced bulge exerts a small forward pull on the moon, which raises its orbit, and an equal and opposite backward pull on the Earth's rotation, which slows it. The exchange is measurable, with laser ranging to reflectors left on the lunar surface giving the recession rate directly and ancient eclipse records giving the change in day length over millennia. Growth bands in fossil corals and shells record the number of days in a year in the distant past, and they indicate substantially shorter days hundreds of millions of years ago, which is independent confirmation from an entirely different discipline. Extrapolating backwards naively puts the moon impossibly close far too recently, which indicates that the rate has not been constant, and the resolution involves the changing arrangement of continents altering how much tidal friction shallow seas produce.

Near very dense objects the effect becomes destructive. Approaching a black hole, the difference in gravity between head and feet grows until it exceeds the strength of tissue and bone, stretching an object into a thin stream, a process given the informal name spaghettification. Where this happens depends on the black hole's mass, and counterintuitively a supermassive one is gentler at its horizon than a stellar-mass one, because the horizon is much further from the centre and gravity varies less steeply there. Stars that stray too close to a supermassive black hole are torn apart in tidal disruption events, which produce bright flares observed in other galaxies and which are now detected regularly by survey telescopes. Comets have been disrupted by passing close to planets, most spectacularly Shoemaker-Levy 9, which was pulled apart by Jupiter and struck the planet as a train of fragments in 1994, an event observed in detail and among the most instructive planetary impacts ever recorded.

Gravity weakening with distance means the near side of a body is pulled harder than the far side, which stretches it along one axis and squeezes the other, producing two bulges rather than one. The effect falls off with the cube of distance, so the nearby moon beats the distant sun. Flexing heats Io enough to make it volcanic, and friction from the dragged bulge is pushing the moon away and lengthening our day.